Nitrogen and carbon assimilation enzyme activities in mungbean (Vigna radiata L. Wilczek) and response to nitrogen application and leaf removal

Nodule enzymes glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH) and phosphoenolpyruvate carboxylase (PEPC) have been suggested to play important roles in the assimilation of fixed nitrogen and carbon. Twelve mungbean (Vigna radiata L. Wilczek) accessions were used to study these enzyme activities and their response to N-application and leaf removal. GS, GOGAT and PEPC activities were found to be stable at 0 to 4 deg C while GDH activity was reduced by low temperatures. PEPC activity was reduced by 85.44 percent at 100 deg C while GS and GOGAT were completely inactivated at 100 C. GDH still showed 38 percent activity at 100 deg C. Mungbean nodule GS showed apparent km values of 12.0 UM for ammonia and 5.0 mM for hydroxylamine. Km values of NADH and alpha-Ketoglutarate of nodule GOGAT were 33.3 UM and 50.0 UM, respectively. Km values observed for GDH were 76.3 UM for NADH and 20 UM for alpha-ketoglutarate. PEPC showed an apparent Km value of 86.0 UM for PEP. Lower km value for and therefore, higher affinity to ammonia of nodule GS indicates its importance in the assimilation of fixed nitrogen. The pH optima obtained were a) 7.0 for GS and GDH; b) 5.0 for GOGAT and c) 8.0 for PEPC. The optimum assay temperatures of GS and PEPC were 37 deg C and 30 deg C, respectively. Significant genotypic variation in the levels of nodule GS, GOGAT and PEPC activities were detected among the mungbean accessions evaluated. However, GDH activity levels were observed to be statistically similar. Highest GS, GOGAT, and PEPC activities were observed in Acc. 658, Acc. 68, and Acc. 638, respectively. Acc 2041 gave the lowest activity for all three nodule enzymes. The genotypes also varied in shoot fresh weight (SFW), shoot dry weight (SDW), number of nodules (NN) and amount of nodule soluble protein (SOLP). Highest SFW and SDW were obtained in Acc. 188 while the highest NN was shown by Acc. 658. SOLP was highest in Acc. 2041. Significant correlation was found between the activities of the nodule enzymes: GSu and GOGATu (r = 0.47); GSu and PEPCu (r = 0.65) and GOGAT and PEPC (r = 0.35)