Photosynthetic heat tolerances and extreme leaf temperatures

Abstract Photosynthetic heat tolerances (PHTs) have several potential applications including predicting which species will be most vulnerable to climate change. Given that plants exhibit unique thermoregulatory traits that influence leaf temperatures and decouple them from ambient air temperatures, we hypothesized that PHTs should be correlated with extreme leaf temperatures as opposed to air temperatures. We measured leaf thermoregulatory traits, maximum leaf temperatures ( T MO ) and two metrics of PHT ( T crit and T 50 ) quantified using the quantum yield of photosystem II for 19 plant species growing in Fairchild Tropical Botanic Garden (Coral Gables, FL, USA). Thermoregulatory traits measured at the Garden and microenvironmental variables were used to parameterize a leaf energy balance model that estimated maximum in situ leaf temperatures ( T MIS ) across the geographic distributions of 13 species. T MO and T MIS were positively correlated with T 50 but were not correlated with T crit . The breadth of species' thermal safety margins (the difference between T 50 and T MO ) was negatively correlated with T 50 . Our results provide observational and theoretical support based on a first principles approach for the hypothesis that PHTs may be adaptations to extreme leaf temperature, but refute the assumption that species with higher PHTs are less susceptible to thermal damage. Our study also introduces a novel method for studying plant ecophysiology by incorporating biophysical and species distribution models, and highlights how the use of air temperature versus leaf temperature can lead to conflicting conclusions about species vulnerability to thermal damage. A free Plain Language Summary can be found within the Supporting Information of this article.