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A Visual Indicator of Physiological Maturity in Soybean Plants¹

Physiological maturity (PM) represents an important growth stage of crop plants, since it is normally associated with maximum accumulation of dry seed weight and yield. Physiological maturity of an individual soybean (Glycine max (L.) Merrill) seed or pod occurs when the seed or pod is completely yellow; however, indicators of PM on a whole plant basis are not available. This investigation was conducted to relate several visual indicators of soybean pod development to PM and yield of field populations. Plants of three cultivars (‘Williams’, ‘Kent’, and ‘Essex’) were grown in a Maury silt loam soil (Typic Paleudalfs, fine, mixed, mesic) in 1977 and 1978 and harvested when 50% of the population had reached or exceeded the following growth stages: 1) a completely expanded green pod at one or more of the four uppermost nodes, 2) one yellow pod per plant, and 3) one normal pod on the main stem that has reached its mature pod color (growth stage R7). A check harvest was made after full maturity. In 1979, seven cultivars representing maturity groups OO to V and both determinant and indeterminate growth habits were grown in the same soil type and harvested at growth stage R7 and at full maturity.

Plants harvested at the green pod growth stage exhibited significantly smaller seed and lower yield compared to those harvested at full maturity. Even though there was no significant reduction in seed size and yield for plants harvested at the yellow pod growth stage, in 1978 there was a consistent trend across cultivars for lower yields at this harvest. There was no reduction in seed size or yield for plants harvested at growth stage R7 in all 3 years compared to harvests at full maturity. Growth stage R7 (one mature pod per plant) was a more accurate indicator of PM than the yellow pod growth stage (one yellow pod per plant), since a greater percentage of pods (64%) and seed (55%) had reached or exceeded complete yellow coloration indicating that those pods and seed were physiologically mature. The appearance of one normal pod on the main stem that has reached its mature pod color (growth stage R7) represents an accurate, easy to recognize visual indicator of PM. This criteria should provide farmers, researchers, and others with a useful tool for determining PM for a single plant or a field population.

Key Words: Glycine max (L.) Merrill • Pod development • Seed size • Seed moisture content • Yield

² Associate professor, professor and technician, Dep. of Agronomy, University of Kentucky, Lexington, KY 40546.

Received for publication June 16, 1980.

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