INFLUENCE OF VESICULAR ARBUSCULAR MYCORRHIZAL INFECTION AND SOIL-PHOSPHORUS LEVEL ON GROWTH AND CARBON METABOLISM OF SOYBEAN

The effect of vesicular-arbuscular (VA) mycorrhizal infection on growth and photosynthesis in nodulated soybean (Glycine max (L.) Merr. cv. Hobbit) plants cultured at high and low levels of soil phosphorus (P) was explored in a 2 .times. 2 factorial experiment. The high- and low-P soils were constituted by adding 200 and 40 .mu.g P (KH2PO4) .cntdot. g-1, respectively, to a low-P soil (8 .mu.g .cntdot. g-1 bicarbonate extractable P). Mycorrhizal (Glomus fasciculatum Thaxter sensu Gerdemann) and non-mycorrhizal inocula were added to each soil, thereby constituting the two mycorrhizal treatments. In plants grown in low-P soil, VA mycorrhizal infection resulted in higher foliar P concentrations (compared with the nonmycorrhizal treatment) and in significantly greater shoot and nodule dry weights. In plants grown in high-P soil, VA mycorrhizal infection had no significant or consistent effect on shoot or root dry weights or on P concentrations, and decreased nodule weight. Photosynthetic rates were not affected by VA mycorrhizal infection or P treatment. These results suggest that in low P grown plants, VA mycorrhizal infection increased the uptake and transport of P to leaves and this, in turn, resulted in greater rates of shoot growth via an increased production of photosynthate, not because of an increase in photosynthesis on a leaf are basis but because of an increase in the rate of expansion of the leaf surface.