Elevated CO2 induces biochemical and ultrastructural changes in leaves of the C-4 cereal sorghum

We analyzed the impact of growth at either 350 (ambient) or 700 (elevated) mu L L-1 CO2 on key elements of the C-4 pathway (photosynthesis, carbon iso tope discrimination, and leaf anatomy) using the C-4 cereal sorghum (Sovghu m bicolor L. Moench.). Gas-exchange analysis of the CO2 response of photosy nthesis indicated that both carboxylation efficiency and the CO2 saturated rate of photosynthesis were lower in plants grown at elevated relative to a mbient CO2. This was accompanied by a 49% reduction in the phosphoenolpyruv ate carboxylase content of leaves (area basis) in the elevated CO2-grown pl ants, but no change in Rubisco content. Despite the lower phosphoenolpyruva te carboxylase content, there was a 3-fold increase in C isotope discrimina tion in leaves of plants grown at elevated CO2 and bundle sheath leakiness was estimated to be 24% and 33%, respectively, for the ambient and elevated CO2-grown plants. However, we could detect no difference in quantum yield. The ratio of quantum yield of CO2 fixation to PSII efficiency was lower in plants grown at elevated [CO2] but only when leaf internal was below 50 mu L L-1. This suggests a reduction in the efficiency of the C-4 cycle when [ CO2] is low, and also implies increased electron transport to accepters oth er than CO2. Analysis of leaf sections using a transmission electron micros cope indicated a 2-fold decrease in the thickness of the bundle sheath cell walls in plants grown at elevated relative to ambient CO2. These results s uggest that significant acclimation to increased CO2 concentrations occurs in sorghum.