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 RF(v/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 RF(v/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