A high-throughput method for measuring critical thermal limits of leaves by chlorophyll imaging fluorescence

Plant thermal tolerance is a crucial research area as the climate warms and extreme weather events become more frequent. Leaves exposed to temperature extremes have inhibited photosynthesis and will accumulate damage to PSII if tolerance thresholds are exceeded. Temperature-dependent changes in basal chlorophyll fluorescence (T-F₀) can be used to identify the critical temperature at which PSII is inhibited. We developed and tested a high-throughput method for measuring the critical temperatures for PSII at low (CT_MIN) and high (CT_MAX) temperatures using a Maxi-Imaging fluorimeter and a thermoelectric Peltier plate heating/cooling system. We examined how experimental conditions of wet vs dry surfaces for leaves and heating/cooling rate, affect CT_MINand CT_MAXacross four species. CT_MAXestimates were not different whether measured on wet or dry surfaces, but leaves were apparently less cold tolerant when on wet surfaces. Heating/cooling rate had a strong effect on both CT_MAXand CT_MINthat was species-specific. We discuss potential mechanisms for these results and recommend settings for researchers to use when measuring T-F₀. The approach that we demonstrated here allows the high-throughput measurement of a valuable ecophysiological parameter that estimates the critical temperature thresholds of leaf photosynthetic performance in response to thermal extremes.