Kinetic response of photosystem II photochemistry in the cyanobacterium Spirulina platensis to high salinity is characterized by two distinct phases

The kinetic response of photosystem II (PS II) photochemistry in Spirulina platensis (Norstedt M-2) to high salinity (0.75 M NaCl) was found to consis t of two phases. The first phase, which was independent of light, was chara cterized by a rapid decrease (15-50%) in the maximal efficiency of PS II ph otochemistry (F-v/F-m), the efficiency of excitation energy capture by open PS II reaction centres (F-v'/F-m'), photochemical quenching (q(P)) and the quantum yield of PS II electron transport (Phi(PS II)) in the first 15 min , followed by a recovery up to about 80-92% of their initial levels within the next 2 h. The second phase took place after 4 h, in which further decli ne in above parameters occurred. Such a decline occurred only when the cell s were incubated in the light, reaching levels as low as 45-70% of their in itial levels after 12 h. At the same time, non-photochemical quenching (q(N )) and Q(B)-non-reducing PS II reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first p hase but increased again in the light in the second phase. The changes in t he probability of electron transfer beyond Q(A) (psi(o)) and the yield of e lectron transport beyond Q(A) (phi(Eo)), the absorption flux (ABS/RC) and t he trapping flux (TRo/RC) per PS II reaction centre also displayed two diff erent phases. The causes responsible for the decreased quantum yield of PS II electron transport during the two phases are discussed.