Gas exchange and chlorophyll fluorescence responses of three south‐westernYuccaspecies to elevated CO2and high temperature

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The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south‐western USA, and the manner in which elevated CO2impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO2are comparatively well understood, little research has assessed plant performance in elevated CO2during extreme (> 45 °C) temperature events. We exposed three species ofYuccato 360 and 700μmol CO2mol–1for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO2on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C3species (Yucca whipplei), a desert C3species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general,Yuccasexposed to elevated CO2showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y.whipplei) showed significant reductions (33%) in CO2saturated maximum assimilation rate (Amax), but the desert species (Y.brevifoliaandY.schidigera) showed no such reductions inAmax. Stomatal conductance was lower in elevated CO2as compared with ambient throughout the temperature event; however, there were species‐specific differences over time. Elevated CO2enhanced photosynthesis inY. whippleiat high temperatures for a period of 4 d, but not forY. brevifoliaorY. schidigera. Elevated CO2offset photoinhibition (measured asFv/Fm) inY. whippleias compared with ambient CO2, depending on exposure time to high temperature. StableFv/FminY. whippleioccurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO2. The value ofΦPSIIremained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions inFv/Fmresulted from thermal energy dissipation in the pigment bed forY. brevifoliaandY. schidigera. The greater efficiency of photosystem II inY. whippleihelped to maintain photosynthetic function at high temperatures in elevated CO2. These patterns are in contrast to the hypothesis that high temperatures in elevated CO2would increase the potential for photoinhibition. Our results suggest that elevated CO2may offset high‐temperature stress in coastalYucca, but not in those species native to drier systems. Therefore, in the case ofY. whipplei, elevated CO2may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.