A Whole‐plant Indicator of Soybean Physiological Maturity1

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Abstract
Loss of green color from the pod has been suggested as an indication of physiological maturity for individual soybean [Glycine max (L.) Merrill] seeds. In this study, we evaluated the usefulness of green pod color loss as an indicator of the physiological maturity of an entire soybean plant. We also studied the relationship of this visual change to other stages of whole‐plant soybean development and maturity already in use or described in the literature. The study was conducted in 1978 and 1979 at St. Paul, Minn. on a well‐drained Waukegan silt loam (sandy‐skeletal, mixed mesic Typic Hapludolls). The 1st year, cvs. ‘McCall’ (Group 00) and ‘Hodgson 78’ (Group I) were used. In 1979, ‘Altona’ (Group 00) and ‘Steele’ (Group I) were added. Seed dry weight was measured at 1‐week intervals for 8 weeks which spanned the maturation phase. The calculated date of physiological maturity was then related to a variety of visual plant changes.Loss of green pod color was found to be the most reliable indicator of physiological maturity across years, cultivars, and maturity groups. Stage R7 according to Fehr and Caviness (occurs when one normal pod on the main stem has reached its mature color) was next most reliable, but occurred before physiological maturity and interacted with years. The ‘all pods brown’ stage occurred midway between physiological maturity and harvest maturity (harvest maturity = average seed moisture at 13%). On the average, early maturing seeds reached physiological maturity 11 days before late maturing seeds. Seed moisture content averaged 44% when the entire plant was physiologically mature. Color of petioles and stems, and color or senescence of leaves were not reliable indicators of physiological maturity.

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