EFFECTS OF DEFOLIATION ON SEED PROTEIN-CONCENTRATION IN NORMAL AND HIGH-PROTEIN LINES OF SOYBEAN

Two high (NC106, NC111) and two normal (NC103, NC107) seed protein con centration lines, derived from two different recurrent selection popul ations of soybean (Glycine max L. Merr.) were subjected to partial def oliation at beginning seed fill (R5) under outdoor pot culture and fie ld conditions. The aim of this study was to test the hypothesis that c apacity to store N in vegetative organs and/or to mobilize that N to r eproductive organs is associated with the high seed protein concentrat ion trait. Symbiotic N-2 fixation was the sole source of N in the pot experiment and the major source of N (met > 50% of the N requirement) in the low N soil used in the field experiment. Seed protein concentra tion and seed yield at maturity in both experiments and N accumulation and mobilization between R5 and maturity in the pot experiment were m easured. The four genotypes did not differ significantly with respect to the amount of N accumulated before beginning seed fill (R5). Remova l of up to two leaflets per trifoliolate leaf at R5 significantly decr eased the seed protein concentration of NC107/111 but had no effect on this trait in NC103/106. Defoliation treatments significantly decreas ed seed yield, whole plant N accumulation (N-2-fixation) during reprod uctive growth and vegetative N mobilization of all genotypes. Differen ces in harvest indices between the high and low protein lines accounte d for approximately 35% of the differences in protein concentration. T he two normal protein lines mobilized more vegetative N to the seed (a verage. 5.26 g plant(-1)) than the two high protein lines (average. 4. 28 g plant(-1)). The two high seed protein lines (NC106, NC111) exhibi ted significantly different relative dependencies of reproductive N ac cumulation on vegetative N mobilization, 45% vs. 29%, in the control t reatment. Whereas, NC103 with normal and NC106 with high seed protein concentration exhibited similar relative dependencies of reproductive N accumulation on vegetative N mobilization, (47% vs. 45%). Collective ly, these results indicate that N stored in shoot organs before R5 and greater absolute and relative contribution of vegetative N mobilizati on to the reproductive N requirement are not responsible for the high seed protein concentration trait.