| Global climate change will likely have significant effects on terrestrial plant communities and agricultural productivity. In order to increase the accuracy of current models and predictions of how plants will respond to future climatic conditions, it is important to understand the fundamental mechanism of any responses of photosynthesis at both the cellular and leaf level. Previous growth chamber experiments on the response of four carbon (C4) photosynthesis to elevated atmospheric carbon dioxide (CO2) concentrations ([Ca]) have shown inconsistent results between studies and may not be relevent to field grown material. Effects of increased atmospheric CO2 concentration and water-stress on photosynthesis of Sorghum bicolor (DK54) grown in a Free-Air Carbon Dioxide Enrichment (FACE) field experiment have been examined. The FACE system provided a unique opportunity to examine the effects of growth under elevated CO2 and other environmental conditions that closely represent field conditions likely to occur in the near future. In developing leaf tissue, prior to emergence from the surrounding whorl of older leaves, ribulose bisphosphate carboxylase/oxygenase (Rubisco; part of the C3 cycle) accumulated before significant amounts of Phosphoenol pyruvate carboxylase (PEPC; part of the C4 cycle) were detectable and was not expressed cell specifically. However, as the leaf tissue emerged from the surrounding whorl and into full sunlight, the C4 syndrome was fully expressed. C4 photosynthesis in young fully expanded leaves was enhanced under elevated [Ca]. This was partially attributed to suppressed rates of photorespiration and decreased CO2 leakage from bundle sheath cells and less overcycling of the C4 pump. Growth under elevated [Ca] also enhanced C4 photosynthesis and reduced nonphotochemical quenching requirements during drought and/or midday conditions. In summary, this work shows that while development of the C4 apparatus was only minimally affected by elevated CO2, future [Ca] will enhance the efficiency of C4 photosynthesis in young leaves. Furthermore, it was clearly demonstrated that C4 photosynthesis will be most responsive to elevated [Ca] when other environmental factors, particularly water-stress, are limiting rates of carbon uptake. |
