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Changes in Soybean Seed Quality from High Temperature during Seed Fill and Maturation¹

Soybean [Glycine max (L.) Merr.] seed production is often located in areas where high temperatures prevail during part or all of the seed-filling and maturation phases of seed growth, which may reduce the subsequent quality of harvested seed. Research was undertaken in 1981 and 1982 to study the effect of various lengths of high temperature during seed filling and maturation on soybean seed quality. Potted soybean (cv. Century) plants were transferred from the field to two growth chambers approximately 5 days before growth stage R5. Plants were exposed to a day/night temperature of 32/28°C (high) for 10, 20, 30, and 40 days after flowering and to a 27/22°C (low) temperature during the rest of the seed-development and maturation period. The seeds were divided into two groups P1 (from main-stem pods) and P2 (from branch and top main-stem pods). Exposure to increasing periods of high temperature during seed fill resulted in a linear decline in seed germination, vigor, and physical quality. The average germination percentages were 84 and 50% when the low and high temperatures were extended to pod maturity, respectively. The percentages of small, etched, discolored, and wrinkled seed were 0.1, 3.6, 2.3, and 0.2%, respectively, under constant low temperature, but the percentages increased to 2.7, 20.1, 26.2, and 2.8% under constant high temperature. The high temperature directly influenced germination, vigor, and physical quality even when applied during the first 10 days of the seed-filling period. Phomopsis sp. (pod and stem blight fungus) seed decay averaged less than 4% and did not account for the reductions observed in germination or vigor. Seedling axes dry weight and final average seed weight were the only measurements that showed any consistent difference between P1 and P2 seeds, with P1 seeds being more vigorous and having greater average seed weight than P2 seeds. The interaction of temperature treatments and seed position on the plant was not significant for any seed and seedling measurements. Factors that shorten the length of high-temperature exposure during seed filling and maturation should reduce seed damage.

Key Words: Etched seed • Germination • Glycine max (L.) Merr. • Heat stress • Leakage • Phomopsis sp. • Physical seed quality • Seed position • Seed size • Vigor

² Research technician, Dep. of Plant Pathology, Seed and Weed Science (formerly, graduate student), and associate professor, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication August 8, 1985.