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Effect of Downward Force on Soybean Hypocotyl Growth¹

Factors affecting hypocotyl elongation in soybeans Glycine max (L.) Merr., have not been well studied, with the exception of soil temperature. The limiting effect of soil resistance has not been examined heretofore. Soybean hypocotyl growth was studied in the laboratory. An apparatus was developed to apply a downward force to the apical hook of dark-grown seedlings. An increase in downward force was accompanied by a decrease in hypocotyl length and an increase in hypocotyl swelling index (mg fresh weight/cm length of the hypocotyl). No difference in hypocotyl length was found between large and small seed of cultivar ‘Corsoy’, but hypocotyl swelling index was greater for large seed at all force levels. In cultivar ‘Amsoy 71’, small seed consistently developed seedlings with longer hypocotyls and had smaller values for the swelling index than did large seed at all force levels; the general response of Amsoy 71 to the inhibitory temperature of 25 C appeared to be independent of the response to downward force. The effect of force on hypocotyl length and swelling index was independent of temperature in cultivar ‘Wayne’, but hypocotyl elongation rate was significantly affected by temperature. Regression equations were obtained for hypocotyl length, swelling index, and hypocotyl elongation rate by using downward force and temperature as independent variables.

Key Words: Soil resistance • Seed size • Temperature • Glycine max (L.) Merr.

² Former graduate research assistant (presently research statistician, Pfizer Genetics, Inc., 1000 Executive Parkway, St. Louis, MO 63141) and professor, Dep. of Botany and Plant Pathology, Iowa State Univ., Ames, IA 50011.

Received for publication April 17, 1978.