Phosphorus Nutrition Influence on Starch and Sucrose Accumulation, and Activities of ADP-Glucose Pyrophosphorylase and Sucrose-Phosphate Synthase during the Grain Filling Period in Soybean

Several lines of evidence indicate that the partitioning of photosynthate between starch and sucrose is influenced by the relative concentrations of inorganic phosphate (Pi) in the cytosol and chloroplast. Two greenhouse experiments were conducted to determine the influence of long-term differences in soil P levels, ranging from deficient to supraoptimum, on leaf starch and sucrose concentrations, and activities of adenosine diphosphate glucose (ADPG) pyrophosphorylase and sucrose-phosphate synthase (SPS) during the grain filling period in soybean (Glycine max [L.] Merr.). It was hypothesized that, compared with optimum P nutrition, leaf starch and sucrose concentrations would be increased and decreased, respectively, for P deficiency and visa versa for supraoptimum P nutrition. Relative to the optimum soil P level, leaf Pi concentration was not altered by P deficiency but was increased two- to fourfold for the supraoptimum soil P treatment. The concentrations of leaf starch and sucrose were not markedly affected by any of the P fertility treatments and were not closely related to the activities of ADPG pyrophosphorylase and SPS. P deficiency resulted in increased activity of both enzymes in one of the experiments. The results indicated that long-term soil P treatments, that caused either large decreases in plant growth (P deficiency) or large increases in leaf Pi concentration (supraoptimum P), did not markedly alter starch and sucrose metabolism. Furthermore, it can be inferred that the method of plant culture and/or imposition of the P treatments is a critical factor in interpreting results of P nutrition studies.