Photosystem II heat tolerances characterize thermal generalists and the upper limit of carbon assimilation.

Abstract

The heat tolerance of photosystem II (PSII) may promote carbon assimilation at higher temperatures and help explain plant responses to climate change. Higher PSII heat tolerance could lead to (a) increases in the high-temperature compensation point (Tmax ); (b) increases in the thermal breadth of photosynthesis (i.e. the photosynthetic parameter Ω) to promote a thermal generalist strategy of carbon assimilation; (c) increases in the optimum rate of carbon assimilation Popt and faster carbon assimilation and/or (d) increases in the optimum temperature for photosynthesis (Topt ). To address these hypotheses, we tested if the Tcrit , T50 and T95 PSII heat tolerances were correlated with carbon assimilation parameters for 21 plant species. Our results did not support Hypothesis 1, but we observed that T50 may be used to 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.