The temperature dependence of the induction kinetics parameters of chiorophyll (Chl) fluorescence in leaves and isolated chloroplasts of young plants of PhaseoliLt vulgaris L. was studied. Theoretical prerequisites for the influence of fluorescence measuring temperature as well as 3 min pretreatment of leaf discs cn both the photophysical and dark processes in the photosynthetic appaiatus were evaluated. The registration manner of fluorescence kinetics detennined the pattem of temperature dependence curve. The characteristic temperature of sharp changes of induction kinetics parameters, maximal and initial fluorescence and Fq, were in ranges of 45-47 and 55-60 °C, respectively. The fitting of experimental data from thermograms registered at weak exciting iiradiance in thermoinactivated leaf discs allowed to estimate the activation energy (FJ of the intemal energy conversion process in excited Chl molecule as 51.2±0.15 kJ moTT The theoretical analysis of /'q measured by a PAM fluorometer showed that Fq registered even at low inadiances markedly exceeded its ideál dark value. The Fq measured at 25 °C was increased by 125±5.3 % by the herbicide DCMU and diminished by -23.8±1.6 % by the elechon acceptor DCBQ. At the experimental conditions ušed there were about 25 % of 1\ from dosed Qg non-reducing reaction centres in the measured Fq value. Thus the thermoinduced increase in Fq observed within the temperature inteiwal of 45-50 °C might be doně by a transition of PS2 reaction centres from a statě capable of reducing Q3 to a Qg-non-reducing statě.
Tobacco (Nicotiana tabacum L.) has been transformed to accumulate different compatible solutes (proline, fructans, or glycine betaine) in order to improve its tolerance to abiotic stress. Photosynthetic activity of wild Type (wt) and transformed tobacco plants before and after freezing stress was studied by measuring chlorophyll (Chl) fluorescence. The JIP test of Chl fluorescence induction was used to analyze in details the functional activity of photosystem 2. No significant differences were found among wild Type and transgenic plants after 12 h of freezing. Both plant Types maintained the same values of the measured parameters [FV/FM, PI(CSM), ABS/RC, TR0/RC, ET/RC] after recovery of stress. The studied Chl fluorescence parameters decreased only for the wild Type plants, stressed for 24 h at -2 °C. The strong inhibition of photosynthetic reactions in the wt plant after 24 h of freezing could not be restored. The evaluated parameters of transgenic plants did not change significantly after 24 h at -2 °C and successfully survived freezing stress. and D. Parvanova ... [et al.].