Selection for Apparent Photosynthesis and Related Leaf Traits in Early Generations of Soybeans1

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Abstract
Evidence suggests that photosynthate production may be yield‐limiting in soybeans [Glycine max (L.) Merr.]. This study was designed to estimate potential for selection of CO2‐exchange rate (CER), specific leaf weight (SLW), leaf thickness (LT), and leaf density (LD) in early generations of soybeans.Two crosses (‘Amsoy’ ✕ ‘Ford;’ ‘Corsoy’ ✕ ‘Hawkeye’) involving adapted cultivars high (Amsoy, Corsoy) and low in leaf CER were used. CER, SLW, LT, and LD were measured in parents, F1s and F2s. CER was measured in selected F3 and F4 lines of one cross, whereas the other variables were measured in all F3 and F4 lines.All traits showed quantitative inheritance without dominance effects. Broad‐sense heritability, based on parental, F1 and F2 variances, was low to moderate for CER (0.36, 0.56); it was low to very low for the other variables. Selection for either high or low CER in the F2 or F3 was ineffective. Gain from hypothetical selection for SLW was about 596 in the F2 and 5‐11% in the F2. Gain from hypothetical F3 selection in LT was about 5%. LD showed poor gain.Use of pure lines from advanced generations may give an entirely different picture of the potential for direct selection for increased photosynthesis than that shown by this study.