Factors Affecting Vesicular-Arbuscular Mycorrhizal Development and Growth of Cotton

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Abstract
The influence of inoculum level, soil fertility, soil temperature and cotton cultivar on vesicular-arbuscular mycorrhizal development and plant growth were determined. In a P-deficient soil, inoculum levels of either 200 or 400 azygospores of Gigaspora margarita/plant significantly stimulated cotton growth, whereas rates of 10, 50 and 100 spores/plant had little or no effect. In a soil deficient in N and P, G. margarita increased plant growth only when sufficient N was added, but the mycorrhizal growth response was not as great as when both N and superphosphate were added to non-mycorrhizal plants. When P was deficient in the soil mix, mycorrhizal plants consistently had a higher content of this element in the foliage than did nonmycorrhizal cotton. Addition of superphosphate (25 .mu.g or more of P/g of soil) to a soil containing 10 .mu.g/g of P produced a growth increase at least equal to the growth response induced by G. margarita or Glomus etunicatus, and rates of 50 and 150 .mu.g/g suppressed root infection. Maximum mycorrhizal development by G. margarita and subsequent cotton growth stimulation occurred at 30 and 24.degree. C, and was slight or absent at 19 or 14.degree. C. Growth stimulation of 5 cotton cultivars inoculated with G. margarita varied considerably; ''Coker 310'' and ''Stoneville 213'' responded best, ''Acala 1517-70'' and ''Deltapine 16'' were marginally influenced, and ''Paymaster 909'' was relatively unaffected.