Phenylenediamine restoration of photosynthetic electron flux in DBMIB-inhibited chloroplasts.

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-substituted p-phenylenediamines accelerate the rate of ferricyanide reduction, a photosystem II photoreaction, under conditions where the radical cations of N-substituted p-phenylenediamines are stable. The P/e2 ratios for these partial reactions are between 0.4 and 0.5; this is taken as evidence that N-substituted p-phenylenediamines are reduced by the chloroplasts close to the outer surface. Both N- and C-substituted p-phenylenediamines are capable of bypassing the site of DBMIB inhibition and restoring electron flow from water to methylviologen. N-substituted p-phenylenediamines appear to be more effective even at high concentrations of DBMIB. The P/e2 ratios for these reactions are on the order of 0.75-1.0; this is taken as evidence that the bypass reaction for N-substituted p-phenylenediamines occurs on the inside of the thylakoid membrane.