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Soybean Hypocotyl Growth Under Field Conditions¹

In soybeans (Glycine max (L.) Merr.) an epigeal emerger, the sole seedling organ responsible for elevating the cotyledons and epicotyl to the soil surface is the hypocotyl. Yet, soybean seedling and, in particular, hypocotyl growth under field conditions have not been well studied.

We measured hypocotyl length and swelling index (mg fresh weight/cm length) of several cultivars and two seed sizes at several planting dates and depths. Genetic variability was found in these two measurements. Cultivar differences in hypocotyl length generally followed the "long" and "short" hypocotyl classification of cultivars based on hypocotyl growth at 25 C. Hypocotyl swelling-index differences generally were independent of the "long" and "short" cultivar classification. Hypocotyl length was highly correlated with rapidity of emergence, whereas hypocotyl swelling index was not highly correated with this measurement. Small seed of ‘Amsoy 71’ and ‘Corsoy’ produced seedlings with longer hypocotyls than did large seed, but there was no effect of seed size on hypocotyl length in ‘Wayne’. Small seed of all cultivars produced seedlings with lower values of hypocotyl swelling-index than did large seed. Depth of planting significantly affected hypocotyl length and swelling index, but the specific soil factors affecting these two measurements were not identified. The consistently larger values of hypocotyl swelling index at deep as compared with shallow plantings, as web as the relatively large values for this measurement in the field as compared with those obtained in the laboratory, suggested that soil resistance is one limiting soil factor.

Key Words: Soil resistance • Seed size • Depth of planting • Glycine max

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.