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

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 F₀ fluorescence were established for the tropical mangrove Rhizophora mangle. Three fluorescence parameters were determined as follows: (i) the temperature at which F₀ began to sharply rise (T_crit); T_crit is the most studied and often the only studied parameter in F₀ heat-stress tests; (ii) the temperature at which maximal F₀ fluorescence occurred (T_max); and (iii) the temperature at which F₀ was 50% of T_max = T₅₀. T_crit was 49.6 °C, T₅₀ 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 T₅₀. The results improve our ability to interpret F₀-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 F₀-temperature response experiments, the widely used heat tolerance parameter T_crit may grossly underestimate the capacity of leaves to prevent irreversible heat damage.