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SUMMARY Mycorrhizal and non‐mycorrhizal onion seedlings were grown in individual soil chambers in which roots were confined to one side of a barrier. External hyphae of Glomus fasciculatus arising from mycorrhizal roots grew into an adjacent volume of soil. 32P was injected into soil at 1‐cm intervals up to a distance of 8 cm from the confined roots. Relatively high levels of radioactivity were subsequently detected in root segments of mycorrhizal plants at all distances from tracer injection. High levels of radioactivity were detected also in leaf segments of mycorrhizal plants. Radioactivity of root or leaf segments of non‐mycorrhizal plants did not exceed background level at any distance from tracer injection. 32P did not move more than 7.5 mm from the point of injection as indicated by gross autoradiography. Absorption of phosphate and its trans‐location to the host by hyphae of G. fasciculatus can extend the phosphate uptake zone of mycorrhizal onions to at least 7 cm from the root surface.

MATERIALS AND METHODSMycorrhizal soybean (Glycine max L. Merr. var. plants have lower resistances to water transport than nonmycorrhizal 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 essentially 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.The vesicular-arbuscular mycorrhizal relationship, one of the most common forms of symbiosis, increases the growth of many host plant species (1-3, 10, 15, 16, 20, 25, 29) including soybean (30, 31). The increased growth has been attributed to an enhanced nutrient uptake by the host plant (2,12,15,(18)(19)(20)26). Mycorrhizal plants take up more phosphorus than nonmycorrhizal controls when relatively unavailable sources of phosphorus are supplied to the roots (12,26). In most cases the addition of more readily available phosphorus eliminates differences in growth and phosphorus uptake (3,12,13,20,26).We have recently shown that VA3 mycorrhizae also decrease the resistance to water transport in whole soybean plants (31). The present study was undertaken to determine where the fungus acts and whether the lowered resistance to water transport can be attributed to increases in the nutrient status of the host brought about by the mycorrhizae.'This work was supported in part by The method used for determining resistance to water transport was that of Boyer (5, 6), which has previously given results that compare well with independent methods (6, 31) and provides consistent determinations of whole plant resistances in mycorrhizal and nonmycorrhizal soybean (31). The method involves measurement of the recovery in water potential of a moderately water-deficient leaf on an intact plant. The time required for recovery, which is determined when no transpiration is occurring, is dependent on the resistance to water transport.For measurement of the recovery of a whole plant, the blade of an intact soybean leaf which was moderately water deficient was sealed in a thermocouple psychrometer chamber that measures the water potential of intact leaves (4). The soil and root system were then submerged in degassed water, excess water was drained away, and the recovery in water potential of the leaf was recorded. After determining that the data conformed to the transfer equation for a plane sheet (5, 9, 1 1), the resistance of the whole plant was calc...