Carbon Accumulation during Photosynthesis in Leaves of Nitrogen- and Phosphorus-Stressed Cotton.

Leaves of cotton (Gossypium hirsutum L.) accumulate considerable dry mass per unit area during photosynthesis. The percentage of C in that accumulated dry mass was estimated as the regression coefficient (slope) of a linear regression relating C per unit area to total dry mass per unit area. Plants were grown on full nutrients or on N- or P-deficient nutrient solutions. In the fully nourished controls, the mass that accumulated over a 9-hour interval beginning at dawn contained 38.6% C. N and P stress increased the C concentration of accumulated mass to 49.7% and 45.1%, respectively. Nutrient stress also increased the starch concentration of accumulated mass, but starch alone could not account for the differences in C concentration. P stress decreased the estimated rate of C export from source leaves, calculated as the difference between C assimilation and C accumulation. The effect of P stress on apparent export was very sensitive to the C concentration used in the calculation, and would not have been revealed with an assumption of unchanged C concentration in the accumulated mass.