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 (T_c) and peak temperatures (T_p) 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.