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Four soybean cultivars differing in the duration of the seed filling period (SFP) were evaluated in the field for N and dry weight accumulation, N_2-fixation and net leaf photosynthetic CO_2 exchange ...rates (CER). N_2-fixation was measured by isotope dilution, using a non-nodulating (non-nod) near-isogenic line of the cultivar Harosoy as a control. There were consistent differences between the non-nod and its sister cultivar for seed yield, N accumulation and N redistribution, but both genotypes had the same SFP duration and final shoot biomass. Reduced N accumulation in the non-nod resulted in less dry weight partitioned to seeds but no reduction in the length of the SFP. Cultivars with long SFP's had greater yield, more N and dry weight accumulation between the beginning of seed fill and maturity, and a slower decline in leaf CER during seed fill, compared to short SFP cultivars. However cultivars varied with respect to vegetative dry weight, N accumulation, N redistribution, the proportion of total N allocated to seeds, and the peak CER, independent of differences in SFP. Neither the amounts of accumulated N or fixed N_2 explained differences in the SFP.show more
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