Cross-tolerance: Salinity Gradients and Dehydration Increase Photosynthetic Heat Tolerance in Mangrove Leaves

1. Understanding the drivers of variation in thermal ecophysiology is essential for characterizing the risks to plant communities posed by the increasing frequencies and intensities of heat waves predicted with climate warming. We evaluated the effects of estuarine salinity and short-term leaf dehydration on photosynthetic (PSII) heat tolerance (P-HT). 2. In the leaves of 12 mangrove species sampled at contrasting salinities along a 20 km estuarine salinity gradient, we measured minimum chlorophyll fluorescence (F-0) with increasing leaf temperature to determine T-crit (the temperature beyond which PSII is destabilized and F-0 rises rapidly) and T-max (the temperature where F0 declines rapidly with catastrophic cell membrane failure). Furthermore, to assess the impacts of leaf dehydration on PHT over short time scales, we re-measured leaves following a bench-drying dehydration treatment. 3. T-crit was similar to 2.51 degrees C higher in leaves of high-salinity-distributed mangrove species, increasing by similar to 0.74 degrees C per practical salinity unit (PSU, ppt) along the estuarine salinity gradient. Tmax was similar to 1.1 degrees C higher in high-salinity-distributed species, increasing by similar to 0.38 degrees C ppt-1. Following dehydration, Tcrit increased by similar to 3.4 degrees C in low-salinity- distributed species; however, no increase in Tcrit was observed in high-salinity-distributed species. Following dehydration, Tmax increased by similar to 1.8 degrees C in both low and high-salinity- distributed mangrove species. 4. Our results illustrate that spatial gradients in salinity, and the variation in growth environments they produce, are associated with gradients in PHT across mangrove species differing in salinity tolerance and within species with broad salinity tolerances. Further, we show that variation in leaf hydration over short time scales influences PHT, with leaf dehydration improving the heat tolerance of PSII. Combined, these results highlight that both tissue and environmental water availability gradients may produce cross-tolerance in PSII, that is, increasing stability at high temperatures. Our results underscore the need for greater resolution of the interactive effects of plant water relations, hydration status and photosynthetic thermal safety across biomes and in a warming world.