2. REVIEW OF LITERATURE

Several reviews and books have appeared recently giving a

comprehensive account of the different aspects of AM fungi (Sieverding, 1991;

Read et al, 1993; Smith and Read, 1997). Internet also acts as great reservoir

of information on current developments and research about AM fungi. At this

juncture, an attempt has been made to bring light on the different aspects of

AM fungi.

The term 'Mycorrhizae' was first coined by Frank (1885). They are

non-pathogenic symbiotic soil fungi which invade on or in the root system of

host plants. Allen (1991) defined a mycorrhiza as a mutualistic symbiosis

between plant and fungus localised in a root or root-like structure in which

energy moves primarily from plant to fungus and inorganic resources move

from fungus to plant.

2.1 Arbuscular Mycorrhizal Fungi (AMF)

There are six different types of mycorrhizal symbionts occur in nature,

viz., Ectomycorrhiza, Ericoid mycorrhiza, Arbuscular mycorrhiza,

Monotropoid mycorrhiza. Orchid mycorrhiza and Arbutoid mycorrhiza.

Among the different types of mycorhizae, AMF are the most prevalent type.

Those are most commonly known endomycorrhizae showing symbiotic

association with the roots of many agricultural crops, shrubs, and many

tropical and temperate trees. AM prevail over a broad ecological range from

aquatic to desert environment. The name vesicular arbuscular mycorrhizal

fungus was introduced by Dangeard (1900). Then Peyronel (1924) was first to

recognize the VAM fungi as Endogone species. Mosse (1956) has

demonstrated experimentally that Endogone species could produce AM fungi

which grew significantly faster in weight and height. There are more than 180

species of AMF (Morton and Redecker, 2002). Taxonomically AMF belong to

the class Zygomycetes, of the order Glomales with three families viz.,

Glomaceae, Acaulosporaceae and Gigasporaceae and with six different

genera viz., Acaulospora, Entrophospora, Gigaspora, Glomus, Scutellospora

and Sclerocystis (Morton and Benny, 1990). The bulk of known species

belongs to the family Glomaceae (Pirozysnki and Dalpe, 1989) which includes

the genera Glomus and Sclerocystis.

Recently, Morton and Redecker (2002) discovered two ancestral classes

of AM fungal species were discovered from deeply divergent ribosomal DNA

sequences and are classified into two new families Archaeosporaceae and

Paraglomaceae. At present, each family consists of one genus, Archaeospora

like spores from sporiferous saccule and Paraglom us consisting of two species

forming spores indistinguishable from those of Glomus species. More recently,

AMF have been separated from the polyphyletic Zygomycota and placed in a

new monophyletic phylum, Glomeromycota composed of four orders with

seven families and ten different genera (SchiiBler et al, 2001).

The characteristic feature of VAM is the presence of two specialized

structures namely vesicles and arbuscules. Both these structures are produced

by internal mycelium but vesicles are usually formed inter and intracellularly.

Vesicles are organs meant for storage purposes, which may not present in

certain forms. The arbuscules are highly ramified minute arborescences

structures produced within few days of infection. The hyphae penetrate

mechanically and enzymatically into cortical cells, where it performs the

exchange of nutrients between the fungus and root.

8

2.2. Vital roles of AMF

Mycorrhizae are vital for uptake and accumulation of ions from soil and

translocation to host plants because of their high metabolic rate and

strategically diffuse distribution in the upper layers of soil. In fact, this fungus

serves as a highly efficient extension of the host root system. Minerals more

than 4 cm distance to the nearest host root can be absorbed by the fungal

hyphae and translocated to roots of mycorrhizal plants. Bieleski (1973)

reported that AM fungi may increase the effectiveness of absorbing capability

of surface host root as much as ten times. Ions such as P, Zn and Cu do not

diffuse readily through soil. Because of this poor diffusion, roots deplete these

immobile soil nutrients from the zone immediately surrounding the root.

Mycorrhizal fungal hyphae extend into the soil, penetrating into nutrient

depletion zone and increased the effectiveness of absorption of immobile

elements by as much as sixty times. The main advantage of mycorrhiza is its

greater soil exploration and increasing uptake of N, P, K, Zn, Cu, S, Fe, Ca,

Mg and Mn supply to the host roots (Li et al., 1991; Smith et al., 1994).

In addition to this AM exhibit other synergistic activities such as

biological control of root pathogens, biological nitrogen fixation, hormone

production and greater ability to withstand water stress (Giller and Cadisch,

1995). The mycorrhizal fungi produce enzymes, auxins, vitamins, cytokinins

and other substances, that increase rootlet size and longevity. They also protect

the rootlets from pathogens. They can also absorb and translocate water to the

host. Two types of mycorrhizal fungal hyphae regulate nutrient movement; the

absorbing hyphae which are fine and highly branched and hyphae that explore

substrates absorbing nutrients released from adjacent soils or organic

substances. In some instances, they also secrete external enzymes capable of

breaking down organic materials or otherwise affecting nutrient availability.

AM fungi alter the kinetic properties of the root, thereby enhancing its nutrient

uptake abihties. Hence it is clear that mycorrhizal fungi play a vital role in

nutrient cycling and productivity of crops (Smith and Read, 1997).

2.3. Diversity of AMF in India

A wide range of AMF distribution is found in India. Bakshi (1974) was

reported for first to give an account of 14 spore types : Glomus macrocarpum,

G. geosporum, G. mosseae, Glomus sp., Sclerocystis coremioides,

Sclerocystis sp., Gigaspora calospora, Acaulospora sp., Endogone gigantea,

E. microcarpum, Endogone 1, Endogone 2, Endogone 3. Gerdemann and

Bakshi (1976) reported two new species viz., Glomus multicauli and

Sclerocystis sinuosa. Bhattacharjee and Mukerji (1982) described the species

Glomus reticulatum from soils of Bangalore. Bhattacharjee et al, (1982)

reported the structure and hyper-parasitism for Gigaspora Candida while

Bhattacharjee and Mukerji (1982a) described the ultrastructure of Sclerocystis

coremioides sporocarp. Mukerji et al, (1983) reported two species of Glomus

viz., G. multisubtensum and G. delhiense both from soils of Delhi. Till this

date, 102 AM fungal species have been reported from India.

The occurrence of AMF in a natural forest was recorded in the old Delhi

Ridge, Saraswathi Range of Haryana (Thapar and Uniyal, 1996), forest soils of

Andhra Pradesh (Manoharachary and Rao, 1991), coastal tropical forest of

Tamil Nadu (Raghupathy and Mahadevan, 1991; Bhaskaran, 1997).

Shervaroyan hills of Tamil Nadu (Raman and Nagarajan, 1995), tea plantations

soil of Nilgiris of Tamil Nadu (Rajeshkumar, 2002; Rajeshkumar and Selvaraj,

2006) and black pepper grown in the forest soils of Kerala (Lekha et al, 1995).

The diversity of AM fungi also studied in the soils of cultivated cereal crops

and medicinal plants in Tamil Nadu (Selvaraj, 1989; Mahesh, 2002; Murugan,

2002; Sankar, 2002; Mani, 2005; Elango, 2005; Suresh and Selvaraj, 2006);

10

and in the coastal regions of Kongan and Shervaroyan hills of Tamil Nadu

(Gopinathan et al., 1991) South East Coast of Tamil Nadu (Nirmala and

Selvaraj, 2007), Coromandel Coast of Tamil Nadu (Raghupathy and

Mahadevan, 1991), Coastal sand dunes at Rameswaram of Tamil Nadu

(Nirmala and Selvaraj, 2007), West Coast of Kerala (Karthikeyan, 2005);

coastal sand dunes at Someshwara, Mangalore, West Coast of India (Beena

et al., 2000) and Western Ghats of Goa (Khade and Rodrigues, 2003). The

occurrence of AMF in arid and semi-arid regions was studied in Tamil Nadu

(Parthipon et al., 1991), deserts (Neeraj and Verma, 1991), and semi-arid grass

lands of Maruthamalai hills of Western Ghats of Peninsular India

(Muthukumar and Udaiyan, 1995).

The diversity of AMF in agricultural fields were also reported:

Leucaena leucocephala in Bangalore (Nalini et al., 1986), tobacco in the

cultivated fields of Tamil Nadu (Abdul Malik, 2000), cultivated medicinal

plants in Tamil Nadu viz., Wedelia chinensis (Sankar, 2002), Cichorium

intybus (Mumgan, 2002), Gloriosa superba (Elango, 2004); tea plantations in

Nilgiris, (Rajesh Kumar, 2002; Kumaran and Santhanakrishnan, 1995),

ornamentals and cultivated plants at Allahabad and adjoining areas (Kehri

et al., 1987), crop fields of Konkan and Sholapur (Dalai and Hippalgaonkar,

1995), in the fields of pearl millet, maize, pigeon pea and chick pea in Gwalior

(Singh and Pandya, 1995), Tamil Nadu (Suresh and Selvaraj, 2006) and

different agro climatic regions of India (Singh and Adholeya, 2002).

The distribution of AM fimgi in stressed ecosystems has also been

reported from coal, lignite and calcite mine soils of India (Mehrotra, 1995),

Kothagudam coal mine site, Andhra Pradesh (Rani et al, 1991), heavy metal

polluted soils of Tamil Nadu (Mahesh, 2002), petro-effluent irrigated fields.

11

soils polluted with industrial and sewage effluents (Reddy et al., 1995;

Mahesh, 2002; Mahesh and Selvaraj, 2007), tannery effluent polluted soils of

Tamil Nadu (Raman et al, 1995; Mahesh, 2002).

Arbuscular mycorrhizal status of tree species from Western Ghats also

been documented (Mohankumar and Mahadevan, 1987). Kandaswamy et al,

(1988), carried out an intensive survey for the prevalence of AM fungi in forest

tree species occurring at different altitudes in Western ghats of Nilgiri district,

Tamil Nadu. Later, Mohankumar and Mahadevan (1988 and 1989) carried out

ecological studies on AM fungal association with plant species from Kalakad

reserve forest area located in the Western Ghats, Tamil Nadu. They

investigated the influence of edaphic factors and seasonal variation on the

distribution of AM fungi in seven well defined ecosystems viz., evergreen,

semi-evergreen, mixed deciduous, teak forests, scrub jungle and grasslands of

high and low attitudes.

AM fungal association with plant species from Western Ghats was

documented by Muthukumar and Manian (1993). Muthukumar and Udaiyan

(2000) surveyed AM fungal association in four vegetation types viz., forest,

grass land, scrub and cultivated lands. They reported mycorrhizal association

in 174 plants out of the total 329 plant species surveyed. Thirty five AM fungal

species belonging to five genera viz., Acaulospora, Gigaspora, Glomus,

Sclerocystis and Scutellospora were recorded. They have also reported the AM

fungal association in 58 of the total 60 medicinal plants of Maruthamalai Hills,

in Western ghats of Southern India (Muthukumar and Udaiyan, 2001). They

reported 10 AM fungal species belonging to four genera viz., Acaulospora,

Gigaspora, Glomus and Scutellospora.

12

Rodrigues and Jaiswal (2001) recorded arbuscular mycorrhizal

association in six plant species growing on sand dune vegetation of Goa. They

reported the presence of three AM fungal genera viz., Acaulospora, Glomus

and Sclerocystis. Sanjay et al. (1997) reported arbuscular mycorrhizal

association in the rhizosphere of 12 plant species growing on the sand dunes in

the west coast of India. A total 16 species of AM fungi were recorded during

the study belonging to four genera viz., Gigaspora, Glomus, Sclerocystis and

Scutellospora. Glomus albidum, Glomus clarum and Scutellospora gregaria

were dominant AM fungi.

Khade and Rodrigues (2002) recorded AM fungal association in

commonly occurring pteridophytes from two sites located in Western Ghat

region of Goa. They recorded a total of 18 AM fungal species belonging to

5 genera viz., Acaulospora, Gigaspora, Glomus, Sclerocystis and

Scutellospora.

These studies indicate that the genus Glomus is ubiquitous in various

ecosystems in India. The distribution of other genera, i.e., Acaulospora,

Entrophospora, Gigaspora, Sclerocystis and Scutellospora is limited,

indicating greater adaptability of Glomus to various soil conditions.

2.4. Diversity of AMF from rhizospliere soils and roots of crop plants

The rhizosphere soil is an imperative one, to assess the AMF diversity

in roots of host plants and also associated with a great variety of plants of

different taxonomic groups (Jeffries, 1987). There are reports on the

association of AM in several cultivated crops (Ammani et al., 1986; Rathi and

Singh, 1990; Johnson et al, 1991; Talukdar and Germida, 1993; Kurle and

Pfleger, 1994; Lakshman and Raghavendra, 1995). Fodder and cereal crops

13

such as maize (Karasawa et al, 2000; Suresh, 2006; Dharmarajan, 2006),

Pennisetum typhoides (Dharmarajan, 2006), S. bicolor (Boemar, 1992;

Suresh, 2006), soyabean (Subba Rao and Krishna, 1987; Khaliel et al, 1992;

Khaliel and Loynachan, 1994), plantation cash crops such as cardamom

(Manjunath and Bagyaraj, 1982; Mallesha and Bagyaraj, 1991; Singh and

Singh, 1992), banana (Girija andNair, 1988), mulberry (Rajagopal et al, 1989;

Katiyar et al, 1995), vegetable crops (Selvaraj, 1989; Muthukumar and

Udaiyan, 1999; Akhilesh Mishra et al, 2002; Mulani et al, 2002) and tuber

crops such as sweet potato (Tholkappian et al, 2000); potato (Iqbal et al,

1990) and rhizomatous medicinal crops (Elango, 2004; Murugan, 2002; Mani,

2005).

There are several reports of mycorrhizal association in oil yielding crops

viz., sesame (Sulochana et al, 1989; Selvaraj, 1989; Manoharachary et al,

1990; Selvaraj and Subramanian, 1995), groundnut (Krishna, 1981), sunflower

(Chandrasekara et al, 1995; Chinnamuthu and Venkata Krishnan, 2001);

coconut (Ramesh, 1984) and safflower (Sulochana and Manoharachary, 1989;

Janakirani and Manoharachary, 1994).

2.5. Host preference of AM fungi.

The AM fungi are not host specific, any plant species can be infected by

an AM fungal species but the degree of AM infection and its effect can differ

with different host endophyte combinations.

It has become more obvious that AM fungi differ greater in their

symbiotic effectiveness (Mosse, 1972). Though a particular AM fungi can

infect and colonize many host plants, it prefers host which exhibits maximum

symbiotic response when colonized by that particular AM fungus, which led to

14

the concept of 'host preference' in AM fungi (Powell, 1982; Bagyaraj et al,

1988, Reena and Bagyaraj, 1990; Vinayak and Bagyaraj, 1990).

Crop species can exert a selective effect in determining AM species

which become predominant in a mixed indigenous population (Schenck and

Kinloch, 1980). Pre-transplant rice plant (Dhillion, 1992) and forage legumes

(Giovannetti and Hepper, 1985) exhibited considerable affinity for

colonization by AM fungi in growth chamber studies and the results of these

studies suggested the presence of host-mycorrhizal specificity. Glomus

gerdemanii infected only one plant species, out of the nineteen plant species

tested (Graw et al, 1979).

The phenomenon of ecological specificity is well known in

ectomycorrhizae (Harley and Smith, 1983), it is, however, rarely reported in

AM except in plants that are selectively breed for it (Gianinazzi, 1991). Apart

from the studies mentioned above, a few studies have reported host

mycorrhizal preference among AM endophytes. It is possible that in a mixed

fungal inoculum, a host exposed to a selection of AM fungi, could be

preferentially colonized by one or more endophytes, suggesting host

mycorrhizal preference of ecological specificity (Louis and Lim, 1987;

McGonigle and Fitter, 1990).

Attempts have been made to use the genetic variability in fungal

efficiency and host response to select AM fungal isolates to improve plant

production (Trouvelot et al, 1986). Variation in the effect of AM colonization

has also been linked with genotype of host plant (Krishna et al., 1985; Lackie

et al, 1987). The isolation of myco-plant mutants (Due et al., 1989) and the

discovery that AM colonization is a heritable trait (Mercy et al., 1990)

15

suggested the possibility of tailoring plant-fungus combinations for maximum

efficiency, leads to development of endo-mycorrhiza specificity with certain

AM fungi (Gianinazzi et ah, 1989). Different levels of host mycorrhizal

affinity existed in Asparagus officinalis, Tagetes erecta, Trifolium pratens,

Sorghum vulgare and Lycopersicum esculentum plants were inoculated with

number of AM species (Hetrick and Bloom, 1986). Subhashini et al. (1988)

reported that genotype differences among forty seven diverse varieties and

exotic germplasm oi Sorghum showing different levels of AM colonization and

phosphorus content in shoot and roots are also varied in different cultivars.

Dhillion (1992) reported that the host-mycorrhizal preference existed between

native prairie grasses and indigenous isolates of AM fungal species. Glomus

geosporum and G. fasciculatum. Host endophyte specificity in the AM

symbiosis was reported in crop plants (Estaun et al., 1987, Bagyaraj et al.,

1989) and in native plants (McGonigle and Fitter, 1990). The concentration of

strigolactones in root exudates coincides with the host specificity of AM fungi

(Akiyama et al., 2005).

2.6. Inoculum production of AMF

The AM inoculum can be produced in a sterile environment through

nutrient film technique, root organ culture and tissue culture (Nopavnovnbodi,

et al, 1988). Large quantities of the inoculam can be. produced by pot culture

technique (Wood, 1984). Cultures of AM fungi on plants growing in

disinfected soil have been the frequently used technique to increase propagule

numbers. A highly susceptible host plant should be used. It should produce

abundant roots quickly and tolerate the high - light conditions required for the

fungus to reproduce rapidly. Trap plants should be screened to ensure that

maximum levels of inoculums were achieved (Bagyaraj and Manjunath, 1980).

The traditional and most widely used approach has been to grow the fungus

16

with the host plant in solid growth medium individually or a combination of

the solid growth media such as soil, sand, peat, vermiculite, perlite, clay or

various types of composed barks (Tiwari and Adholeya, 2002).

Plants with mycorrhizal associations predominate in most natural

ecosystems, so inoculum of mycorrhizal fungi is present in most soils. The

quantity inoculum of mycorrhizal fungi which are compatible with a host plant

in soils can be measured by bioassay experiments. In these experiments,

seedlings are grown in intact soil cores or mixed soil samples for sufficient

time to allow mycorrhizas to form, then roots are sampled, processed and

assessed to measure mycorrhiza formation (McAfee and Fortin 1986, Warcup

1991, Brundrett and Abbott, 1995).

Several host plants including Sudan grass {Sorghum bicolor var.

Sudanese), bahia grass {Paspalum notatum), guinea grass (Panicum

maximum), cenchrus grass {Cenchrus ciliaris), cloves {Trifolium

subterraneum), strawberry (Fragaria sp.). Sorghum (Sorghum vulgare),

maize {Zea mays), onion {Allium cepa) and Coleus {Coleus sp.) have been

used for their suitability to multiply AM fungal inoculum. Sreenivasa and

Bagyaraj (1988b) reported that rhodes grass {Chloris gayana) is the best host

for mass multiplication of Glomus fasciculatum.

2.7. Selection of an efficient AM fungi and their impact on plant growtli

Species of AM fungi that can either directly or indirectly increase plant

growth by improving soil conditions (Kapoor and Mukerji, 1990). Direct

benefits are usually related to the enhancement of phosphate uptake by the

plant, however, in some soils zinc, copper and ammonium are also important

(Stribley, 1987). Indirect benefits may include increased soil aggregation or

stabilization of soil associated with hyphae formed in the soil. The selected

17

inoculants should have the ability to colonize roots rapidly following

inoculation, absorb phosphate from the soil, transfer phosphorus to the plant,

increase plant growth and persist in the soil as required (Raman and

Mahadevan, 1996). The selection of AM fungi for possible use in agriculture

has been first reported by Abbott and Robson (1982). Govinda Rao et al.

(1983) suggested that several fungi could be screened for symbiotic response

using a test-host through pot culture followed by micro-plot and then by field

trails. Many studies have led to the selection of AM fungi for economically

important agricultural crop and medicinal plants such as Allium cepa (Ramana

and Babu, 1999); Ananas comosus (Jaizme-Gega and Azcon, 1995); Arachis

hypogeae (Vijaya Kumar and Bhiravamurthy, 1999); Acacia nilotica (Reena

and Bagyaraj, 1990); Azadirachta indica (Sumana and Bagyaraj, 1999);

Cajanus cajan (Ahiabar and Hirata, 1994); Capsicum (Jizma-Gega and

Azcon, 1995); Colacasia esculenta (Ganesan and Mahadevan, 1994);

Cyamopsis tetragonoloba (Mathur and Vyas, 1996); Coleus aromaticus

(Selvaraj et al., 1996); Citrus (Vinayak and Bagyaraj, 1990); Eleusine

coracana (Tewari et al., 1993); Glycine max (Vasuvat et al., 1987);

Lycopersicum esculentum (Mallesha et al., 1994); Leucaena leucocephala

(Byrareddy and Bagyaraj, 1988); Cassava (Sivaprasad et al., 1990; Ganesan

and Mahadevan, 1994); Musa acuminata (Jaisme-Vega and Azcon, 1995);

Oryza sativa (Secilia and Bagyaraj, 1994; Ammani and Rao, 1996); Phaseolus

mungo (Vasuvat et al., 1987); Polianthus tuberosa (Gaur et al., 1998),

Trifolium (Gazey et al., 1992); Tamarindus indica (Reena and Bagyaraj,

1990); Tectona grandis (Rajan et al., 2000); Triticum aestivum (Asif e al.,

1995); Vigna mungo (Rao and Rao, 1996); Vigna radiata (Rao and Rao,

1996); Vigna unguiculata (Ahiabor and Hirata, 1994), Zea mays (Boucher

et al., 1999) and Zizyphus mauritiana (Mathur and Vyas, 1996).

18

Umamaheswar Rao et al, (1995) reported that AM fungi play a vital

role in crop production. The soils showed a positive correlation with relative

humidity and organic content of soils and negative correlation with

atmospheric temperature. Mukherjee and Rai (2000) reported that the highest

grain yield in Triticum aestivum at levels of phosphorus by inoculation of

Pseudomonas striata followed by Glomus fasciculatum. Udaiyan and

Sugavanam (1996) reported that inoculation of Glomus fasciculatum with

plants of Casuarina equisetifolia, results in the higher growth and biomass.

Sitaramaiah et al. (1998) reported that AMF inoculated plants increased

vegetative growth, total chlorophyll content and uptake of nutrients like,

nitrogen, phosphorus, potassium, calcium and magnesium in maize plants.

Ghandour (1992) reported that mycorrhizal association is beneficial for plant

growth and nutrient absorption particularly nutrient are limiting factor for plant

growth. Mudalagiriyappa et al, (1997) analysed that AMF inoculation

significantly increased in dry matter production, improved growth and net

assimilation rate. Geethakumari et al., (1994) observed that there was an

interactive pattern between phosphorus rich soil and mycorrhizal fungi. They

also reported that there was a positive influence of mycorrhizal inoculation in

the growth and yield of cowpea due to better uptake of nutrient especially

phosphorus by the plants.

Bagyaraj and Manjunath (1980) observed enhanced plant growth,

nodule formation, dry matter production, root colonization and nutrient uptake

in AMF inoculated leguminous plants. Tilak et al.. (1995) observed that the

phosphate solubilizing micro organism interact well with AMF and resulted in

synergistic effect with higher yield and pronounced growth, due to the

mobilization of phosphorus by AM fungi. Thakur and Panwar (1995) reported

19

that the AMF inoculation increased the root, shoot and total dry matter

production in mungbean. It also enhanced the growth and developmental traits

such as plant height, leaf area and net photosynthetic rate. Iqbal, et al, (1990)

analysed that dual inoculation of AMF in potato enhanced higher AMF

colonization improved plant growth and developed resistance to infection by

Rhizoctonia solani.

Gupta and Janarthanan (1991) reported that artificial inoculation of

palmorosa grass (Cymbopogon martini) with Glomus aggregatum showed

enhanced growth and biomass production. Dey et al, (2005) reported that

highest fruit weight, fruit length and fruit diameter with the inoculation of

AMF. Chinnamuthu and Venkata Kiishnan (2001) reported that combined

application of vermicompost with AMF significantly increased the yield of

sunflower. Samanta et al, (2003) reported the increasing plant height and

number of leaves in combined application of phosphate solubilizer with AMF.

Vankassel et al, (1985) observed the synergistic interaction between soyabean

and maize treated with AMF, and they reported the increased growth rate,

more uptake of nutrients, increased supply of phosphorus and growth

hormones.

2.7.1. Nutritional aspects of AM fungi

The benefits are greatest in phosphorus deficient soils and decrease as

soil phosphate levels increase (Schubert and Hayman, 1986). Very high and

very low phosphorus levels may reduce mycorrhizal infection/colonization

(Koide, 1991). It is well estabhshed that:

• infection by mycorrhizal fungi is significantly reduced at high soil

phosphorus levels (Amijee et al, 1989; Koide and Li, 1990)

20

• the addition of phosphate fertilization results in a delay in infection as

well as a decrease in the percentage of infection of roots by mycorrhizae

• an increase in the level of soil phosphate results in reduction in

chlamydospore production by the fungus. These spores are involved in

root infection and spread of the fungus through the soil profile (Koide,

1991).

Abbott and Robson (1979) reported that levels of soil phosphorus is

greater than that required for plant growth eliminated the development of the

arbuscles of vesicular-arbuscular types of mycorrhizae. Arbuscles are

structures produced within the host plant cells by the VA mycorrhizae. These

structures are responsible for the transfer of absorbed nutrients from the fungus

to the plant. The level of phosphorus in the plant also has been shown to

influence the establishment of VA mycorrhizae with high levels inhibiting

colonization by mycorrhizae.

Plant responses to AMF infection have received several explanations, of

which phosphorus uptake has consistently proved to be the primary factor.

Mechanisms suggested for improved phosphorus uptake by AMF infected root

include; (1) infection and proliferation of AM hyphae in the host roots; (2)

better distribution of absorbing network; (3) greater surface area and faster

extension rate of active hyphae involved in absorption and (4) more favourable

geometry of hyphae relative to roots and exploration of smaller pore spaces in

soil. Factors that increase surface area of the root such as root hairs and

mycorrhizal hyphae, will be advantageous to the absorption of nutrients

present in low concentration in soil solution. Root growth rate and radius are

the two most important root parameters for phosphorus uptake where uptake

increases with root radius because of increased surface area. Phosphorus

21

uptake depends on narrower absorbing structures; the possibility that AMF

strains with narrow hyphae may be more effective in promoting growth of

clover and Sorghum inoculated with G. fasciculatum and Gigaspora

margarita. Sorensen et al, (2005) investigated that high soil phosphorus level

or high soil inoculum was mostly responsible for the limited response of

increased mycorrhiza formation on plant growth and nutrient concentration.

Elke Neuman et al, (2004), investigated that the accumulation of more

phosphorus in shoots and increased biomass production in AM inoculated

plants oiSorghum bicolor.

Nitrogen uptake from soil by AM fungi can be affected by a number of

factors, which intum influence the predominant available forms of nitrogen

i.e., N H / and NO3. Thus factors such as organic matter content, soil texture

(clay content), microbial mineralization and nitrification can greatly influence

nitrogen uptake via extrametrical hyphae (Arines, 1990).

Nitrogen has been reported to stimulate as well as suppress root

colonization by AM fiingi. There have been several reports on the suppression

of root colonization by nitrogen (Buwalda and Cole, 1982). Chambers et al.

(1980) reported that both NO3' and NH4" depressed root colonization by AM

fungi and suggested that the suppressive effect of NH4 was due to drop in

rhizosphere pH. Thompson (1986) also found that nitrogen source influenced

AM colonization primarily through pH modification. In contrast, nitrogen

application has been reported to increase AM fungal colonization in Leucaena

(Aziz and Habte, 1989) and lettuce (Hepper, 1983). Sylvia and Neal (1990)

found that the suppressive effect of phosphorus on root colonization was

evident only in nitrogen sufficient plants. They also reported that plants with

nitrogen stress affect the resistance of the host colonization by AMF. AM

22

increase nitrogen accumulation in plant roots and the various mechanisms

suggested include; (i) direct uptake of combined nitrogen by AM fungi;

(ii) improvement of symbiotic biological nitrogen fixation can have indirect

AM activity based on the supply of phosphate; (iii) interplant transfer of

nitrogen benefit the non-fixing plants growing nearby and (iv) enzymatic

activities involved in nitrogen metabolism.

There are many reports on the effects of AM fungi on concentration and

the amounts of potassium in the plants and these results are inconsistent and

difficult to interpret (Sieverding and Toro, 1988). The ability of the

extrametrical AM fungal hyphae to uptake and transport potassium has also

been demonstrated in compartmented pots (George et al., 1992). Significant

differences in growth response of soybean to different geographical isolates of

G. mosseae seemed to be more related to impart potassium rather than

phosphorus nutrition of the host (Bethlenfalvay et al, 1989). The uptake and

transport rate of calcium is very low compared to phosphorus. A substantial

contribution of hyphae delivery to the host plant is not likely under most cases

because of high mobility of Ca " and SO/" in the soil.

The numerous reports on the enhancement of zinc and copper uptake by

AM plants can be attributed to the uptake and transport in external hyphae to

the host plant. The hyphal contribution to the uptake of zinc ranged from 16-25

per cent compared to 13 to 20 per cent for phosphorous in maize grown in

calcareous soil (Kothari et al., 1991). In the same soil ( Li et al., 1991)

demonstrated that the delivery of copper from the hyphae compartment ranged

from 52 to 60 per cent of the total copper uptake under restricted rooting space.

In contrast, manganese uptake and concentration in plants are either unaffected

but more often are lower in AM plants (Lambert and Weidensaul, 1991). The

23

decrease in concentration of manganese in plants is most likely an indirect

effect caused by AM induced changes in the rhizosphere microorganisms

among the population of manganese reducers (Kothari et al., 1991). The role of

AM on boron nutrient of the host plant is either lacking or inconclusive. AM

may decrease boron concentration in host plants (Kothari et al., 1991). Plants

have varying mechanisms for mobility, chelating and reducing ferric (Fe) in

order to facilitate uptake of iron. Treeby (1992) indicated that AM fungi may

facilitate the iron uptake in acidic but not in alkaline soils.

2.7.2. Biochemical aspects of AMF

Increasing attention is given to the biochemical aspects of AMF

inoculated plants in order to study the chlorophyll content, soluble sugars,

soluble proteins, amino acids and enzymes. Manjunath and Bagyaraj (1981)

reported that the rate of photosynthesis is higher in AMF treated plants than

non inoculated plants. Since photosynthesis is often a sink limited process with

excess carbon dioxide fixation, the enhancement may be mediated not only

through mineral nutrients improved by fungi to the host plant, but also by the

role of soil hyphae as organs of export to the soil. Cytokinin and chlorophyll

contents of plants have been found to increase by AM fungal association

(Allen et al., 1980). Panwar (1991) observed the beneficial effect of

Azospirillum brasilense and AMF on growth and yield under pot culture

condition. Azospirillum inoculation enhanced the root growth whereas AMF

enhanced the shoot growth resulting in altering the shoot and root ratios. Dual

inoculation enhances total chlorophyll content, photosynthetic rate, in vivo

nitrate reductase activity (NRA) and glutamine synthetase activity (GS). Dual

inoculation of AMF and Azospirillum caused synergistic effect on growth of

wheat. The chlorophyll content was considerably increased due to the uptake

of nutrients and release of growth regulating substances like auxin, gibberellin

24

and cytokinin (Coxwell and Johnson, 1985). AMF enhanced the mineral

uptake particularly magnesium, copper, iron, sulphur, and zinc which leads to

increase in the chlorophyll content by the formation of grana in chloroplast

(Adepedu and Akapa, 1977). An increase in the activity of nitrate reductase

was regarded as an indirect effect of the AM fungus and not due to the activity

of enzymes (Oliver et al., 1983).

Increasing attention is being given to study of the biochemical processes

involved in the mycorrhization symbiosis at the root level. Difference in

quantitative and qualitative expression of proteins has been shown in ecto

mycorrhizas (Hilbert and Martin, 1988; Martin and Hilbert, 1991) and in

arbuscular mycorrhizas (Arines et al, 1993; Dumas et al., 1989; Pacovsky,

1989; Schellenbaum et al, 1992; Wyss et al, 1990).

Coxwell and Johnson (1985) reported that the effect of AMF and

nitrogen source increase growth and transport of amino acid composition of

host plants. Nemec and Meredith (1981) found that G. etunicatum inoculated

Citrus limon leaves had higher total amino acids than the control. Krishna and

Bagyaraj (1984) found that proteins and amino acids increased in

G. fasciculatum inoculated Arachis hypogeae roots. Dumas et al (1994)

observed higher protein content in AM fungi inoculated Nicotiana tobaccum

and onion {Allium cepa) roots than the control. There was no appreciable

difference in free amino acid composition among the different treatments

except proline, arginine which were higher in the mycorrhizal plants than in all

other treatments. In Trifolium basicola infected plants, the free amino acids

level increased in the initial stage and decreased in later stages compared with

the control plants. High amount of protein was found in G. mosseae infected

Trifolium pratense roots (Arines et al, 1993).

25

Protein content was much higher in mycorrhizal than in

non-mycorrhizal root extracts of tobacco and onion (Dumas et al., 1989). The

protein content was 6-fold higher in mycorrhizal plants than non-mycorrhizal

plants of red clover roots. Five polypeptides with 16, 17, 18, 22 and 30 kDa

were found only in AM roots, and were considered to be arbuscular

mycorrhizins (Pacovsky, 1989). Wyss et al. (1990) have also found some

polypeptides in both the low and high molecular weight ranges, which were

immuno-precipitated with an antiserum against soluble nodulins or membrane-

bound nodulins. Mycorrhizal infection causes changes in polypeptide

accumulation of either decreasing or increasing synthesis of new proteins

(Martin and Hilbert, 1991). Mycorrhiza formation increases the expression of

low molecular weight proteins as suggested by the results obtained in tobacco

(Dumas et al., 1990), soybean (Pacovsky, 1989) and mulberry (Kumaresan,

1997). Arines et al. (1993) compared protein patterns in non-mycorrhizal and

VA-mycorrhizal roots of red clover. Soluble protein content was higher in

mycorrhizal than in non-mycorrhizal roots. Several additional polypeptides

appeared in mycorrhizal extracts, after separation by DEAE cellulose

chromatography and electrophoretic analysis.

Singh et al, (1984) suggested that the AM fungi increase the better

absorption of minerals especially phosphates and nitrogen which leads to the

biosynthesis of amino acids such as valine, glycine and proline. These are the

essential amino acids require for the synthesis of proteins. Ramesh et al,

(2000) reported that individual inoculation of AM fungi in Pennisetum

pedicellatum enhanced the enzyme activity of alkaline phosphatase,

superoxide dismutase (SOD), chitinase and acid-B-glycerophosphatase. These

are various enzymes act as pathogen related protein involved in the defense

mechanism and also act as synthesis of new proteins and scavenging the

26

nucleic acids (RNA and DNA) in host plants. Subba Rao (1993) reported that

the Glomus mosseae enhanced the accumulation of enzymes and amino acids

resulting in the development of resistance in the host plant. In plants, amino

acids are synthesized by organic compounds (glutaric acid) and inorganic

nitrogen. The inorganic nitrogen enters into plant either in the form of nitrate

or ammonia and it incorporated into glutamic acid. It serves as precursor for

synthesis of other amino acids and proteins.

Bago et al. (2000) suggested that the glyoxylate cycle may be central to

the flow of carbon in the AM symbiosis. Bago et al. (2003) carried out a study

in the analysis of storage and structural carbohydrates labeling after ' Cs

glucose provided to AM roots and concluded that glycogen performs four roles

in AM symbiosis viz., sequestration of hexose taken from host, long term

storage in spores, translocation from intra-radical mycelium to extra-radical

mycelium and buffering of intracellular hexose levels throughout the life cycle.

Fatty acids derived from abundant phospholipids of AM fungi (located

in membrance structures) and neutral lipids (located in storage structures) are

potentially useful for estimating the biomass of infective AM propagules

(Olsson et al., 1995). In addition some fatty acids have potential as specific

markers for AM fungi. For example, fatty acids 16:1 © 5c, 18:1 o 7c, 20:3,

20:4 and 20:5 have been detected either exclusively or in higher amounts in

AM spores in the roots of plants colonized by AM fungi, prompting attempts

for identification, characterization or differentiation of AM fimgi on the basis

of fatty acid profiles (Graham et al, 1995; Jansa et al, 1999; Olsson, 1999;

Madan et al, 2002). The AM fungal symbiosis is responsible for huge flexes

of photosynthetically fixed carbon from plants to the soil.

27

Carbon is transferred for the plant to the fungus as hexose, but the main

form of carbon stored by the mycobiont at all stages of its life cycle is

triacylglycerol (Cox et al, 1975; Beilby and Kidbuy, 1980; Jabaji-Hare, 1988;

Gasper et al, 1994; Bago et al, 2002). Lipid bodies have been observed in

arbuscular trunks, intercellular hyphae, extraradical spores and germ-tubes by

electron microscopic observation (Bonfante-Fasolo, 1984; Bonfante-Fasolo et

al, 1994). Recently, proposed models of carbon movement in the AM

symbiosis include translocation of fungal lipids as a central route of carbon

flow during both symbiotic and spore germination phases of the fungal life

cycle (Bago et al, 1999a and b; Bogo et al, 2000; Lammers et al, 2001).

Madan et al (2002) reported the fatty acid methyl ester analysis in the

spores of four AM fungi Glomus coronatum, G. mosseae, Gigaspora

margarita and Scutellospora calospora and found out that 16:1 co 5c to be the

dominant fatty acid.

van Aarle and Olsson (2003) compared the occurrence of fatty acid with

that of arbuscules and vesicles (lipid storage organs). The fatty acid 16:1 ©5

was used as a indicator for both AM fungal phospholipids and neutral lipids in

roots and in soil. The formation of arbuscules and the accumulation of AM

fungal phospholipids in intraradical mycelium closely related. In contrast, the

neutral lipids of G. intraradices increased continuously in the intraradical

mycelium and it decreases in the vesicles after initial rapid root coonization by

the fungus. The amount of neutral lipids is usually higher than that of

phospholipids in AM fungi, since these fungi store a large proportion of their

energy carbon as neutral lipids (Olsson and Johansen, 2000). The intraradical

mycelium utilizes of hexoses from the host, which are metabolized to yield

neutral lipids (Pfeffer et al, 1999). Neutral lipids thus have a central role in

28

carbon metabolism and transport in AMF which are probably the main

respiratory substrate in the extraradical mycelium (Bago et al, 2000).

Triacylglycerols are the main type of neutral lipids found in large amounts in

AM fungal spores and vesicles (Beilby and Kidby, 1980; Cooper and Losel,

1978; Nagy et al., 1980). In contrast, neutral lipids appear to be rare in

arbuscules (Nemec, 1981). The neutral lipids in glomalean fungi are enriched

in 16:1c fatty acids compared with the phospholipids (Graham et al., 1995;

Olsson and Johansen, 2000). This fatty acid is not normally found in other

fungi (Muller et al, 1994) and has been used as a indicator for AM fungal

lipids in different growth systems (Green et al., 1999; Larsen et al., 1998). This

fatty acid accumulates in roots during AM fungal colonization (Graham et al.,

1996; Graham et al., 1995; Olsson et al., 1995) and the amount accumulated is

correlated total root colonization (Olsson et al., 1997). The phospholipids fatty

acid 16:1 co 5 in soil systems are rather high, the amount of neutral lipid fatty

acid 16:12 co 5 is a sensitive indicator for the amount of AM fungal myceHa

(Olsson, 1999).

Subba Rao (1995) suggested that Glomus mosseae is known to enhance

the enzyme activity and arginine accumulation, which develops resistance in

host plants. Cliquet and Stewart, (1993) reported that, the AM fungi are well

known to bring about physiological changes in plants by means of increasing

enzymatic activity. Pan war (1992) reported that nitrate reductase (NR) activity

enhanced significantly in the mycorrhizal inoculated plants. The synergistic

effect was noticed when both inoculants were combined together. The

inoculation with exotic AM fungi increased the root and shoot NR activity

(about 188 % and 38% respectively) than the un-inoculated plants of

Juniperus oxycedrus even under stressed conditions (Alguacil, 2006).

29

Many species of AM fungi exhibit variation in enzyme banding patterns

both in terms of total buffer soluble proteins (Seviour et al, 1985) and specific

enzymes (Alfenas et al., 1984; Backhaus et al, 1984; Maghrabi and Kish,

1985), it is essential that there should be similarities between isolates of the

same species for identification. AM fungi can be identified within roots by

differences in the mobility of specific fungal enzymes during polyacrylamide

gel electrophoresis (Hepper et al., 1986). Gianinazzi-Pearson and Gianinazzi,

(1978), observed that the involvement of mycorrhiza phosphatase in plant

growth stimulation by the enzyme alkaline phosphatase. The polyphosphate

granules present in the fungal vacuoles are responsible for the activities of

alkaline phosphatase (Gianinazzi et al, 1979). Dodd et al. (1987) found that

the high concentrations of acid as well as low alkaline phosphatase activity in

the rhizosphere of VA-mycorrhizal plants, may be due to the direct fungal

secretion or an induced secretion by the plant roots. Peroxidase, which is

generally composed of a number of isozymes, is capable of catalyzing several

different types of oxidative reactions. Peroxidase may differ in biochemical

properties such as specific activity, substrate affinity, cofactors and sensitivity

to inhibitors (Sen and Hepper, 1986).

The possibility of isozyme variation at the isolate level is of interest

because AM fungi are now being described which belong to the same taxon

but which differ either in their adaptation to the environment, for instance in

their tolerance to heavy metals (Gildon and Tinker, 1981) or pH (Wang et al,

1985) or in their physiological effects on a host plant (Stahl and Smith, 1984;

Haas and Krikun, 1985). Hepper et al. (1986) found that the mobilities of three

enzymes, esterase (EST), glutamate oxaloacetate transminase (GOT) and

peptidase (PEP), during polyacrylamide gel electrophoresis have been used to

identify individual species of vesicular-arbuscular mycorrhizal fungi in roots.

The enzyme banding patterns obtained from mycorrhizal root extracts were

30

compared with those of uninfected roots. Chlamydospores, external mycelium

and internal mycelium (obtained by enzymic digestion of host root tissue) were

used as controls to confirm the position of the fungal specific enzyme bands on

the gels. Glomus caledonium and Glomus mosseae could be detected in leek

{Allium porrum L.) roots against the host background by the mobility of bands

of EST, GOT and PEP activity and it was possible to detect both

G. caledonium and G. mosseae in leek roots which had been grown in the

presence of a mixed inoculum, by staining for any of these three enzymes.

Glomus mosseae could be identified in maize {Zea mays L.) roots by location

of GOT and PEP activity but the major bands of EST, GOT and PEP activity in

G. caledonium had the same mobility as those of maize bands, and so this

fungus could not be easily identified in this host using these experimental

conditions. Glomus fasciculatum type E3 had a characteristic PEP band which

was separable from leek and maize host roots and this fungus could also be

identified in maize roots by the position on the gel of fungal GOT activity.

Oliver et al. (1983) reported that when root extracts of two cultivars of

cereals infected by three species of Glomus were subjected to electrophoresis.

They exhibited bands of alkaline phosphatase activity which could be

attributed to the fungal partner. It has also been shown that the external

mycelium of two species of Glomus contained acid phosphatases which had

different electrophoretic mobilities (Maskall et al, 1982).

The involvement of mycorrhiza phosphatase in plant growth stimulation

has been suggested (Gianinazzi-Pearson and Gianinazzi, 1978). This enzyme

was characterized as alkaline phosphatase (Gianinazzi-Pearson and Gianinazzi,

1978) and considered to be related to the phosphate metabolism of the fungus

because alkaline phosphatase activities were present within the fungal vacuoles

where the polyphosphate granules were observed (Gianinazzi et al, 1979).

31

Dodd et al. (1987) found that the high concentrations of acid as well as

low alkaline phosphatase activity in the rhizosphere of VA-mycorrhizal plants,

may be due to the direct fungal secretion or an induced secretion by the plant

roots. Plants containing many kinds of acid phosphatase isoenzyme in their

tissues (Paul and Williams, 1987) and their activity of special isoenzymes is

enhanced by phosphorus starvation.

Peroxidase, which is generally composed of a number of isozymes, is

capable of catalyzing several different types of oxidative reactions. Peroxidase

may differ in biochemical properties such as specific activity, substrate

affinity, cofactors and sensitivity to inhibitors. Whether electrophoretic

mobilities of enzymes from different geographical isolates of a particular

species of fungus are the same has only been briefly studied using two isolates

of G. mosseae (Sen and Hepper, 1986).

Tisserant et al. (1998) found that a series of glass house experiments

was used to determine mycorrhiza-specific isozymes (MSIs) produced by five

species of Glomus colonizing roots of desert shrub legumes viz., Anthyllis

cytisoides. Thymus vulgaris and Allium porrum. Extracts of colonized roots

were electrophoresed on non-denaturing polyacrylamide gels (PAGE) and

stained for 10 different enzymes. Staining protocols for esterase, glutamate

oxaloacetate transminase, alkaline phosphatase and malate dehydrogenase

provided MSIs for the mycorrhizas formed by different AM fungi that had

colonized roots of the three host plants. There was no apparent correlation

between levels of colonization or arbuscular intensities, at or between each

sampling, and the presence of MSIs. The development of colonization by the

AM fungi differed little between the three plants when assessed with two

methods of estimating fungal biomass. The variety of MSIs detected might

reflect the diversity of metabolic activities of these Glomus species and,

32

possibly, differing ecological functions. The high-level induction of two

alkaline phosphatase MSIs in the mycorrhizas of Anthyllis cytisoides colonized

by Glomus microaggregatum BEG56 was used to track the fate of this fungus

when the same plant was inoculated and transplanted into a semi-arid site in

south-east Spain. The probable fungal origin of the isozyme was indicated by

detection of the same isozyme in the extraradical mycelium formed by Glomus

microaggregatum BEG56 on Allium porrum.

Rosendahl (1992) tested the influence of three VAM fungi, Glomus

species on activity of enzymes in the roots of Cucumis sativus. Cucumber

plants were grown in a split-root system, in which colonized and un-colonized

roots of a single plant could be separated. The activity of the host root malate

dehydrogenase (MDH), glucose-6-phosphate dehydrogenase (Gd), glutamate

oxaloacetate transminase (GOT) and glutamate dehydrogenase (GDH) was

measured on a densitometer after separation of the host and fungal enzymes on

polyacrylamide gel. The results showed that only minor changes in the activity

of the host root enzymes occurred after VAM inoculation. Gd was stimulated

by VAM and phosphorus of GDH in the host plant when both parts of the root

system were colonized.

Changes in respiration and in the activity of specific plant enzymes

occur in plants attacked by fungal pathogens. Increase in the specific activity

of malate dehydrogenase and in the number of enzyme bands in plants diseased

with Fusarium oxysporum has been reported (Reddy and Stahmann, 1975).

This increase in enzyme activity in diseased plants is believed to be connected

to the accumulation of metabolites on the diseased tissue. In studying the

influence of VAM symbiosis on the host plant metabolism, Dehne (1986)

observed that glycolytic enzymes and various dehydrogenases were stimulated

in VAM plants. The enzymes involved in phosphate metabolism (Capaccio and

33

Callow, 1982; Gianinazzi-Pearson and Gianinazzi, 1976 and 1978; Krishna

et al, 1983) and nitrogen metabolism (Carling et ai, 1978; Oliver et al, 1983;

Smith et al, 1985) have also been studied. Direct measurement of the

glutamate synthetase activity in the internal mycelilum of the fungus suggested

that increased activity was at least in part due to the activity of fungal enzymes

(Smith et al, 1985). An increase in the activity of nitrate reductase was

regarded as an indirect effect of the VAM fungus and not due to the activity of

fungal enzymes (Carling et al, 1978; Oliver et al, 1983). Most of the above

mentioned studies have been performed with extracts of mycorrhizal roots

which included both plant enzymes and enzymes from the fungal symbiont

(Dehne, 1986; Dodd et al, 1987). In some studies fungal component was

separated from the plant by degrading the roots enzymatically to measure the

activity of the internal mycelium of the fungus (Cappaccio and Callow, 1982;

Smith et al, 1985). However, the enzymatic digestion of the host material

resulted in a dramatic decline in activity of the fungal succinate dehydrogenase

(McGee and Smith, 1990).

Isozyme techniques have featured in taxonomic and population genetic

studies of VAM (Hepper et al, 1988; Rosendahl, 1989). The isozymic patterns

of malate dehydrogenase (MDH) which is essentially an enzyme of TCA cycle

can be used for the identification and differentiation of many species / genera

of VAM fungi (Shankar and Varma, 1993).

Hepper et al (1988) reported that variation between different isolates of

three species of AMF was investigated by using the electrophoretic mobilities

of six enzymes extracted from resting spores. Some intra-specific variation was

observed but after numerical analysis of data, isolates of Glomus monosporum

and G. mosseae formed a cluster which was clearly distinguishable from

G. clarum and G. caledonium. A single isolate of G. manihotis fitted within

34

the G. clarum cluster. Isozyme analysis allowed unidentified or tentatively

identified isolates to be compared with fungi which had been classified on the

basis of spore morphology.

Sen and Hepper (1986) found that six species of Glomus have been

characterized by subjecting spore extracts to polyacrylamide gel

electrophoresis and selective enzyme staining. The banding patterns of six

enz)mies viz., esterase, glutamate oxaloacetate transminase, hexokinase, malate

dehydrogenase, peptidase and phosphoglucomufase were diagnostic for each

species and recommendations of suitable enzymes to distinguish between these

fungi were given.

2.8. Seagrasses

Seagrasses are the marine flowering plants which represent the only

group of higher plants that have established so successfully in the subtidal and

tidal environments of World oceans expect the polar region. Eventhough the

importance of seagrasses was realized in the beginning of the 20 century, only

in the past two decades, the seagrasses attracted the attention of the marine

scientific community.

2.8.1. Taxonomical, biological and ecological studies on Indian seagrasses

Studies on seagrasses of India were started during early 1970s. Qasim

and Bhattathiri (1971) estimated the primary productivity of the Laccadive

seagrasses. Untawale and Jagtap (1975) recorded H. becarii from the Mandovi

estuary, Goa for the first time. Balasubramanian and Wafer (1975) studied the

primary productivity of some seagrass beds of the Gulf of Mannar.

Lakshmanan and Rajeswari (1979a, b) brought out the wealth of the seagrasses

of the Krusadai island and studied the anatomy of the seagrasses of

Hydrocharitaceae and embryology oi Syringodium isoetifolium. Occurrence of

35

Thalassia hemprichii was reported from the Krusadai and Rameswaram

islands by Lakshmanan and Rajeswari (1982) and Haloduleprinifolia from the

Porto Novo coast by Velusamy and Kannan (1985). Ravikumar et al. (1990)

reported Halodule wrightii for the first time in India. Ravikumar and Ganesan

(1990) described a new species viz., Halophila ovalis sub sp. ramamurthiana

from the East Coast of India. Parthasarathy et al. (1991) described the

distribution of seagrasses along the Tamilnadu coast and Jagtap (1991)

described the distribution of seagrasses along the Indian coast. Ramamurthy et

al. (1992) have also explored the wealth of seagrasses of the Coromandal

Coast with taxonomic illustrations. Primary production of the seagrass,

Cymodocea serrulata and its contribution to the total productivity of the

Lakshadweep island were worked out by Kaladharan and David Raj (1989).

Das (1996) made detailed studies on the seagrass habitats of the Andaman and

Nicobar coasts. Kannan et al. (1989) reported on the morphological diversity

of Halophila ovalis from the Porto Novo marine environment. Recently, the

status of seagrasses in different regions of India with their special floral and

faunal values has been prepared by Kannan et al. (1999).

2.8.2. Productivity of seagrasses

The highly productive areas of the oceans are the upwelling regions on

the continental shelf, the shallow protected bays of all coasts, salt marshes,

sea-tree forests (mangroves) and seagrass meadows. Among these, seagrasses

contribute significantly to the productivity of coastal areas of both temperate

and tropical waters (Phillips and McRoy, 1980). Larkum and West (1983) have

stated that seagrasses have primary productivities comparable to those of

terrestrial crops, though more recent studies set a more realistic upper limit

while still acknowledging that the seagrasses are amongst the most productive

of submerged ecosystems. Fortes (1986) and Estaction and Fortes (1988)

36

reported the production rate of E. acoroides as 1.4 g Cm" day"' in Cape

Bolinao, Northern Phihppines and 1.08 g Cm' day'' in North Basis Bay,

Southern Philippines respectively. These are fairly comparable to those of

cultivated crops like wheat, com, rice, etc. on world average basis (McRoy and

McMillan, 1977).

Kaladharan and David Raj (1989) have stated that Cymodocea plants

produce 6.4 g cm' day" in Laccudives under natural light conditions and at the

same time production of planktons is only 0.931 gm cm"'' day"' which is

6 times lesser than that of C. serrulata production. Due to greater radiation

input and much longer growing seasons, tropical seagrasses can have still

greater productivities.

2.8.3. Biomass of seagrasses

Seagrasses biomass refers to above and below sediment plant materials

and the standing crop refers to the above sediment plant materials only and

their values are expressed as gram dry weight per square meter (g dry wt. m")

(Hillmann et ah, 1989). Majority of seagrasses literature reports about standing

crop values and a few studies report about all components of the seagrasses

biomass because of the difficulty in recovery root material and it will be quite

difficult to sample adequately the below ground biomass because of the depth

of penetration of the root system (Jones, 1968; Zieman, 1972, 1975).

Brouns (1987) reported that little seasonal variation in the above ground

biomass of five species of seagrasses {Thalassia hemprichii, Cymodocea

serrulata, C. rotundata, Halodule universes and Syringodium isoetifolium) in

Papua New Guinea waters.