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. We developed and tested a high-throughput method for measuring photosynthetic critical thermal limits at low (CTMIN) and high (CTMAX) temperatures to achieve pragmatic and robust measures of thermal tolerance limits using a Maxi-Imaging fluorimeter and a thermoelectric Peltier plate temperature ramping system. Leaves exposed to temperature extremes accumulate damage to photosystem II (PSII). Temperature-dependent changes in basal chlorophyll fluorescence (T-F0) can be used to identify the critical temperature at which PSII is damaged. We examined how experimental conditions: wet vs dry surfaces for leaves and temperature ramp rate, affect CTMIN and CTMAX across four species. CTMAX estimates were not different whether measured on wet or dry surfaces, but leaves were apparently less cold tolerant when on wet surfaces. Temperature ramp rate had a strong effect on both CTMAX and CTMIN that was species-specific. We discuss potential mechanisms for these results and recommend settings for researchers to use when measuring T-F0. The system described and tested here allows high-throughput measurement of critical temperature thresholds of leaf photosynthetic performance for characterising plant function in response to thermal extremes.