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 photosynth ate between starch and sucrose is influenced by the relative concentra tions of inorganic phosphate (Pi) in the cytosol and chloroplast. Two greenhouse experiments were conducted to determine the influence of lo ng-term differences in soil P levels, ranging from deficient to suprao ptimum, on leaf starch and sucrose concentrations, and activities of a denosine diphosphate glucose (ADPG) pyrophosphorylase and sucrose-phos phate synthase (SPS) during the grain filling period in soybean (Glyci ne max [L.] Merr.). It was hypothesized that, compared with optimum P nutrition, leaf starch and sucrose concentrations would be increased a nd decreased, respectively, for P deficiency and visa versa for suprao ptimum P nutrition. Relative to the optimum soil P level, leaf Pi conc entration was not altered by P deficiency but was increased two- to fo urfold for the supraoptimum soil P treatment. The concentrations of le af starch and sucrose were not markedly affected by any of the P ferti lity treatments and were not closely related to the activities of ADPG pyrophosphorylase and SPS. P deficiency resulted in increased activit y of both enzymes in one of the experiments. The results indicated tha t long-term soil P treatments, that caused either large decreases in p lant growth (P deficiency) or large increases in leaf Pi concentration (supraoptimum P), did not markedly alter starch and sucrose metabolis m. Furthermore, it can be inferred that the method of plant culture an d/or imposition of the P treatments is a critical factor in interpreti ng results of P nutrition studies.