EFFECTS OF GROWTH TEMPERATURES OF 5-DEGREES-C AND 25-DEGREES-C ON LONG-TERM RESPONSES OF PHOTOSYSTEM-II TO HEAT-STRESS IN ATRAZINE-RESISTANT AND SUSCEPTIBLE BIOTYPES OF ERIGERON-CANADENSIS

When leaves of atrazine-resistant (AR) and atrazine-sensitive (S) plan ts of Erigeron canadensis (L.) Cronq, grown at 25 degrees C were expos ed to continuously rising temperatures, the heat-induced changes in th e initial level (F-0) 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 F-0 v. T curves for the leaves of the S biotype grown at 5 degrees C were considerably lower, in contrast with the leaves of the AR biotype, for which these values were very similar to those at 25 degrees C. For the warm and c old-acclimated AR biotype, the temperature dependences of the fluoresc ence quenching parameters were not greatly different, in contrast with the S plants. The different growth temperatures resulted in a shift i n the optimal thermal interval of CO2 fixation between the cold and wa rm-acclimated S biotype, whereas this interval was nearly the same for the AR biotype grown at the two temperatures. The results suggest tha t, besides the D-1 protein mutation, the growth temperature independen ce of the thylakoid fluidity, over and above a higher susceptibility t o heat stress, may cause the limited capacity of acclimation to temper ature in AR plants.