Role of Edaphic Factors on VAM Fungal Colonization and Spore Populations in Certain Tropical Wild Legumes

Four nodulating annual tropical wild legumes, viz., Alysicarpus monilifer, Desmodium triflorum, Indigofera linnaei and Tephrosia purpurea from three different regions in the Western Ghats ecosystem were investigated to assess their mycorrhizal status. The response ofvesicular-arbuscular mycorrhizal (VAM) root colonization and spore number to edaphicfactors such as soil moisture, pH and available Nand P was analysed. Though the spore number varied significantly both within and between sites, a uniformly high degree ofroot colonization was observed for all the plants in the present study. The spore number recorded was high, rangingfrom 15 to 165 spores gl soil. Spores of sixteen VAM fungal species belonging to Acaulospora, Glomus and Scutellospora were isolated from the rhizosphere soils. Soil moisture generally had a positive influence on VAM colonization and sporulation except in I. linnaei. The pH correlated negatively with root infection in I. linnaei and T. purpurea, but had no influence in the other two species. The effect of 2-pH on sporulation varied with host species and sites. No general correlation existed between available soil nutrients, root colonization and spore number but the influence ofNand P was counteractive on VAM infection. The present study indicates that the response of root colonization and spore number to edaphic factors is a localised rather than a generalised phenomenon. INTRODUCTION Vesicular-arbuscular mycorrhizal (VAM) fungi are ubiquitous and are an important factor in regulating and cycling of nutrients in most natural ecosystems. Information is lacking on how the composition and distribution of these fungi are affected by climatic and edaphic conditions. Researches in the past were directed mainly to understanding the general phenomenon of the mycorrhizal symbiosis such as differences in T. MUTHUKUMAR, K. UDAIYAN AND S. MANIAN their distribution, effect on host plants and their ability to colonize roots and sporulate. Such studies have demonstrated the sensitivity of individual species/isolates of VAM fungi to soil and environmental conditions (Mosse et al. 1981) . Spore germination, colonization ofhost roots and the ability of VAM fungi to influence the growth and physiology of the host are affected by edaphic factors (Daniels Hetrick 1984). Most of these studies were conducted in pot experiments under simulated field conditions, which have a limited predictive value under natural conditions although they provide a basic understanding of the physiology and ecology of the VAM systems (Bethlenfalvay el al. 1982). The intricacy of various interacting factors in natural ecosystems makes it difficult to assess the relationship between the host plant and its VAM endophyte (Bowen and Bevege 1976; Ross and Gillam 1973). Even as the distribution and abundance ofVAM fungi are known to vary with climatic and edaphic environments, the factors which control their actual distribution are poorly understood (Azizah Chulan and Omar 1991). Our ability to manipulate mycorrhizal symbiosis for agricultural benefits would be limited without a clear understanding of the role of edaphic factors on VAM fungal colonization and sporulation. In the present study, four annual nodulating legumes from three different regions in the Western Ghats ecosystem were selected for assessment of their mycorrhizal status and the response of VAM system to edaphic conditions. Since the response of VAM fungi may vary with host age, season etc., it seemed worthwhile to determine the response of VAM fungi in these annual legumes which are available for a short period during the monsoon. The aims of the present study are to: 1) assess the mycorrhizal status of the legumes under investigation, 2) find out the relationship between edaphic factors and VAM fungal root colonization and spore density and 3) use the results from this study as a reference for the introduction of exotic VAM fungal species in future experimental studies. MATERIALS AND METHODS Study Area and Plants Investigated The studies were conducted at three different regions in Western Ghats. Site A, an Acacia dominated jungle of Maruthamalai forest, is located at 76og3'N and 11°4'E. It is an offshoot of Western Ghats. Site B is Cymbo pogoncaesius dominated grassland at the foot of Maruthamalai hills. Site C, a Teciona grandis dominated dry deciduous forest at Siruvani, is a part of Nilgiri Biological Reserve, located at 76°37'N and 10 58'E. Four annual nodulating legumes Alysicarpus monilifer DC., Desmodium triflorum W. & A., Indigofera linnaei Ali and Tephrosia purpurea Pers., were examined for VAM association and their rhizosphere soils examined for VAM spores. Soil samples were collected at 550m, 426.72m and 500m MSL from the respective study sites. Total annual rainfall during the study period was 134.6mm at sites A and Band 238mm at site C. Collection ofSamples Roots and rhizosphere soil samples were collected from the three sites between August and October 1991, during the monsoon when the plants were plentiful. An average of five samples for each species were sampled. Plant roots were dug out, washed thoroughly to remove the adhering soil particles and fixed in FAA (5ml formalin, 5ml glacial acetic acid and 90ml 70% ethanol). Rhizosphere soil samples of the respective plant species (collected from different individuals) were thoroughly mixed to form a composite soil sample. The composite samples were packed individually in polyethylene bags for transport to the laboratory and stored at 4°C for future analyses. Analysis ofSoil Physiochemical Properties The soil moisture (dried 24h at 105°C) and pH (1: 1, soil: water) were determined soon after the soil samples were brought to the laboratory. The available nutrients, nitrogen and phosphorus were analysed using standard procedures of Jackson (1958) and Misra (1968). Determination of Mycorrhizal Status Clearing and Staining ofRoots The root samples of each species were gently washed free of FAA and cut into approximately 1 cm long segments. The root segments of the individuals of a species were mixed to form a composite sample. The composite sample was cleared by boiling in 10% KOH and the boiling time varied according to the thickness of the root 34 PERTANlKAJ. TROP. AGRIC. SCI. VOL. 17 NO.1, 1994 ROLE OF EDAPHIC FACfORS ON VAM FUNGAL COLONIZATION AND SPORE POPULATIONS segments. Cleared roots were acidified (5N Hel) and stained with 0.05% trypan blue in lactophenol (Phillips and Hayman 1970). Fifty root segments from each composite sample were examined for the presence of VAM structures. The percentage root colonization by VAM fungi was estimated using the root-slide technique of Read el al. (1976). VAM Fungal Spore Enumeration VAM fungal spores were recovered from soil by a slight modification of the wet sieving and decanting technique of Gerdemann and Nicolson (1963). One hundred grams of soil from each composite sample were dispersed in 500 ml of water to form a uniform suspension. The heavier soil particles were allowed to settle for 10-20 min. The suspension was then passed through a series of sieves ranging from 710-38m. The residues from the sieves were washed into beakers. After the settling of the heavier particles, the suspension was filtered through a Whatman No. I filter paper. To make spore count easier, lines at a distance of 2 mm were drawn on the filter paper and the intact spores were counted under appropriate magnification (x100). Isolation and Identification of VAM Fungal Spores Intact spores were picked up using a wet needle and mounted in lactophenol for identification. The species ofVAM fungi were identified according to the sporocarpic and spore characters such as spore size, colour, spore walls, hyphal attachments and other morphological characters (Morton 1988; Schenck and Perez 1987). Statistical Analyses Two factor analysis of variance was used to compare the variation of root colonization and spore number between and within sites. Pearson's coefficient correlation was used to determine the degree of association between root colonization and spore number with edaphic factors. Data on root colonization and spore number were subjected to linear regression analysis (Zar 1984)