Recurrent Selection for Percent Protein in Soybean Seed — Indirect Effects on Plant N Accumulation and Distribution

Six cycles of recurrent selection in two soybean [Glycine max (L.) Merr.] populations increased percent protein in the seed 1.2 and 3.3 percentage units (2). We subsequently evaluated seasonal accumulatioann dd istribution in these populationsto relate changes in N accumulationw ith the observedc hangesi n seed percent protein. Cycles 0, 2, 4, and 6 of selection were tested in 2 years, two locations, with four or five replications per environment. Plants were separated into leaves, petioles, stems, roots, and pods at 70, 91, 105, 118, and 133 days after emergence for Kjeldahl N and dry weight determination. In Population I, selection indirectly increased total N accumulation in vegetative parts. This increase was primarily the result of increased N accumulationin the leaves. Leaf area index, total stem N, total petiole N, and percent N in the petioles also increased with selection while seed yield changed little. The increase in leaf N with selection increased the amount of N potentially available for translocation to the seed and was the apparenct ause for the increase in percent protein in seed. In Population I, selection indirectly decreased total vegetative N accumulation. This decrease resulted from decreased N accumulation in leaves. Number of leaves, leaf area index, and seed yield also decreased with selection while percent N in the petioles increased. The decrease in leaf N with selection decreased the amount of, N potentially available for translocation to the seed, and thus did not explain the increase in seed percent protein. Carbon supply to the seed, althoughn ot directly measured, may have been reduced by genetic changes in leaf N accumulation and leaf area index to bring about increased seed protein percent and decreased seed yield. The observed changes in N accumulation and distribution with selection show that morphological-physiological traits can be genetically related to percent protein of seed and seed yield. Such relationships may have use in breeding program