Phosphate Regulation of Nitrate Assimilation in Soybean

Abstract

It is known that phosphorus deficiency results in alterations in the assimilation of nitrogen. An experiment was conducted to investigate mechanisms involved in altered 15NO−3 uptake, endogenous 15N translocation, and amino acid accumulation in soybean (Glycine max L. Merrill, cv. Ransom) plants deprived of an external phosphorus supply for 20 d in solution culture. Phosphorus deprivation led to decreased rates of 15NO−3 uptake and increased accumulation of absorbed 15N in the root. Both effects became more pronounced with time. Asparagine, the primary transport amino acid in soybean, accumulated in large excess in roots and stems. In roots of phosphorus-deprived plants, concentrations of ATP and inorganic phosphate declined rapidly, but dry weight accumulation was similar to or above that of the control even after 20 d of treatment. Arginine accumulation in leaves was greatly enhanced, even though 15N partitioning into the insoluble reduced-N fraction of leaves was unaffected. The results suggest that decreases in NO−3 uptake in low phosphorus plants could be caused by feedback control factors and by limited ATP availability. The decline in endogenous N transport from the root to the shoot may be associated with changes in membrane properties, which also result in parallel effects on hydraulic conductance and the upward flow of water through the plant.