Stomatal and mesophyll conductances to CO2 in different plantgroups: Underrated factors for predicting leaf photosynthesisresponses to climate change?

The climate change conditions predicted for the end of the current century are expected to have an impact on the performance of plants under natural conditions. The variables which are foreseen to have a larger effect are increased CO₂ concentration and temperature. Although it is generally considered CO₂ assimilation rate could be increased by the increasing levels of CO₂, it has been reported in previous studies that acclimation to high CO₂ results in reductions of physiological parameters involved in photosynthesis, like the maximum carboxylation rate (V_c,max), stomatal conductance (g_s) and mesophyll conductance to CO₂ (g_m). On the one hand, most of the previous modeling efforts have neglected the potential role played by the acclimation of g_m to high CO₂ and temperature. On the other hand, the effect of climate change on plant clades other than angiosperms, like ferns, has received little attention, and there are no studies evaluating the potential impact of increasing CO₂ and temperature on these species.

In this study we predicted responses of several representative species among angiosperms, gymnosperms and ferns to increasing CO₂ and temperature. Our results show that species with lower photosynthetic capacity – such as some ferns and gymnosperms – would be proportionally more favored under these foreseen environmental conditions. The main reason for this difference is the lower diffusion limitation imposed by g_s and g_m in plants having high capacity for photosynthesis among the angiosperms, which reduces the positive effect of increasing CO₂. However, this apparent advantage of low-diffusion species would be canceled if the two conductances – g_s and g_m – acclimate and are down regulated to high CO₂, which is basically unknown, especially for gymnosperms and ferns. Hence, for a better understanding of different plant responses to future climate, studies are urged in which the actual photosynthetic response/acclimation to increased CO₂ and temperature of ferns, gymnosperms and other under-evaluated plant groups is assessed.