Effect of Temperature on the Rate of Soybean Hypocotyl Elongation and Field Emergence1

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Abstract
Rate of hypocotyl elongation of ‘Cutler’ and ‘Lee 68’ soybean (Glycine max (L.) Merr.) cultivars was measured at several temperatures and incorporated into a prediction model for initial soybean growth. The rate of hypocotyl elongation was found to be a function of temperature and the length of the hypocotyl. Hypocotyl elongation was extremely slow at 10 C, and the seed did not germinate at 40 C. The rate of hypocotyl elongation reached a maximum at approximately 30 C and increased with increasing hypocotyl length for both cultivars. Elongation rates of Lee 68 were consistently higher than those of Cutler. Prediction equations were developed from the data and used to predict the hours to 50% emergence as a function of soil temperature and depth of planting. These equations provided a reliable estimate of emergence times for each cultivar when compared with the actual data under a range of soil temperature conditions in the field and in the laboratory. Theoretical results from the equations indicated that the optimum time to plant soybeans so that emergence would occur in the least amount, of time was when the soil temperature was in the range of 25 to 35 C, provided no other factors were limiting emergence.

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