Heat tolerance parameters
We calculated three temperature-response parameters of Fv/Fm: TCrit, the critical temperature beyond which Fv/Fmdeclines and leaves are irreversibly heat damaged; T50, the temperature at which PSII functionality is reduced by 50%; and b, the steepness of the decrease in Fv/Fmaround T50. T50 and b were determined by fitting the data with Eqn 1 as:
\(F_{v}/F_{m}=\frac{F_{v}/F_{m\mathrm{,\ }\text{Control}}}{1+e^{b\times\left(T_{\text{Leaf}}-T_{50}\right)}}\)Eqn 1
where Fv/Fm,Control is the average Fv/Fm of control leaves that were not heat treated, and TLeaf is the incubation temperature (°C) of the leaf disks. Curves were fitted with the ‘nls_multstart’ function from the ‘nls.multstart’ package (Padfield & Metheson 2018) in R version 3.6.1 (R Core Team 2019). The curves thus generated enabled us to generate species-level parameter estimates with their standard errors. TCrit was calculated as the temperature at which a line describing the slope of the Fv/Fmdecline at T50 intersected with a horizontal line indicating Fv/Fm,Control.