PHOSPHATE REGULATION OF NITRATE ASSIMILATION IN SOYBEAN

It is known that phosphorus deficiency results in alterations in the a ssimilation of nitrogen. An experiment was conducted to investigate me chanisms involved in altered (NO3-)-N-15 uptake, endogenous N-15 trans location, and amino acid accumulation in soybean (Glycine max L. Merri ll, cv. Ransom) plants deprived of an external phosphorus supply for 2 0 d in solution culture. Phosphorus deprivation led to decreased rates of (NO3-)-N-15 uptake and increased accumulation of absorbed N-15 in the root. Both effects became more pronounced with time. Asparagine, t he primary transport amino acid in soybean, accumulated in large exces s in roots and stems. In roots of phosphorus-deprived plants, concentr ations of ATP and inorganic phosphate declined rapidly, but dry weight accumulation was similar to or above that of the control even after 2 0 d of treatment. Arginine accumulation in leaves was greatly enhanced , even though N-15 partitioning into the insoluble reduced-N fraction of leaves was unaffected. The results suggest that decreases in NO3- u ptake in low phosphorus plants could be caused by feedback control fac tors and by limited ATP availability. The decline in endogenous N tran sport from the root to the shoot may be associated with changes in mem brane properties, which also result in parallel effects on hydraulic c onductance and the upward flow of water through the plant.