Bidirectional Selection in Alfalfa for Activity of Nodule Nitrogen and Carbon-Assimilating Enzymes

The nodule enzymes phosphoenolpyruvate carboxylase (PEPC) and glutamate synthase (GOGAT) are important in the assimilation of symbiotically fixed N2 in alfalfa (Medicago sativa L.). The objectives of this study were: (i) to determine the response to one cycle of bidirectional selection for nodule PEPC and GOGAT activities within six alfalfa germplasms, and (ii) to determine the effect of selection for nodule enzyme activity on other traits previously found to be associated with increased N2 fixation and on the associations among those traits. Four subpopulations (SPO, SP1, SP2H, and SP2L) were obtained for each of the six germplasms. The SP1 subpopulation for each germplasm was derived from the unselected base SPO subpopulation by sequential selection for seedling vigor, increased shoot, nodule, and root mass, and high acetylene-reduction activity (ARA). The SP2H and SP2L were developed by intercrossing plants selected from the SP1 for high and low nodule enzyme activity, respectively. Plants from the SPO, SP1, SP2H, and SP2L subpopulations of the six germplasms were evaluated simultaneously in the glasshouse for PEPC and GOGAT activities, ARA, nodule mass and soluble protein concentration, shoot and root dry weights, and N concentration in the shoots and roots. Selection for traits to form the SP1 subpopulation was effective in increasing ARA, nodule mass, and shoot and root dry weight above the SPO. Averaged over the six germplasms, the SP1 subpopulation had about 5 and 7% greater PEPC and GOGAT activities, respectively, than did the SPO. Bidirectional selection for nodule enzyme activity resulted in the PEPC and GOGAT activities of the SP2H subpopulations being 20 and 18%, respectively, greater than those of the SP2L. The SP2H subpopulation had greater nodule soluble protein and root N concentrations than did the SP2L. The ARA, nodule mass, root dry weight, and shoot N concentration traits were not affected by selection for enzyme activity, but selection for low nodule enzyme activity reduced shoot dry weight. This study demonstrated that it was possible to concurrently select for two enzyme activities involved in N and C assimilation in the same plant organ. Joint contribution from the Minnesota Agric. Exp. Stn. and the USDA-ARS, St. Paul. Paper no. 15 072.