In face of climate change catastrophes, understanding the thermal limits and optimal physiological thermal window food crop is of particular urgency. This research aims to evaluate: 1) how physiological performances of plant will change with increasing chronic and acute heat stress; 2) if the examined parameters form a hierarchy in terms of thermal tolerance; and 3) the optimal thermal window and critical temperatures of the examined plants with response to chronic and acute heat stress. Six temperate vegetables were subjected to chronic and acute heat stress and a suite of physiological parameters were evaluated. Dose responses were observed in shoot fresh weight, photosynthetic gas exchange, photosynthetic oxygen evolution, electron transfer rate, photo-and non-photochemical quenching with significant drop in performance as early as 28˚C for selected species. Conversely, ratio of variable to maximum fluorescence (F v /F m ) was not affected by heat stress until 46˚C in chronic heat stress. Examining the temperature at which a measured parameter's performance dropped by 50% compared to control (LT 50 ), a distinct hierarchy of the indices was observed for Canasta, recombinant inbred line 141, Lactuca serriola and Lactuca sativa (L. "Salinas"): shoot fresh weight, representing the highest integrated level of photosynthesis was the most sensitive to thermal stress (28˚C -30˚C), followed by oxygen evolution (35˚C -45˚C) while non-photochemical and photochemical quenching which is subcellular function of stress alleviation had a much higher capacity failure temperature (47˚C -60˚C). It is expected that F v /F m ratio, a measurement of sub-cellular structural integrity, will approach that of non-photochemical and photochemical quenching, if not exceeding it. By examining the photosynthetic parameters via their hierarchy of biological organization, it can be inferred that plants like Arugula and recombinant inbred line 192 are already operating near their thermal limit and have less energetic investment into heat stress mediation whereas L. serriola prioritizes thermal tolerance at the expense of photosynthesis efficiency.

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