Phosphorus-deficiency effects on response of symbiotic N2 fixation and carbohydrate status in soybean to atmospheric CO2 enrichment

The impact of phosphorus (P) deficiency on response of symbiotic N-2 fixation and carbohydrate accumulation in soybean (Glycine max [L.] Merr.) to atmospheric CO2 enrichment was examined. Plants inoculated with Bradyrhizobium japonicum MN 110 were grown in growth chambers with controlled atmospheres of 400 and 800 mu L CO2 L-1 and supplied either 1.0 mM-P (P-sufficient) or 0.05 mM-P (P-deficient) nitrogen (N)-free nutrient solution. When plants were supplied with sufficient P, CO2 enrichment significantly increased whole plant dry mass (83%), nodule mass (67%), total nitrogenase activity (58%), and N (35%) and P (47%) accumulation at 35 days after transplanting (DAT). Under sufficient P supply, CO2 enrichment significantly increased starch concentrations in nodules compared to the normal atmospheric CO2 treatment. Under normal CO2 levels (400 mu L L-1) nonstructural carbohydrate concentration (starch plus soluble sugar) was significantly higher in leaves of P-deficient plants than in leaves of P-sufficient plants in which nonstructural carbohydrate concentration exhibited a strong diumal pattern. Under deficient P supply whole plant dry mass, symbiotic N-2-fixation parameters, and N and P accumulation were not enhanced by atmospheric CO2 enrichment. Phosphorus deficiency decreased nonstructural carbohydrate accumulation in nodules at the end of a 10-day period in which functional activity was developing by 86% relative to P-sufficient controls. While P deficiency elicited significant increases in the nonstructural carbohydrate concentration in leaves, it caused significant decreases in the nonstructural carbohydrate concentration in nodules over the diurnal cycle from 30 to 31 DAT. Collectively, these results indicate that the lack of a symbiotic N-2-fixation response to atmospheric CO2 enrichment by P-deficient plants may be related to the decreased carbohydrate status of nodules.