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Carbohydrate Levels in Soybean Leaves During Reproductive Growth¹

The carbohydrate status of a soybean [Glycine max (L.) Merrill] leaf during reproductive growth is related to the interaction between the production of carbohydrate by photosynthesis and its utilltation by the developing seeds and other active sinks. This interaction between source and sink plays an important role in the production of yield. Leaf samples were collected from field grown soybeans, ‘Cutler 71,’for 3 years to investigate changes in levels of free sugars and starch during reproductive growth. Shade cloth that reduced the incident solar radiation by 60% was installed over plants at the beginning of the seed filling period for 2 years and leaf samples were collected from the 6th and/or 12th leaf position at weekly intervals. Changes in free sugar levels in the leaves during seed filling varied from an increase to no change to a decrease across the $ years. Starch leveis increased to a maximum and, in 2 of the 3 years, declined as the plant approached maturity. The increase in starch was associated with an increase in specific leaf weight. Leaves that were ready to abscise contained up to 8.5% starch. Shading reduced yield (19 to 32%) but had no effect the level of free sugars in the leaves. However, the shade caused an immediate reduction (within 5 to 8 days) the concentration of starch, to 13 to 28% below the control, and this difference in concentration was maintained throughout the remainder of the filling period. Although soybean leaves accumulate starch during the seed filling period, some of the starch is remobili~ed prior to leaf abscission and the amount in the abseised leaves does not represent a significant quantity relative to the seed yield.

Key Words: Glycine max (L.) Merrill • Free sugars • Starch • Shade • Yield • Photosynthesis

² Professors of agronomy and technical assistant, respectively.

Received for publication October 30, 1979.

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