Effects of phosphorus deficiency on carbon and nitrogen assimilation and utilization in whole soybean plants

Research was conducted to examine effects of P deficiency on carbon assimilation and utilization in whole soybean plants. Phosphorus deficiency caused increased starch concentrations in leaves, stems and roots throughout a diurnal cycle. High starch concentrations in source leaves were caused by the increased partitioning of concurrently fixed carbon into starch during the day and during the dark to light transition phase and a lack of starch degradation during the light to dark transition phase. The starch degradation rate over the dark phase, once initiated, was not affected by P deficiency, suggesting existence of an alternative pathway for starch degradation and the export of degradation products from chloroplasts. The accumulation of starch in all organs resulted from decreased starch utilization over the dark phase. Greater accumulation rates of starch than structural biomass in these organs during onset of P deficiency suggested that photosynthetic capacity was affected to a lesser extent than plant growth.; To assess the effects of P deficiency on NO{dollar}sb3sp-{dollar} assimilation and utilization, changes in NO{dollar}sb3sp-{dollar} uptake rate and NO{dollar}sb3sp-{dollar}, soluble reduced N (SRN) and insoluble reduced N (IRN) contents in whole plants were examined during onset of and recovery from P deficiency. Although P deficiency decreased unit NO{dollar}sb3sp-{dollar} absorption rate, NO{dollar}sb3sp-{dollar} absorption capacity was greatly enhanced during recovery from P deficiency. This increase was associated with increased utilization of NO{dollar}sb3sp-{dollar} and increased plant growth. The concentrations of NO{dollar}sb3sp-{dollar} and SRN increased under P deficiency and during onset of P deficiency. Increasing external P levels resulted in rapid utilization of NO{dollar}sb3sp-{dollar} and SRN and sharp increases in IRN concentration and whole plant IRN pool, suggesting protein synthesis was stimulated under this condition. Increased SRN accumulation in the P-deficient source leaf was partly due to an increase in free amino acid-N, particularly arginine-N and asparagine-N. Increased arginine and asparagine concentrations in the xylem sap suggested that transport of these two compounds from the roots to the leaves may be one of the factors contributing to their increased accumulation in the leaves of P-deficiency. The results indicate that P nutrition has an important role in controlling N uptake and assimilation.