Regulation of Assimilate Partitioning in Soybean

Abstract

Incresed concentrations of nitrate in a nutrient solution (2,5, and 10 millimolar KNO3) were correlated with increased shoot:root ratios of non-nodulated soybeans (Glycine max [L.] Merr.) grown in sand culture. While altering the pattern of C and N partitioning, the N treatments did not affect whole plant photosynthesis over the study period. To determine the mechanism responsible for the observed changes in assimilate partitioning, detailed C and N budgets were worked out with plants from each N treatment over three consecutive 4-day periods of midvegetative growth. The information for the C and N budgets from the 2 and 10 millimolar NO3- treatments was combined with data on the composition of xylem and phloem exudates to construct a series of models of C and N transport and partitioning. These models were used to outline a ''chain-reaction'' of cause-and-effect relationships that may account for the observed changes in assimilate partitioning in these plants. The proposed mechanism identifies two features which may be important in regulating the partitioning of N and other nutrients within the whole plant. (a) The concentration of N in the phloem is highly correlated with the N concentration in the xylem. (b) The amount of N which cycles through the root.sbd.from phloem imported from the shoot to xylem exported by the root.sbd.is regulated by the root''s requirement for N: only that N in excess of the root''s N requirements is returned to the shoot in the xylem. Therefore, roots seem to have the highest priority for N in times of N stress.