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

Legumes need large amounts of N to grow satisfactorily. Under low NO3- availability in the soil, many legumes meet their N requirements by N-2 fixation in association with rhizobia. Both NO3- uptake and N-2 fixation decrease as temperature exceeds optimal growth conditions, but the mechanisms of regulation of N-2 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 degrees C. Common bean was grown in symbiosis with each of six rhizobial strains belonging to four different species and varying in N-2 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 N-2 fixation resulting in lower proportion of ureide-N in nodules and xylem sap of nodulated plants in relation to amide-N and alpha-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 N-2 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. (C) 2013 Elsevier B.V. All rights reserved.