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Effects of elevated CO₂ on the tolerance of photosynthesis to acute heat stress in C₃, C₄, and CAM species¹

² Department of Environmental Sciences, University of Toledo, Toledo, Ohio 43606 USA ³ Department of Biology, Washington & Lee University, Lexington, Virginia 24450 USA

ABSTRACT

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 C₃, C₄, and CAM species at current or elevated (370 or 700 ppm) CO₂ and at species-specific optimal growth temperatures and at 30°C (if optimal 30°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 C₃, but decreased in C₄ (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 C₃ species. Thus, benefits of elevated CO₂ to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C₄ species, so effects of elevated CO₂ on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.

Key Words: C₃ • C₄ • CAM • carbon dioxide • global climate change • photosystem II • thermotolerance