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Performance of Semideterminate and Indeterminate Soybean Genotypes Subjected to Defoliation¹

The percentage reduction in seed yield from defoliation has been shown to differ between indeterminate and determinate soybean [Glycine max (L.) Merr.] cultivars, but no information is available on the response of semideterminate cultivars to loss of leaf tissue. The objective of our study was to compare the performance of semideterminate and indeterminate genotypes when subjected to defoliation. Ten pairs of near-isogenic lines were used, one line of each pair being semideterminate and the other indeterminate. The lines were completely defoliated at R2 (full bloom) or R5 (beginning seed) during 2 years. Seed yield, 100-seed weight, time of maturity, plant height, and lodging of treated plots were compared with untreated controls. The average yield reduction from the R2 treatment was significantly greater for the semideterminate than for the indeterminate genotypes by 10.9 percentage units in 1981 and 10.2 in 1982. Yield loss from the R5 treatment was significantly greater for the semideterminate genotypes by 7.9 percentage units in 1981 and 10.4 in 1982. The average reduction in 100-seed weight was only 1.6 percentage units greater for the semideterminates than for the indeterminates, which indicated that reduction in seed number was primarily responsible for the difference in yield loss between the stem-termination types. The maximum difference between stem-termination types for the change in other agronomic characters was 1 day in time of maturity, 5 cm in height, and 0.7 score for lodging. A distinction between semideterminate and indeterminate cultivars may be useful in assessing the yield loss from defoliation that occurs during reproductive development.

Key Words: Glycine max (L.) Merr. • Hail injury • Insect injury • Yield reduction • 100-seed weight • Maturity • Plant height • Lodging

² Professor and graduate research assistants, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication March 16, 1984.