Correlated influence of cation concentration and excitation intensity on PS II activity-I. Influence of high salt...

A correlated influence of cation concentration and excitation energy level on PS II Abbreviations: AMPD = 2-amino-2-methyl-1,3-propanediol; DCIP = 2-6 dichlorophenol indophenol; DCIPH2 = DCIP in reduced state; DCIPr = rate of DCIP-reduction; DCMU = 3-(3,4 dichlorophenyl)1-1 dimethyl-urea; Fv = variable fluorescence; Fcat = Fv in the presence of added cations; Ft = Fv without added cations; HEPES = N-2-hydroxyethylpiperazine N\t'2-sulfonic acid; PS II = photosystem II; Vcat = DCIPr in presence of added cations; Vt = DCIPr without added cations. activity is demonstrated.In low light conditions (under 60 Wm(-2)) Mg(++) effect on DCIP reduction rate (DCIPr) saturates at rather low concentrations (2-10 mM). Higher concentrations induce a quenching of the effect, as already observed by several authors. In high light conditions (1000 Wm(-2)) however, Mg(++) is increasingly effective on DCIPr up to concentrations of 200 mM.Na(+) induced variations of DCIPr are weak in low light conditions and slightly positive for 100-600 mM in strong light; no quenching occurs.Modifications in variable fluorescence do not follow those of DCIPr in all cases, especially in high light.These results allow us to distinguish three different effects of cations on the photochemistry of PS II: one on the spill-over, another on the turnover rate of the centers and the last on the cation exchange through the thylakoid membrane.