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The heat tolerance of 8 temperate- and 1 subtropical-origin C-3 species as well as 17 tropical-origin ones, including C-3, C-4, and CAM species, was estimated using both F-0-T curve and the ratio of chlorophyll fluorescence parameters, prior to and after high temperature treatment. When leaves were heated at the rate of ca. 1 degrees C min(-1) in darkness, the critical temperature (T-c) varied extensively among species. The T-c’s of all 8 temperate-origin species ranged between 40-46 degrees C in winter (mean temperature 16-19 degrees C), and between 32-48 degrees C in summer (mean temperature ca. 30 degrees C). Those for 1 subtropical- and 12 tropical-origin C3 Species ranged between 25-44 degrees C and 35-48 degrees C, and for 1 CAM and 4 C4 species were 41-47 and 45-46 degrees C, respectively. Acclimating three C-3 herbaceous plants at high temperature (33/28 degrees C, day/night) for 10 d in winter caused their Tc’s rising to nearly the values measured in summer. When leaves were exposed to 45 degrees C for 20 min and then kept at room temperature in darkness for 1 h, a significant correlation between RFv/m (the ratio of F-v/F-m before and after 45 degrees C treatment) and Tc was observed for all tested temperate-origin C3 species as well as tropical-origin CAM and C-4 species. However, F-0 and F-v/F-m of the tropical-origin C-3 species were less sensitive to 45 degrees C treatment, regardless of a large variation of T-c thus no significant correlation was found between their RFv/m and Tc. Thus T, might not be a suitable index of heat tolerance for plants with wide range of environmental adaptation. Nevertheless, T-c’s of tropical origin C-3 species, varying and showing high plasticity to seasonal changes and temperature treatment, appeared suitable for the estimation of the degree of temperature acclimation in the same species.