K. Udaiyan

Arbuscular mycorrhizas of plants growing in the Western Ghats region, Southern India

Abstract A survey of the arbuscular mycorrhizal (AM) status of plants growing in the Western Ghats region of Southern India was undertaken. Root and soil samples of plants growing in the four vegetation types forest, grassland, scrub, and cultivated land or plantation were examined. Of the 329 species (representing 61 families) examined, 174 were mycorrhizal. AM association was recorded in 81 species for the first time, including species from several families assumed to be non-mycorrhizal, e.g. Amaranthaceae, Capparaceae, Commelinaceae, Cyperaceae and Portulacaceae. AM fungal spores of 35 species belonging to Acaulospora, Gigaspora, Glomus, Sclerocystis and Scutellospora were recorded. AM fungal species richness was found to be highest in scrub and lowest in agricultural and plantation soils. Mean colonization levels were dependent on plant life-form, life-cycle pattern and vegetation type.

Mycorrhiza in sedges--an overview.

Most terrestrial plants associate with root-colonising mycorrhizal fungi, which improve the fitness of both the fungal and plant associates. However, exceptions exist both between and within plant families failing to associate with mycorrhizal fungi or in the incidence and the extent of mycotrophy, which may vary greatly. Sedges are important pioneers of disturbed habitats and often dominate vegetations like wetlands, and arctic and alpine vegetations, in which the mycorrhizal inoculum in the soil is often low or absent. In the past, sedges were often designated as non-mycorrhizal, though limited reports indicated the presence of mycorrhiza in certain species. However, studies since 1987 indicate widespread occurrence of mycorrhiza in sedges. Based on these studies, the family Cyperaceae is no longer a non-mycorrhizal family, but the mycorrhizal status of its members is greatly influenced by environmental conditions. Further, sedges appear to have several morphological adaptations to thrive in the absence of mycorrhizal association. Though mycorrhizal associations have been noted in many sedge species, the ecological role of this association is not well documented and no clear generalisation can be drawn. Similarly, the role of mycorrhizal fungi on sedge growth and nutrient uptake or non-nutritional benefits has yet to be fully ascertained. This paper reviews the current information available on the incidence of mycorrhiza in sedges and the possible reasons for low mycotrophy observed in this family.

Arbuscular mycorrhizal morphology and dark septate fungal associations in medicinal and aromatic plants of Western Ghats, Southern India

We investigated roots of 107 medicinal and aromatic plants (MAPs) in the Western Ghats region of Southern India for arbuscular mycorrhizal (AM) and dark septate endophyte (DSE) associations. Of the 107 MAPs belonging to 98 genera in 52 families examined, 79 were AM and 38 harbored a DSE association. Typical Arum- and Paris-type mycorrhizas are first reported in the presumed nonmycorrhizal family Amaranthaceae. Similarly, DSE associations are recorded for the first time in nine plant families and 37 plant species. Thirty MAPs had both AM and DSE associations. The number of MAPs having Arum-type mycorrhiza was greater than those having Paris-type. This was more prominent among herbaceous plants than in trees where the Paris-type was predominant. Similarly, the Arum-type was more prevalent in annuals than in perennials. DSE associations were more frequent in herbs and perennials compared to other MAPs.

Growth response and nutrient utilization of Casuarina equisetifolia seedlings inoculated with bioinoculants under tropical nursery conditions

We investigated the role of tetrapartite associations between an arbuscular mycorrhizal (AM) fungus (Glomus geosporum), phosphate solubilizing bacteria (Paenibacillus polymyxa), Frankia and Casuarina equisetifolia on growth, nutrient acquisition, nutrient utilization and seedling quality of C. equisetifolia. Seedlings of C. equisetifolia were grown in an Alfisol soil and inoculated with G. geosporum, P. polymyxa and Frankia either individually or in combinations. Inoculation of bioinoculants stimulated seedling growth, the efficiency of nutrient uptake and improved seedling quality. However, microbial inoculation generally reduced the efficiency of nutrient utilization in dry matter production (nutrient use efficiency). Inoculation of P. polymyxa or Frankia increased the extent of AM colonization, which resulted in the accumulation of the nutrients. Seedlings inoculated with Frankia and G. geosporum had more, and heavier nodules compared to seedlings inoculated with Frankia alone. Dual inoculation of microbes was more effective than individual inoculations. The growth response of seedlings to inoculation involving all the microbes was greater than the response to either individual or dual inoculations. The results of this study showed that the tetrapartite association could improve the growth, nutrient acquisition and seedling quality of C. equisetifolia under tropical nursery conditions.

Influence of native endomycorrhiza, soil flooding and nurse plant on mycorrhizal status and growth of purple nutsedge ( Cyperus rotundus L.)

A study was made to investigate the effects of native endomycorrhizal fungi, soil flooding and nurse plant (onion) on the mycorrhizal status, growth and phosphorus (P) uptake of purple nutsedge (Cyperus rotundus L.). Above ground (AG) and below ground (BG) dry weight of mycorrhizal purple nutsedge plants were significantly lower than the non-mycorrhizal plants, but ratios of BG:AG dry weight were the same except when grown with onion. Mycorrhizal infection was characterized by vesicles and hyphae, both intraradical and extramatrical, but lacked arbuscules. Percentage root colonization of purple nutsedge and onion was significantly higher when the two grew together, but plants growing alone had less infection. Flooding of soil inhibited mycorrhizal formation in purple nutsedge, and mycorrhizal infected plants produced smaller tubers than uninfected plants. AG tissue P concentration was lower in mycorrhizal purple nutsedge but BG P concentration had no significant variations between infected and uninfected plants. P utilization efficiency (PUE) was similar for mycorrhizal and non-mycorrhizal purple nutsedge. Plants in flooded soil had the maximum AG and BG dry weights, tuber dry weight and P content, but had lower AG:BG dry weight ratio and PUE.

Vesicular-arbuscular mycorrhizae in tropical sedges of southern India

Twenty-four species of sedges (representing six genera) from different vegetation types in Western Ghats, South India, were examined for vesicular-arbuscular mycorrhizal (VAM) associations. All the sedges had VAM fungal infections with hyphae and vesicles, but arbuscules were observed only in 42% of the total species. The VAM fungal colonization varied considerably between species, ranging from 9 to 62%. Root colonization was positively and negatively correlated with root diameter and root hair length, respectively. The number of VAM fungal spores in the rhizosphere varied from 5 to 86 g-1 soil. No significant relationship was found between spore numbers or root colonization and either soil pH or moisture. Four Glomus spp., one Acaulospora sp., one Sclerocystis spp., and one Gigaspora sp. were identified among the VAM fungal spores. The results reflect a high incidence of VAM in sedges occurring in Western Ghats and emphasize the need to assess the VAM status of plant species from different ecosystems to understand their mycorrhizal status.

The role of seed reserves in arbuscular mycorrhizal formation and growth of Leucaena leucocephala (Lam.) de Wit. and Zea mays L.

Abstract We show here that seed reserves in Leucaena leucocephala (Lam.) de Wit. and Zea mays L. (maize) are important for mycorrhizal formation and seedling growth. Seed reserve removal reduced mycorrhizal formation markedly in Leucaena but not in maize, except at 15 and 45 days after seed reserve removal. Partial or total removal of seed reserves decreased plant growth and tissue nutrient concentrations in both hosts. Nodule number in Leucaena, which was related positively to plant biomass and mycorrhizal infection levels, was reduced when one or both cotyledons were severed. Leucaena seedlings without or with partial seed reserves had higher nutrient use efficiencies throughout seedling growth. But such an effect was observed only initially in maize. Seed reserve removal increased the specific absorption rates of nutrients in both hosts. Phosphorus absorption rate was significantly and positively related to root infection levels in both Leuceana and maize. Though the growth rates of plants without seed reserves were low initially, these plants had higher growth rates during later stages. We conclude that seed reserves are not only important for seedling growth, but also for mycorrhizal formation and nodulation.

Arbuscular mycorrhizas in cycads of southern India

Abstract. Root and soil samples of three potted or ground-grown cycads (Cycas circinalis, C. revoluta, Zamia sp.) were collected between November 1999 and June 2000 and surveyed for arbuscular mycorrhizal (AM) colonization and spore populations. AM fungi were associated with all root systems and rhizosphere samples examined. Root colonization was of a typical Arum type and AM colonization levels differed significantly between species and between potted and ground-grown cycads. Mycorrhizal colonization levels were inversely related to root hair number and length. Spores of nine morphotypes belonging to three genera (Acaulospora, Glomus, Scutellospora) were extracted from soil. The percentage root length colonized by AM fungi was not related to soil factors, but total AM fungal spore numbers in the rhizosphere soil were inversely related to soil nitrogen and phosphorus levels. AM fungal spore numbers in the soil were linearly related to root length colonized. The co-occurrence of septate non-mycorrhizal fungi was recorded for the first time in cycads. These observations and the relationship between plant mycorrhizal status and soil nutrients are discussed.

Biostatic effect of fumigation and pesticide drenches on an endomycorrhizal-Rhizobium-legume tripartite association under field conditions

The effects of formaldehyde fumigation and pesticide drenching with Bavistin, Cuman, Copperthom, Sulfex, Furadon, and Termix at recommended rates on vesicular-arbuscular mycorrhizal (VAM) colonisation and Rhizobium sp. nodules were assessed regularly for a period of 90 days in the legumes Cajanus cajan, Dolichos biflorus, Vigna mungo, and V. unguiculata under field conditions. The fumigant and the pesticides initially reduced VAM fungal colonisation and the number of spores in all plants. Following the initial decrease there was a slow recovery, but by 90 days after emergence, root colonisation was either parallel to or still lower than the control, and the number of spores was still well below control levels for all species except C. cajan, which had more VAM spores than the control in all treatments except fumigation and Furadon. Although the number of nodules did not differ from control levels at 30 days after emergence, differences were evident during the later stages of plant growth for all species except V. unguiculata. The effect of pesticides on VAM fungi and root nodulation varied with the associated host plant species. Plant tissue P and VAM colonisation were significantly correlated in all host plants. The pesticide treatments had no marked effect on plant growth, but accumulations of nutrients in pesticide-treated plants were lower than those in untreated plants. Growth and nutrient status of the legumes varied with VAM fungal colonisation.

The effect of burning on soil enzyme activities in natural grasslands in southern India

Soil amylase, cellulase, invertase and phosphatase activities were determined following wildfire in a savanna type grassland in southern India. Activities of these enzymes increased substanially in burned soils compared to those in adjacent unburned soils. Surface soil (0–10 cm) exhibited a greater increase in enzyme activities than subsurface soil (10–20 cm). Amylase activity was more pronounced following fire than the other enzyme activities.