As the air's CO2 content continues to rise, many plants will respond by reducing their stomatal apertures, because plants need not open their stomates as wide as they do at lower atmospheric CO2 concentrations to allow for an inward diffusion of CO2 capable of maintaining the same rate of photosynthesis. Consequently, plants typically exhibit greater water-use efficiencies at elevated CO2 concentrations, as documented elsewhere on our website (see Water Use Efficiency in our Subject Index). In this summary, we thus review some of the recent literature pertaining to elevated CO2 effects on the stomatal conductances of grasses.
In a three-month growth chamber study performed on both C3 and C4 grasses, Volin et al. (1998) determined that a 300 ppm increase in atmospheric CO2 concentration significantly reduced the stomatal conductances of all species tested. Similarly, LeCain and Morgan (1998) documented CO2-induced stomatal conductance reductions in six different C4 grasses exposed to twice-ambient atmospheric CO2 concentrations for 39 days.
In a community-level study, Volk et al. (2000) grew plant assemblages characteristic of the calcareous grasslands of northwest Switzerland for three months at 600 ppm CO2, finding that elevated CO2 consistently reduced stomatal conductances in these communities, regardless of soil moisture status. In another community-level study, Huxman and Smith (2001) exposed naturally-growing Mojave Desert vegetation to a 200 ppm increase in atmospheric CO2 during a typical wet year. For this situation they found elevated CO2 to have no significant effect on stomatal conductance within an annual grass species, although it significantly decreased the stomatal conductance of a perennial forb.
In summary, it appears that elevated CO2 reduces the stomatal conductances of most grass species under both wet and dry soil moisture conditions. Thus, as the air's CO2 concentration continues to increase, it is likely that lower stomatal conductances will lead to enhanced plant water-use efficiencies in grasslands, likely allowing them to expand their ranges into barren deserts.
References
Huxman, T.E. and Smith, S.D. 2001. Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niņo year in the Mojave Desert. Oecologia 128: 193-201.
LeCain, D.R. and Morgan, J.A. 1998. Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD-ME and NADP-ME C4 grasses grown in elevated CO2. Physiologia Plantarum 102: 297-306.
Volin, J.C., Reich, P.B. and Givnish, T.J. 1998. Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group. New Phytologist 138: 315-325.
Volk, M., Niklaus, P.A. and Korner, C. 2000. Soil moisture effects determine CO2 responses of grassland species. Oecologia 125: 380-388.