PHOTOSYNTHESIS TO ACUTE HEAT STRESS IN C3, C4, AND CAM SPECIES1

Determining the effect of elevated C02 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 C02 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) C02 and at species-specific optimal growth temperatures and at 30?C (if optimal ^ 30?C). We then as sessed thermotolerance of net photosynthesis (P?), stomatal conductance (gsl), leaf internal [C02], and photosystem II (PSII) and post-PSII electron transport during AHS. Thermotolerance of Pn in elevated (vs. ambient) C02 increased in C3, but decreased in C4 (especially) and CAM (high growth temperature only), species. In contrast, elevated C02 decreased electron transport in 10 of 11 species. High C02 decreased gst in five of nine species, but stomatal limitations to Pn increased during AHS in only two cool season C3 species. Thus, benefits of elevated C02 to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C4 species, so effects of elevated C02 on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.