Mesophyll conductance to CO2 is the most significant limitation to photosynthesis at different temperatures and water availabilities in Antarctic vascular species

The impact of climate change on Antarctic plants is not only associated to the effect of increased temperature but also strongly modulated by water availability (WA) illustrating the importance of this factor in predicting responses to warming. The aim of this study was to evaluate the effect of temperature and WA on the photosynthetic performance and photosynthetic limitations of the only two Antarctic vascular species: Deschampsia antarctica and Colobanthus quitensis. We hypothesize that: the ability of Antarctic plants to increase their net CO2 assimilation (AN) in response to raising growth temperature would be constrained by mesophyll conductance (gm); and decreases in water availability may counteract any benefit in carbon gain obtained upon increasing temperature. To address this issue, both species were grown (Tg) and measured (Tm) at three different temperatures (5, 10 and 16 °C). Furthermore, two different irrigation conditions (well-watered, WW, and water-deficit, WD) were applied at 16 °C. Gas-exchange measurements showed that AN and their underlying diffusive (gs and gm) and biochemical components (Vcmax) were mainly determined by Tg and, to a lesser extent by Tm. Warmer conditions favor AN of both species, although D. antarctica requires higher increases of temperature to show the same response. Changes in AN in response to either temperature or WA are due to proportional concomitant changes of stomatal and mesophyll conductances, and carboxylation capacity. However, gm remains the most important limitation at any environmental condition. Reduced WA can completely counteract any benefit to photosynthesis induced by raising temperature, suggesting that these species may present a quite homeostatic photosynthetic response to the climate changes predicted for the Antarctic region.