RESUMO

The effects of a) glucose, b) mannose and 2-deoxyglucose (glucose analogues capable of triggering hexokinase-mediated photosynthesis repression) and c) sodium citrate on chlorophyll fluorescence parameters of the blue-green alga Spirulina platensis (Nordst.) Geitl. were investigated. The addition of glucose evoked, at first, the lowering of all photochemical fluorescence quenching parameters (F_V/F_M, F_V'/F_M', q_P and Φ_{PS II}). The decrease correlated with glucose concentration. The inhibition of photochemical activity relaxed after the third day of the experiment, perhaps, as the consequence of glucose utilization. If the concentration of glucose in the cultural medium was high enough (50 mM), on the seventh day all the indices of photochemical quenching began to diminish again, at which time q_P reduction was more significant than that for F_V'/F_M'. The relief of glucose repressive effects was light-dependent. Glucose analogues caused the decrease in only one photochemical quenching parameter, q_P. Unlike mannose, the effect of 2-deoxyglucose was more pronounced and did not weaken till the end of experiments. Nonphotochemical quenching fluorescence parameters (q_N and NPQ) were also reduced under monosaccharide treatment. The influence of sodium citrate on chlorophyll fluorescence parameters was negligible. Therefore, the multiplicity of glucose effects in the course of inhibition of S. platensis photosystem II activity was demonstrated, while hexokinase-dependent repression mechanism was playing only a partial role and resulting in the blocking of electron flow from photosystem II reaction centers down the electron transport chain.