Influence of Phosphorus Nutrition on Growth and Carbon Partitioning in Glycine max

Soybean plants (Glycine max [L.] Merr var Amsoy 71) were grown in growth chambers with high-phosphorus (high-P) and low-phosphorus (low-P) culture solutions. Low-P treatment reduced shoot growth significantly 7 days after treatment began. Root growth was much less affected by low-P, there being no significant reduction in root growth rate until 17 days had elapsed. The results suggest that low-P treatment decreased soybean growth primarily through an effect on the expansion of the leaf surface which was diminished by 85%, the main effect of low-P being on the rate of expansion of individual leaves. Low-P had a lesser effect on photosynthesis; light saturated photosynthetic rates at ambient and saturating CO(2) levels were lowered by 55 and 45%, respectively, after 19 days of low-P treatment. Low-P treatment increased starch concentrations in mature leaves, expanding leaves and fibrous roots; sucrose concentrations, however, were reduced by low-P in leaves and increased in roots. Foliar F-2,6-BP levels were not affected by P treatment in the light but in darkness they increased with high-P and decreased with low-P. The increase in the starch/sucrose ratio in low-P leaves was correlated primarily with changes in the total activities of enzymes of starch and sucrose metabolism.