duced from sorghum, agave, stevia, and other sources of nat-

grain; and increasingly from noncaloric synthetic sweeteners.Su12Fisu(F(ssupopehifroratu

Sugarcane is a perennial tropical grass with highly efcientC4 photosynthesis, high yields, and the ability to providehlotocola

yi-t,stsfn7etse

sugar from beets, and the beet sugar industry itself, originated24arvests for several years without replanting. It is suitable forw-technology hand planting and harvest as well as amenableautomation. Production is heavily concentrated in theuntries of South America and Asia (Table 1). Brazil has thergest area devoted to sugarcane production for both sugar

from this work. His student Karl Achard developed processingmethods for sugar extraction from beet and made the rst se-lections of higher sugar-type beets. The blockade of shipmentsof cane sugar to Europe by the British during the Napoleonicwars stimulated the industrialization of sugar production fromgar production is a global, agroindustrial enterprise, with3 countries producing sugar (43 beet, 71 cane, and 9 both,gure 1). In 2013, approximately 20% of the worlds sugarpply was derived from beets and nearly 80% from sugarcaneAOSTAT, 2013). The most recent estimate for world sugarucrose) production is 160 million metric tons. Sugar con-mption has been growing at roughly the same rate as worldpulation or 2% per year. There are substantial differences inr capita sugar consumption among nations worldwide. It isghest in Europe and lowest in China and Africa. Sucrosem cane and beets is also converted into ethanol and a widenge of consumer products and feedstock chemicals, substi-ting for nonrenewable petroleum.Encyclopedia of Agricult0Sugar Beets

Sugar beet (B. vulgaris spp. vulgaris, L.), a genus of the familAmaranthaceae (formerly Chenopodiaceae), is one of the dverse and useful group of cultivars from the same species thaincludes Swiss chard, fodder beet, and red beet (McGrath2011). The rst modern sugar beets originated as selectionmade in the middle of the eighteenth century from fodder beegrown in then German Silesia, but food and medicinal uses othe genus are much older. A precursor is known to have beeused as food as early as dynastic times in ancient Egypt. In 174a German chemist, Andreas Marggraf, demonstrated that thcrystals formed after a crude extraction from pulverized roowere identical to sugarcane crystals (sucrose). Attempts to derivural sweeteners; from high fructose syrup derived from maize Origin and History of Sugar Cropsof the worlds supply of sucrose, the sugar of commerce.Small amounts of sucrose and alternative sweeteners are pro-

nean to arid regions, irrigation is necessary.Sugar CropsSR Kaffka, University of California, Davis, CA, USADA Grantz, University of California, Riverside, CA, USA

r 2014 Elsevier Inc. All rights reserved.

GlossaryBagasse Fibrous material remaining after crushing andextracting juice from sugarcane stalks.Biomass Dry matter produced by plants.Bolting The formation of seed stalks by sugar beet.C3 Photosynthetic system of most plants of temperateregions.C4 Photosynthetic system of many important tropicalcrop plants, including grasses, such as maize, sorghum, andsugarcane.Evapotranspiration The loss of water from a given area byevaporation from the soil surface and by transpiration fromplants.

Overview

Sucrose, the common sugar of commerce, is synthesized inmost plants as a temporary storage product for photo-synthetically reduced carbon, and it is the principal form ofcarbon transported in plants. Sucrose is typically convertedinto starch for long-term storage, especially in the seeds ofplants, but accumulates to an exceptional degree in un-modied form in sugar beet (Beta vulgaris L.) and sugarcane(Saccharum ofcinarum L.). These two crops produce nearly alland ethanol and dominates world sugarcane production. Acrop of temperate and Mediterranean regions (Figure 1), sugarbeet is a biennial crop grown as an annual. For the most part,it is intensively farmed and is one of the most efcient cropplants. Over time, production technology has come to includea long list of innovations in plant breeding, mechanization,pest management, and fertilizer practice. Sugar beet-producingregions lie north and south of the 30th parallels. It must begrown in rotation with other complimentary crops. Most sugarbeet and sugarcane production is rain fed, but in Mediterra-Mega grams per ha (Mg ha1) An equivalent to metrictons per ha; when multiplied by 0.446 equals short tons(2000 pounds) per acre.Photosynthesis The process by which green plants utilizethe suns energy to produce carbohydrate from carbondioxide and water.Photosynthetically active radiation The spectral range ofsolar radiation from 400 to 700 nm used by plants forphtotosynthesis.Stand The number and distribution of plants afteremergence from seed or vegetative cuttings.Sucrose (C12H22O11) The sugar of world commerce.Ton 2000 lb, when multiplied by 1.102 equals 1 MT (mg).ure and Food Systems, Volume 5 doi:10.1016/B978-0-444-52512-3.00150-9

Figure 1 Sugar production from sugarcane and sugar beet by country. Reproduced from FAOSTAT, 2013. Food and Agriculture Organization ofthe United Nations. Available at: http://faostat.fao.org/default.aspx (accessed 18.02.14).

Table 1 Top 20 countries ranked by sugarcane production during the period 200112. Yields of recoverable sugar are approximately 812% ofthese fresh cane yields

Country Area harvested (ha) Cane production (ton) Cane yield (tons ha1 year1)

Average 200111 Average 200111 Average 200111

Brazil 6 715 370 507 308 916 74.7India 4 417 324 296 284 500 66.9China 1 496 251 98 746 750 65.6Thailand 1 008 002 64 130 700 63.3Pakistan 1 020 691 50 762 858 49.7Mexico 669 208 49 023 541 73.3Colombia 362 281 35 358 300 99.6Australia 407 179 33 548 691 82.3Philippines 389 918 31 078 416 79.9United States of America 377 878 29 764 208 78.5Indonesia 380 629 26 013 750 68.9Argentina 313 960 23 055 833 73.4Cuba 615 858 20 130 000 32.5South Africa 315 416 19 962 458 63.2Guatemala 211 612 19 213 150 91.3Vietnam 287 233 16 147 566 56.4Egypt 135 577 16 098 700 118.7Venezuela 127 957 9 135 769 71.6Peru 69 807 8 518 708 121.9Myanmar 148 384 8 027 825 53.9

Source: Reproduced from FAOSTAT (http://faostat.fao.org/default.aspx).

Sugar Crops 241

the increased availability of newly introduced beverages, in-cluding coffee, cocoa, and, in England especially, tea, all

gration of large numbers of workers, especially from China

pressures and reduce water-use efciency, creating other sea-sonal limits to efcient production. Where successful sugar

242 Sugar Cropstypically consumed with sugar. Sugarcane provided nearly allsugar in world commerce until the commercialization of sugarbeet in the nineteenth century.

Impacts of sugarcane production had substantial impacts onthe global labor force. The development of a global sugarmarket based on sugarcane was closely associated with devel-opment of a global market in slave and later indentured labor.Sugarcane production throughout the Americas was dependenton importation of African slaves beginning in the early sixteenthcentury (James, 2004). By the eighteenth century a majorcomponent of international commerce was the exchange ofslaves for sugar, molasses, and rum. Slavery was abolished overa prolonged period, ending in the British colonies by 1838, inUnited States sugarcane-producing areas in 1863, and in Brazilbeets, especially in France, through a more intensive search forsweeter beets, an innovative plant breeding program, and theconstruction of many crude factories in France and elsewhere inEurope. After the battle of Waterloo (1815) and the lifting ofthe British blockade, the incipient sugar beet industry in Francedeclined for a time, but effectively a new crop had been createdand the efcacy of sugar extraction from the beet had beendemonstrated. The beet industry not only continued to expandin Europe, which remains the center of the industry, but alsodeveloped in other countries in the Middle East and NorthAfrica; Central Asia and Japan; and North and South America.The rst successful commercial factory in the United States wasconstructed by E. H. Dyer Alvarado, in California, in 1879. Soonafter, sugar beet culture and factories expanded in many states.

Sugarcane

In contrast to sugar beet, sugarcane has been used as asweetener in India and China for nearly three millennia. Anearly ancestor of commercial sugarcane spread from Asia intoPapua New Guinea, which became a center of diversity and amajor site of domestication. Selected sugarcanes spreadthroughout the Pacic Basin from around CE 8000, witheventual reintroduction of domesticated clonal materials toSouth and Southeast Asia. Domesticated clones spreadthroughout the Pacic islands with exploration and migration,reaching Hawaii by CE 750 (Barnes, 1974). Sugarcane wasintroduced to Europe from India in the fourth century CE byAlexander the Great and again from the Middle East by re-turning Crusaders in the Middle Ages. It came to the Americaswith Columbus in CE 1493. Successful production was es-tablished in Haiti and the Dominican Republic (formerlyHispaola) in 1506, in Puerto Rico in 1515, and in Mexico by1520 (Barnes, 1974; James, 2004).

Sugar was a luxury in European economies until extensivecultivation of sugarcane began in the Caribbean islands andCentral and South America (James, 2004; Galloway, 1989).This allowed a more than hundred-fold increase in Englishsugar consumption between the mid-1600s and mid-1900s.Sugar use increased along with the Industrial Revolution andthe broad appeal of inexpensive and rapidly prepared caloriesto support urban factory workers who no longer producedtheir own food. Per capita consumption was also fostered bybeet-based industries have developed, diverse adjustments tothese effective physiological limits to crop growth have beenmade. In addition, pest and disease management issues, likethe threat of insect-vectored virus diseases, interact with thecrops agroecological requirements in locally diverse ways toset other practical limits to crop production that are regionallyspecic (Section Crop Management). These factors result in alarge number of different cropping patterns worldwide.

Sugarcane

Sugarcane is a tropical plant with a highly efcient nicotina-mide adenine dinucleotide phosphate-malic enzyme type C4and India. Some of these laborers were eventually repatriated,but most remained as settlers after their period of indenturedservitude. These social and economic forces had substantialimpact on the demographics of sugarcane-producing coun-tries, contributing to large present day communities of African,Indian, and Chinese descent in many current and formersugarcane-producing countries. More recently, labor in sugar-cane enterprises has been on a more conventional wage basis,or by small holders working their own or leased plots.

Production Environments

Sugar Beets

Cultivated sugar beet is a crop of temperate and Mediterraneanregions predominantly and has a C3 photosynthetic system. Itis biennial and when the growing plant undergoes prolongedexposure to cold temperatures (approximately 90 days at57 1C, followed by warmer temperatures and longer days),seed stalk production (bolting) takes place. Wild beet rela-tives (B. vulgaris, spp. maritima) do not require vernalization toower, only increasing day length. Sugar beet seed will ger-minate and emerge at low soil temperatures (45 1C), butemergence is much greater at temperatures greater than 10 1C.Mature plants tolerate modest freezing temperatures, but ex-tended exposure to temperatures below approximately 4 to 5 1C results in cell disruption and death leading to rotting,requiring harvest and storage before severely freezing tem-peratures occur. These limits affect the length of the growingseason of beets in northern latitudes with cold winters. Thefarthest northern production regions with sugar industries arelocated in Finland and Sweden in Europe. In Mediterraneanlocations, like Californias central valley and parts of Turkey,Egypt, and Morocco, sugar beets grow year round, but over-wintered crops must be harvested by late spring to preventvernalization-induced owering from occurring. In arid desertregions with irrigation, like Californias Imperial Valley andparts of the Middle East, extremely hot temperatures increasethe susceptibility of roots to pathogens like root rots and insectby 1888. With the abolition of slavery, and advent of wagelabor, production in many areas declined precipitously.

This situation was addressed in many production systemsby institution of indentured contract labor, fostering immi-

bleaching of leaves in some clones. At both high and lowtemperatures, stomatal control of water loss may be disrupted

du

Sugar Crops 243photosynthetic pathway. This photosynthetic pathway is gen-erally associated with adaptation to high temperature and highlight environments, and to efcient use of water as well aslight, while maintaining high productivity (Sage and Kubien,2007). The optimal environment for cultivation of sugarcanewas described as one having a long warm growing seasonwith adequate rainfall, fairly dry and cool but frost-freeripening and harvesting season, [and] freedom from tropicalstorms (Mangelsdorf, 1950). However, with irrigation thisideal changes to one with an absence of clouds, and diurnaland seasonal gradients in temperature, but still free from frostor hurricanes. These requirements are met primarily in low-land subtropical areas and in the tropical highlands, and thehighest yields per land area are achieved in such environments(Table 2), with high irradiance, irrigation, and a cool dryseason to stimulate sugar accumulation, known as ripening.

A major limitation to the expansion of sugarcane as a sugaror biofuel crop is its natural limitation to the latitudes wherenative Palmaceae (palm trees) are found (Figure 1). This palmzone, approximately 301 N to 301 S, accommodates the lim-ited cold tolerance of commercial sugarcane clones and avoidsthe occurrence of freezing night temperatures (Ming et al.,2001) that absolutely limit the current production region.Visible cold damage is not generally observed above 0 1C(Irvine, 1983), when cold chlorosis may be observed asbleached stripes across the leaf lamina. In Louisiana andFlorida, risk of freezing nights dictates short growing seasonsof approximately 9 months, compared with 12 months intropical and subtropical environments. A large number of

Table 2 Top 10 countries ranked by yield of sugarcane per land area812% of these fresh cane yields

Country Area harvested (ha)

Peru 69 807Egypt 135 577Senegal 7 307Ethiopia 21 775Malawi 22 041United Republic of Tanzania 20 125Zambia 23 875Burkina Faso 4 458Colombia 362 281Chad 3 797

Source: Reproduced from FAOSTAT (http://faostat.fao.org/default.aspx).producing countries experience freezing temperatures duringthe off-season.

Even in the mild, subtropical, marine environment of Ha-waii, sugarcane exhibits a strongly bimodal growth pattern,with substantial reduction in the cool (but not cold) winters.Physiological acclimation to progressively cooling tempera-tures may extend this range. Even moderate chilling may bedeleterious (Moore, 1987). In sugarcane elds in Hawaii, ex-cursions below average nocturnal temperatures of only a fewdegrees Celsius were sufcient to depress stomatal conduct-ance and to substantially inhibit photosynthesis for severaldays (Grantz, 1989). Chilling effects on mesophyll functiondominated these responses, and conductance and photo-synthesis were uncoupled. This was most pronounced insummer when rates were greatest and least acclimated to(Grantz, 2014).Under production conditions, owering of sugarcane is to

be avoided as it reduces growth, consumes sugar, and reducesyield. Most modern commercial clones are not heavilyowering in their adapted production environments. Flower-ing is required for traditional breeding but is often difcult toinduce. Shortening days are required, but not sufcient.Conditions with photoperiods just exceeding 12 h, moderateday and night temperatures, and reduced fertility and watersupply have been considered essential (James, 1980). For ex-ample, in Hawaii, a breeding station on the windward side ofthe island of Oahu provided a far more favorable environmentfor owering than locations at the same latitude on theleeward side.

The Sugar beet Crop

Sugar Beet Breeding and Genetics

In countries with intensive agriculture, hybrid varieties areused for production. Hybrid creation in sugar beets is complexchilling. The effect was also observed in spring, but not inwinter, although rates were lower in winter relative to theirlevels in spring or summer. Extreme heat may also limit pro-duction. Sugarcane trials in the low desert of California, wheresummer temperatures exceed 45 1C, resulted in mid-summer

ring the period 200112. Yields of recoverable sugar are approximately

Cane production (ton) Cane yield (ton ha1 year1)

8 518 708 121.916 098 700 118.7

837 038 114.62 399 600 112.12 370 833 107.52 113 750 104.82 498 333 104.6445 833 100.0

35 358 300 99.6376 916 99.2and is made difcult by both physiological and genetic char-acteristics particularly associated with sugar beets (Bosemark,2006). These have to do with the crops biennial character,self-incompatibility, multigerm seed formation in the Betagenome, and the wide range of environments, pest, and dis-eases affecting the crop in diverse production regions. Creatinga new hybrid is a multiyear process. Sugar beets are self-sterile,but mating between close genetic relatives is possible. Cyto-plasmic male sterility is found commonly in the Beta genome(Owen, 1945) and is used for hybrid production. However, toobtain offspring from male sterile plants that are themselvesmale sterile, maintainer lines (called O-types) must be pro-duced as part of the hybrid development process, adding tothe complexity of hybrid creation and plant breeding in gen-eral. In 1950, Savitsky (1952) laboriously identied a few

plants that formed single embryos (monogerms) which havebecome the basis for the seed industry worldwide outside ofRussia, where separate lines were identied. All commercialhybrid seeds are monogerm.

In both Europe and the United States, sugar beet varietyimprovement and seed production are carried out primarily byprivate companies. However, the US Department of Agri-culture (USDA) developed most of the varieties grown in therst half of the twentieth century in the United States andcurrent variety development often uses genetic lines derivedfrom USDA research (Panella et al., 2013). Both diploid (n9)and triploid hybrids are available, but increasingly, diploidhybrids are being developed due to greater ease and betteradaptability to rapid incorporation of new traits (Bosemark,2006), most recently resistance to a virus disease (Beet necroticyellow vein virus) called rhizomania (vectored by Polymyxabetae) that spread rapidly to all growing regions of the world inthe 1980s (Wisler and Duffus, 2000). Plant biotechnology,such as marker-assisted selection, has been used to assist thedevelopment of new sugar beet hybrids (Panella et al., 2013;McGrath et al., 2007). In North America, herbicide-tolerantsugar beets are now grown commercially, but their adoption inother parts of the world has been delayed by regulatoryrestrictions.

For most of the world, sugar beet seed is produced in

Most countries have a variety of testing programs to ensurethe use of cultivars that are productive and well adapted tolocal conditions. Sugar beet breeding, together with improve-ments in agronomic and pest management practices, hasallowed continuous yield and efciency improvements overtime (especially root yield, (Figure 2; Panella et al., 2013;Jaggard et al., 2012; Zimmerman and Zeddies, 2000). Incountries with less intensive agriculture or where larger num-bers of small-scale producers grow beets using less than opti-mal technology, yields have not risen as much indicating thatthere is substantial room for yield increases if supportingconditions are established.

Sugar Beet Industry Organization

Worldwide, sugar beet production, sugar processing, andmarketing are carefully integrated. In North America, all thecompanies are grower-owned cooperatives. In other regions,either privately held or state-owned companies are found. Ofnecessity, there is a closer and more cooperative relationshipamong growers and companies than is found with many otheragronomic commodities, which results in careful organizationof all aspects of production from area planted, scheduling ofharvest of individual elds, and through sale of the nal

80

(196ult.

244 Sugar Cropsnarrow latitudinal ranges (441461 N) in northern Mediterra-nean areas in Italy and Southern France that have appropriateday length conditions and mild winter temperatures for seedproduction. Similarly in the United States, sugar beet seed isproduced most efciently in the Willamette Valley of Oregonat similar latitude. In all these locations, winter temperaturesare low, but the roots do not freeze, allowing seed producers tomanipulate the plants biennial habit, and day-length con-ditions support selection of types less likely to ower (bolt)during the growing season.

1950 1960 1970 19

Aver

age

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yie

lds

(t ha

1 )

0

20

40

60

80

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120California USA France Germany Spain Iran

Figure 2 Root yield trends in selected sugar beetproducing countriesOrganization of the United Nations. Available at: http://faostat.fao.org/defaYear

1990 2000 2010 2020

12011). Reproduced from FAOSTAT, 2013. Food and Agricultureaspx (accessed 18.02.14).product. Commonly, contracts between sugar processors andgrowers contain quality incentives (see Section Sugar BeetGrowth and Management; Figure 8).

Sugar beet root yields (Figure 2) and sucrose concen-trations vary widely. This variance is most strongly related tothe climate where they are produced, especially the length ofthe growing season, local soil types, and the level of agri-cultural development in each region. A surprisingly diverseset of production, harvest, and processing arrangements ispossible. In temperate sugar beet production regions like

Northern Europe and most of North America, beets are plan-ted as early as possible in spring after the danger of severe frost(usually March to April) and harvested in autumn (starting inSeptember) for as long as soil conditions and the onset ofcontinuously freezing weather allow. During a typical, con-centrated autumn harvest campaign in Europe, factories pro-cess large amounts of raw beets to produce crystallized sugarand varying amounts of thick juice (sucrose syrup) that isstored and can be crystallized subsequently at a slower rateduring the rest of the year. In the Red River Valley Region ofMinnesota and North Dakota in North America, the earlyonset of extremely frigid temperatures shortens harvest of ap-proximately 1 month but allows roots to be frozen in massivepiles in autumn and processed until late spring, often for 200days or more. In Mediterranean or semiarid to arid regionswith milder winters, beets can grow for more than 6 months oreven year round, and longer harvest campaigns are possible. Inwarm locations, roots must be processed shortly after harvestdue to losses of sucrose from root respiration and to patho-gens during storage.

Sugar Beet Growth and Management

A rapidly growing sugar beet crop is capable of high rates ofsucrose accumulation. Dry matter (DM) accumulation andsugar yield are directly proportional to the amount of solar

grows rapidly and may reach 30 cm or more by the time therst true leaf is developed. During the rst 30 days, growth isconned primarily to its leaves and brous roots. After ap-proximately 30 days both top and storage root growth pro-ceeds rapidly, with tops reaching near maximum fresh weightin 6090 days and canopy closure occurring at a leaf areaindex (LAI) of 3 (Milford, 2006). Subsequently, with favorableclimate, top growth remains fairly constant but storage rootscontinue to grow rapidly for another 2040 weeks (for a 10-month crop). As the crop develops, an increasing amount ofDM accumulates in roots. While leaf number and area mayremain relatively constant, in areas with longer growing sea-sons, roots consist of larger amounts of crown materials, sothere is a tendency for impurities to accumulate as well. Theseimpurities reduce sugar recovery from roots in factories(Harvey and Dutton, 1993).

As the storage root increases in size, there is a constanttranslocation of sucrose from the leaves to the root where it isstored primarily in concentric rings of vascular tissues derivedfrom secondary cambium initiated early in the roots devel-opment and in root parenchyma cells that increase in numberand enlarge during growth. Bell et al. (1996) and Milford(2006), summarizing a large number of studies, reported thatDM partitioning in roots is regulated by the cells withinthe root and is independent of the photosynthate supply(Figure 3, Bell et al., 1996; Milford, 2006). On a fresh weightbasis, the sucrose content of the root remains relatively con-

Vascular Par

e

of sf thion

Sugar Crops 245zonePhloem

Free space sucrosconcentration

Figure 3 Longitudinal and transverse views of the secondary structurezones of the cambial rings visible in the root cross section on the right ophysiology. In: Draycott, P. (Ed.), Sugar Beet. Oxford: Blackwell PublicatSons.radiation absorbed by the crop (Jaggard and Qi, 2006). Otherconditions being equal, the longer the growing season, thelarger the yield potential. Under appropriate conditions, theplant develops quickly from seed with the seedling emergingfrom the soil within 510 days after planting undersuitable soil temperature and moisture conditions. The taprootstant, unless suitable external factors cause the concentrationto change. In temperate regions, these usually increase towardharvest but are also reported to decrease in warmer locations(Figure 4).

The largest average sugar beet yields come from Californiaand France (Figure 2). Californias Mediterranean to semiarid

enchymazone

ugar beet root. Sugar concentration is highest in cells in the vasculare gure. Reproduced from Milford, G.F.J., 2006. Plant structure andLtd, pp. 3049 (Chapter 3), with permission from John Wiley and

7st

246 Sugar Crops11

12

13

14

15

16

17

18

19

7 12 17 22 27 32 3

Gro

ss s

ugar

con

tent

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Week of harveclimate allows a long growing season, combined with high-quality soils and irrigation. The worlds apparent commercialrecord yield has come from a eld in the Imperial Valley in2012 and equaled 28.0 Mg ha1 of sucrose from a crop grownover a 300-day period from October to August (177 Mg ha1

of roots at 15.89% sucrose; based on processor data). Averagecrops in this region reach 55% of the biomass and sugar yieldsof record crops. Over the total growing season, the record cropaccumulated an estimated 147 kg total DM ha1 day1

(44.1 Mg total DM per 300 days) and 93.3 kg sucrose ha1

day1. DM and sucrose accumulation is not uniformthroughout the growing season. Initially it is relatively slowand then accelerates (Figure 5). Peak DM and sucrose accu-mulation rates can be double the average reported for thecropping season as a whole and most likely exceed rates of200 kg sucrose ha1 day1.

Figure 4 Root sucrose (gross sugar) concentration with time during harvetemperatures, sucrose concentrations are maintained during the harvest perithe harvest period. Reproduced from Barbanti, L., Zavanella, M., Venturi, G.,to contrast them. In: Proceedings International Institute for Beet Research SuBelgium: International Institute for Beet Research.

Days since start of season80 100 120 140 160 180 200 220 240 260 280

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10

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TotalRootLeaf

January July

Figure 5 Dry matter accumulation (tons ha 1) in a very high-yielding September-planted and July-harvested crop fertilizedwith 250 kg N per ha, from the Imperial Valley of California (Kaffka,2007).Sugar beet rening produces several products in addition tosucrose. These include molasses, dry root pulp, and mono-sodium glutamate, an amino acid salt used to enhance theavor of foods. The sugar beet pulp left after sucrose extractionis used widely in the dairy and beef cattle industries as a feedsupplement due to its highly digestible ber and energy con-tent. The tops of beets can be fed, grazed, or returned to soil asan organic matter and nutrient addition. Beet roots containapproximately 1% N, 0.1% P, and 1.1% K on a DM basis,although this varies (Cariolle and Duvall, 2006). An 80 Mg hacrop removes approximately 200 kg N, 20 kg P, and 200250 kg K ha1, although actual amounts vary with yield andgrowing conditions. Because beets are efcient at accumulatingphotosynthate in a useful form, they are also efcient con-vertors of agricultural inputs, such as water and nitrogen. One

42 47 52

Netherlands

Belgium

N.-C. Spain

South Spain

N.-C. Italy

Morocco

California

st campaigns. In temperate countries or locations with cold autumnod, but in warmer Mediterranean locations, they tend to decline during2007. Losses in sugar content along the harvest campaign and meansmmer Congress, Marrakech, Morocco. pp. 165175. Brussels,of the reasons sugar beet requires relatively low use of fertilizernitrogen is its efciency in recovering residual soil nitrogenfrom previous crops or decomposed organic matter. Beets re-cover more of their N requirements from soils than othercrops. The crop has been shown to require 2550% less fer-tilizer nitrogen than maize (Zea mays L.) (Hills et al., 1983).

Many interacting inuences affect the balance between rootgrowth and canopy growth. Sugar beets can redistribute Nwithin the plant recovered from soils or take up additional Nfrom soils. Redistribution buffers variations in soil N supply.As long as crops have the amount of N needed for optimumsugar yields, they are capable of sustaining fast rates of rootgrowth and sugar production throughout the growing seasonwithout additional supply from the soil or fertilizer. A rapidincrease in root sucrose content is correlated with cool nighttemperatures in the fall of the year coupled with a nitrogendeciency. It has been repeatedly established that sugar beetsrequire only modest levels of N to produce the highest sugaryields and that beets appear to recover more of their requiredN from soil reserves than other crops (Hills et al., 1983). Ulrichand Hills (1990), working in California, established a methodof plant testing to identify surplus or deciency and suggested

a1

+ro

ed fre a

Sugar Crops 247that 1200 mg N per kg DM in mature leaf petioles was a re-liable indicator for sufciency of non-N limiting sugar beetgrowth. Generally, nitrogen fertilization is required forprotable sugar beet production. However, sugar yield issensitive to the absolute amount and the timing of N avail-ability, requiring sufcient amounts early for maximumvegetative growth, and also to a period of N deciency beforeharvest for proper sugar accumulation in the storage roots. Thehighest sugar yields, a function of root yield and sucroseconcentration, usually are achieved with a fertilizer rate lowerthan that which maximizes root yields (Figure 6; Hills et al.,1982; Cariolle and Duvall, 2006). Excess N fertilizer results inlarger total DM accumulation, but lower total gross andextractable sucrose yields, and could lead to losses of N to theenvironment. Milford et al. (1988), Armstrong and Milford

N fertilizer (kg h0 50 100 150

Yiel

d (t h

a1 )

fresh

weig

ht

40

50

60

70

80

90

100Tops

Roots

Figure 6 Root and sugar yields of a typical sugar beet crop. ReproducBulletin 1891. Oakland, CA: University of California, Division of Agricultu(1983), and Hills et al. (1983) all reported that beets requirelower levels of N than many other crops for maximum sugaryields and that sugar beet can serve as a nitrogen-scavengingcrop to prevent possible nitrate pollution of groundwater. Nfertilizer use has tended to decline with increasing yields.Milford (2006) and Milford et al. (1988) suggested that en-vironmental or agronomic factors that affect the size and rateof development of the shoot inuence sucrose accumulationin the root.

When N becomes decient before harvest, leaf initiationand expansion is slowed relative to photosynthesis, andphotosynthetically produced sucrose accumulates in roots asstorage rather than as new vegetative growth. This is illustratedby results from California, where production occurred over adiverse set of climate conditions, allowing comparison ofcrops in different locations at the same time of year. Kaffkaet al. (2001) found that very high sucrose concentrations oc-curred in sugar beet roots harvested in October from a highelevation growing region with a continental climate, wherearidity results in very high levels of photosynthetically activeradiation, but where higher elevations (1200 m) also correl-ated with night time temperatures at or near freezing inautumn. Naturally high organic matter soils provided largeamounts of N mineralized from soil organic N. They suggestedthat leaf initiation and expansion (favored by excess N) wassuppressed by cold temperatures at the end of the growingseason to a greater degree than photosynthesis, favoring su-crose accumulation. Root storage tissues continue to developin sugar beet as long as assimilate is available. Similarly, inCalifornias milder central valley at the same time of year,sucrose concentrations in sugar beet roots typically declined inOctober under conditions with milder average temperatures,as deep-rooted crops recovered residual soil N supplies underconditions of declining water stress (Figure 4). Similar be-havior with respect to beet sucrose concentration in autumn iswidely reported from other temperate (Cariolle and Duvall,2006) and Mediterranean regions with mild temperature

)200 250 300

Perc

ent s

ucro

se

14.0

14.5

15.0

15.5

16.0

16.5

Suga

r yie

ld (t

ha1 )

11

12

13

14

15

16ots

(Sugar yield)

rom Hills, F.J., Sailsbery, R., Ulrich, A., 1982. Sugarbeet Fertilization,nd Natural Resources.conditions in late autumn that encourage additional cropgrowth (Barbanti et al., 2007).

Improvements in plant breeding and seed technology haveled to increasing levels of seed emergence and establishment,with the result that hand labor long associated with sugar beetproduction, especially around stand establishment, has beeneliminated where modern agricultural technology is available.Over time, monogerm seed, improved weed control, improvedplanters, and seed treatments that reduce losses to pathogensand insect pests during the vulnerable period of crop emer-gence and establishment have reduced the need for large seedpopulations and hand thinning of seedlings. Planting to astand and 7080% emergence and establishment have becomecommon in growing areas with advanced agricultural prac-tices. Ideal plant populations are 75 00080 000 plants per haon 5076 cm rows (Jaggard and Qi, 2006).

Most sugar beet production in temperate regions is basedon rainfall and stored soil moisture. Variable weather at timesresults in water stress and yield limitations when rainfall isinadequate or poorly timed with crop demand. In arid andMediterranean areas, careful and timely irrigation is essentialfor economic yields. Furrow or sprinkler irrigation is possible,

and some drip irrigation systems are being used. Irrigationwater requirements in locations where irrigation supplementsgrowing season rainfall are small (100200 mm per year), butin climates where irrigation is necessary, they commonly rangefrom 600 mm of water per ha per season in a cool climatewhere the soil is lled with plentiful winter rain to as much as1200 mm per ha in an arid location with a long growingseason like the Imperial Valley of California (Dunham, 1993;Hills et al., 1990). DM accumulation and sugar yield are alinear function of transpiration, although both are more easilyand commonly correlated with evapotranspiration (ET)(Dunham, 2003). Actual irrigation amounts depend on thedepth of the rooted prole, soil water-holding capacity andavailable water at the start of irrigation, planting and harvestdates, the length of the growing season, and climate during thegrowing season. Diverse estimates of water-use efciency havebeen measured reecting this varying set of inuences.

Water-use efciency for both total DM yield and sugar yieldtends to decrease in warmer climates with higher temperaturesand light intensities, where season-long irrigation is required,compared with locations with supplemental irrigation. Em-pirically determined water-use efciency values for total DMproduction (qet) and for sugar production (set) are summarizedby Dunham (2003) and modied with additional data byLangner (1996). For total DM these ranged from 0.0068 Mgcm1 ha1 in Great Britain for a spring-planted crop to0.0023 Mg cm1 ha1 in Californias arid summer for thesame. For sugar yield, at the same locations, values ranged

1 1

et al., 1999; Hills et al., 1990). When grown on soils that havebeen preirrigated or that have large amounts of available waterin the soil prole, maximum yields can be achieved at levelsless than 100% irrigation (Figure 7), (Morillo-Velarde andOber, 2006; Langner, 1996). In irrigated regions, if beets canbe produced during the winter period, very high levels ofwater-use efciency can be achieved (Table 3).

Sugar beet is a halophytic species that requires Na(Draycott and Christiansen, 2003) and tolerates salinity. It isconsidered one of the most tolerant crops (Maas, 1990) andcan be produced by using low-quality water resources, likesaline tile drainage water, in part, and on salt-affected soils(Kaffka et al., 2005, 1999; Moreno et al., 2001). When usingdrainage or other wastewater, care must be taken to accountfor N present in the irrigation water, because that reduces su-crose yields, even if it does not affect total DM.

Controlling pests and diseases is important for protablecrop production. Sugar beet is slow to establish and is sus-ceptible to weed competition in its early stages. Moderateweed infestation is controlled by crop rotation and a com-bination of chemical and mechanical methods. In the UnitedStates, herbicide-tolerant sugar beet is now being grown but isnot widely used elsewhere due to regulatory restrictions. Sugarbeet is susceptible to preemergence and postemergence seed-ling rots known collectively as damping-off diseases. Otherimportant diseases that must be controlled in areas where theyoccur are as follows: curly top, a virus disease transmitted bythe sugar beet leafhopper; sugar beet yellows, a virus complex

(D

c

lied

unn

y)e)ly)

, Sco

248 Sugar Cropsfrom 0.004 to 0.0013 Mg cm ha . Intermediate values werealso reported from environments with growing conditions lessmild or extreme (Table 3). Beets are deep rooted, with manyreports of soil water depletion to 2 m and some to 3 m (Kaffka

Table 3 Diverse estimates of water-use efciency for total dry matter

Location Year(s) Soil type Estimated ET(or range app(mm)

Spring-planted trials

Suffolk, UK 197984 Sandy loam 450Jena, Germany 1984 Deep Loess 500Utah, USA 1980 Silt loam

Nebraska, USA 1966 Very ne sandyloam

800 Cloudy s

Washington, USA 1976 Silt loam 640730

California, USA 1980 Clay loam 900California, USA 1987 Clay loam 9801140

Fall-planted trials

California, USA 197273 Silty clay 9001200

California, USA 1996 Clay loam 430600 (Ma440870 (Jun5601000 (Ju

Abbreviation: nr, Not recorded.Source: Reproduced from Dunham, R.J., 2003. Water use and irrigation. In: Cooke, D.A.(Chapter 8), pp. 279309.consisting of one or more different aphid-transmitted viruses;powdery mildew (Erysiphe polygoni DC) and Cercospora leaf-spot (Cercospora beticola Sacc.), diseases caused by leaf fungi;rhizomania caused by a virus (beet necrotic yellow vein)

M) and sugar production for irrigated sugar beets.

)Water-use efciency (Mg cm1 ha1) Reference

DM (Qet) Sucrose (Set)

0.0068 0.004 Dunham (1989)0.0061 nr Roth et al. (1988)0.0055 0.0022 Davidoff and Hanks

(1989)y 0.0058 nr Brown and Rosenberg

(1971)0.0027nr 0.0020.0016 Miller and Aarstad

(1976)0.0023 0.0013 Ghariani (1981)0.002 0.00090.0007 Howell et al. (1987)

nr 0.0020.0016 Ehlig and Lemert(1979)

0.0055 0.00220.0017 Langner (1996)0.0037 0.00250.00140.0036 0.00220.0014

tt, R.K. (Eds.), The Sugar Beet Crop, Science into Practice. London: Chapman and Hall

liemerespowersum

Sugar Crops 249transmitted by a soilborne fungus (P. betae Keskin); and sugarbeet cyst nematode (Heterodera schachtii Schmidt) and root-knot nematodes (Meloidogyne sp.). Strategies for the control ofthese diseases involve development of resistant varieties, at-tention to time of planting, isolation of new plantings from

Ratio (App30 40 50 60

Sucr

ose

yield

(mg/h

a)

8

9

10

11

12

13

14

15

Figure 7 Sucrose yield in successive harvests from a fall-planted, sumratio of irrigation to measured crop evapotranspiration. There was little rJuly, decit irrigation resulted in the largest sucrose yields. Sugar beetscapacity to use residual soil N. Reproduced from Langner, P., 1996. ConCalifornia. MS Thesis, University of California, Davis.old sugar beet elds that can serve as sources of virus inoculumin locations with year-round growing climates and practices,the selective use of fungicides, soil fumigation, and carefulattention to crop rotation (Wisler and Duffus, 2000; Whitneyand Duffus, 1986). Varietal resistance, including from trans-genic sources, may become increasingly important (McGrathet al., 2007). But currently there are limits. Foliar diseases, rootrots, and insect predation are reasons why beets have not beenwidely grown in regions with humid, hot temperature con-ditions during the growing season.

Although diverse production conditions result in differingplanting and harvesting practices, sugar extraction and pro-cessing is remarkably similar in most beet factories throughoutthe world. Because sugar factories are expensive industrialfacilities, it is prudent to operate them for as many days aspossible. In northern temperate growing regions, beets areharvested in early to late autumn as day length and particularlytemperature decline. Severe frost damages roots and causesrotting, so freezing weather with temperatures sufcient tofreeze soil marks the end of the harvest period. Consequently,factories are designed to maximize the amount of beets that canbe washed, sliced into cossettes, and diffused each day. Still,many more tons of beets are needed to support a modern sugarfactory than can be processed only during sometimes shortharvest periods in autumn. The problem of extending beetsupplies to factories is solved variably, depending on interactinglocal circumstances. In most of Northern Europe, some sugarbeet roots are stored along eld margins under cover and de-livered in late autumn as factory capacity allows. In the north-ern tier of US states (Idaho, Montana, Wyoming, and Michigan)cold weather beginning in early autumn allows recently har-vested roots to be stored outside in piles with ventilation to

d water : ET(crop))70 80 90 100 110

November to JulyNovember to JuneNovember to May

-harvested trial in California's San Joaquin Valley, compared with thense to irrigation when the crop was harvested in May. In June ande able to recover water from the soil prole in a way similar to theirptive water use of fall planted sugarbeets in the San Joaquin Valley ofmaintain cool but not frozen conditions. In the Red River Valleyregion of Minnesota and North Dakota (USA), exceptionallycold weather beginning early in autumn allows massive piles ofrecently harvested roots to be maintained throughout winter.Frozen beets are then processed during the winter monthsbefore temperatures warm sufciently in the spring to thawthe piles. Processing campaigns can last 200 or more days.

In Mediterranean or warmer regions, beets cannot be storedwithout signicant losses of sucrose to respiration, changes insucrose to other hexose sugars that cannot be used to makecommercial sugar, and losses to pathogens in storage piles.Consequently, daily harvesting and processing is normal. Inmany locations, this limits the length of time that a factory canoperate. For example, in the Imperial Valley of California(mentioned previously), harvest begins in April followingplanting the previous September and October and lasts untillate July or early August, when desert temperatures become toohot to maintain beets in elds without signicant loss to rootrots or other pests and diseases. In less extreme semiarid orMediterranean locations, to allow factories to operate forlonger periods, complementary planting and harvesting dis-tricts can be combined to allow daily harvest over a multi-month period. This occurred in California in the San JoaquinValley, where both summer and winter temperatures allowedcrops to grow year round. In that case, harvest began in Aprilafter winter rains using beets sown the previous May andcontinued until late October or early November, harvesting

250 Sugar Cropsbeets from different regions with planting dates matched toexpected harvest periods.

Creating a uniform, pure product (commercial sugar is99.9% sucrose) from a variable feedstock is a challenging taskon an industrial scale. Typical sugar beet composition is de-picted in Figure 8, although all of these percentages can varylocally and seasonally. Sugar beets vary in quality as a functionof many interacting factors, including agronomic practices,harvest conditions, location, and time of year. The goal of thesugar beet industry, both farmers and sugar manufacturers, isto maximize the recovery of sugar from beet roots. Sugar re-covery is inhibited by the presence of large amounts of Ncompounds, like amino acids and by minerals like K and Nathat are part of the ash content (Harvey and Dutton, 1993).Conversion of sucrose into its constituent sugars (fructose andglucosereferred to as inversion) during the diffusion processreduces commercial sugar yields and may lead to creation of

Dry matter = 25%Water = 75%

Total insolublesolids-5%

Total soluble solids-20%

Sugar-16% Non-sugar, soluble solids-4%

Solublenitrogenousorganiccompounds-1.8%

Soluble N-freeorganiccompounds-1.4%

Solubleminerals-0.8%

Draycott and Christiansen, 2003

Figure 8 Composition of a typical sugar beet root at harvest. Totalinsoluble solids fraction is used as livestock feed (beet pulp). Adaptedfrom Draycott, A.P., Christiansen, D.R., 2003. Nutrients for Sugar BeetProduction. Cambridge: CABI Publishing, pp. 242.other sugars that interfere with crystallization. The larger themolasses fraction, the less sucrose that can be recovered fromthe gross sugar stored in roots at harvest.

There is a long history in the sugar beet industry of trying tocorrelate crop production practices with decreased loss tomolasses. Particular attention has been paid to the relation-ship between N fertility and fertilizer management with im-purities in roots. There are a large number of regression-basedformulas relating sugar recovery to amino N, Na, and K con-stituents in roots at harvest. These have been used to test beetsat harvest and to encourage growers to produce higher qualitybeets through incentive payments. Formulas have varied overtime and by region, with no particular formula proving to beuniversally applicable. The general consensus is that rootquality remains elusively complex and a subject for ongoingresearch and empirical experimentation (Dutton andHiuijbregts, 2006; Draycott and Christiansen, 2003).

On delivery to a factory, beets are rst washed to makethem free of soil and stones. Those processed immediately arethen sliced into cossets and conveyed to large diffusers thatdissolve sucrose and other soluble constituents from the plant.

tration and where plant water decit may be avoided. Thebroad adaptation is reected in wide phenotypic diversity

among Saccharum genotypes. For example, stalk diameterranges from a few mm in some S. spontaneum genotypes tomore than 10 cm in some S. ofcinarum chewing canes. Similardiversity is observed in genome structure (Hogarth, 1987;DHont et al., 1996).

Saccharum spontaneum exhibits much greater diversity thanS. ofcinarum, which does not exist outside of cultivation.Saccharum spontaneum is found in wild populations acrossAsia, North Africa, and the Middle East (Tew and Cobill,2008). This diversity is reected in observed variation in stressresistance (Moore, 1987) and genomic diversity (Zhang et al.,Dissolved sugar and other constituents are then passedthrough a series of steps that result in a puried sucrose syrupand byproducts, primarily molasses, monosodium glutamate,and proteins and amino acids (McGinnis, 1982). Residualber (called Marc or beet pulp) is a valuable livestock feed, notonly for dairy cattle, but also for other livestock species, andeven for human nutrition, and is dried for feeding purposes.Sugar beets have very little lignin, and the ber is composedprimarily of cellulose, hemicellulose, and pectin, in roughlyequal amounts. Molasses may be added back to pulp if desiredto further increase its energy and protein content.

The Sugarcane Crop

Sugarcane Breeding and Genetics

Sugarcane is a member of the Saccharum complex (Sreenivasanet al., 1987). These tall, perennial, and tropical grasses includethe originally domesticated sugarcane, S. ofcinarum; con-generic species, such as Saccharum robustum, Saccharum sinensis,Saccharum barberi, and Saccharum spontaneum; and variousinterspecic hybrids as well as related genera, such as Erian-thus, Ripidium, and Miscanthus. Saccharum ofcinarum appearsto have derived from selection and domestication of S. robus-tum. The early domesticated clones represented selection formulticolored stalks, high sap sucrose concentration, relativelylow ber, large diameter stalks, and limited tillering andowering. The resulting noble canes were those grown inhousehold gardens in Papua New Guinea, were carried byPolynesians to the Hawaiian Islands, and formed the basis ofearly commercial production and crop improvement efforts.

Sugarcane improvement, as with sugar beet and othercrops, is a constant requirement for the continued viability ofthe industry. Modern sugarcane improvement began with se-lection of high yielding S. ofcinarum clones in Java in thenineteenth century CE. These noble canes from the Javanesebreeding collection initially dominated global commercialproduction. These were subsequently crossed with the wildspecies, S. spontaneum, to begin production of the modernhybrid, nobilized cultivars in the early twentieth century. Thenoble canes were supplanted by the resulting interspecichybrids, which exhibited improved disease resistance, broaderenvironmental adaptation, and increased yield.

Wild Saccharum occurs across a surprisingly large range ofrainfall and altitude (Irvine, 1983), frequently along nonsalinewaterways where open vegetation allows high light pene-

production from sugarcane over this period was approximately1.7 billion tons (Table 1). More than 90% was produced in

Sugar Crops 2512012). This diversity has contributed considerably to sugar-cane improvement for the past century or more. Saccharumspontaneum exhibits lower stalk sucrose than the noble canes,higher ber, thinner stalks, and sufciently common oweringand tillering to make it a potentially invasive species. Incommercial canes, sucrose accumulation and robust growth ofthick stalks are derived mostly from S. ofcinarum, whereasgenes for vigor, broad environmental tolerance, high ber, andabundant tillering are derived mostly from S. spontaneum.

The sugarcane genome is a subject of considerable currentinvestigation using modern genetic techniques. The Saccharumspecies are autopolyploids, with copy numbers (ploidy level)ranging from 5x to 16x. The genome of S. ofcinarum is rela-tively uniform among genotypes, being autooctaploid (x10;2n8x80 chromosomes), with only a few possible ex-ceptions (Zhang et al., 2012). In contrast, S. spontaneum ismore variable (x8; 2n36128), although approximatelythree-quarters of genotypes contain some multiple of 8x (Minget al., 2001). Saccharum ofcinarum and S. spontaneum appear tohave diverged approximately 1.52.0 million years ago (Jan-noo et al., 2007).

The genome size differs between species, with ploidy leveland with monoploid chromosome number. The monoploidgenome size of S. ofcinarum is estimated to be approximately985 Mb (million base pairs) and of S. spontaneum approxi-mately 843 Mb, with much greater variation in S. spontaneum(Zhang et al., 2012). The full polyploidy genome within theSaccharum complex ranges from 212 Gb/C (billion base pairsper diploid cell; Zhang et al., 2012). Genome size is a usefulsurrogate for chromosome number (Zhang et al., 2012).

Commercial sugarcane germplasm is largely derived fromcrosses of S. ofcinarum x S. spontaneum, with repeated back-crossing to S. ofcinarum. Current sugarcane clones containapproximately 90% S. ofcinarum and 10% S. spontaneumgermplasm (Ming et al., 2001; DHont et al., 1996). In crossesof female S. ocinarum x male S. spontaneum, an unusualchromosomal transmission is often observed in which thediploid, somatic complement (2n) of the female is retainedalong with the haploid, gametic (1n) complement of the male.This so-called 2n n transmission and the unpredictablepairing of the variable number of homologous chromosomesin such crosses (Ming et al., 2013) result in considerablecomplexity in the genomes of commercial clones. Similar toother vegetatively propagated species, sugarcane is hetero-zygous at most loci, with no existing inbred lines that wouldbe useful for crop improvement.

Autopolyploidy is observed in several important crop spe-cies, including sugarcane and sugar beet (Ming et al., 2013;Zeven, 1979). Despite the common occurrence of this genomicreplication, it presents challenges for modern genetic analysisand the search for useable markers for selection. Identicationof quantitative trait loci (QTL; a genomic marker) for pheno-typic traits of interest is complex, as several alleles may seg-regate in various combinations, and individual genes maycontribute only marginally to phenotypic traits of interest.

A study of QTLs for two independent traits sugar contentand plant height found that multiple copies of favorablealleles, in unlinked regions of the genome, had less thanadditive effects (Ming et al., 2013). This suggests that one copy,among the multiple potential locations in the polyploidthe top 20 producing countries (Table 2), concentrated intropical and subtropical areas. Central and South Americaproduced 53.1% of global output with an additional 36.9%from Asia and 5.3% from Africa. The remainder was distrib-uted with 2% or less from the Caribbean region, Oceania(including Australia), North America, and Europe (0.0003%).

The largest single producer during this period was Brazil(Table 2), with more than 500 million tons of cane pro-duction, reecting vigorous government investment in an in-tegrated sugar and biofuel economy based on sugarcane. Indiaand China produced approximately 300 million and 100million tons, respectively. Total production is closely related toarea harvested, although differences in yield per land area areobserved.

Average yields were largest in Central and South Americaand Oceania (Table 1). Of the top 20 producers (Table 2),only three have yields at or above 100 tons ha1 year1 Peru(122 tons ha1 year1), Egypt (119 tons ha1 year1), andColombia (100 tons ha1 year1). Only 10 countries achievedlong-term average yields of 99 tons ha1 or above, mostlycharacterized by prolonged dry periods and long growingseasons (Waclawovsky et al., 2010). Many major producers,such as Australia (82 tons ha1 year1), the United States(78 tons ha1 year1), and Brazil (75 tons ha1 year1), ex-hibited lower average yields. Production areas, such as thegenome, may be sufcient to provide physiologically relevantprotein synthesis. Additional copies may potentially be ma-nipulated in pursuit of other desirable traits with minimalnegative consequences. This may simplify development oftransgenic lines, as insertion of one copy of a novel allele maybe sufcient to drive improved phenotype, despite the pres-ence of other loci with less favorable alleles. However, mul-tiple copies of important genes may confer environmentalstability (Ming et al., 2013) as observed in other species. Inhighly selected commercial clones, favorable alleles (Lam et al.,2009) have been xed at several loci, so that identication ofQTLs is more difcult than in less selected genotypes.

The recently sequenced genome of Sorghum bicolor(Paterson et al., 2009) has proven useful as a template tounderstand the sugarcane genome (Ming et al., 1998, 2013;Dufour et al., 1997). The sugarcane monoploid size is similarto that of the haploid Sorghum genome (Zhang et al., 2012).Sugarcane diverged from Sorghum less than 10 million yearsago (Jannoo et al., 2007; Lam et al., 2009). QTLs in anS. spontaneum x S. ofcinarum segregating population, for bothhigh and low sugar content (Ming et al., 2013), all mapped toone of eight distinct regions of the Sorghum genome. Loci ofinterest in the larger sugarcane genome may reect a smallnumber of ancestral genes, simplifying manipulation duringcrop improvement.

Sugarcane Industry Organization

Sugarcane production took place in nearly one hundredcountries (FAOSTAT, 2013) during 200112. Another 10countries, mostly small producers, ceased commercial pro-duction during or before this period. Annual global sugar

252 Sugar Crops1970

35 000 00030 000 00025 000 000

15 000 00020 000 000

10 000 0005 000 000

500 000

400 000

300 000

200 000

1980 19900northern coastal deserts of Peru and the inland desert valleysof Southern California, arid and high irradiance environments,may provide substantial opportunities for very high yields andefcient use of limiting resources, such as land, water, andnitrogen.

Sugarcane production in the United States increased overthe past 50 years (Figure 9) by 12%, although land area de-voted to sugarcane increased by 28.7% (19722012). By 2011,the US industry had 4547% of planted area in Florida andLouisiana, cool short season production environments, and5.6% in southern Texas. Sugar production was 51% in Floridaand only 38% in Louisiana and 5.6% in Texas. During thisperiod the exceptionally productive Hawaiian industry de-clined by 87.3%. By 2011, Hawaii had 1.9% of US sugarcaneplantings but 4.5% of US production. Hawaiian yields peakedat 220.0 tons ha1 year1 shortly before the collapse of theindustry on all but one of the sugar-producing islands. By2013, only one sugar enterprise remained, on the island ofMaui. These observations suggest that agronomic factors are

100 000

0

100

80

60

40

20

0

1970

1970 1980 1990 200

1980 1990 20

Figure 9 Fifty year trends in sugarcane production in the United States. Da(www.nass.usda.gov).Series1

Series1

2000 2010

Cane produced (tons)

Area harvested (ha)

2020not always decisive in determining the viability of a pro-duction system.

The sugarcane industry exists in a diverse matrix of coun-tries, cultures, and economic systems. The organization ofproduction reects this diversity. In some areas, mills andproduction elds are owned by a single entity, commonlycalled a plantation system. This may represent large privateenterprises or cooperative associations. A signicant benet ofsuch organization is that improvements in both eld oper-ations (yield, production costs, sugar content) and mill oper-ations (prevention of cane deterioration, recovery percentage,factory costs, and marketing) accrue to the benet of bothgrower and rener, and there are inherent incentives for im-provement and investment in both operations. However, inmuch of the world, the industry is based on a large number ofindependent producers providing cane to mills that are oper-ated by an unrelated entity. The result is an unavoidableconict of interest in which incentives to increase cane qualityand sugar recovery may not accrue to both parties, and

0 2010 2020

Series1

00 2010 2020

Cane yield (ton ha1 year1)

ta obtained from the USDA National Agricultural Statistics Service

delivered by mechanized harvesters from nearby elds that arededicated to seed production. This set aside from production isa signicant cost of sugarcane establishment.

Stalk segments are planted in furrows that may be sub-sequently lled to result in at planting. This may providemore even water availability and is suitable for many mech-anized operations. However, erosion due to water runoff andpressure from some soilborne pests may be exacerbated, andcrop waterlogging in poorly drained elds may be increased.For these reasons, i.e., to channel runoff, to elevate cane abovemoisture and moisture-attracted pests, and to raise soil tem-peratures, cane is often planted in ridges separated by well-maintained furrows. Irrigation may then be provided throughthe furrows if available and as required. If moisture availabilityon the ridges is inadequate, the cane may be planted in thefurrows and ridges later constructed along the plant line, al-though this disruptive activity may slow growth.

Row spacing reects a balance between increased costs toproduced seed cane (planting stalks) and the time required forthe crop canopy to fully cover the ground for maximal inter-ception of sunlight. Productivity is closely related to seasonalintercepted solar radiation. In practice, row separation is usu-ally between 1 and 2 m, with narrower spacing under poorer

Sugar Crops 253Sugarcane Growth and Management

Sugarcane production methods depend on soil type, rainfall,drainage, and availability of labor and mechanized equip-ment. Adapted genotypes exhibit high productivity, whereasthe perennial life cycle allows economically efcient pro-duction, with one or more ratoon (stubble) crops. Replantingfrom vegetative stalk segments is required only after severalgenerations when yields decline due to soil compaction, dis-ease or pest pressure, reduced plant population, or cumulativeharvest damage.

Ratoon production varies with environment, cultivar, har-vesting techniques, and management, ranging from no ratooncrop to four or more. From two to ve appears to be a com-mon range among current sugar production areas. Sugar yieldeventually declines, necessitating a fallow period and re-planting with vegetative stalk pieces. The length of the ratoonperiod is ultimately an economic decision, balancing re-planting costs with foregone sugar yields but may be skewedby considerations of labor supply, availability of new andimproved cultivars, and international sugar market conditions(Ellis and Merry, 2004).

Planting costs are substantial in sugarcane, due to its vege-tative mode of reproduction. Before replanting, the seed bed isusually worked to remove the previous crop and any weeds andto optimize soil tilth. Sugarcane is planted from vegetative stalksegments. These may be long sections with only the tops re-moved, or they may be shorter sections, with one to three budsper section. Both types of planting material may be hand cut ordistortions of industry expansion and investment are common(Todd et al., 2004).

These conicts have been addressed through a variety ofpayment schemes (Todd et al., 2004), each with advantagesand disadvantages with respect to the economic viability of thenational sugarcane industry. Payments for cane delivery maybe based on the average quality of cane delivered to the mill,which removes individual growers incentives to improvequality. They may be based on the quality of an individualgrowers cane, which incentivizes the individual. This results incompetition, either between the growers for a xed share ofrevenues in xed revenue-sharing systems, or between indi-vidual growers and with the mill operator for a share of totalrevenues in variable revenue-sharing systems. This latter op-tion may spread risk and reward most equitably but is com-plex to administer and requires quality testing at multiplepoints in the production pathway.

An example of a management decision that may dependwholly on industry organization and payment scheme is lengthof the harvest and milling season. A longer season makes mostefcient use of high cost-xed equipment at the mill but in-evitably reduces cane quality and sugar recovery by extendingthe harvest season to nonoptimal periods. Depending ongrower incentives and ownership of elds and mills, this im-proves protability for all participants, or only for the millowners. In some production areas, a blended system providesindividual incentives that are calculated on the basis the averagequality of cane delivered during each phase of the harvest (Toddet al., 2004), rather than over the entire harvest season.growing conditions or where erosion is a threat (e.g., hillsides)and wider spacing where rapid growth and canopy closure isexpected. In some cases, available equipment dictates rowconguration.

In irrigated and highly favorable production environments,cane is harvested at approximately 12 months of age, althoughthis may vary from 9 months in temperate climates withunsuitable winter temperatures to 2436 months in cooler butnonchilling environments or dryland situations with water-limited growth. Sugarcane exhibits a sigmoidal growth curve,as observed in sugar beet and other crops (Figure 10). Thesheer magnitude of the developing canopy has led to this ex-ponential middle portion of the growth curve to be called theperiod of Grand Growth.

300

140

120

100

80

60

40

20

00 100 200 400 500 600

Figure 10 Biomass production in sugarcane during regrowthfollowing harvest. This model from Simoes et al. (2005) describes the4th and 5th rattoon of an early variety with vigorous regrowth in Brazil.Vertical axis is above-ground biomass (tons ha 1); horizontal axisis days after cutting. The curve is generated from the equation givenby Simoes et al. out to 480 days. BM120 exp( (4.6981))exp( (0.010098))Days.

254 Sugar CropsIn many production areas, sugarcane is harvested by hand,with cane knives or machetes and carried in bundles of 1020 kg by individuals to collection points at the side of theeld. Increasingly, harvest is mechanized, using machines thattrim the tops and then sever the stalk near the base, leavingneatly aligned piles of intact stalks laid across the row. Thistype of whole-stalk or soldier harvester imposes fewer cutsurfaces and reduces risks of quality deterioration but requiresburning in the piles and some additional logistical challenges.It is used to prepare seed cane for whole-stalk planting in someareas.

The alternative is a billet-cutting, or combine, harvester. Thisapproach does not require burning of the trash, which is blownoff during harvest by fans on the harvester. However, efciencyis improved by burning in the eld. The cane is cut at the top toremove green portions, severed near the ground as above, andthen chopped into convenient lengths before ejection into areceiving wagon pulled alongside. A highly mechanized systemwas in use throughout Hawaii until the recent decline of thatindustry. Fields were burned in place, then push-raked withlarge tracked vehicles into piles, which were loaded by ineldcranes into trucks that were driven on private cane haul roads tothe factory. Productivity and lodging in this system exceeded thecapacity of available harvest equipment.

Burning of the cane requires that irrigation be terminatedin a timely fashion and that rainfall does not occur during thecritical period. The primary benet of burning is to removeleafy eld trash, both reducing the volume and weight ofmaterial to be hauled to the mill and increasing the efciencyof mill operations. Ineld burning has the additional benetof driving off or destroying pests and potentially dangerouswildlife, before entry of harvest crews. Disadvantages are therequirement for suitable weather and increasing concerns byneighboring populations regarding the hazards and nuisanceof smoke inhalation.

Harvest method and the amount of soil and leafy trashcarried to the mill have a large inuence on sugar recovery.Growth conditions and management, particularly harvest date,may result in a wide range of juice content and soluble solids(Brix, %). High-quality sugarcane stalks typically have a Brix ofapproximately 18%, of which approximately 90% is sucrose.Additional factors within the mill associated with crushing andextraction, removal of impurities, and crystallization also havea large effect on sugar recovery. In general approximately 10%of fresh cane weight is recovered as sugar.

Sugarcane water requirements are substantial and highlydependent on environment and crop growth stage. Sugarcaneis traditionally grown in the humid tropics, where ET is re-duced by high relative humidity and where water is availablefor extended periods of the growing season. As cultivation hasmoved to higher radiation but drier environments, the risk ofcrop water decit increased, and in many areas supplementalor full irrigation has been applied. Although sugarcane isconsidered a heavy user of water, the water requirements de-pend on the environment. It is most useful to consider waterrequirements relative to alternative crop choices. For example,in the hot, inland Imperial Valley of California, water re-quirements calculated from the crop coefcients provided bythe UN FAO Irrigation and Drainage Paper No. 56 (Allen et al.,1998) indicated that approximately 1.8 m of water wasrequired, about the same as the existing dominant crop in thearea, alfalfa (Medicago).

Sugarcane is sensitive to soil water depletion, with shootextension growth declining as available soil moisture declinesby less than 10%. Stomatal closure is slightly less sensitive,beginning to decline approximately 15% depletion (Nableet al., 1999). In parallel experiments in large pots, the moredrought tolerant, but closely related plant, Sorghum main-tained both shoot growth and stomatal conductance untilapproximately 50% depletion.

Successful cultivation of sugarcane has been achieved withmany types of irrigation systems, from furrow to sprinkler anddrip. Under some conditions, ood irrigation of at-planted canemay be appropriate. With the large inputs of water required bysugarcane, sufcient attention must be paid to drainage. Thismust provide for an unsaturated root zone to 60 cm or more(Ellis and Merry, 2004). Drainage must not lead to excessivelosses of irrigation water, particularly as may occur with furrowsystems in sandy elds, but must allow enough drainage throughthe prole to avoid buildup of salts. Available commercialsugarcane clones are not tolerant of saline conditions. Underconditions of poorly drained soils with at gradients, it may benecessary to provide drainage aids. These may be closely spaceddrainage ditches or even buried drain tile, which carry percolatedsoil solution to nearby ditches and away from the eld.

In many production areas, runoff from sugarcane elds,with its burden of nitrates, phosphates, dissolved carbon, andpesticides, is an environmental threat to adjacent ecosystemsand is highly regulated. These relatively recent pressures, aswell as the long-standing imperative to control soil erosionlosses, make control of water inputs and drainage an import-ant component of management.

Fertilizer requirements depend on the environment, pre-vious crop, and residual soil fertility. In heavier, nonpeat soils,substantial N fertilization is required for sustained yields. As inmost cropping systems, nitrogen is the nutrient that is mostlikely to become limiting in sugarcane production (Irvine,2004). In general, approximately 100 kg N ha1 is required forthe plant crop and approximately 150 kg ha1 for each suc-cessive ratoon crop. Nitrogen applications must be accom-panied by sufcient irrigation or rainfall to facilitate uptake.Because of the potential for losses due to deep drainage,runoff, and volatilization, application of nitrogen is typicallydelayed until rapid crop growth has begun and immediateuptake and utilization are most likely to occur. In productionfor sugar, nitrogen is applied early and the plant is allowed todeplete soil reserves. This along with dry, cool weather leads toripening, or accumulation of sugar rather than ber in thestalk. For bioenergy, later application of nitrogen will sustainthe maximum period of biomass accumulation.

Sugarcane is understood to be less N efcient than manycrop species, suggesting that alternative practices might beconsidered. There is recent evidence that Saccharum may differfrom even closely related cereal crops, in preferentially util-izing ammonium rather than nitrate from the soil N pool(Robinson et al., 2011). As nitrate fertilizers are labile in theenvironment, increased use of other, particularly more re-duced, forms of N may reduce the environmental impact ofsugarcane and energy cane cultivation and reduce the netgreenhouse gas balance of associated biofuels.

usually with the intent to raise soil pH, as levels in most soilsare sufcient. Addition of lime may increase the availability of

Sugar Crops 255phosphorous and reduce the availability of aluminum. Po-tassium is not often decient, and oversupply of this elementwill carry over into the mill, passing through clarication toaccumulate in the molasses where it impedes crystallization ofsugar and reduces recovery (Irvine, 2004).

Pest management in sugarcane reects the fact that sugar-cane is typically grown in a monoculture. This accentuates therisk of epiphytotics, although sugarcane is not an extremelyvulnerable crop. On a global basis, sugarcane is susceptible toa number of bacterial diseases, leading to stunted growth,occluded xylem, and foliar symptoms and a variety of fungaldiseases from rusts, mildews, and smuts to rots that consumethe sugar storage tissues in the stalk. Effective production re-quires vigilance, as unexpected diseases may appear suddenly.Although seed pieces (setts) may be treated with fungicide, theprincipal defense against known diseases is through geneticresistance acquired in sugarcane-breeding programs.

In some environments, insect pressure may be severe andmay require treatment. Sugarcane is subject to attack by stalk-boring insects, and buried seed pieces are vulnerable to a widevariety of pests that may destroy the viable buds. Nematodesmay be a problem in sandy soils. Leaf feeders are typically nota major challenge to sugarcane culture except that some, suchas leafhoppers, may vector serious viral diseases. Whereavailable, the use of biological control has often proveneffective. As with pathogens, the most effective defense isgenetic resistance derived in the breeding programs.

Weeds are a serious challenge in sugarcane culture becauseof the relatively slow canopy closure. This provides weeds aprolonged period of full sunlight and minimal competitionfrom the developing crop. In particular, a wide variety ofgrasses are difcult to remove once the cane crop becomesestablished. If weeds can be suppressed in the early season,eventually the dense sugarcane canopy, with LAI up to seven,will provide effective suppression of most weeds. Tillage andpreemergent herbicides are the most successful controlstrategies.

Sugarcane is grown in a large number of countries under avery wide range of conditions. Although the ancestral chewingcanes were strictly tropical, with introgression of highly diversewild relatives, sugarcane has now spread, often under irri-gation, to environments characterized by cool winters and veryhot summers. Sugarcane remains a dominant source of naturalsweetener in global markets.

Future Prospects for Sugar Crops

Sugar Beet

With the use of sugar beets, the yield of sugar achieved in mostof the principal beet-producing countries has increasedUnder some conditions, deciencies of potassium, calcium,and phosphorous will have to be addressed, based on soil test.Other nutrients are essential but at much lower levels and arenot commonly limiting, including magnesium, sulfur, iron,boron, manganese, zinc, copper, nickel, molybdenum, andchloride. Calcium is typically applied as lime before planting,substantially over the last several decades. Most of this yieldprogress is due to increasing root yields (Figure 2), althoughsugar content in beets has increased less or been largely stable(Jaggard et al., 2012). Yield increases can be attributed toseveral interacting factors, including improvements in plantbreeding techniques and information technology, improvedseed quality and agronomic practices, and planting and har-vesting machinery. Where adopted, all these practices have ledto improved overall efciency in the sugar beet industry, fromcrop production to factory operations (Parkin and De Bruijn,2010). Increasing levels of atmospheric CO2 have also con-tributed to increasing yields over time. Weigel and Man-derschied (2012), Jaggard et al. (2007), Demmers-Derks et al.(1998), and others have correlated signicant increases in rootyield and total biomass under enriched CO2 conditions whenfertilization levels were sufcient to support increased cropgrowth. Part of the effect was attributed to warmer summertemperatures (Jaggard et al., 2007). Crop nitrogen use ef-ciency was also improved in many industrialized productionareas (Figure 11). Increasing levels of atmospheric CO2 shouldsupport slowly increasing yield trends if other agronomicfactors are adjusted appropriately for higher yield expectations.Donatelli et al. (2003) in a modeling study suggested that inregions where temperatures increase due to climate change,irrigation may be needed to sustain yield increases. Similarly,Jones et al. (2003) predicted signicant yield increases but alsoincreased yield variance due to more frequent occurrences ofdrought in parts of Europe.

Comparisons of yields and yield trends among developedand developing regions indicate that there is still substantialtechnical opportunity for improvement in beet crop yields inmany areas of the world. For example, yields in California aretwo or more times greater than those in Egypt and Iran, withareas with similar climates. In recent years, there has been asignicant investment made in molecular genetics and the useof transgenic traits for herbicide tolerance and pest and diseaseresistance has taken place. Currently, herbicide-tolerant sugarbeets are grown in the United States and Canada but have notbeen accepted for use in other countries due to differing per-ceptions of risk and regulatory cultures. Nonetheless, the po-tential value of molecular breeding to improve cropproductivity, reduce pesticide use, and increase overall ef-ciency is widely recognized within the industry and is expectedto eventually become part of hybrid development programs(Lathouwers et al., 2005).

Part of the value of molecular breeding techniques is theirpotential to improve environmental performance in sugar beetproduction systems, especially where intensive agriculturalpractices are carried out (McGrath et al., 2007). Mrlnder et al.(2003) identied several additional new technologies for im-proving the environmental performance of beet production,including conservation tillage, and integrated productionstrategies to reduce inputs and use of pesticides and fertilizersper unit yield and off-farm emissions. Kaffka et al. (2005,2001, 1999) had demonstrated the use of precision agri-cultural techniques and the capacity of sugar beets to use low-quality water for irrigation to improve salinity management insalt-affected areas and to recover nutrients where groundwaterand surface water are affected by eutrophication. In general,there has been an improvement in the efciency of sugar beet

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256 Sugar Cropsproduction over time. The labor invested in producing a hec-tare of sugar beets has declined, whereas yields have increased.

Both sugarcane and sugar beet provide a readily ferment-able source of carbohydrates for conversion to ethanol, otherhigher alcohols, and biochemical feedstocks. The efciencyattributed to sugarcane as a biofuel source is due in part to theuse of bagasse for the production of electricity needed topower the sugar or ethanol renery. In contrast, sugar beetresidues are commonly fed to livestock because they are ahighly digestible feed for ruminant livestock. Because the berportion of beet roots is digestible and low in lignin, they can

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Figure 11 Yield and N fertilizer-use trends from France. Yields have inccommon throughout the sugar beet industry in regions with developed,2006. Nutrition nitrogen. In: Draycott, P. (Ed.), Sugar Beet. Oxford: BlackWiley and Sons.ugar

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14also be converted to C6 and C5 sugars using enzymes (Santek,et al., 2010). This increases the overall sugar yield from beetsavailable for fermentation to alcohols and makes beets apromising bioenergy feedstock (Panella and Kaffka, 2011),which compares favorably to both sugarcane and strictly cel-lulosic feedstocks (Figure 12). In Europe, some of the sugarproduced at reneries in excess of sugar market requirements isconverted to ethanol (Klenk and Kunz, 2008). Beet roots alsodigest readily when used in anaerobic digesters and are usedfor this purpose on a wide scale in Germany and elsewhere inEurope (Demirel and Scherer, 2008).

Sugarcane

Well-adapted sugarcane clones are considerably closer thanmost crop species, including those with the same photo-synthetic pathway, to achieving theoretical maximal yields ofapproximately 6% of the energy available in sunlight (Zhuet al., 2008). The maximum reported efciency in the eld isonly approximately 4.3%. Maximum theoretical potentialproductivity of sugarcane is approximately 280 tons ha1

year1, and with reduction for radiation striking bare groundduring incomplete canopy closure, approximately 220 tonsha1 year1 of above-ground biomass. With a harvest index ofapproximately 0.8 (leaving only roots, stubble, and some leafytrash in the eld), potential above-ground dry biomass yield is177 tons ha1 year1, equivalent to 360380 tons ha1 year1

of fresh millable stalk material (Waclawovsky et al., 2010).This is substantial relative to other potential crops.

Yields of sugarcane have increased steadily in many pro-duction areas, by approximately 1% year1, reaching ap-proximately 150 tons ha1 year1 of above-ground drybiomass under optimal experimental conditions in irrigated,high irradiance conditions in Brazil (Waclawovsky et al.,

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ed, whereas N fertilizer use has declined. This phenomenon issive agricultural systems. Reproduced from Cariolle, M., Duvall, R.,l Publication Ltd, pp. 169184 (Chapter 8), with permission from John2010). Record commercial yields were lower, approximately120 tons ha1 year1, and typical grower yields yet lower(Table 1), globally averaging approximately 35 tons ha1

year1 dry biomass or 72 tons ha1 year1 of millable caneover the rst decade of the current millenium. In sugarcane, asin other crops, closing the gap between record and typicalyields is an important goal for sugarcane production both forsugar and for bioenergy.

As producers around the world seek to expand productionof sugarcane for sugar and bioethanol by expanding pro-duction into marginal areas, efforts will intensify to developtolerance to drought, heat, and chilling. Efforts in Brazil toextend production to the south (higher latitudes), and in theUnited States to expand production northward and into thewestern inland valleys, have stimulated considerable efforts toenhance chilling tolerance in high-yielding clones. Geneticimprovement of tolerance to heat and chilling appears feasibleby selection among current germplasm, by further crossingwith related wild species, particularly S. spontaneum, whichexhibits considerable tolerance to abiotic stresses (Moore,1987), and with the closely related genus, Miscanthus.

Sugarcane may represent a bridge crop for biofuel pro-duction from other sources. Current commercial clones pro-vide sugar in high yields for direct fermentation. These clones

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Selection for high sugar has potentially created a geneticbottleneck that has narrowed the genetic resources from whichhigh biomass and low- or high-lignin clones might be selected.This may have reduced maximum potential productivity incurrent commercial germplasm. This may be relieved as se-lection for total biomass in pursuit of efcient lignocellulosicbioenergy feedstocks is undertaken. Higher ber clones thatstill contain relatively high sugar have been termed Type IEnergy Canes (Tew and Cobill, 2008). These were advocatedsome time ago by Alexander (1985). Selection within currentbreeding programs will most likely identify improved Type I

near-term technology. Light blue, current or simple technology, mid-blue (neavailable-the theoretical conversion limit). Reproduced from Kaffka, S.R., Zhamodeling support biofuel production from beets in California. Proceedings o(accessed 29.04.14). lignocellulose sugar/starchretical ethanol yield

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ts can be produced with high yields and efciency using current orcultivars, with somewhat higher ber and lower sugarthan conventional selections within these programs, but im-proved total biomass production Tew and Cobill, 2008).These may be a stop-gap solution toward maximum biofuelproduction.

In the long run, dedicated clones are more likely to out-perform these compromise candidates in terms of biomassproduction, stress tolerance, and yields of lignocellulosic bio-fuel. These Type II clones are more likely to require separatecrossing and selection programs. The emphasis will be onbiomass production with stress tolerance. There appears to beconsiderable potential for enhanced productivity. Lignin andcellulose are coregulated at the level of gene expression(Ragauskas et al., 2006) with studies in other species sug-gesting that repressing lignin may increase cellulose synthesisand digestibility. Further, stress tol