CONTROL OF STEADY-STATE PHOTOSYNTHESIS IN SUNFLOWERS GROWING IN ENHANCED CO2

Control coefficients were used to describe the degree to which ribulos e bisphosphate carboxylase/oxygenase (Rubisco) limits the steady-state rate of CO2 assimilation in sunflower leaves from plants grown at hig h (800 mu mol mol(-1)) and low (350 mu mol mol(-1)) CO2. The magnitude of a control coefficient is approximately the percentage change in th e flux that would result from a 1% rise in enzyme active site concentr ation. In plants grown at low CO2, leaves of different ages varied con siderably in their photosynthetic capacities. In a saturating light fl ux and an ambient CO2 concentration of 350 mu mol mol(-1), the Rubisco control coefficient was about 0.7 in all leaves, indicating that Rubi sco activity largely limited the assimilation flux. The Rubisco contro l coefficient for leaves grown at 350 mu mol mol(-1) CO2 dropped to ab out zero when the ambient CO2 concentration was raised to 800 mu mol m ol(-1). In relatively young, fully expanded leaves of plants grown at high CO2, the Rubisco control coefficient was also about 0.7 at a satu rating light flux and at the CO2 concentration at which the plants wer e grown (800 mu mol mol(-1)). This apparently resulted from a decrease in the concentration of Rubisco active sites. In older leaves, howeve r, the control coefficient was about 0.2. Because, on the whole, Rubis co activity still largely limits the assimilation flux in plants grown at high CO2, the kinetics of this enzyme can still be used to model p hotosynthesis under these conditions. The relatively high Rubisco cont rol coefficient under enhanced CO2 indicates that the young sunflower leaves have the capacity to acclimate their photosynthetic biochemistr y in a way consistent with an optimal use of protein resources.