Plant heat tolerance characterizes carbon assimilation strategy

The heat tolerance of photosystem II (PSII) may promote carbon assimilation at higher temperatures and may help explain plant responses to climate change. PSII heat tolerance could lead to 1) increases in the high temperature compensation point (Tmax); 2) increases in the thermal breadth of photosynthesis (i.e. the photosynthetic Ω parameter) to promote a thermal generalist strategy of carbon assimilation; 3) increases in the optimum rate of carbon assimilation Popt and promote faster carbon assimilation; and/or 4) increases in the optimum temperature for photosynthesis (Topt). To address these hypotheses, we tested if the Tcrit, T50 and T95 metrics of PSII heat tolerance were correlated with each carbon assimilation parameter for 21 species. Hypothesis 1 was not supported, but we observed that T50 may estimate the upper thermal limit for Tmax at the species-level, and that community mean Tcrit may be useful for approximating Tmax. The T50 and T95 heat tolerance metrics were positively correlated with Ω in support of hypothesis 2. We found no support for hypotheses 3 or 4. Our study shows that high PSII heat tolerance is unlikely to improve carbon assimilation at higher temperatures, but may characterize thermal generalists with slow resource acquisition strategies.