On the relationship between Photosystem II thermotolerance and irreversible heat damage in leaves of Rhizophora mangle

Abstract Global temperatures are on the rise and may, together with more frequent and more intense heat waves, adversely affect plant performance and survival. Measurements of Photosystem II (PSII) chlorophyll a fluorescence are a widely used tool to assess the high‐temperature limits of PSII, but information on how decreases in PSII competence relate to permanent leaf tissue damage and plant survival is limited. Temperature response curves of dark‐level F 0 fluorescence were established for the tropical mangrove Rhizophora mangle . Three fluorescence parameters were determined as follows: (i) the temperature at which F 0 began to sharply rise (T crit ); T crit is the most studied and often the only studied parameter in F 0 heat‐stress tests; (ii) the temperature at which maximal F 0 fluorescence occurred (T max ); and (iii) the temperature at which F 0 was 50% of T max = T 50 . T crit was 49.6 °C, T 50 51.3 °C and T max 54.5 °C. Leaf heating experiments that included 2 weeks of post‐culture of leaves showed that severe tissue damage and sustained reductions in F v / F m , a measure of maximum quantum yield of PSII chemistry in the dark‐adapted state, were associated with temperatures close to T max , but not T crit and T 50 . The results improve our ability to interpret F 0 ‐temperature response curves in the context of heat‐induced leaf mortality, draw attention to the significance of T max in assessing permanent heat damage in leaves of Rhizophora mangle and highlight the fact that in F 0 ‐temperature response experiments, the widely used heat tolerance parameter T crit may grossly underestimate the capacity of leaves to prevent irreversible heat damage.