The effects of salt stress (0–0.8MNaCl) on excitation energy transfer from phycobilisomes to photosystem
I (PSI) and photosystem II (PSII) in the cyanobacterium Spirulina platensis were investigated. Salt stress
resulted in a significant decrease in photosynthetic oxygen evolution activity and PSII electron transport
activity, but a significant increase in PSI electron transport activity. Analyses of the polyphasic fluorescence
transients (OJIP) showed that, with an increase in salt concentration, the fluorescence yield at the
phases J, I and P declined considerably and the transient almost leveled off at 0.8MNaCl. Analyses of the
JIP test demonstrated that salt stress led to a decrease in the maximal efficiency of PSII photochemistry,
the probability of electron transfer beyond QA, and the yield of electron transport beyond QA. In addition,
salt stress resulted in a decrease in the electron transport per PSII reaction center, but an increase in the
absorption per PSII reaction center. However, there was no significant change in the trapping per PSII
reaction center. Furthermore, there was a decrease in the concentration of the active PSII reaction centers.
Analyses of 77K chlorophyll fluorescence emission spectra excited either at 436 or 580nm showed that
salt stress inhibited excitation energy transfer from phycobilisomes to PSII but induced an increase in
the efficiency of energy transfer from phycobilisomes to PSI. Based on these results, it is suggested that,
through a down-regulation of PSII reaction centers and a shift of excitation energy transfer in favor of
PSI, the PSII apparatus was protected from excess excitation energy.