Abstract

Relative growth rates, basal and acclimated thermotolerance, membrane damage, fluorescence emission, and relative levels of free and conjugated ubiquitin and HSP70 were compared after 2 h of treatment at different temperatures between Prosopis chilensis and Glycine max (soybean), cv. McCall, to evaluate if the thermotolerance of these two plants was related to levels of accumulation of heat shock proteins. Seedlings of P. chilensis germinated at 25°C and at 35°C and grown at temperatures above germination temperature showed higher relative growth than soybean seedlings treated under the same conditions. The lethal temperature of both species was 50°C after germination at 25°C. However, they were able to grow at 50°C after germination at 35°C. Membrane damage determinations in leaves showed that P. chilensis has an LT50 6°C higher than that of soybean. There were no differences in the quantum yield of photosynthesis (Fv/Fm), between both plants when the temperatures were raised. P. chilensis showed higher relative levels of free ubiquitin, conjugated ubiquitin and HSP70 than soybean seedlings when the temperatures were raised. Time-course studies of accumulation of these proteins performed at 40°C showed that the relative accumulation rates of ubiquitin, conjugated ubiquitin and HSP70 were higher in P. chilensis than in soybean. In both plants, free ubiquitin decreased during the first 5 min and increased after 30 min of heat shock, conjugated ubiquitin increased after 30 min and HSP70 began to increase dramatically after 20 min of heat shock. From these data it is concluded that P. chilensis is more tolerant to acute heat stress than soybean.