Maintenance of High Photosynthetic Rates during the Accumulation of High Leaf Starch Levels in Sunflower and Soybean 1

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Sunflower (cv. “Mammoth Greystripe”) and soybean (Merr. cv. “Amsoy 71”) leaves were exposed to continuous light for at least 52 hours in an attempt to determine the relationship between leaf starch levels and photosynthetic rates. Immature rapidly expanding and relatively mature slowly expanding sunflower leaves were studied. After 52 hours continuous light, the rapidly expanding leaves accumulated high starch levels (3.3 milligrams per square centimeter, 43% of dry weight) with only about a 10% decline from the initial photosynthetic rate of 42 milligrams CO2 per square decimeter per hour. Under the same conditions, the slowly expanding leaves accumulated less starch, but the photosynthetic rate declined 30%. Soybean leaves, which were slowly expanding, accumulated less starch than sunflower leaves (2.1 milligrams per square centimeter, 34% of dry weight), and their photosynthetic rates declined only about 10% after 54 hours continuous light.

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