title: Salt stress induces a decrease in excitation energy transfer from phycobilisomes to photosystem II but an increase to photosystem I in the cyanobacterium Spirulina platensis.
abstract: The effects of salt stress (0-0.8M NaCl) 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 0.8M NaCl. 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 Q(A), and the yield of electron transport beyond Q(A). 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.
authors.affiliation: Photosynthesis Research Center, Key Laboratory of Photobiology, Institute of Botany, Chinese Academy of Sciences, Beijing, PR China.
authors.forename: Tao
authors.initials: T
authors.lastname: Zhang
authors.forename: Hongmei
authors.initials: H
authors.lastname: Gong
authors.forename: Xiaogang
authors.initials: X
authors.lastname: Wen
authors.forename: Congming
authors.initials: C
authors.lastname: Lu
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D004579:Electron Transport / Q000187:drug effects; D004735:Energy Transfer / Q000187:drug effects*; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis / Q000187:drug effects; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D045524:Phycobilisomes / Q000378:metabolism*; D012965:Sodium Chloride / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D053545:Spirulina / Q000187:drug effects / Q000378:metabolism*; D013312:Stress, Physiological / Q000187:drug effects*; D013696:Temperature
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D045524:Phycobilisomes; D002734:Chlorophyll; D012965:Sodium Chloride; D010100:Oxygen