Sites of action of sulphite and bisulphite in the photosynthetic apparatus of sugar maple leaves as studied by photo-acoustic and modulated fluorescence methods

The photosynthetic activity of intact sugar maple leaves has been assessed in the presence of exogenous sulphite, bisulphite and sulphate under varying light conditions using photo-acoustic and modulated fluorescence methods. In the light, bisulphite was found to be more toxic than the other two, sulphate being the least toxic of all. Interestingly, the vitality index, R_rd, measured as the ratio of the modulated fluorescence decrease (F_d) to the steady-state fluorescence (F_s), which indicates the efficiency of the whole photosynthetic activity, was more affected than the total photosynthetic energy storage (PES) of PSII and PSI during linear and cyclic electron transport, and F_v/F_m (PSII activity). The severity of the damage appeared to be a function of light intensity. Bisulphite treatment in darkness resulted in a dramatic decrease in R_rd, a moderate decrease in PES and a marginal decrease in F_v/F_m. As for sulphite, the effect was negligible with a tendency for enhanced activity. It is inferred that the Calvin cycle is a good candidate for the primary site of bisulphite and sulphite action. Recovery of activity, especially at the R_rd level, obtained in the presence of ascorbate, glutathione and L-cysteine, indicated a contribution of O₂ free radicals to the observed inhibition of the photosynthetic activity in the light.