SITES OF ACTION OF SULFITE AND BISULFITE IN THE PHOTOSYNTHETIC APPARATUS OF SUGAR MAPLE LEAVES AS STUDIED BY PHOTOACOUSTIC AND MODULATED FLUORESCENCE METHODS

The photosynthetic activity of intact sugar maple leaves has been asse ssed in the presence of exogenous sulphite, bisulphite and sulphate un der varying light conditions using photo-acoustic and modulated fluore scence methods. In the light, bisulphite was found to be more toxic th an the other two, sulphate being the least toxic of all. Interestingly , the vitality index, R(fd), measured as the ratio of the modulated fl uorescence decrease (F-d) to the steady-state fluorescence (F-s), whic h indicates the efficiency of the whole photosynthetic activity, was m ore affected than the total photosynthetic energy storage (PES) of PSI I and PSI during linear and cyclic electron transport, and F-v/F-m (PS II activity). The severity of the damage appeared to be a function of light intensity. Bisulphite treatment in darkness resulted in a dramat ic decrease in R(fd), a moderate decrease in PES and a marginal decrea se in F-v/F-m. As for sulphite, the effect was negligible with a tende ncy for enhanced activity. It is inferred that the Calvin cycle is a g ood candidate for the primary site of bisulphite and sulphite action. Recovery of activity, especially at the R(fd) level, obtained in the p resence of ascorbate, glutathione and L-cysteine, indicated a contribu tion of O-2 free radicals to the observed inhibition of the photosynth etic activity in the light.