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Running head: Reduced root branching improves drought tolerance 1
Jonathan Lynch 2
Department of Plant Science, The Pennsylvania State University, University Park, PA, 16802 3
USA 4
Telephone 814-863-2256 5
email: [email protected] 6
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Plant Physiology Preview. Published on June 15, 2015, as DOI:10.1104/pp.15.00187
Copyright 2015 by the American Society of Plant Biologists
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Reduced lateral root branching density improves drought tolerance in maize 8
Ai Zhan1,2, Hannah Schneider2, Jonathan P. Lynch2* 9
1State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest 10
A&F University, Yangling, Shaanxi, 712100, China 11
2Department of Plant Science, The Pennsylvania State University, University Park, PA, 16802 12
USA 13
Summary: Maize genotypes with reduced lateral root branching density have superior water 14
capture, growth, and yield under drought. 15
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Financial sources: This research was supported by the Howard G. Buffett Foundation and 17
the Agriculture and Food Research Initiative of the USDA National Institute of Food and 18
Agriculture competitive grant number 2014-67013-2157 to JPL. 19
* Corresponding author. Email: [email protected], Tel.: +1 8148632256 20
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Abstract 22
An emerging paradigm is that root traits that reduce the metabolic costs of soil exploration 23
improve the acquisition of limiting soil resources. Here we test the hypothesis that reduced 24
lateral root branching density will improve drought tolerance in maize (Zea mays) by reducing 25
the metabolic costs of soil exploration, permitting greater axial root elongation, greater rooting 26
depth, and thereby greater water acquisition from drying soil. Maize recombinant inbred lines 27
with contrasting lateral root number and length (FL: few but long; MS: many but short) were 28
grown under water stress in greenhouse mesocosms, in field rainout shelters, and in a 29
second field environment with natural drought. Under water stress in mesocosms, lines with 30
the FL phenotype had substantially less lateral root respiration per unit axial root length, 31
deeper rooting, greater leaf relative water content, greater stomatal conductance, and 50% 32
greater shoot biomass than lines with the MS phenotype. Under water stress in the two field 33
sites, lines with the FL phenotype had deeper rooting, much lighter stem water δ18O signature 34
signifying deeper water capture, 51 to 67% greater shoot biomass at flowering, and 144% 35
greater yield than lines with the MS phenotype. These results entirely support the hypothesis 36
that reduced lateral root branching density improves drought tolerance. The FL lateral root 37
phenotype merits consideration as a selection target to improve the drought tolerance of 38
maize and possibly other cereal crops. 39
Keywords: Lateral root branching; water stress; respiration; rooting depth; maize 40
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Introduction 42
Suboptimal water availability is a primary limitation to crop productivity in both developed and 43
developing countries (Lynch, 2007; Lobell et al., 2014). Climate change as well as decreased 44
freshwater availability are likely to increase the frequency and severity of crop water stress in 45
the future, causing significant yield loss (Tebaldi and Lobell, 2008; Brisson et al., 2010; IPCC, 46
2014). This will be a major obstacle to sustaining an increased human population, which is 47
projected to reach 9.6 billion by 2050 (Lee, 2011). Therefore, the identification and 48
understanding of traits improving crop drought tolerance have been the focus of the 49
development of more drought-tolerant crops and cropping systems. 50
Root architecture regulates water and nutrient acquisition by positioning root foraging activity 51
in specific soil domains in time and space (Lynch, 1995; Gregory, 2006; Lynch, 2011). 52
Genotypic variation for root traits and their functional implications for soil resource acquisition 53
and improved yields under nutrient and water stress conditions have been reported in many 54
crops. In the case of phosphorus (P), the most immobile macronutrient, whose availability is 55
therefore greatest in the topsoil, the ‘topsoil foraging’ ideotype appears to be particularly 56
important for genotypic adaptation to low phosphorus soils (Lynch and Brown, 2001; Lynch, 57
2011; Richardson et al., 2011). For superior acquisition of water and nitrate, which are highly 58
mobile in the soil, the ‘steep, cheap, and deep (SCD)’ ideotype has been proposed, consisting 59
of architectural, anatomical, and physiological traits accelerating subsoil exploration (Lynch, 60
2013; Lynch and Wojciechowski, 2015). One element of the SCD ideotype is a low density of 61
lateral roots per length of axial root and greater lateral root length of crown roots, as traits that 62
would reduce inter-root competition, improve the metabolic efficiency of soil exploration, and 63
accelerate the elongation of axial roots. 64
Lateral roots originate from a small number of differentiated cells situated in the subapical 65
zone of the axial root. The development of lateral roots has been studied in detail in the model 66
plant Arabidopsis thaliana (Nibau et al., 2008; Péret et al., 2009). Multiple genes, ABA, and 67
auxin are important in pre-branch site formation, lateral root initiation, and lateral root 68
emergence (Swarup et al., 2008; Zhao et al., 2014). For example, in the pre-branch site 69
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formation stage, the position of lateral root primordium was demarked by auxin response DR5 70
driven luciferase, which expression was dependent on AUXIN RESPONSE FACTOR 6, 7, 8, 71
and 19, and on auxin repressor protein IAA8, 19, and 28 (Moreno-Risueno et al., 2010; De 72
Rybel et al., 2010). A newly discovered adaptive mechanism termed lateral root 73
hydropatterning was also involved in regulating pre-branch sites (Bao et al., 2014). In addition, 74
transcription factors such as LBD 16, 18, and 29 which belong to the LBD/ASL family, 75
positively regulate lateral root formation (De Smet et al., 2010). In cereals, a number of genes 76
and growth regulators that regulate lateral root formation have been reported, including the 77
rice crl1 mutants in rice which reduced lateral root number by 70% (Inukai et al., 2005), and in 78
maize, lateral root initiation is inhibited when auxin transport is disrupted by the rum1 mutation 79
(Woll et al., 2005). 80
Lateral roots typically comprise the major portion of root systems, accounting for 81
approximately 90% of the total root length (Pierret et al., 2006; Zobel et al., 2007). The 82
formation of lateral roots increases the sink strength of the root system, promoting the 83
development of greater root length and thereby greater soil resource acquisition (Varney and 84
Canny, 1993; Postma et al., 2014). However, root construction and maintenance requires 85
metabolic investment, which can exceed 50% of daily photosynthesis (Lambers et al., 2002). 86
Thus, the metabolic costs of the construction and maintenance for additional lateral roots, 87
either calculated in units of carbon or in terms of other limiting resources, may reduce the 88
growth of other roots, like axial roots (Borch et al., 1999; Borch et al., 2003; Lynch and Ho, 89
2005; Walk et al., 2006), potentially slowing axial root elongation into deep soil strata. This is 90
especially important for the acquisition of water, whose availability is greater in deeper soil 91
strata in most soils. A plant that is able to access water in deep soil domains at reduced 92
metabolic cost will have superior productivity, because it will have more metabolic resources 93
available for further resource acquisition, growth, and reproduction. Evidence in support of 94
this hypothesis comes from empirical and modeling studies for maize under water and 95
edaphic stress (Lilley and Kirkegaard, 2011; Jaramillo et al., 2013; Uga et al., 2013; Chimungu 96
et al., 2014a; Lynch, 2014; Saengwilai et al., 2014a; Chimungu et al., 2014b; Saengwilai et al., 97
2014b). In addition, increased lateral root branching places roots closer together, which may 98
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increase competition for water among roots of the same plant, effectively reducing the uptake 99
efficiency per unit root length (Postma et al., 2014). The frequency and length of lateral 100
branching determines the balance between the capture of mobile and immobile resources. 101
Mobile resources are captured more efficiently by fewer but longer laterals capable of 102
exploring larger volumes of soil with greater spatial dispersion among roots. Results from the 103
functional-structural plant model SimRoot indicate that the optimal lateral root branching 104
density for N capture is less than that for P capture in maize (Postma et al., 2014). Previous 105
research showed that reduced lateral root branching density improves N capture under N 106
deficiency (Zhan and Lynch, 2015). The few/long (FL) lateral root phenotype is therefore an 107
element of the SCD ideotype for efficient water capture, because sparse lateral branching 108
should conserve internal resources, reduce competition for water among neighboring lateral 109
roots, and explore a greater volume of soil than the many/short (MS) lateral root phenotype. 110
The objectives of this research were to test the hypotheses that: (1) reduced lateral root 111
branching density decreases the respiration of maize roots; (2) maize genotypes with few but 112
long lateral roots have greater rooting depth under water stress conditions, resulting in greater 113
water acquisition, and improving both plant growth and yield. To test these hypotheses we 114
compared the performance of maize recombinant inbred lines (RILs) sharing a common 115
genetic background but having contrasting lateral root branching phenotypes under water 116
stress in greenhouse mesocosms and two field environments. 117
Results 118
Lateral root branching and root length 119
Most genotypes selected for this study displayed stable lateral root branching density 120
phenotypes, except MO327. In two replications of the Pennsylvania (PA) field site in water 121
stress (WS) conditions, MO327 displayed the MS (i.e. Many/Short) lateral root phenotype, 122
rather than the FL (i.e. Few/Long) lateral root phenotype. For purposes of this study, MO327 123
was classified as the FL phenotype in all figures and statistics, which did not substantially 124
affect statistical analyses (Table S1). 125
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In greenhouse mesocosms (GH), water stress (WS) significantly decreased lateral root 126
branching density in crown and primary roots (Fig. 1). Compared with FL (i.e. Few/Long 127
lateral root phenotype) lines, MS (i.e. Many/Short lateral root phenotype) lines had 128
significantly greater lateral root branching density in crown roots, but no significant difference 129
was found in primary and seminal roots. In the two field sites, lateral root branching density of 130
crown roots in MS lines was significantly greater than that of in FL lines in both WS and WW 131
(well watered) conditions (Fig1B-C). Water stress significantly decreased lateral root 132
branching density in crown roots, although no difference was found in primary and seminal 133
roots. 134
Under WS In greenhouse mesocosms, the average axial root lengths of crown, primary, and 135
seminal roots of the FL lines were greater, by 34%, 73%, and 71%, respectively, compared to 136
the MS lines (Fig. 2). Total root length of FL and MS lines had equivalent value in either WS or 137
WW, but WS significantly decreased total root length in both FL and MS lines (Fig. 2). 138
Lateral root branching effects on respiration and rooting depth 139
Water availability and genotype had significant effects on specific root respiration and root 140
respiration of lateral roots per unit axial root length (Table 1). Water stress decreased specific 141
root respiration in the mesocosms by 37%. Under WS conditions in mesocosms, specific root 142
respiration of FL lines was 47% less than MS lines (Table 1). Specific root respiration was 143
positively correlated with lateral root branching density of crown roots under water stress (R2 = 144
0.86, p = 0.0005, Fig. 3). Root respiration of axial and lateral roots per unit axial root length 145
was significantly affected by water treatment (Table 1). Root respiration of axial root per unit 146
axial root length showed no significant difference among genotypes in either WS or WW, but 147
root respiration of axial roots per unit axial root length in WS was 37% less than that in WW. 148
Root respiration of lateral roots per unit axial root length in WS was 43% less than in WW 149
(Table 1). Under WS, root respiration of lateral roots per unit axial root length in FL lines had 150
141% less respiration than MS lines. Root respiration of lateral roots per unit axial root length 151
was positively correlated with the lateral root branching density of crown roots under WS (R2 = 152
0.95, p < 0.0001, Fig, 3). 153
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Water stress decreased root length density in the GH, and in the field in Arizona (AZ) and PA, 154
and the FL lines had greater root length density in deep soil layers than MS lines (Fig. 4). 155
Under WS in AZ, and PA, FL lines had significantly greater D95 (D95 is the depth above which 156
95% of total root length is located in the soil profile) than MS lines (Table S2). FL lines under 157
water stress had a D95 value of 118cm, 54cm, and 55cm in GH, AZ, and PA compared to 88cm, 158
43cm, and 45cm in MS lines, respectively. Lateral root branching density of crown roots was 159
negatively correlated with D95 under WS in all three environments (Fig. 5). 160
Lateral root branching effects on leaf relative water content and δ18O uptake 161
Water treatment and genotype had a significant impact on leaf relative water content (LRWC) 162
in PA and mesocosms (Tables S2). Under nonstressed conditions, LRWC in the GH and AZ 163
was not significantly different between FL and MS lines. Under water stress, the LRWC of FL 164
lines in GH and PA were significantly greater, by 8% and 13%, than the MS lines. Under WS, 165
LRWC was positively correlated with rooting depth in GH (R2 = 0.80, p = 0.0017) and PA (R2 = 166
0.92, p = 0.0001,Fig. 6). 167
Under WS conditions, analysis of soil water isotopic signature (δ18O) in both AZ and PA 168
showed progressively lighter isotope signature of water with increasing depth (Fig. 7). In AZ, 169
the majority of change in this signature was found in the top three soil layers (0-10 cm, 10-20 170
cm, and 20-30 cm) (approximately 1.97‰), while in PA, this change was mainly found in the 171
top two soil layers, 0-10 cm, and 10-20 cm (approximately 4.19‰). No significant difference 172
was found in the deepest three soil layers, which were aggregated as ‘deep water’ for 173
subsequent analyses. The values of stem water δ18O of the eight genotypes varied by 3.25‰ 174
in AZ and 3.45‰ in PA (Table 2). The FL lines in AZ and PA had a 46% and 44% lighter stem 175
water signature, respectively, than the MS lines. Soil water δ18O values were used in an 176
isotopic mixing model to determine water sources contributing to the δ18O signature for stem 177
water, assuming that any water acquired below 30 cm depth was ‘deep water’. Under WS in 178
AZ and PA, the FL lines mainly absorbed ‘deep water’, averaging 66% and 66% of stem water, 179
respectively, while the MS lines had greater reliance on the two most shallow soil layers 180
(Table 2). Lateral root branching density of crown roots was negatively correlated with the 181
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δ18O signature for stem water in AZ (R2 = 0.83, p = 0.0011), and PA (R2 = 0.54, p = 0.0224) 182
(Fig 8). 183
Lateral root branching effects on plant growth and yield 184
In all three environments, WS significantly decreased CO2 exchange rate and stomatal 185
conductance (Fig. 9). Under WS, FL lines had significantly greater leaf CO2 assimilation than 186
MS lines, by 58% in GH, 42% in AZ, and 79% in PA. Stomatal conductance in GH, AZ, and PA 187
were 84%, 73%, and 65% greater in FL lines than in MS lines, respectively, under WS 188
conditions (Fig. 9). 189
Relative shoot dry weight in GH, AZ, and PA were significantly influenced by water treatment 190
and in the two field sites were influenced by genotype (Tables S2). Under WS, the FL lines 191
had 50%, 51%, and 67% greater relative shoot dry weight at 42 days after planting in the GH, 192
and at anthesis in AZ and PA, respectively, than MS lines (Fig. 10). Relative shoot dry weight 193
was negatively correlated with lateral root branching density of crown roots (GH: R2 = 0.86, p 194
= 0.0006; AZ: R2 = 0.51, p = 0.0279; PA: R2 = 0.45, p = 0.0402, Fig. 11A). In PA, the lateral 195
root branching density of crown roots was negatively correlated with yield (R2 = 0.50, p = 196
0.0307, Fig. 11B). Under WS, compared to MS, FL lines improved yield by 144% (Fig. 11B). 197
Discussion 198
We hypothesized that reduced lateral root branching density would decrease the metabolic 199
cost of soil exploration, thereby improving water acquisition, plant growth and yield under 200
water stress. Our results from greenhouse mesocosms and two field environments entirely 201
support the hypothesis that under water stress, root phenotypes with few but long lateral roots 202
have less specific root respiration, greater rooting depth, greater acquisition of deep soil water, 203
improved plant water status, leaf photosynthesis, stomatal conductance, and hence greater 204
plant growth and yield. These results support the inclusion of this lateral root phenotype in the 205
SCD ideotype for optimal acquisition of water and N (Lynch, 2013). 206
In order to impose terminal drought by progressive reduction of soil water content, we used 207
greenhouse mesocosms, reduced irrigation in AZ, and automated rainout shelters in PA. The 208
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combination of results from three distinct environments is noteworthy. Mesocosms are 209
simplified, controlled environments, yet permit detailed analysis of root distribution by depth 210
and intact root respiration as entire root systems can be excavated. The field environments 211
include variable environmental factors such as temperature, rainfall, soil biota, and soil 212
physical properties that may affect results, and the two field environments had contrasting soil 213
physical properties. The fact that results from these contrasting water stress environments are 214
in agreement with each other suggests that potentially confounding factors of any given 215
environment are not driving the results. In addition, we used RILs which share a common 216
genetic background (i.e. all lines descend from the same two parents), without artificially 217
induced mutations or transformation events. Each RIL is a distinct genotype, and comparison 218
of several RILs allows the analysis of a phenotype in distinct genomes, thereby minimizing the 219
risk of confounding effects from pleiotropy, epistasis, or other genetic interactions (Zhu and 220
Lynch, 2004). RILs are particularly valuable in the analysis of phenotypic traits governed by 221
multiple genes as is the case for lateral rooting in maize (Zhu et al., 2005b; Burton et al., 222
2014). 223
We have proposed that reduced lateral root branching density may be a useful adaptation to 224
drought by reducing the metabolic costs of soil exploration (Lynch, 2013). The metabolic costs 225
of soil exploration by root systems are substantial, and can exceed 50% of daily 226
photosynthesis (Lambers et al., 2002). The fewer roots are initiated, the fewer carbon and 227
other resources need to be invested in root growth and maintenance, which could save 228
photosynthate and improve the growth of shoots, other roots, and may enhance reproduction 229
(Lynch, 2007). Root respiration associated with growth, maintenance, and ion uptake are 230
major components of root metabolic costs (Lambers et al., 2002; Lynch and Ho, 2005). In the 231
present study, decreasing lateral root branching of crown roots from 11 to 3 branch cm-1 was 232
associated with 58% reduction of specific root respiration and 71% reduction of lateral root 233
respiration per unit axial root length (Fig. 3). Empirical and modeling results indicate that the 234
optimal density of lateral branching of maize roots decreases at low N availability (Postma et 235
al., 2014; Zhan and Lynch, 2015). In the present study, results from mesocosms and one field 236
site show that reduced lateral root branching density increases rooting depth and improves 237
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plant water status and stomatal conductance (the lack of effect of lateral root branching 238
density on LRWC and stomatal conductance in AZ was due to rainfall at the time of sampling) 239
under drought conditions. Reduced lateral branching directly reduces the respiratory costs 240
associated with sustaining more lateral roots, thereby permitting the axial root to elongate 241
faster. An indirect benefit of reduced lateral branching is that for mobile resources like water 242
and nitrate, greater spatial dispersion of lateral roots increases the soil volume explored per 243
unit of root cost, and reduces resource competition among roots of the same plant, which 244
improves the metabolic efficiency of soil exploration (Postma et al., 2014). This has practical 245
implications, since in many rainfed or drought environments, the topsoil dries before the 246
subsoil, and, as drought progress, roots must exploit increasingly deeper soil strata to capture 247
water. Therefore genotypes with deep root systems would have the capability to capture 248
water from deep soil strata and resist water stress (Lopes and Reynolds, 2010; Wasson et al., 249
2012; Lynch and Wojciechowski, 2015). 250
Reduced lateral root branching density is important for drought tolerance because this 251
phenotype determines the balance between the capture of mobile and immobile resources 252
(Lynch, 2013). Greater lateral root branching increases the rate at which a soil domain is 253
depleted of resources, especially for immobile resources like P. For example, results from a 254
recent modeling study showed that a greater density of lateral branches in the topsoil can 255
improve P uptake from low P soil in wheat by 142% (Heppell et al., 2015). However, for highly 256
mobile resources, like N and water, depletion zones are larger and the greater lateral root 257
branching creates overlapping resource depletion zones around roots of the same plant, 258
thereby decrease resource capture efficiency (Ge et al., 2000). Therefore, lateral root 259
phenotypes to optimize mobile resources should be long and dispersed along the axial roots. 260
Results from the structural-functional simulation model SimRoot have shown that the optimal 261
density of lateral branching of maize roots at low N availability is less than that at low P 262
availability (Postma et al., 2014), a result later confirmed at low N in field and mesocosm 263
studies (Zhan and Lynch, 2015). Here we show that reduced lateral root branching density 264
improves plant water capture under water stress (Table 2, Fig. 6). 265
An additional benefit to reducing root cost is that extra resources from reduced root metabolic 266
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demand can contribute to growth and yield (Lynch, 2014), which are competing sinks for 267
current photosynthate. In the present mesocosm study, decreasing lateral branching of crown 268
roots from 11 to 3 branches cm-1 was associated with a 11% increase of relative shoot 269
biomass (Fig 11A). In the field experiments, decreasing lateral branching of crown roots from 270
11 to 4 branches cm-1 in AZ, and from 12 to 5 branch cm-1 in PA was associated with 40% and 271
37% increase of relative shoot biomass, respectively (Fig. 11A). Simulation results indicate 272
that without root maintenance respiration, maize plants had up to 72% greater growth under 273
limiting nutrient supply (Postma and Lynch, 2010; Postma and Lynch, 2011). Therefore 274
reduced root carbon demand in FL genotypes may be beneficial by increasing carbohydrate 275
availability (Fig. 11A). These results support the hypothesis that genotypes with less costly 276
root tissue could develop the extensive, deep root systems required to fully utilize soil water 277
resources in drying soil without as much yield penalty. 278
Hydrogen and oxygen stable isotope analysis provides an effective approach for studying root 279
water uptake. Normally, natural discrimination by evaporation against heavy isotopes 280
increases the concentration of heavy isotopes of oxygen in water at the soil surface (Durand 281
et al., 2007). Under dry conditions, this results in a relative enrichment in heavy isotopes of 282
elements of water (D and 18O) in the topsoil while deeper soil strata maintain the average 283
isotopic composition of regional precipitation (Durand et al., 2007, Fig. 7). No hydrogen and 284
oxygen isotope fractionation occurs during soil water uptake by root systems (Ehleringer and 285
Dawson, 1992), so the water absorbed by plant roots can be considered as the mixture of 286
water acquired from different soil depths. In the present study, stem water δ18O signatures 287
showed that the FL phenotype had lighter isotope signatures and greater dependency on 288
deep soil water than the MS phenotype (Table 2). The difference in the depths of root water 289
acquisition between the FL and MS genotypes could be attributed to their rooting depth (Table 290
S2, Figs. 4,5). 291
Studies have shown that lateral root formation for embryonic (seminal and primary roots) and 292
post-embryonic (nodal roots, including crown and brace roots) is controlled by multiple 293
pathways or different sensitivities to signals in lateral root formation (Hochholdinger and Feix, 294
1998; Hochholdinger et al., 2001). In maize, the lrt1 mutant is deficient in the initiation of 295
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lateral roots in the primary roots, seminal roots, and the crown roots emerging from the 296
coleoptilar node, however crown roots from subsequent nodes have normal lateral root 297
formation (Hochholdinger and Feix, 1998). In addition, slr1 and slr2 maize mutants display 298
reduced elongation of lateral roots from roots of the embryonic root system and normal lateral 299
root formation from roots in the post-embryonic root system (Hochholdinger et al., 2001). In 300
the present study, primary and seminal roots did not show the same lateral root branching 301
phenotypes as the crown roots, and phenotypes for branching densities were intermediate, 302
rather than in distinct groups of MS or FL (Fig. 1). These results are evidence that lateral 303
branching density for the embryonic and postembryonic root system is under distinct genetic 304
control. However, during vegetative growth of the plant, the crown roots capture the majority 305
of the soil resources (Lynch, 2013). The SCD ideotype proposes an increased lateral 306
branching density of seminal roots to optimize P and ammonium capture during seedling 307
establishment and a decreased lateral root branching density of crown roots to improve 308
capture of nitrate and water during vegetative growth (Lynch, 2013). Results from this study 309
support the SCD ideotype. The FL lateral branching phenotype on crown roots improved plant 310
water status, plant growth, and yield in WS conditions. The genotypes selected for this study 311
did not have clear MS or FL lateral branching phenotypes for primary and seminal roots, 312
which had intermediate branching densities. To further examine the SCD ideotype for primary 313
and seminal roots, additional studies should be conducted using genotypes contrasting for FL 314
and MS of primary and seminal roots. 315
Lateral branching is a heritable trait (Zhu et al., 2005b) and genetically controlled (Doebley et 316
al., 1995; Takeda et al., 2003). Genotypes selected for this study generally displayed stable 317
lateral root branching density phenotypes regardless of treatment or environment. One 318
exception to this was one genotype, MO327, in two replications of the PA field site in WS 319
conditions, which displayed the MS phenotype, rather than the FL phenotype. For purposes of 320
this study, all figures and statistics were performed with MO327 classified as the FL 321
phenotype, which minimally impacts statistics (Supplementary Table S1). Although genotypes 322
remained stable in terms of phenotypic classes for lateral root branching density throughout 323
the experiment, shifts of lateral branching density were observed in the data. For example, in 324
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WS conditions in PA the average lateral root branching density (branch cm-1) was 5 for FL and 325
9.8 for MS, whereas in AZ the average lateral root branching density (branch cm-1) was 6.3 for 326
FL and 11 for MS. This plasticity response may reflect varying soil and environmental 327
conditions at the field sites, which is to be expected as previous studies have shown that 328
genetic variation exists for plasticity in root traits (Zhu et al., 2010). 329
Root depth is one of the most important traits for plant resistance to water stress (Wasson et 330
al., 2012; Lynch and Wojciechowski, 2015). Modeling studies indicate that selection for 331
deeper, more effective roots could significantly improve capture of water and nitrogen in 332
wheat (Manschadi et al., 2006; Asseng and Turner, 2007; Lilley and Kirkegaard, 2011). In rice, 333
maximum root length, root depth, and basal thickness are correlated with yield under water 334
stress (Champoux et al., 1995; Li et al., 2005). When introduced into a shallow-rooting rice 335
cultivar, DRO1 improved yield under drought conditions by increasing rooting depth (Uga et 336
al., 2013). Root depth also has been positively correlated with yield in soybean (Cortes and 337
Sinclair, 1986). Our results in greenhouse mesocosms and two field experiments clearly show 338
that the FL phenotype increases rooting depth (Fig. 4 and Fig. 5), improves water capture 339
from deep soil (Table 2, Fig. 6, and Fig. 8), and improves plant water status, growth and yield 340
(Fig. 6, Fig. 10, and Fig. 11). Although the present study focuses on maize, we suggest that 341
the phenotype of few long lateral roots would improve water capture in other species, like 342
sorghum which has a root system architecture similar to that of maize (Lynch, 2013). Other 343
poaceae species have the same basic root structure as maize and may also benefit from this 344
phenotype, like wheat, rice, and barley, although greater density of nodal roots in tillering 345
species may change the relationship of lateral root branching density and resource capture. 346
Our results are entirely supportive of inclusion of reduced lateral root branching as a 347
component of the SCD ideotype (Lynch, 2013) for improved capture of N (Zhan and Lynch, 348
2015) and water (this paper) when those resources limit growth. The SCD ideotype applies to 349
both water and N capture, since both of these soil resources are often localized in deep soil 350
strata under limiting conditions. 351
Plant breeders rarely select for root traits because they are challenging to phenotype, many 352
traditional metrics of root phenotypes are actually phene aggregates with low heritability, and 353
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root phenotypes often display plasticity in response to soil conditions (Tuberosa et al., 2002; 354
Malamy, 2005; York et al., 2013; Lynch, 2014). As shown here and in previous literature, 355
genotypic differences in lateral root number and length exist in maize (Zhu et al., 2005b; 356
Trachsel et al., 2011; Lynch, 2013; Burton et al., 2014). Previous studies indicate that lateral 357
branching is a heritable trait (Zhu et al., 2005b) and genes affecting lateral branching have 358
been identified in several species, including maize (Doebley et al., 1995) and rice (Takeda et 359
al., 2003), making lateral branching and length a feasible target for plant breeding. Our results 360
from three distinct environments, greenhouse mesocosms and two field sites, are entirely 361
consistent with the hypothesis that the few/long lateral root phenotype increases rooting depth 362
by reducing root metabolic costs, resulting in greater water acquisition from deep soil strata, 363
and improved plant growth and yield under water stress. We suggest that lateral root number 364
and length deserves consideration as a root phenotype to improve drought tolerance in crop 365
breeding programs. 366
Materials and Methods 367
Greenhouse mesocosm experiment 368
Plant materials 369
Eight recombinant inbred lines (RILs) of maize (Zea mays L.), genotypes MO067, MO079, 370
MO086, MO134, MO295, MO321, MO327, and MO362 from the intermated B73 × MO17 371
population (IBM) were obtained from Dr. Shawn Kaeppler (University of Wisconsin, Madison, 372
WI, USA) (Genetics Cooperation Stock Center, Urbana, IL, USA). Our previous screening for 373
lateral root branching and length in this population indicated that RILs MO067, MO079, 374
MO086, and MO327 had few but long lateral roots (FL phenotype), and RILs MO134, MO295, 375
MO321, and MO362 had many but short lateral roots (MS phenotype) (Trachsel et al., 2011; 376
Trachsel et al., 2013a). Thus, in the present study we consider RILs MO067, MO079, MO086, 377
and MO327 to have the FL phenotype, and RILs MO134, MO295, MO321, and MO362 to 378
have the MS phenotype. 379
Experimental design 380
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The greenhouse experiment was a randomized complete block design, the factors were two 381
water treatments (well watered: WW, and water stress: WS) and eight RILs (MO067, MO079, 382
MO086, MO134, MO295, MO321, MO327, and MO362), with four replicates. 383
Growth conditions 384
Plants were grown from March 19 to April 30, 2014 in a greenhouse located on the campus of 385
The Pennsylvania State University in University Park, PA, USA (40°48′ N, 77°51′ W) under 386
constant conditions (14/10 h at 28/24°C day/night, 40-70% relative humility). Seeds of 8 387
genotypes were surface-sterilized in 0.05% NaOCl for 30 min and imbibed for 24 h in aerated 388
1 mM CaSO4, then were placed in darkness at 28 ± 1°C in a germination chamber for two 389
days. Three seedlings of similar size were transplanted to mesocosms consisting of 390
polyvinylchloride (PVC) cylinders 15.7 cm in diameter and 155 cm in height, with plastic liners 391
made of 4 mil (0.116 mm) transparent hi-density polyethylene film, which was used to 392
facilitate root sampling, then thinned to one seedling per mesocosm five days later after 393
planting. The growth medium consisted of (by volume) 50% medium size (0.5–0.3 mm) 394
commercial grade sand (Quikrete Companies Inc., Harrisburg, PA, USA), 35% horticultural 395
size #3 vermiculite, 5% perlite (Whittemore Companies Inc., Lawrence, MA, USA), and 10% 396
topsoil. The topsoil was collected from the Russell E. Larson Agricultural Research Center in 397
Rock Springs, PA (Fine, mixed, semiactive, mesic Typic Hapludalf, pH=6.7, silt loam). To 398
ensure a consistent bulk density, a uniform volume (29 L) of the soil mixture was used in each 399
mesocosm. Mineral nutrients were provided by mixing the media with 70g per column of 400
OSMOCOTE PLUS fertilizer consisting of (in %): N (15), P (9), K (12), S (2.3), B (0.02) Cu 401
(0.05), Fe (0.68), Mn (0.06), Mo (0.02), and Zn (0.05) (Scotts-Sierra Horticultural Products 402
Company, Marysville, Ohio, USA). Two days before planting, each cylinder was irrigated with 403
4.5 L of deionized water. In the first four days, plants received 100 ml of deionized water every 404
day. And then 200ml of deionized water was irrigated in the WW treatment every 2 d, and WS 405
treatment received no further irrigation. Additional light was provided with 400 W metal-halide 406
bulbs (Energy Technics, York, PA, USA) for 14 hours per day to a maximum illumination of 407
1200 μmol photos m-2 s-1. Average daytime temperature in the greenhouse was 408
approximately 28 °C. 409
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18
Root respiration and root harvest 410
Three days before harvesting, the ‘head space’ approach of sampling air flow over the soil 411
surface was used in this study to measure the intact root respiration. In short, a PVC plate 412
was placed on top of the pot to seal off the root system from the shoot. An air pump provided 413
a stable flow of air through the ‘head space’ compartment of the pot. The airflow rate was 414
1200 µmol s-1. The measurements were conducted in early morning with a Li-6400 portable 415
Infrared Gas Analyzer (Li-Cor Biosciences, Lincoln, NE, USA). Intact root respiration was 416
measured for a short time (ca.5 min, Fan et al., 2003; Zhu et al., 2005a). In the present study, 417
we assumed that the amount of natural soil and the respiration of microbes was the same in 418
all cylinders (Bouma et al., 1997a; Bouma et al., 1997b), and used the intact root system plus 419
media respiration as a proxy for total root respiration. The intact root system respiration 420
values were divided by the total root length obtained by WinRhizo scanning (described below) 421
to obtain the specific root respiration per unit of root length (µmol CO2 m-1 root length s-1). 422
At harvest (30 April 2014), the plastic sleeve was removed from the supporting PVC cylinder, 423
cut open, and roots were separated from the soil by vigorous rinsing at low pressure with 424
water. Root respiration of axial and lateral roots were measured. Three representative 10cm 425
root segments from the third crown root were excised 20 cm from the base. Lateral roots of 426
axial roots were removed with a Teflon blade (Electron Microscopy Sciences, Hatfield, PA, 427
USA). Excised axial and lateral root samples were patted dry and placed in a 40 ml custom 428
chamber connected to the Li-6400 IRGA (LI-COR, Lincoln, NE, USA), separately. The 429
temperature of the chamber was maintained at 26 ± 1°C by using a water bath while 430
respiration was measured. Carbon dioxide evolution from the root segments was recorded 431
every 5 seconds for 180 seconds. Axial root length of crown and seminal roots was collected 432
from three representative root samples representing the average growth of each root class. 433
Root number in each whorl of crown roots and seminal roots were counted manually. Average 434
axial root length of crown roots was calculated by using a weighted average from all roots. 435
Roots from each 20 cm soil layer were collected, and lateral root number from three 436
representative roots was obtained by scanning with image analysis software (WinRhizo Pro, 437
Régent Instruments, Québec, Canada) as described below. Total root length of each plant 438
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was the sum of root length in each layer. 439
Plant measurements and shoot dry weight 440
One day before harvesting, leaf relative water content (LRWC) was measured. To measure 441
LRWC, four fresh leaf discs (1 inch in diameter) were collected from the third fully expanded 442
leaf and weighed immediately to determine fresh weight (FW). After which the discs were 443
immediately hydrated to full turgidity by soaking them in distilled water for 8 h. After 8 h, the 444
discs were patted dry and weighed again to determine turgid weight (TW). The discs were 445
then dried at 70 °C for 72 h, and dry weight determined (DW). Leaf RWC was calculated 446
according to the equation: LRWC (%) = 100× (FW – DW) / (TW – DW). 447
The CO2 exchange rate and stomatal conductance of the third fully expanded leaf was 448
measured with a Li-6400 portable photosynthesis system (Li-Cor Biosciences, Lincoln, NE, 449
USA) using a red-blue light at a photosynthetically active radiation intensity of 1200 μmol 450
photons m-2 s-1, CO2 concentration of 400 ppm, and leaf temperature of 25°C. The 451
measurements were conducted between 9 am and 11 am. At harvest, shoots were collected 452
and dried at 70 °C until constant weight for biomass determination. 453
Field studies 454
Field conditions, experimental design, and plant materials 455
Field experiments were carried out from April to July 2014 at the Apache Root Biology Center, 456
Willcox, AZ, USA (AZ) (32°2′0" N, 109°41′30" W) and from May to August 2014 at the Russell 457
Larson Research and Education Center of the Pennsylvania State University in Rock Springs, 458
PA, USA (PA) (40°42′37" N, 77°57′07" W). The soils at the experimental sites were a Grabe 459
loam (coarse-loamy, mixed, thermic Typic Torrifluvent) in AZ and a Hagerstown silt loam (fine, 460
mixed, mesic Typic Hapludalf) in PA. 461
The two experiments were arranged in a split-plot design replicated four times with two water 462
treatments (WS and WW). The main plots were composed of two moisture regimes and the 463
subplots were eight genotypes (RILs MO067, MO079, MO086, MO134, MO295, MO321, 464
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MO327, and MO362) in each experiment. In AZ, the experiments were planted in five 6 m row 465
plot with 25 cm distance between plants, and 75 cm wide between rows. The WS treatment 466
was initiated starting 40 days after planting by withholding water application in AZ. In PA, each 467
subplot consisted of three rows, each row was 3 m long, 25 cm between plants and 75 cm 468
between rows. The drought treatment was initiated 30 days after planting using an automated 469
rainout shelter in PA. The shelters (10 by 30 m) were covered with a clear greenhouse plastic 470
film (0.184 mm) and were automatically triggered by rainfall to cover the plots, excluding 471
natural precipitation. Adjacent non-sheltered control plots were rainfed, to maintain the soil 472
moisture close to field capacity throughout the growing season, drip-irrigation was applied 473
when necessary. Soil water content in AZ and PA were monitored using soil moisture probes 474
(PR2, Dynamax Inc., Houston, TX, USA) and TRIME FM system (IMKO Mocromodultechnik 475
GmbH, Ettlingen, Germany), respectively, both in WS and WW treatments. At each location, 476
the recommended fertilizer rate was applied before planting. Pest control and irrigation were 477
carried out as needed. 478
Shoot and roots were evaluated 10 weeks after planting at AZ and 12 weeks after planting at 479
PA (anthesis stage). Three representative, adjacent plants were randomly selected in the 480
same row for shoot dry weight per replicate, and dried at 70 °C for 72 h before being weighed. 481
Roots were excavated by removing a soil cylinder ca. 40 cm diameter and 25 cm depth with 482
the plant base as the horizontal center of the soil cylinder. The excavated root crowns were 483
cleaned by vigorous rinsing at low pressure with water. The clean roots were subsequently 484
used to measure lateral root number. All nodal roots emerging belowground were classified 485
as crown roots. Three 5 cm root segments were taken 5 cm from the base of each whorl of 486
crown, primary and seminal roots, and lateral root number of corresponding roots was based 487
on counts. 488
Plant measurements 489
Two days before harvest, CO2 exchange rate and stomatal conductance of the ear leaf was 490
measured with a Li-6400 portable photosynthesis system (Li-Cor Biosciences, Lincoln, NE, 491
USA) using a red-blue light at PAR intensity of 1800 μmol photons m-2 s-1, constant CO2 492
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21
concentration of 400 ppm, and leaf temperature of 28 °C. The measurements were conducted 493
between 9:00 and 11:00 am. In both field experiments, LRWC was measured as described 494
above, except that nine fresh leaf discs were collected from the ear leaf for three 495
representative plants per plot (three fresh leaf discs per plant). At physiological maturity, grain 496
yield was collected in PA. Yield was not collected in AZ because of uneven anthesis of these 497
RILs in response to the temperature and photoperiod regime at this location. 498
Shoots and roots were evaluated and excavated 10 weeks after planting at AZ and 12 weeks 499
after planting at PA (anthesis stage). Three representative, adjacent plants were randomly 500
selected in the same row for shoot dry weight per replicate, and dried at 70 °C for 72 h before 501
being weighed. 502
Root harvest and rooting depth 503
Roots were excavated by removing a soil cylinder ca. 40 cm diameter and 25 cm depth with 504
the plant base as the horizontal center of the soil cylinder. The excavated root crowns were 505
cleaned by vigorous rinsing at low pressure with water. The clean roots were subsequently 506
used to measure lateral root number. All nodal roots emerging belowground were classified 507
as crown roots. Three representative 5 cm root segments were taken 5 cm from the base of 508
each whorl of crown, primary and seminal roots, and lateral root number of corresponding 509
roots was based on counts. 510
In both field sites, soil cores were collected at flowering to determine root distribution in the 511
soil profile. A soil coring tube (Giddings Machine Co., Windsor, CO, USA) 5.1 cm in diameter 512
and 60 cm long was used for sampling, the core was taken within a planting row midway 513
between two plants. Soil core was subdivided into 10 cm segments and roots were extracted 514
from each segment. Extracted root samples were scanned using a flatbed scanner (Epson, 515
Perfection V700 Photo, Epson America, Inc. USA) at a resolution of 23.6 pixel mm-1 (600 dpi) 516
and analyzed using image processing software WinRhizo Pro (Regent Instruments, Québec, 517
Canada). Percentages of root length at each depth were calculated in each soil core. Depth 518
above which 95% (D95) of root length is located was calculated by linear interpolation between 519
the cumulative root lengths (Trachsel et al., 2013b). 520
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Soil and plant sampling of δ18O analysis 521
In both AZ and PA, soil samples for δ18O analysis were collected adjacent to plants in the WS 522
treatment at flowering stage using a 5 cm diameter soil core. Soil cores were taken to the 523
maximum achievable depth of 60 cm. The cores were separated into 10 cm increments: 10, 524
20, 30, 40, 50, and 60 cm. In each replicate three cores were collected from different positions 525
and mixed as one sample for each depth. The maize stems were collected at the same time, 526
approximately 8-10 cm of the stem was collected just above ground level and the epidermis 527
was immediately removed. Soil and maize stem samples were put in a snap vials, sealed with 528
parafilm to prevent evaporation, and refrigerated immediately. Cryogenic vacuum distillation 529
(Ehleringer and Osmond, 1989) was used to extract soil water and crop stem water. In 530
cryogenic vacuum distillation, two glass tubes were attached to a vacuum pump. The sample 531
was placed in one tube and frozen by submerging the tube in liquid nitrogen, and then both 532
tubes were evacuated by vacuum pump to create a closed U-shape configuration. After that, 533
the tube containing the sample was heated to 100°C, while the collection tube was still 534
immersed in liquid nitrogen to collect evaporated water. Samples were weighed and oven 535
dried after extraction to ensure the extraction time was sufficient to vaporize all the water in 536
the samples. The water samples were analyzed using a PICARRO L2130-i δD/δ18O Ultra 537
High Precision Isotopic Water Analyzer (PICARRO Inc, CA, USA). Results were expressed as 538
parts per thousand deviations from the Vienna Standard Mean Ocean Water (VSMOW). To 539
determine the percent contribution of soil water from different depths to the signature of water 540
within the plant tissue, an isotopic mixing model was used (Phillips et al., 2005). IsoSource 541
Version 1.3.1 (Phillips and Gregg, 2003) was used to evaluate the relative contribution of each 542
soil layer to tissue water signature. The fractional increment was set at 1%, and tolerance at 543
0.1. 544
Statistical analysis 545
The experimental data were statistically analyzed by ANOVA, and Tukey’s Honest Significant 546
Difference method (α = 0.05) was used for multiple comparisons with SAS 8.0 software (SAS 547
Stat. Inst. Cary, NC, USA). Linear regression analysis and Pearson correlation coefficients 548
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23
were calculated using Sigmaplot software (SigmaPlot 10.0, Systat Software Inc., CA, USA). 549
Acknowledgements 550
We thank the China Scholarship Council for support of Ai Zhan, and Jennifer Yang, Robert 551
Snyder and Johan Prinsloo for technical support. 552
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Walk TC, Jaramillo R, Lynch JP (2006) Architectural tradeoffs between adventitious and basal 703
roots for phosphorus acquisition. Plant Soil 279: 347–366 704
Wasson P, Richards R, Chatrath R, Misra SC, Prasad SVS, Rebetzke GJ, Kirkegaard JA, 705
Christopher J, Watt M (2012) Traits and selection strategies to improve root systems and 706
water uptake in water-limited wheat crops. J Exp Bot 63: 3485–3498 707
Woll K, Borsuk LA, Stransky H, Nettleton D, Schnable PS, Hochholdinger F (2005) Isolation, 708
characterization, and pericycle-specific transcriptome analyses of the novel maize lateral 709
and seminal root initiation mutant rum1. Plant Physiol 139: 1255–1267 710
York LM, Nord EA, Lynch JP (2013) Integration of root phenes for soil resource acquisition. 711
Front Plant Sci 4: 1–15 712
Zhan A, Lynch JP (2015) Reduced frequency of lateral root branching improves N capture from 713
low N soils in maize. J Exp Bot 66: 2055–2065 714
Zhao Y, Xing L, Wang X, Hou YJ, Gao J, Wang P, Duan CG, Zhu X, Zhu JK (2014) The ABA 715
receptor PYL8 promotes lateral root growth by enhancing MYB77-dependent transcription of 716
auxin-responsive genes. Sci Signal 7: ra53–ra53 717
Zhu J, Lynch JP (2004) The contribution of lateral rooting to phosphorus acquisition efficiency in 718
maize ( Zea mays ) seedlings. Funct Plant Biol 31: 949-958 719
Zhu J, Kaeppler SM, Lynch JP (2005a) Topsoil foraging and phosphorus acquisition efficiency 720
in maize (Zea mays L.). Funct Plant Biol 32: 749–762 721
Zhu J, Kaeppler SM, Lynch JP (2005b) Mapping of QTL for lateral root branching and length in 722
maize (Zea mays L.) under differential phosphorus supply. Theor Appl Genet 111: 688–695 723
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Zhu J, Zhang C, Lynch JP (2010) The utility of phenotypic plasticity of root hair length for 724
phosphorus acquisition. Funct Plant Biol 37: 313–322 725
Zobel R, Kinraide T, Baligar V (2007) Fine root diameters can change in response to changes in 726
nutrient concentrations. Plant Soil 297: 243–254 727
728
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Table 1. Root respiration of eight maize genotypes under two water treatments. 729
Specific root respiration per unit root length (ηmol CO2 cm-1 RL s-1), root respiration of axial roots per unit axial root length (axial root 730
respiration) (ηmol CO2 cm-1ARL s-1), root respiration of lateral roots per unit axial root length (lateral root respiration) (ηmol CO2 cm-1 731
ARL s-1) of eight maize recombinant inbred lines (RILs) with contrasting lateral root branching density (FL: few but long lateral roots; 732
MS: many but short lateral roots) under well watered (WW) and water stress (WS) conditions in greenhouse mesocosms. Data are 733
means (n=4). The same letters within each column are not significantly different at α = 0.05 (Tukey’s Honest Significant Difference 734
method). ANOVA table (in the lower part of the table) for above parameters as influenced by soil moisture regime (T), genotype (G), 735
and phenotype (P) with associated F-values and probabilities (NS, not significant; *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001) are shown. 736
Classification based on LRBD RILs
Specific root respiration (ηmol CO2 cm-1 RL s-1)
Axial root respiration(ηmol CO2 cm-1ARL s-1)
Lateral root respiration (ηmol CO2 cm-1 ARL s-1)
WS WW WS WW WS WW
FL 67 164.44 bc 292.63 a 10.24 ab 17.82 a 3.50 b 14.11 a
79 157.81 c 296.45 a 9.27 b 17.72 a 4.03 b 12.13 a
86 149.62 c 253.62 a 12.76 ab 17.34 a 5.27 b 14.11 a
327 159.31 c 241.37 a 11.21 ab 18.70 a 5.31 b 13.03 a
MS 134 233.19 ab 259.49 a 9.79 ab 19.71 a 11.35 a 13.59 a
295 237.43 a 243.42 a 13.53 ab 21.55 a 12.10 a 15.56 a
321 236.21 a 279.96 a 15.37 a 18.15 a 12.24 a 11.26 a
362 221.95 ab 268.77 a 10.50 ab 14.93 a 7.92 ab 14.45 a
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ANOVA
Treatment (T) 71.76***
3.42**
9.45**
57.61***
1.67NS
1.35NS
89.69***
5.32***
27.37***
Genotype (G)
Phenotype (P)
G × T 5.46*** 0.97NS 5.02***
P × T 21.24*** 0.16NS 21.64***
737
738
739
740
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Table 2. Stem water isotopic signature (δ18O) of eight maize genotypes and proportional water use by depth. 741
Means of δ18O of stem water (n = 4 ± SE) measured for eight maize genotypes contrasting in lateral root branching density (LRBD; FL: 742
few but long lateral roots; MS: many but short lateral roots) and proportional water use by depth from different soil layers (‘deep’ is the 743
aggregate of three deep soil layers) under water stress conditions at anthesis in Arizona (AZ) and Pennsylvania (PA). 744
Classification based on LRBD RILs
δ18O of stem water Proportional water use by depth (%)
AZ PA AZ PA
10 cm 20 cm deep 10 cm 20 cm deep
FL 67 -7.66 ± 0.17 -9.69 ± 0.11 10.20 10.20 61.15 9.65 25.58 64.78
79 -7.89 ± 0.21 -9.94 ± 0.51 9.28 8.90 65.58 8.18 25.83 66.00
86 -7.62 ± 0.13 -9.76 ± 0.35 10.98 10.70 59.28 9.18 25.60 65.28
327 -8.46 ± 0.36 -9.91 ± 0.40 5.60 5.15 80.10 8.23 24.43 67.33
MS 134 -5.39 ± 0.23 -6.49 ± 0.26 26.85 41.60 11.20 56.38 27.03 16.65
295 -5.21 ± 0.22 -6.98 ± 0.0.27 36.45 35.80 9.15 41.68 41.43 23.90
321 -5.59 ± 0.21 -7.01 ± 0.37 29.88 28.80 14.70 47.55 32.60 19.85
362 -5.44 ± 0.25 -6.87 ± 0.11 30.65 33.03 12.85 48.53 32.15 19.33
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745
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Figure Legends 746
Figure 1. Lateral root branching density in greenhouse mesocosms (A), in the field in 747
Arizona (B) and Pennsylvania (C) under well watered (WW) and water stress (WS) 748
conditions. Bars show means of four replicates of the four genotypes in each phenotype 749
class in WW or WS ± SE. Bars with the same letters are not significantly different within 750
the same panel (α = 0.05). 751
Figure 2. Average axial root length of crown, primary, and seminal roots (A), and total root 752
length (B) in greenhouse mesocosms under well watered (WW) and water stress (WS) 753
conditions. Bars show means of four replicates of the four genotypes in each phenotype 754
class in WW or WS ± SE. Bars with the same letters are not significantly different within 755
the same panel (α = 0.05). 756
Figure 3. Correlation between lateral root branching density of crown roots and specific 757
root respiration per unit root length (A), and root respiration per unit axial root length (B) 758
under water stress in greenhouse mesocosms. Each point is the mean of four replicates of 759
each genotype. 760
Figure 4. Root length density by soil depth of maize RILs in greenhouse mesocosms (A), 761
and in the field in Arizona (B) and Pennsylvania (C) under water stress (WS, circles) and 762
well watered (WW, triangles) conditions. The data shown are the mean of four replicates 763
of the four genotypes in each phenotype class in WS or WW ± SE. The average value of 764
D95 for four FL (dash arrow) and four MS (solid arrow) genotypes under WS are shown in 765
each panel. 766
Figure 5. Correlation between lateral root branching density of crown roots and rooting 767
depth (D95) in greenhouse mesocosms (A), in the field in Arizona (B) and Pennsylvania 768
(C) under water stress. Each point is the mean of four replicates of each genotype. 769
Figure 6. Correlation between D95 and leaf relative water content in greenhouse 770
mesocosms (GH), in the field in Arizona (AZ) , and Pennsylvania (PA) under water stress 771
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38
conditions. Each point is the mean of four replicates of each genotype. 772
Figure 7. Mean oxygen isotope composition (δ18O) of soil water along the soil profile in 773
the field in Arizona (A) and Pennsylvania (B) under water stress conditions. Values are the 774
means ± SE of four observation points. Bars with the same letters are not significantly 775
different within the same panel (α = 0.05). 776
Figure 8. Correlation between lateral root branching density and stem water isotopic 777
signature (δ18O) in the field in Arizona (AZ) and Pennsylvania (PA) under water stress 778
conditions. Each point is the means of four replicates of each genotype. 779
Figure 9. CO2 exchange rate (A-C) and stomatal conductance (D-F) of FL and MS 780
phenotypes in greenhouse mesocosms (A and D), in the field in Arizona (B and E) and 781
Pennsylvania (C and F) under well watered (WW) and water stress (WS) conditions. Bars 782
show means of four replicates of the four genotypes in each phenotype class in WW or 783
WS ± SE. Bars with the same letters are not significantly different within the same panel (α 784
= 0.05). 785
Figure 10. Relative shoot dry weight (% of greatest shoot dry weight within each location) 786
of FL and MS phenotypes in greenhouse mesocosms (GH), in the field in Arizona (AZ), 787
and Pennsylvania (PA) under water stress (WS) and well watered (WW) conditions. Bars 788
show means of four replicates of the four genotypes in each phenotype class in WW or 789
WS ± SE. 790
Figure 11. Correlation between lateral root branching density of crown roots and A: 791
relative shoot dry weight (% relative to greatest shoot dry weight within each location) in 792
greenhouse mesocosms, in the field in Arizona (AZ) and Pennsylvania (PA), B: relative 793
yield (% relative to greatest yield) in PA under water stress conditions. Each point is the 794
mean of four replicates of each genotype.795
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796
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Supplementary Table S1. Analysis of the effect of plasticity of MO327 genotype of results in water stress (WS) conditions 1
at the PA field site (LRBD: lateral root branching density , LRWC: leaf relative water content). A) All replications of 2
genotype MO327 classified as FL; B) All replications of MO327 classified as MS; C) MO327 classified as MS for replicates 3
2 and 3 and FL for replicates 1 and 4; D) MO327 removed from dataset. 4
5
6
Relative SDW (%)
Relative Yield (%)
CO2 exchange rate (μmol CO2 m-2 s-
1)
Stomatal conductance (mol H2O m-2
s-1)
LRWC (%)
Crown LRBD
(branches cm-1)
Primary LRBD
(branches cm-1)
Seminal LRBC
(branches cm-1)
D95 (cm)
MS FL MS FL MS FL MS FL MS FL MS FL MS FL MS FL MS FL A 31 52.4 19.2 47.4 15.2 27.9 0.1 0.2 77.1 86.6 10 5 6.2 8.2 5.7 4.6 45.1 55.3B 34.8 51.8 24.2 46.7 17.2 27.2 0.1 0.2 79.1 86.8 9.8 6.1 5.6 6.9 5.5 4.7 46.9 54.9C 28.4 42.9 14.6 32.4 16.5 27.2 0.1 0.2 78.3 86.6 10 5.4 6.9 6.1 5.8 4.3 46.1 55.1D 31 52.4 19.2 47.4 15.2 27.9 0.1 0.2 77.1 86.6 10 5 6.2 8.2 5.7 4.6 45.1 55.3
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Supplementary Table S2. Summary of analysis of variance for rooting depth (D95), leaf relative water content (LRWC) 7
(%), shoot dry weight (SDW) and yield (Y) as influenced by soil moisture regimes (treatment (T)), genotypes (G), and 8
lateral root phenotypes (P) in greenhouse mesocosms (GH), Arizona (AZ), and Pennsylvania (PA). The associated F-9
values and probabilities (NS, not significant; *, P<0.05; **, P<0.01; ***, P<0.001) are shown. 10
Source of variation
GH AZ PA
D95 LRWC SDW D95 LRWC SDW D95 LRWC SDW Y
Treatment (T) 0.42NS 65.01*** 1949*** 9.70** 8.22** 18.59*** 57.84*** 432.96*** 469.97*** 118.53***
Genotype (G) 24.98*** 4.16** 2.26* 13.55*** 1.50NS 2.21* 5.95*** 7.71*** 7.52*** 2.57*
Phenotype (P) 95.58*** 22.21*** 1.61NS 86.50*** 1.06NS 17.94*** 39.50*** 57.08*** 17.76*** 8.71**
G × T 10.68*** 5.89*** 4.72*** 4.50*** 2.43* 2.15* 4.33*** 9.55*** 5.60*** 1.62NS
P × T 30.59*** 34.64*** 9.52** 26.15*** 5.05* 4.71* 27.62*** 69.88*** 13.50*** 8.24**
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