Analysis of chlorophyll fluorescence induction curves in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea as a function of magnesium concentration and NADPH-activated light-harvesting chlorophyll [formula omitted] phosphorylation

Abstract

The effect of Mg 2+ concentration and phosphorylation of light-harvesting chlorophyll a b- protein on various chlorophyll fluorescence induction parameters of isolated pea thylakoids has been studied. (1) Lowering the Mg 2+ concentration from 3 to 0.4 mM decreases only the variable fluorescence ( F v) and the area above the induction curve while at the same time increasing the slow exponential component of the rise ( β max). (2) A further decrease in Mg 2+ concentration from 0.4 to 0 mM decreases the initial ( F 0) fluorescence level such that the ratio F v F m increases slightly as does the area above the induction curve and β max. (3) Thylakoid membranes, phosphorylated at 5 mM Mg 2+, show an equal decrease in F v and F 0, no change in the area above the induction curve and an increase in β max. At 2 mM Mg 2+, however, phosphorylation induced a more extensive quenching of F v so that the F v F m ratio was lowered and the area above the induction curve decreased while β max increased. (4) When phosphorylated membranes were subsequently suspended in an Mg 2+-free medium the effect on F 0 due to phosphorylation was found to be additive to that due to the absence of Mg 2+. The effect of membrane phosphorylation on fluorescence is discussed in relation to the control of excitation energy distribution and shows that different mechanisms operate depending on the background Mg 2+ levels. At high Mg 2+ the phosphorylation seems to affect the absorption cross-section of Photosystem II while at lower Mg 2+ levels there is an additional effect of increased spillover from Photosystem II to I.