Effects of elevated CO2 on the tolerance of photosynthesis to acute heat stress in C3, C4, and CAM species

Determining the effect of elevated CO2 on the tolerance of photosynthesis to acute heat stress (AHS) is necessary for predicting plant responses to global warming because photosynthesis is heat sensitive and AHS and atmospheric CO2 Will increase in the future. Few studies have examined this effect, and past results were variable, which may be related to methodological variation among studies. In this study, we grew 11 species that included cool and warm season and C-3, C-4, and CAM species at current or elevated (370 or 700 ppm) CO2 and at species-specific optimal growth temperatures and at 30 degrees C (if optimal not equal 30 degrees C). We then assessed thermotolerance of net photosynthesis (P-n), stomatal conductance (g(st)), leaf internal [CO2], and photosystem II (PSII) and post-PSII electron transport during AHS. Thermotolerance of P-n in elevated (vs. ambient) CO2 increased in C-3, but decreased in C-4 (especially) and CAM (high growth temperature only), species. In contrast, elevated CO2 decreased electron transport in 10 of 11 species. High CO2 decreased g(st) in five of nine species, but stomatal limitations to P-n increased during AHS in only two cool-season C-3 species. Thus, benefits of elevated CO2 to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C-4 species, so effects of elevated CO2 on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.