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Phosphorus stress effects on assimilation of nitrate.

An experiment was conducted to investigate alterations in uptake and assimilation of NO(3) (-) by phosphorus-stressed plants. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were deprived of an external phosphorus (P) supply for 12 days. On selected days, plants were exposed to (15)NO(3) (-) during the 12 hour light period to determine changes in NO(3) (-) assimilation as the P deficiency progressed. Decreased whole-plant growth was evident after 3 days of P deprivation and became more pronounced with time, but root growth was unaffected until after day 6. Uptake of (15)NO(3) (-) per gram root dry weight and translocation of absorbed (15)NO(3) (-) out of the root were noticeably restricted in -P plants by day 3, and effects on both increased in severity with time. Whole-plant reduction of (15)NO(3) (-) and (15)N incorporation into insoluble reduced-N in the shoot decreased after day 3. Although the P limitation was associated with a substantial accumulation of amino acids in the shoot, there was no indication of excessive accumulation of soluble reduced-(15)N in the shoot during the 12 hour (15)NO(3) (-) exposure periods. The results indicate that alterations in NO(3) (-) transport processes in the root system are the primary initial responses limiting synthesis of shoot protein in P-stressed plants. Elevated amino acid levels evidently are associated with enhanced degradation of protein rather than inhibition of concurrent protein synthesis.