Plants grown in elevated CO2 environments often exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by long-term rates of photosynthesis that are somewhat lower than what would be expected on the basis of measurements made during short-term exposure to CO2-enriched air. These downward adjustments result from modest long-term decreases in the activities and/or amounts of the primary plant carboxylating enzyme rubisco. Acclimation is said to be present when the photosynthetic rates of long-term CO2-enriched plants are found to be lower than those of long-term non-CO2-enriched plants when the normally CO2-enriched plants are measured during brief exposures to ambient CO2 concentrations. Nevertheless, rates of photosynthesis displayed by plants grown and measured at elevated CO2 concentrations are often greater than those exhibited by plants grown and measured at ambient CO2 concentrations, leading to enhanced growth in CO2-enriched air.
Roberts et al. (1998) conducted a FACE experiment in southern California, USA, exposing Adenostoma fassciculatum shrubs to atmospheric CO2 concentrations of 360 and 550 ppm while they studied the nature of gas-exchange in this chaparral species. After six months of CO2 fumigation, photosynthetic acclimation occurred. However, because of reductions in stomatal conductance and transpirational water loss, the CO2-enriched shrubs exhibited leaf water potentials that were less negative (and, hence, less stressful) than those of control plants. This CO2-induced water conservation phenomenon should enable this woody perennial to better withstand the periods of drought that commonly occur in this southern California region, while the photosynthetic down regulation it exhibits should allow it to more equitably distribute the limiting resources it possesses among different essential plant physiological processes.
Huxman and Smith (2001) measured seasonal gas exchange during an unusually wet El Niņo year in an annual grass (Bromus madritensis ssp. rubens) and a perennial forb (Eriogonum inflatum) growing within FACE plots established in the Mojave Desert, USA, which they maintained at atmospheric CO2 concentrations of 350 and 550 ppm. The elevated CO2 consistently increased net photosynthetic rates in the annual grass without inducing photosynthetic acclimation. In fact, even as seasonal photosynthetic rates declined post-flowering, the reduction was much less in the CO2-enriched plants. However, elevated CO2 had no consistent effect on stomatal conductance in this species. In contrast, Eriogonum plants growing at 550 ppm CO2 exhibited significant photosynthetic acclimation, especially late in the season, which led to similar rates of net photosynthesis in these plants in both CO2 treatments. But in this species, elevated CO2 did reduce stomatal conductance over most of the growing season. Hence, although both desert plants exhibited different stomatal and photosynthetic responses to elevated CO2, both experienced significant CO2-induced increases in water use efficiency and biomass production, thus highlighting the existence of different, but equally-effective, species-specific mechanisms for responding positively to atmospheric CO2 enrichment in a desert environment.
In another study conducted at the Mojave Desert FACE site, Hamerlynck et al. (2002) determined that plants of the deciduous shrub Lycium andersonii grown in elevated CO2 displayed photosynthetic acclimation, as maximum rubisco activity in the plants growing in the CO2-enriched air was 19% lower than in the plants growing in ambient air. Also, the elevated CO2 did not significantly impact rates of photosynthesis. Leaf stomatal conductance, on the other hand, was consistently about 27% lower in the plants grown in the CO2-enriched air; and during the last month of the spring growing season, the plants in the elevated CO2 plots displayed leaf water potentials that were less negative than those exhibited by the control plants growing in ambient air. Hence, as the CO2 content of the air increases, Lycium andersonii will likely respond by exhibiting significantly enhanced water use efficiency, which should greatly increase its ability to cope with the highly variable precipitation and temperature regimes of the Mojave Desert. Moreover, the acclimation observed within the shrub's photosynthetic apparatus should allow it to reallocate more resources to producing and sustaining greater amounts of biomass. Thus, it is likely that future increases in the air's CO2 content will favor a "greening" of the American Mojave Desert.
In summary, the few studies of the acclimation phenomenon that have been conducted on chaparral and desert plants indicate that although it can sometimes be complete, other physiological changes, such as the reductions in stomatal conductance that typically produce large increases in water use efficiency, often more than compensate for the sometimes small to negligible increases in photosynthesis.
References
Hamerlynck, E.P., Huxman, T.E., Charlet, T.N. and Smith, S.D. 2002. Effects of elevated CO2 (FACE) on the functional ecology of the drought-deciduous Mojave Desert shrub, Lycium andersonii. Environmental and Experimental Botany 48: 93-106.
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.
Roberts, S.W., Oechel, W.C., Bryant, P.J., Hastings, S.J., Major, J. and Nosov, V. 1998. A field fumigation system for elevated carbon dioxide exposure in chaparral shrubs. Functional Ecology 12: 708-719.