PHOSPHORUS EFFECT ON PHOSPHATASE-ACTIVITY IN ENDOMYCORRHIZAL MAIZE

Success of a mycorrhizal symbiosis is influenced by the availability o f phosphorus (P) in the soil. Maize (Zea mays L. cv. Great Lakes 586) plants were grown under five different levels of soil P, either in the presence or absence of formononetin or the vesicular-arbuscular mycor rhizal (VAM) fungus Glomus intraradices Schenck and Smith. We detected physiological differences in mycorrhizal roots very early in the deve lopment of symbiosis, before the onset of nutrient-dependent responses . Under low P levels, VAM roots accumulated a greater shoot dry weight (13%), root P concentration (15%) and protein concentration (30%) tha n non-VAM roots, although root growth was not statistically significan tly different. At higher P levels, mycorrhizal roots weighed less than non-VAM roots (10%) without a concomitant host alteration of growth o r root P concentration. Mycorrhizal colonization decreased as soil P i ncreased. Formononetin-treatment enhanced colonization of the root by G. intraradices and partially overcame inhibition of VAM colonization by high soil P concentrations. This is the first report that formonone tin improves root colonization under high levels of soil P. Acid phosp hatase (ACP) and alkaline phosphatase (ALP) activities were closely re lated to the level of fungal colonization in corn roots. ACP activity in corn roots responded more to soil P availability than did ALP activ ity (38% more). These results suggest that ACP was involved in the inc reased uptake of P from the soil, while ALP may be linked to active ph osphate assimilation or transport in mycorrhizal roots. Thus, soil P d irectly affected a number of enzymes essential in host-endophyte inter play, while formononetin enhanced fungal colonization.