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Interactive effects of water stress and elevated CO2 on growth, photosynthesis, and water use efficiency

Hsiao, TC, RB Jackson
Carbon Dioxide and Environmental Stress
Journal Volume/Pages: 
Academic Press, San Diego, pp. 3-31, (Y Luo & HA Mooney, eds.)

Of all the physical stresses in the global environment, water deficit is probably the most important in determining plant growth and productivity world wide. At the same time, plant water use and growth are strongly influenced by climatic conditions and CO2 concentration in the atmosphere. Of particular interest is the fact that as the level of CO2 is raised above the present ambient level, photosynthesis is commonly enhanced and transpiration is often reduced, resulting in a higher efficiency of water use, and plant growth and productivity are generally increased. Limited data also show that elevated levels of CO2 may facilitate plantsĀ¹ adjustment to drought. The rise in atmospheric CO2 due to fossil fuel burning and other anthropogenic activities will continue for decades and centuries to come, although the extent of the rise is uncertain and a matter of debate. The broad consensus is that this rise will result in hotter and drier environments in many parts of the world, which would also affect plant productivity in addition to the effects of rising CO2. How water deficits and elevated CO2 interact to impact plant productivity and water use efficiency (WUE) is a pivotal question in the consideration of future changes of natural and managed terrestrial ecosystems. For natural communities, differences in WUE under elevated CO2 may determine the success in adaptation and competition of plant species, and ultimately in community succession, in environments of generally warmer temperature and more frequent drought (Ehleringer and Cerling, 1995). For managed communities, where water is the major limiting factor, productivity is determined by WUE of the crop or tree, or of individual species making up the community, and the amount of water available. This productivity in turn affects population dynamics at higher trophic levels. Conversely, since the standing biomass of plants is a major sink for atmospheric CO2, the growth and succession of plant communities in turn play a role in modulating the future rise in CO2.

More than 90% of the dry matter (biomass) produced by the plant comes from assimilated CO2. Hence, productivity depends on the capture of photosynthetically active radiation (PAR) by the plant and the use of the radiation for photosynthesis. This chapter focuses on three critical aspects of crop productivity as affected by water deficit and elevated CO2. The first is expansive growth of leaves and roots. Leaf growth underlies canopy development and hence PAR capture. Next comes photosynthesis and its adjustment to the environment. The last aspect discussed is WUE.