A study on Arbuscular Mycorrhizal Fungal spore abundance and species composition was undertaken in 25 medicinal plants in the Anamalai Hills, Western Ghats. The fungal spores found in the soil sample include Acaulospora, Glomus, Gigaspora, Sclerocystis and Scutellospora, genus. Among the rhizosphere soil examined, a maximum spore count of 696 spores/100 gram soil. The maximum spore population was observed during the winter season and percentage of colonization was found to be high during the summer season. The spore population was observed maximum in Tribulus subramaniamii. There is a correlation between spore abundance and colonization with respect to seasonal patterns were observed in this study.
Keywords- AMF, Mycorrhiza, Spore population and Root Colonization.
Arbuscular Mycorrhizal Fungi (AMF) are a ubiquitous group of soil fungi colonizing the roots of plants belonging to more than 90% of plant families (Brundett, 1991). Enhanced plant growth due to AMF association is well documented (Bagyaraj, 1984). In the past few decades, AMF have emerged as potential biofertilisers, a cheap, environmentally friendly alternative to expensive chemical fertilizers. AM fungi are obligate symbiotic soil fungi which colonize the roots of the majority of plants. The occurrence of this association has been often reported from very difficult environment (Arctic & Desert) to normal agro ecosystems. The association of this symbiotic relationship between the plants and fungus is widely reported and recognized by various researchers in various parts of the world. It is estimated that 90% of the terrestrial plants forms association with fungal partner for beneficial network of both plant and fungus.
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It is difficult to distinguish the relative contributions of the different types of propagules to colonization of the root systems of plants growing in any particular field situation that is to the 'infectivity' of the soil. The density of the spores in soil can be determined, but although this sometimes shows a correlation to the extent of root colonization (Suresh and Nagarajan, 2011).
Medicinal herbs are known as sources of phyto-chemicals, or active compounds that are widely sought after worldwide for their natural properties. They are useful source of essential oil and have been used for a long time in the perfumery, cosmetic, food and pharmaceutical industry.
The potential for increasing medicinal plant growth by effective management of AM strains reinforces the need to determine AM colonization of plants and the species composition of the AM community. Since most of the AM fungi are host specific it is necessary to find out the specificity of the organism. Earlier studies have indicated host preferences of mycorrhizal fungi (Miller et al., 1987), thus suggesting the need for selecting efficient AMF for a particular host (Sailo and Bagyaraj, 2005). In this present study an attempt has been made to find out the occurrence and percentage of colonization of AM fungus in 25 medicinal plants was assessed.
MATERIALS AND METHODS
1. Study area
The study area Anamalai hills is a significant segment of the Western Ghats, which lies between 10° 13' to 10° 33' N in latitude and 76° 49' to 77° 21' E. The vegetation of this region, harbor many endemic species and is a unique ecological tract rich in biodiversity. The forest tract of Anamalais exhibiting a wide diversity in terrain, elevation and climate supports diverse vegetations of striking differences. The main range of Anamalai hills has a general direction from North West to south east, with an elevation ranging from ca 900 m to 2500 mts.
2. Collection and analysis of samples
The rhizosphere soil and root samples of 10 Asteraceae members were randomly collected during Jan 2008 to Dec 2008 in a polythene bag and stored in refrigerator in Kongunadu Arts and Science College Laboratory for further analysis. The rhizosphere soil samples were wet-sieved for spores using the method described by (Gerdeman and Nicolson, 1963). Hundred grams of soil from each plant rhizosphere were independently suspended in 150 ml water, stirred with a magnetic stirrer for 10 min, sieved using 40, 70, 100 and 150 mm sieves with tap water, filtered onto a filter paper, and then placed in a 9 cm Petri dish for examination under a binocular stereomicroscope. The intact, healthy AMF spores with shining appearances were considered to be alive and counted in the four sieved samples. The spores were identified using keys adopted by Schenck and Perez (1990) and Raman and Mohankumar (1988).
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For the convenience of the study the period has been classified into three seasons namely winter (Jan-Apr), summer (May-Aug) and rainy (Sep-Dec).
Result and Discussion
This study describes the distribution of VAM fungi in the rhizosphere soil of medicinal plants. Both plants and rhizosphere soils were collected during January to December of 2008. Twenty five common herbaceous plants of indigenous medicinal value belonging to 17 families from 4 different sites were screened for AM association. All the plants were found to be positive AM colonization and were belonging to different genera. The spore population and percentage of root colonization was observed on the pattern of seasonal variation. The season was classified as Summer, Winter and rainy for the convenience of the study. The maximum spore population was found to be maximum during the winter season. The maximum population was found in Tribulus subramaniamii of Zygophyllaceae. The minimum spore population was seen in Euphorbia hirta during the rainy season.
The root colonization was observed maximum during the summer season in Zizyphus jujube of Rhamnaceae and minimum was observed Fluggea leucopyarus of Euphorbiaceae. The root colonization was maximum during the summer season this may be due to the increased activity of the plant and requirement for the nutrients and water stress. The decline in the infection was observed during the rainy season. This may be due to the inactive condition of the plants and washing away of the spores by rain water or may be because of the animal disturbances.
The widely accepted concept of increased sporulation and colonization in winter and summer and declining towards the rainy season was observed in the present study. Again, we are not able to conclude the exact reason for the increase in SP during summer and found very less during rainy season may be due to inactiveness of the plant or less stress conditions of the plants during these seasons.
The degree of variations in root colonization was varied from species to species and even within the species of same family. This shows the host specificity of the AMF and impact of abiotic factors such as climate, soil and disturbance by animals. The climatic variation influences the selection of AMF or regulates the incidence of certain specific strains in the soil (Requena et al., 1996). Throughout the study, a correlation was seen in the spore populations and temperature. This pattern of correlation was reported by Suresh and Nagarajan, (2009). A detailed study with more parameters such as soil characters, soil nutrients, animal and human disturbance levels are required to know exact reasons for these variations. This observation is in accordance with in situations where density of spores is positively correlated with the extent of root colonization, both may increase during the growing season of annual plants. Decreases in density with depth of soil could well be associated with the decline in density of roots and mycorrhizas (Hayman, 1970). Based on the spores the study revealed that the genus Glomus is dominating the rhizosphere of all times of sampling. Out of 5 genus observed the glomus is the most common genus found associated with almost all the plant species. Previously many researchers have reported the dominance of the genus Glomus worldwide. This could attribute to the survival ability of Glomus in many agro ecological conditions of Anamalai Hills. In aggregating with these present observations, various workers reported the ubiquitous nature of Glomus species in various tropical soils (Gerdemann, 1968). Thus the abundance and colonization looks rich in the soils will be the reason for the wide distribution of species richness of the plant community.