Leaf carbon assimilation in a water-limited world

Over the past 150 years the amount of CO2 in the atmosphere has been increasing, largely as a result of land-use change and anthropogenic emissions from the burning of fossil fuels. It is estimated that the atmospheric [CO2] will reach 70Pa by the end of the 21st Century. The most important consequence of this rise in [CO2] is warming the surface temperature of the Earth by 0.4-0.6 degrees C per decade throughout the 21st Century. Increasing [CO2] along with associated changes in temperature will most likely alter the structure and function of agro-ecosystems, affecting their productivity and their role as stable sinks to CO2 sequestration. Both CO2 and temperature are key variables affecting plant growth, development and functions. Moreover, because of the future scenario of higher temperature and evaporative demand, drought occurrences will be increased in frequency, intensity, and erratic pattern. The combination of elevated temperatures and the increased incidence of environmental stress will probably constitute the greatest risk caused by climate change to the agro-ecosystems in arid or semiarid areas of the world. The purpose of this paper is to review the exchange of carbon driving the main ecophysiological processes of plants in response to climate change and environmental stresses. Drought and salinity first affect the acquisition of CO2 by increasing stomatal and mesophyll resistances, and only after cause irreversible damages to the biochemical apparatus. Heat stress denatures thylakoid membranes, but this action may be counteracted by the synthesis of many isoprenoids in the chloroplasts from carbon freshly fixed by photosynthesis. There is rising concern about the impact of environmental stress on tree growth with this future scenario of global climate change. The combination of elevated temperatures and the increased incidence of environmental stress (particularly drought and salinity) will probably constitute the greatest risk caused by global climate change to the forest ecosystems in arid or semiarid areas of the world.