Phenylenediamine restoration of photosynthetic electron flux in DBMIB-inhibited chloroplasts

Abstract

Phenylenediamines have been studied and compared as to their effectiveness in stimulating photosynthetic electron flux in DBMIB-inhibited chloroplasts. It has been found that N-substituted as well as C-substitutedp-phenylenediamines accelerate the rate of ferricyanide reduction, a photosystem II photoreaction, under conditions where the radical cations of N-substitutedp-phenylenediamines are stable. TheP/e ₂ ratios for these partial reactions are between 0.4 and 0.5; this is taken as evidence that N-substitutedp-phenylenediamines are reduced by the choloroplasts close to the outer surface. Both N- and C-substitutedp-phenylenediamines are capable of bypassing the site of DBMIB inhibition and restoring electron flow from water to methylviologen. N-substitutedp-phenylenediamines appear to be more effective even at high concentrations of DBMIB. TheP/e ₂ ratios for these reactions are on the order of 0.75–1.0; this is taken as evidence that the bypass reaction for N-substitutedp-phenylenediamines occurs on the inside of the thylakoid membrane.