Divergent Selection for Nodule Aspartate Aminotransferase and Asparagine Synthetase Activities in Alfalfa

Abstract

Root nodule aspartate aminotransferase (AAT) and asparagine synthetase (AS) catalyze synthesis of the two primary nitrogenous compounds translocated from alfalfa (Medicago sativa L.) nodules. The objectives of this research were to (i) determine the heritability nodule AAT and AS activities, (ii) evaluate the effect of selection for AAT and AS activities on characteristics associated with N2 fixation, and (iii) assess relationships among nodule characteristics, N concentration, and plant dry weight. One nondormant and two moderately dormant alfalfa populations were divergently selected for nodule AAT and AS activities. Plants were grown in a greenhouse under N2‐dependent conditions and the first regrowth at early bloom was measured for nodule fresh weight, nodule soluble protein concentration, plant dry weight, N concentration, acetylene reduction activity (ARA), and nodule activities of AAT, AS, glutamate synthase (GOGAT), and phosphoenolpyruvate carboxylase (PEPC). Realized heritabilities for AAT and AS were 0.64 and 0.15, respectively, after one cycle of phenotypic selection. Nodule fresh weight responded negatively to divergent selection for AAT and AS activities in populations that had previously been selected for high nodule enzyme activity. Simple correlations among the activities of AAT, AS, GOGAT, and PEPC and nodule soluble protein concentration ranged between r = 0.60 and r = 0.87 (P = 0.01). Specific activities of nodule AAT and AS did not appear to be desirable selection criteria for improving N2 fixation because correlated responses of ARA, plant dry weight, and N concentration were variable or nonsignificant.