Effects of growth temperatures of 5 and 25°C on long-term responses of photosystem II to heat stress in atrazine-resistant and susceptible biotypes of Erigeron canadensis

Abstract

When leaves of atrazine-resistant (AR) and atrazine-sensitive (S) plants of Erigeron canadensis (L.) Cronq. grown at 25°C were exposed to continuously rising temperatures, the heat-induced changes in the initial level (F0) of modulated fluorescence indicated an enhanced heat sensitivity of the chloroplasts in the leaves of the AR biotype. The critical (Tc) and peak temperatures (Tp) of the F0 v. T curves for the leaves of the S biotype grown at 5Q°C were considerably lower, in contrast with the leaves of the AR biotype, for which these values were very similar to those at 25°C. For the warm and cold-acclimated AR biotype, the temperature dependences of the fluorescence quenching parameters were not greatly different, in contrast with the S plants. The different growth temperatures resulted in a shift in the optimal thermal interval of CO2 fixation between the cold and warm-acclimated S biotype, whereas this interval was nearly the same for the AR biotype grown at the two temperatures. The results suggest that, besides the D1 protein mutation, the growth temperature independence of the thylakoid fluidity, over and above a higher susceptibility to heat stress, may cause the limited capacity of acclimation to temperature in AR plants.