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Variability and Selection for Leaf Photosynthesis in Advanced Generations of Soybeans¹

Selecting crop genotypes with increased leaf photosynthesis may be one approach to greater yields. The objectives of this study were to determine whether leaf photosynthesis varied in a population of advanced-generation lines, to assess the effectiveness of maintaining selected differences in leaf photosynthesis, and to determine the correlation between photosynthesis and other leaf traits in a population of near-homozygous soybean [Glycine max (L.) Merr.] lines.

Leaf total photosynthetic rate (TPs), as determined by ¹⁴CO₂ uptake, specific leaf weight (SLW), total soluble protein content (TSP), chlorophyll content (CHL), and time to physiological maturity (PM) were measured during seed filling in 110 field-grown, F₆-derived lines from a cross of ‘Amsoy’ x ‘Ford’, two soybean cultivars differing in leaf photosynthetic capacity, and the parents. Significant differences in TPs were discernible among these F₇ lines, so 14 high and 14 low photosynthesizing lines subsequently were selected and grown in the F₈. TPs and PM were evaluated in the selected lines.

The high and the low TPs groups remained significantly different in the F₈, but the difference between the two groups decreased 50% compared with the selected difference. This decrease was attributable to a 16% increase in TPs by the low group.There was considerable overlap between the two groups for individual lines. This overlap could be accounted for by differences in PM.

None of the leaf traits was strongly correlated with TPs, and only CHL and TSP were significantly related.

We conclude that effective selection for leaf photosynthesis can succeed within a soybean population of homozygous lines of similar developmental stage and that photosynthesis per se needs to be measured.

Key Words: CO₂-exchange • Specific leaf weight • Total soluble protein • Chlorophyll • Glycine max

² Research assistant, former undergraduate student, and professors, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication December 15, 1980.

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