Proc. lndian Acad. Sci. (Plant Sci.), Vol. 98, No. 1, February 1988, pp. 55-59. 9 Printed in India. Vesicular-arbuscular mycorrhizal associations of sesamum M VIJAYALAKSHMI and A S RAO Department of Botany, Nagarjuna University, Nagarjunanagar 522 510, India MS received 4 April 1987; revised 14 November 1987 Abstract, Extensive mycorrhizal infection of sesamum under field conditions was observed. The plants were found to be mycorrhizal even in the early stages. There was a progressive increase in mycorrhizal infection up to 6 weeks, then der slightly. A gradual increase in vesicle count per 1 cm segment as the plants aged was noticed. Species of Glomus, Gigaspora and Sclerocystis were found in the rhizosphere soil. Among the spore types recorded, chlamydospores of Glomus, especially Glomus mosseae were predominant. Keywords. Sesamum indicum; vesicular-arbuscular mycorrhizae; spores; sporocarps; Glomus spp.; Gigaspora spp.; Sclerocystis pakistanica. 1. Introduction Vesicular-arbuscular mycorrhizae (VAM) ate known to increase phosphorus uptake and growth in a number of agricultural crops, especially in soils low in available phosphorus (Gerdemann 1975; Hayman 1980). VAM can also improve uptake of copper (Timmer and Leyden 1980), zinc (Lambert et al 1979) and sulphur (Gray and Gerdemann 1973). Plants infected with mycorrhizal fungi exhibit significant yield increases over non-mycorrhizal plants (Khan 1975; Saif and Khan 1977). As there were no studies of mycorrhizas on sesamum (Sesamum indicum L.), the identity oÿ the fungal partners, their development in root system and their spore count in corresponding rhizosphere were investigated. 2. Materials and methods The sesamum crop was planted in small field plots during 1986 rabi (January-March). As the rabi season was warm and dry, the crop was grown under irrigation. The soil in which plants raised was a red laterite with ph 7.2, organic matter 0"28% and MHC20.8%, and is known to harbour VAM fungi (Parvathi et al 1984; Vijayalakshmi and Rao 1987). At 15-day intervals from the time of sowing until harvest (after 75 days), the plants were removed at random, their roots were excised, washed carefully, cut into 1 cm segments, cleared, stained and examined for V.AM fungi (Phillips and Hayman 1970). The presence or absence of the fungus and the number of vesicles in 50 root segments of 1 cm length collected at random from a total of 5 plants were estimated on each occasion. The percentage of mycorrhizal infection was calculated as follows: No. of infected segments VAM infection(%)= x I00. Total no. of root segments examined Spore types of VAM fungi and their abundance in rhizosphere were recorded simul- 55
56 M Vijayalakshmi and A S Rao taneously with root infection for 5 replicates, after extraction by the wet-sieving and decanting technique (Gerdemann and Nicolson 1963). 3. Results Mycorrhizal infection was mostly confined to finer lateral roots in the initial stages. Profuse external hyphae with prominent angular projections were common on the root surface (figure 1). Entry was mostly through the epidermal cells (figure 2). Mycorrhizal infection with internal hyphae, arbuscules (figure 3) and vesicles (figure 4) could be seen in 15-day old plants. The per cent root segments infected by VAM fungi increased progressively until 6 weeks (table 1), later there was an apparent decline which may be due to the absence of infection in newly formed roots. The incidence of arbuscules in root segments was initially high but declined slightly at later stages. Vesicle count per 1 cm segment progressively increased with increasing plant age. A progressive increase in spore population in rhizosphere soil as the plants aged was found. Chlamydospores of Glomus species were predominant among the spore types recorded. Of the 3 species of Glomus identified viz. Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, Glomus macrocarpum Tul. and Tul. (figure 5) and Glomus multicaule Gerdemann and Bakshi (figure 6), G. mosseae was quite abundant in soil samples. Sporocarps of Sclerocystis pakistanica Iqbal and Bushra (figures 7 and 8) were also found but not abundant. These VAM fungi were identi based on the descriptions given by Mosse and Bowen (1968), Gerdemann and Trappe (1974), El- Figures I-4. I. External mycelium over root surface. 2. Entry of VAM fungi into the root. 3. Arbuscules in root cortex. 4. Vesicles/spores in root cortex.
VAM of sesamum 57 Table 1. Mycorrhizal development in S. indicum. Spores and sporocarps in rhizosphere soil (50 g) Average no. Plant Root colonization a of internal Unidentified age vesicles Glomus Gi9aspora spore (days) External Internal cm-1 root spp. spp. S. pakistanica b types 15 39.1 (5.4) 42.5(4.9) 8.2(1.9) 45(15'0) 9(1.6) -- 15(4.3) 30 50-0 (7"2) 56.8(6.5) 12.5(2.8) 87(12.0) 10(3-2) 1(0-6) 41(8.5) 45 63.3(11.1) 48-3(9-8) 19.1(2.1) 115(14.5) 15(4.1) 1(0-6) 44(7.9) 60 49.7 (12.0) 36.8 (14.2) 19.9 (3.6) 208 (21.8) 18 (2.0) 2 (1.6) 39 (6.4) 75 25-6 (9.3) 30.0 (7.8) 20.5 (2.1) 239 (28.6) 20 (5.0) 2(1.4) 44 (6"8) aper cent root segments examined, bsporocarps. Numbers in parentheses denote SD values (n = 5). I! Ioo H 5 loop 7 Figures 5--8. 5. Sporocarp of G. macrocarpum. 6. Chlamydospore of G. multicaule. 7. Radial arrangement of spores around central plexus of hyphae in S. pakistanica. 8. Chlamydospores of S. pakistanica. Giahmi ee al (1976), Gerdemann and Bakshi (1976) and Iqbal and Perveen (1982). Besides the above mentioned spore types, azygospores of two species of Gigaspora were isolated, but not in sufficient numbers for confirming the identification. 4. Discussion This study confirms the formation of VA mycorrhizas in sesamum in a tropical soil in India. The roots were found to be mycorrhizal even in early stages, in contrast to
58 M Vijayalakshmi and A S Rao other hosts like groundnut, pigeon pea and black gram in the same soil (Rao and Parvathi 1982). As reported by Rao and Parvathi (1982), ah apparent decline in root infection by VAM fungi as the plants aged in the above crops was also noticed in sesamum and the cause for the decline in root infection by VAM fungi needs investigation. The extensive occurrence of internal vesicles with simple or funnel shaped attachments indicates that the VAM fungus may be a species of Glomus. This observation is confirmed by the predominant occurrence of chlamydospores of G. mosseae and G. macrocarpum in the rhizosphere soil. Earlier studies suggest that some species of VAM fungi were more efficient in phosphorus uptake than others (Jensen 1982; Krishna and Dart 1984; Schubert and Hayman 1986). The present study indicated the occurrence of a wide variety of endophytes in a tropical red lateritic soil and attempts are now being made to obtain monoxenic cultures for determining their efficacy on nutrient uptake of this important oil yielding crop. Acknowledgements The authors wish to thank Dr I R Hall, Invermay Agricultural Research Centre, New Zealand for providing literature on the taxonomy of Endogonaceae. One of the authors (MV) is grateful to the Council of Scienti and Industrial Research, New Delhi for financial assistance. References El Giahmi A A, Nicolson T H and Daft M J 1976 Endomycorrhizal fungi from Libyan soils; Trans. Br. Mycol. Soc. 67 164-169 Gerdemann J W 1975 Vesicular-arbuscular mycorrhizae; in The Development and Function ofroots (eds) J G Torrey and D T Clarkson (London: Academic Press) pp 575-591 Gerdemann J W and Bakshi B K 1976 Endogonaceae of India: Two new species; Trans. Br. Mycol. Soc. 66 340-343 Gerdemann J W and Nicolson T H 1963 Spores of mycorrhizal Endo(4one species extracted from soil by wet-sieving and decanting; Trans. Br. Mycol. Soc. 46 235-244 Gerdemann J W and Trappe J M 1974 The Endogonaceae in the Pacific Northwest; Mycol. Mem. 5 76 Gray L E and Gerdemann J W 1973 Uptake of sulphur-35 by vesicular-arbuscular mycorrhizae; Plant Soil 39 687-689 Hayman D S 1980 Mycorrhiza and crop production; Nature (London) 287 687-688 lqbal S H and Perveen B 1982 Some species of Sclerocystis (Endogonaceae) from Pakistan; Trans. Mycol. Soc. Jpn 21 57-63 Jensen A 1982 Influence of four vesicular-arbuscular mycorrhizal fungi on nutrient uptake and growth in Barley (Hordeum vulgare); New Phytol. 90 45-50 Khan A G 1975 The effect of vesicular-arbuscular mycorrhizal associations on the growth of cereals. II Effects on wheat growth; Ann. Appl. Biol. 80 27-36 Krishna K R and Dart P J 1984 Effect of mycorrhizal inoculation and soluble phosphorus fertilizer on growth and phosphorus uptake of pearl millet; Plant Soil 81 247-256 Lambert D H, Baker D E and Cole H Jr 1979 The role of mycorrhizae in the interactions of phosphorus with zinc, copper and other elements; Soil Sci. Soc. Am. J. 43 976-980 Mosse B and Bowen G D 1968 A key to the recognition of some Endogone spore types; Trans. Br. Mycol. Soc. 51 469--483 Parvathi K, Venkateswarlu K and Rao A S 1984 Occurrence of VA mycorrhizas on different legumes in a laterite soil; Curr. Sci. 53 1254-1255 Phillips J M and Hayman D S 1970 Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection; Trans, Br. Mycol. Soc. 55 158 161
VAM of sesamum 59 Rao A S and Parvathi K 1982 Development of vesicular-arbuscular mycorrhiza in groundnut and other hosts; Plant Soil 66 133-137 Saif S R and Khan A G 1977 The effect of vesicular-arbuscular mycorrhizal associations on growth of cereals. III. Effects on barley growth; Plant Soil 47 17-26 Schubert A and Hayman D S 1986 Plant growth responses to vesicular-arbuscular mycorrhiza. XVI. Effectiveness of different endophytes at different levels of soil phosphate; New Phytol. 103 79-90 Timmer L W and Leyden R F 1980 The relationship of mycorrhizal infection to phosphorus-induced copper deficiency in sour orange seedlings; New Phytol. 85 15-23 Vijayalakshmi M and Rao A S 1987 Development of vesicular-arbuscular mycorrhizal fungi in sunflower; Proc. Indian Natl. Sci. Acad. (in press)