EVALUATION OF RELATIVE CONTRIBUTION OF INITIAL AND VARIABLE CHLOROPHYLL FLUORESCENCE MEASURED AT DIFFERENT TEMPERATURES

The temperature dependence of the induction kinetics parameters of chl orophyll (Chl) fluorescence in leaves and isolated chloroplasts of you ng plants of Phaseolus vulgaris L. was studied. Theoretical prerequisi tes for the influence of fluorescence measuring temperature as well as 3 min pretreatment of leaf discs on both the photophysical and dark p rocesses in the photosynthetic apparatus were evaluated. The registrat ion manner of fluorescence kinetics determined the pattern of temperat ure dependence curve. The characteristic temperature of sharp changes of induction kinetics parameters, maximal and initial fluorescence F-m and F-0, were in ranges of 45-47 and 55-60 degrees C, respectively. T he fitting of experimental data from thermograms registered at weak ex citing irradiance in thermoinactivated leaf discs allowed to estimate the activation energy (E(a)) of the internal energy conversion process in excited Chl molecule as 51.2+/-0.15 kJ mol(-1). The theoretical an alysis of F-0 measured by a PAM fluorometer showed that F-0 registered even at low irradiances markedly exceeded its ideal dark value. The F -0 measured at 25 degrees C was increased by 125+/-5.3 % by the herbic ide DCMU and diminished by -23.8+/-1.6 % by the electron acceptor DCBQ . At the experimental conditions used there were about 25 % of F-v fro m closed Q(B) non-reducing reaction centres in the measured F-0 value. Thus the thermoinduced increase in F-0 observed within the temperatur e interval of 45-50 degrees C might be done by a transition of PS2 rea ction centres from a state capable of reducing Q(B) to a Q(B)-non-redu cing state.