Characterization of PSII photochemistry in salt-adapted cells of cyanobacterium Spirulina platensis

The changes in pigment composition, photosynthesis and PSII photochemistry were investigated in cells of Spirulina platensis adapted to salt stress (< 0.75 M NaCl). A decrease in the phycocyanine/chlorophyll and no significant change in the carotenoid/chlorophyll ratio were observed in salt-adapted c ells. Salt stress inhibited the apparent quantum efficiency of photosynthes is and PSII activity while stimulating PSI activity and dark respiration si gnificantly. Salt stress also resulted in a decrease in overall activity of the electron transport chain, which could not be restored by diphenylcarba zide, an artificial electron donor to the reaction centres of PSII. Measure ments of the polyphasic fluorescence rise in fluorescence transients includ ing phases O, J, I and P showed that salt stress had no effect on the fluor escence yield at phase O but decreased the fluorescence yield at phases J, I and P. Analyses of the: JIP test developed from the polyphasic rise of fl uorescence transients showed that salt stress led to a decrease in both the maximum quantum efficiency of PSII photochemistry and the maximum quantum efficiency of electron transport beyond the primary quinone electron accept or. However, salt stress induced no significant changes in the probability of transporting an electron beyond Q(A), the trapping flux per PSII reactio n centre, or the electron transport flux per PSII reaction centre. A theore tical analysis of fluorescence parameters indicated a decrease in the rate constant of excitation energy trapping by PSII reaction centres. In additio n, salt stress induced an increase in the complementary area above the fluo rescence induction curve in the presence of 3-(3,4-dichlorophenyl)-1,1-dime thylurea, suggesting an increase in the proportion of closed PSII reaction centres in salt-adapted cells. Based on these results, it is suggested that modifications in PSII photochemistry in salt-adapted Spirulina cells maint ained a high conversion efficiency of excitation energy, such that no signi ficant change was observed in either the trapping flux or the electron tran sport flux per PSII reaction centre.