Analysis of chlorophyll fluorescence quenching by DBMIB as a means of investigating the consequences of thylakoid membrane phosphorylation

Abstract

The effect of Mg 2+ concentration and phosphorylation of the light harvesting chlorophyll a b protein on the ability of DBMIB to quench chlorophyll fluorescence of isolated pea thylakoids has been studied. Over a wide range of Mg 2+ concentrations (5−0.33 mM), the observed changes in fluorescence yield are mirrored by similar changes in the quenching ability of DBMIB, indicating that the cation-induced phenomenon involves alterations in radiative lifetimes. In contrast, phosphorylation at 10 mM Mg 2+ brings about a lowering of the chlorophyll fluorescence yield, while having no effect on the quenching capacity of DBMIB. This result can be interpreted as a phosphorylation-induced decrease in PS II absorption cross-section. At Mg 2+ levels between 5 and 1 mM, phosphorylation leads to a change in the quenching of fluorescence by DBMIB, when compared with non-phosphorylated thylakoids. At these cation levels, the degree of DBMIB-induced quenching cannot wholly account for the observed changes in chlorophyll fluorescence due to phosphorylation. It is concluded that the phosphorylation- and Mg 2+-induced changes in fluorescence yield are independent but inter-related processes which involve surface charge screening as emphasised by the change in cation sensitivity of the DBMIB quenching before and after phosphorylation.