The kinetic response of photosystem II (PS II) photochemistry in
Spirulina platensis(Norstedt M2 )
to high salinity (0.75 M NaCl) was found to consist of two phases. The first
phase, which was independent of light, was characterized by a rapid decrease
(15–50%) in the maximal efficiency of PS II photochemistry
(Fv
/Fm), the efficiency of
excitation energy capture by open PS II reaction centres
(Fv′/Fm′),
photochemical quenching (qp) and
the quantum yield of PS II electron transport (Φ
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 decline in above
parameters occurred. Such a decline occurred only when the cells were
incubated in the light, reaching levels as low as 45–70% of their
initial levels after 12 h. At the same time, non-photochemical quenching
(qN) 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 phase but increased again in the light in the second phase.
The changes in the probability of electron transfer beyond
QA (ψo)
and the yield of electron transport beyond
QA (φ
Eo), the absorption flux (ABS/RC) and the trapping
flux (TRo /RC) per PS II reaction centre also
displayed two different phases. The causes responsible for the decreased
quantum yield of PS II electron transport during the two phases are discussed.
The Small, Methionine-Rich Chloroplast Heat-Shock Protein Protects Photosystem II Electron Transport during Heat Stress