Nutrient status and mycorrhizal enhancement of water transport in soybean.

Mycorrhizal soybean (Glycine max L. Merr. var. Harosoy-63) plants have lower resistances to water transport than nonmy-corrhizal plants after 4.5 weeks of growth. Although resistances of whole plants differ by 40%, there were no differences in the resistances of stems plus leaves, indicating that the major effect of the mycorrhizae was to reduce the resistance of the roots. Since the fungitoxicant, p-chloronitrobenzene, had no effect on resistances to water transport, reduced resistances were probably not caused by a direct modification of the transport pathway by the fungus. Differences in resistance between mycorrhizal and nonmycorrhizal soybean were essentialy eliminated by the application of nutrients to the soil. Thus, lowered resistances of mycorrhizal roots growing in soil with low levels of nutrients probably resulted from the enhanced nutrient status of the plant brought about by the fungus. Mycorrhizal infection increased growth at both low and high nutrient levels.