Elevated CO2 induces biochemical and ultrastructural changes in leaves of the C4 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 isotope discrimination, and leaf anatomy) using the C-4 cereal sorghum (Sovghum bicolor L. Moench.). Gas-exchange analysis of the CO2 response of photosynthesis indicated that both carboxylation efficiency and the CO2 saturated rate of photosynthesis were lower in plants grown at elevated relative to ambient CO2. This was accompanied by a 49% reduction in the phosphoenolpyruvate carboxylase content of leaves (area basis) in the elevated CO2-grown plants, but no change in Rubisco content. Despite the lower phosphoenolpyruvate carboxylase content, there was a 3-fold increase in C isotope discrimination 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 other than CO2. Analysis of leaf sections using a transmission electron microscope 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 suggest that significant acclimation to increased CO2 concentrations occurs in sorghum.