The photosynthetic rate of individual cotton leaves declines rapidly within 15-20 days following completion of expansion. During the last half of fruit development, the canopy photosynthetic activity declines rapidly posing a potential yield limitation. We have evaluated the probable cause of the photosynthetic rate decline with increasing leaf age. Our results indicate that the soluble protein concentration declines rapidly, coincident with protein demands by the developing fruit. The activity of RuBP carboxylase is decreased due to concentration reductions and is responsible for the photosynthetic rate decline. Nitrogen management may be able to alleviate the problem.