The Effect of Analogues of Dibromothymoquinone and of Bromonitrothymol on Photosynthetic Electron Flow

Photosynthesis, Inhibitors, Herbicides, Dibromothymoquinone, Nitrophenols, Chloroplast Alkyl substituted derivatives of halogenated p-benzoquinones, of halogenated /7-nitrophenols, and of 2,4-dinitrophenols were tested in the inhibition of photosynthetic electron flow in chloro­ plasts. The effect of the compounds on photoreductions by photosystem I or II, on a TMPD bypass in NADPH formation and the reversibility of the inhibition by dithiothreitol is used to distin­ guish between an inhibition site before or after plastoquinone function, i. e. between a DBMIB versus a DCMU inhibition pattern. It is shown, that different isopropyl and /-butyl substituted halogenated p-benzoquinones are as effective and specific as DBMIB in the inhibition of plastoquinone function. Alkyl substituted /^-nitrophenols, with an additional halogenor nitro-group at C-2, are shown to be effective electron flow inhibitors. The new potent nitrophenol derivatives inhibit at the site of DCMU action, nevertheless they do not contain the basic chemical element essential for inhibi­ tion common to DCMU and its many herbicidal analogues. Small changes in the ring-substitution can alter the inhibition pattern from a DCMU typ to a DBMIB typ inhibition.