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Physiological Maturity in Soybean¹

A visual indicator of physiological maturity (maximum dry seed weight) in soybean [Glycine max (L.) Merrill] would be useful to researchers and soybean producers. Experiments were conducted utilizing two cultivars in the greenhouse and six cultivars in the field to characterize the color of soybean seeds and pods at physiological maturity. In greenhouse experiments where plants were exposed to ¹⁴CO₂ as they neared maturity, it was found that the ¹⁴C content of the seed decreased as the yellow coloration of the seed coat increased. Practically no ¹⁴CO₂ was recovered from yellow seeds, regardless of the color of the pod, suggesting that the seed stops importing assimilates and is physiologically mature when it is completely yellow (free of green color). Normally, yellow seeds were found only in yellow or brown pods. The respiration rate of seed, measured as CO₂ evolution, declined rapidly as the seedcoat began to turn yellow and reached a low level when the seed were completely yellow at a seed moisture content of 55 to 60%.

Physiological maturity of an entire plant, estimated from dry seed weight data in the field, occurred when 26%, of the seed were yellow and 35% of the pods were yellow or brown. The seed moisture content ranged from 54 to 62%. The date of physiological maturity estimated in the field was slightly early in comparison to complete physiological maturity (all seed completely yellow) due to the sampling variation associated with estimating dry seed weight and the variation in pod maturity on an individual plant.

The data suggest that the occurrence on the main stem of one normally colored mature pod per plant represents a useful and acceptable indicator of physiological maturity, even though all the pods or seed were not completely yellow at this stage of development.

Key Words: Harvest maturity • Seed development • ¹⁴CO₂ labeling • Glycine max (L.) Merr.

² Associate professor, associate professor, graduate assistant, former undergraduate student (current address, Dep. of Agronomy, Univ. of Wisconsin, Madison, Wis.) and former post-doctoral fellow (current address, MSU-DOE Plant Research Laboratory, East Lansing, MI 48824).

Received for publication October 19, 1978.

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