As the CO2 content of the air increases, most plants exhibit physiological modifications. One common change is a reduction in stomatal conductance. With greater concentrations of CO2 in the air, plants need not open their stomates as wide as they do at lower CO2 concentrations; for the greater gradient in CO2 concentration between the CO2-enriched air and the air within the substomatal cavities of the plant leaves typically allows greater amounts of CO2 to diffuse through their stomates, in spite of the CO2-induced reduction in stomatal conductance. Hence, with a greater flux of CO2 into their leaves, and a reduced flux of water out of their leaves, plants growing in CO2-enriched air typically experience substantial increases in water-use efficiency. In this summary we review the results of some recent studies of this phenomenon in grassland plants.
Szente et al. (1998) studied two grasses and two broad-leaved species common to loess grasslands in the vicinity of Budapest in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm. After 231 days, they reported that the grasses and broad-leaved species exhibited CO2-induced increases in seasonal plant water-use efficiencies of 72 and 366%, respectively. Clark et al. (1999) likewise noted that mixed grassland species from a New Zealand pasture exposed to an atmospheric CO2 concentration of 700 ppm consistently displayed greater water-use efficiencies than species growing in ambient air, due mostly to CO2-induced increases in photosynthesis. On the other hand, in a two-year CO2-enrichment study of a tallgrass prairie ecosystem located in Kansas, USA, Adams et al. (2000) found that plants in open-top chambers fumigated with twice-ambient levels of atmospheric CO2 exhibited significantly reduced rates of stomatal conductance and transpirational water loss, which also enhanced plant daily average water-use efficiency throughout the entire study, but via the reduced water loss route as opposed to the enhanced photosynthesis route observed by Clark et al.
As is evident from these studies, it is clear that as the atmospheric CO2 concentration increases, CO2-induced reductions in stomatal conductance or CO2-induced increases in photosynthetic carbon fixation, or both of these phenomena, will likely lead to significant increases in plant water-use efficiency in grassland species. Thus, grasslands will likely expand their ranges into areas where they previously could not live due to low soil moisture contents. Consequently, carbon sequestration by grasslands is likely to become more substantial as CO2-driven increases in water-use efficiency allow them to expand their ranges.
References
Adams, N.R., Owensby, C.E. and Ham, J.M. 2000. The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie. Photosynthesis Research 65: 121-129.
Clark, H., Newton, P.C.D. and Barker, D.J. 1999. Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental Botany 50: 233-242.
Szente, K., Nagy, Z. and Tuba, Z. 1998. Enhanced water use efficiency in dry loess grassland species grown at elevated air CO2 concentration. Photosynthetica 35: 637-640.