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

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Abstract
Sunflower (cv. ''Mammoth Greystripe'') and soybean (Merr. cv. ''Amsoy 71'') leaves were exposed to continuous light for at least 52 h 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 h continuous light, the rapidly expanding leaves accumulated high starch levels (3.3 mg/cm2, 43% of dry wt) with only about a 10% decline from the initial photosynthetic rate of 42 mg CO2/dm2 per h. 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 mg/cm2, 34% of dry wt), and their photosynthetic rates declined only about 10% after 54 h continuous light. In sunflower and, to a lesser extent, in soybean, the accumulation of large amounts of leaf starch was not necessarily associated with an appreciable decline in photosynthetic rate. However, in sunflower, the stage of leaf maturity was a major determinant in the photosynthetic response to continuous, relatively high light with its associated starch accumualtion.