Photosynthetic Units and Carbon Assimilation in Leaves of Grain Sorghum under Different Light Intensities

The Cyt f and P700 contents in leaves of three Sorghum, varieties were measured, in relation to their carbon assimilation, under different light intensities during growth. At the maximum irradiation used (1,800 μE m−2 s−1) the ratio of P700 to Cyt f was close to unity, whereas under low irradiation (450 μE m−2 s−1) the ratio of P700 to Cyt f ranged from two to three. A strikingly positive correlation existed between the P700 contents of the leaves and their rates of carbon dioxide fixation, dry matter production and Cyt f contents, only when the plants were grown under high light intensities. The P700 content of the leaves in plants grown under low irradiation was unrelated to the contents of Cyt f. Thus, at a high light intensity there is a close relationship between the Cyt f and P700 levels, but at low intensities the amounts of electron carriers and the reaction centre are independent.