Membrane charge affecting electron donation to PS II in chloroplasts.

Pretreatment of chloroplast with 0.75 mM of EDTA inhibits markedly electron flow at pH above 8.5. This inhibition can be reversed by adding donors to PS II or by addition of salts to the reaction medium.Restoration of electron flow in EDTA-treated chloroplasts by salts depends clearly on the valency of the cation used. The efficiency observed is: C(3+)>C(2+)>C(+), which is indicative of screening of negative charges on the membrane. However, maximal restoration of electron flow depends also on the presence of a relatively low concentration of Cl(-) which is known to be required at the oxygen evolution site. Charge density in the region of Q was measured in control and EDTA-treated chloroplasts. The calculated charge densities were: -1.1 μC/cm(2) and -2.0 μC/cm(2) for control and EDTA-treated chloroplasts respectively.It is concluded that EDTA-treatment, by dissipating ° pH and by chelating Mg(2+), causes an increase in the negative charge density on the thylakoid membrane which includes a site (or sites) closely related to water donation.