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Effect of Freezing Temperatures during Soybean Seed Maturation on Seed Quality¹

Soybean [Glycine max (L.) Merr.] seed produced from late maturing cultivars or following delayed planting of early maturing cultivars is often exposed to frost prior to maturation and harvest. Since little information is available regarding the effect of freezing temperatures on seed germination, freezing experiments were conducted with intact plants, and detached pods in controlled environmental chambers. Pods at three developmental stages (green, yellow, and brown) were exposed to temperatures of –2 to –12 C for up to 32 hours. There was a curvilinear relationship (R² = 0.94) between seed moisture and the freezing temperature of seed with the freezing point decreasing from –2 C at a moisture percentage of 68% (green pods) to –20 C at 30% moisture (brown pods).

Seed exposed in detached pods supercooled 2 to 5 C below the freezing point temperature at all development stages. Immature seed in green pods exposed on the plant froze at their freezing point (–2 C); however, seed in yellow or brown pods exposed on the plant supercooled 2 to 4 C below their freezing point. Seed injury did not occur when the seed remained in the supercooled state; however, ice formation resulted in seed injury and death.

Immature soybean seed in green pods (65% moisture) were not injured by freezing temperatures of approximately –2 C; however, seed in yellow pods (physiological maturity, 55% moisture) showed significant reductions in seed germination and vigor following an 8-hour exposure at –7 C. Germination of seed in brown pods (approximately 35% moisture) was reduced only by exposure to –12 C. These results indicate that the temperature required to cause freezing injury to soybean seed decreases during seed development and maturation. When soybean seed have reached physiological maturity (maximum dry weight) temperatures substantially below 0 C are required to cause injury.

Key Words: Supercooling • Physiological maturity • Seed development • Freezing point temperature • Germination • Vigor

² Former graduate assistant, associate professor, professor, Dep. of Agronomy and professor, Dep. of Agricultural Engineering, Univ. of Kentucky, Lexington, KY 40546-0091.

Received for publication August 10, 1981.

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