The transition temperature of the leaf polar lipids and the critical temperature for chill-induced inhibition of photosynthesis was determined for three altitudinal ecotypes of the wild tomato Lycopersicon hirsutum. Photosynthesis was measured as CO(2)-dependent O(2) evolution at 25 degrees C after leaf slices were exposed to chilling temperatures for 2 hours at a moderate photon flux density of 450 micromoles per square meter per second. The transition temperature of the leaf polar lipids was detected from the change in the temperature coefficient of the fluorescence intensity of trans-parinaric acid. Chill-induced photoinhibition was evident in the three tomato ecotypes when they were chilled below a critical temperature of 10 degrees , 11 degrees , and 13 degrees C, respectively, for the high (LA1777), mid (LA1625), and low (LA1361) altitudinal ecotypes. The temperature differential, below the critical temperature, required to produce a 50% inhibition was also similar for the three ecotypes. A transition was detected in the leaf polar lipids of these plants at a temperature similar to that of the critical temperature for photoinhibition. The results show that the three tomato ecotypes are similar with respect to their critical temperature for chilling-induced photoinhibition and the rate of their response to the chilling stress. They are, thus, similarly sensitive to chilling.