Regulation of N2 fixation and NO3−/NH4+ assimilation in nodulated and N-fertilized Phaseolus vulgaris L. exposed to high temperature stress

• N2 fixation and NO3− assimilation are decreased significantly by temperatures above 34 °C.
• Decreases in N2 fixation are associated with accumulation of ureide-N in the nodule.
• N2 fixation is determined by the limitation in NH4 assimilation in the nodule rather than nitrogenase activity.
• Rhizobial strains play a role in activating enzymes of NH4 assimilation in nodules.

Legumes need large amounts of N to grow satisfactorily. Under low NO3− availability in the soil, many legumes meet their N requirements by N2 fixation in association with rhizobia. Both NO3− uptake and N2 fixation decrease as temperature exceeds optimal growth conditions, but the mechanisms of regulation of N2 fixation and NO3−/NH4+ assimilation under high temperature stress are not completely understood. We describe an experiment in which physiological mechanisms regulating N metabolism of common bean (Phaseolus vulgaris L.) are investigated in plants submitted to daily maximum temperatures of 28, 34 and 39 °C. Common bean was grown in symbiosis with each of six rhizobial strains—belonging to four different species and varying in N2 fixation effectiveness—or fertilized with NO3− until flowering. Harvest measurements included the activities of shoot, stem and root NO3− reductase (NR), nodule glutamine synthetase (GS), NADH-dependent glutamate synthase (GOGAT), nitrogenase, phosphoenol pyruvate carboxylase (PEPcase), N-export rates by nodules and concentration of N compounds in the xylem sap. Higher temperatures inhibited N2 fixation resulting in lower proportion of ureide-N in nodules and xylem sap of nodulated plants in relation to amide-N and α-amino-N. Higher temperatures consistently reduced the activity of NR in leaves of N-fertilized plants. Higher temperatures also decreased N exported from nodules and activities of nitrogenase, GS, GOGAT and PEPcase. The rate of decreases varied in plants with different strains. Furthermore, the activities of GS and GOGAT were more strongly affected by high temperatures than the activity of nitrogenase. There was a remarkable increase in the concentration of NH4+-N and ureide-N in the nodules when GS and GOGAT activities decreased. Therefore, the results provide evidence that N2 fixation in common bean submitted to heat stress is limited by NH4+ assimilation via GS-GOGAT rather than by decreased activity of nitrogenase. Rhizobial effectiveness determined the degree of down-regulation of GS-GOGAT activity in nodule tissues.