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Temperature Effects on Hypocotyl Elongation of Soybeans¹

Effects of temperature on hypocotyl elongation of soybean [Glycine max (L.) Merr.] cultivars were studied in unlighted growth chambers. We evaluated the relationship between length of exposure to 25 C and amount of inhibition of hypocotyl elongation. Seeds of four cultivars were germinated in sand and in paper towels at each of three temperatures (20, 25, and 30 C), then transferred at daily intervals to a different temperature for the remainder of an 11-day growth period. Initial final temperature sequences used were 20 25 C, 30 25 C, 25 20 C, and 25 30 C. Hypocotyl lengths of ‘Hawkeye’ seedlings (a long hypocotyl cultivar) were similar in the four temperature regimes. Hypocotyl lengths of ‘Clark,’ ‘Ford,’ and ‘Amsoy’ seedlings (short hypocotyl cultivars) decreased with increased length of exposure to 25 C.

The length of exposure to 25 C needed for classification of cultivars was evaluated. Seedlings of Clark, Ford, Amsoy, and Hawkeye were grown at constant temperatures of 20, 25, and 30 C and measured daily up to 10 days after planting. Hypocotyl growth rates of all cultivars were comparable at 20 and 30 C. At 25 C, Hawkeye grew at a faster rate than the other cultivars beginning 3 days after planting, and differences among cultivars increased with days after planting. Differences among cultivars were most fully expressed 10 days after planting.

Inhibition of hypocotyl elongation was studied at constant temperatures between 20 and 32 C, at 1-C increments. Inhibition of short hypocotyl cultivars occurred at all temperatures between 21 and 28 C, with maximum inhibition at 25 C.

Our results indicate that the amount of inhibition of hypocotyl elongation is influenced by length of exposure to inhibiting temperatures between 21 and 28 C. This range of inhibiting temperatures permits the use of ordinary laboratory facilities for large-scale testing of soybean genotypes in a breeding program.

Key Words: Glycine max (L.) Merr. • Seedling emergence • Temperature inhibition

² NSF Graduate Trainee, Associate Professor of Agronomy and Collaborator, PSRD, ARS, USDA, and Associate Professor of Botany and Plant Pathology, Iowa State University, Ames, Iowa 50010.

Received for publication October 2, 1972.