Effects of salt stress on PSII function and photoinhibition in the cyanobacterium Spirulina platensis

The changes in PSII function and susceptibility to photoinhibition in the cyanobacterium Spirulina platensis adapted to 0.75 mol/L NaCl were evaluated by use of chlorophyll fluorescence. In salt-stressed cells, photochemical quenching (q(p)) decreased while the proportion of Q(B)-non-reducing PSII reaction centres increased. However, salt stress had small effect on the maximal efficiency of PSII photochemistry (F(v)/F(m)) and the efficiency of excitation energy capture by open PSII reaction centres (F(v)/F(m')). These results suggest that the decreased quantum yield of PSII electron transport (Φ(PSII)) in salt-stressed cells was mainly due to the decreased qp, which can be explained by an increase in the proportion of O(B)-non-reducing PSII reaction centres. On the other hand, increased susceptibility to photoinhibition was observed in salt-stressed cells when exposed to high light (600 μmol m^-2 s^-1). Such increased susceptibility was reflected by a greater decrease in F(v)/F(m), F(v')/F(m'), q(p) and Φ(PSII), as well as in a greater increase in the proportion of Q(B)-non-reducing PSII reaction centres. No significant difference in non-photochemical quenching (q(N)) was observed between control cells and salt-stressed cells. Our results suggest that an increase in the proportion of Q(B)-non-reducing PSII reaction centres induced by salt stress may be responsible for increased susceptibility of PSII to photoinhibition.