INCREASING SOYBEAN PRODUCTIVITY THROUGH SELECTION FOR NITROGEN-FIXATION

Nitrogen fixation and accumulation are major factors of soybean [Glycine max (L.) Merr.] production. Investigations were carried to detemine whether higher N2 fixation could be considered as a criterion for the improvement of productivity. A selection for N2 fixation, which was based on acetylene reduction estimates, was carried out at Growth Stage R5. A set of 210 F2 plants issued from three crosses and developed in nutrient solutions was studied in phytotronic chambers, under temperature and mineral N nutrition conditions that were favorable for the maximum expression of the symbiosis. The seed yield of the F4-derived lines was evaluated under normal population density on a Typic Udifluvent loamy-sand soil (pH 6.0, organic matter 1.5%, and total N 1 g kg(-1). Seed yield and N2 fixation were expressed with respect to the mean value of the genotypes with the same date of maturity. Seed yields of the F4- derived lines were significantly correlated with the N2 fixation of the F2 parent plants. This correlation was observed for all of the plants investigated including all crosses (r = 0.294**, significant at P = 0.01) and for each of the three crosses: Weber X Maple Arrow (r = 0.433**), Weber X Jiling 14 (r = 0.394*, significant at P = 0.05), Weber X Kingsoy (r = 0.597**). The seed yield of the F4 lines was also significantly correlated with nodule volume and nodule dry weight, particularly at each cross level. High nodulation ability and high N2 fixation rates appear to increase soybean seed yield. Early breeding for these traits may therefore be expected to result in a genetic gain in seed yield in the progeny.