An attempt to characterize the mechanism of inhibition of photosynthetic electron transport in isolated pea chloroplasts by the herbicide 4,6-dinitro-o-cresol (DNOC) by a comparison with the effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) revealed the following:1. The percent inhibition of oxygen evolution by a given herbicide concentration is the same at various light intensities except at very low intensities where the percent inhibition becomes larger. The same results are obtained with the herbicide DCMU.2. The concentration of DCMU causing 50% inhibition of oxygen evolution decreases with de creasing chloroplast (and thus of chlorophyll) concentration. W ith DNOC, the relative decrease is much less than with DCMU. At the inhibited molecule, there appears to be a cooperative binding of DCM U with two binding sites and a noncooperative binding of DNOC with only one binding site.3. The chlorophyll a fluorescence induction is influenced by DNOC in the same characteristic way as it is by D C M U : both herbicides cause a faster rise in fluorescence yield than in control chloroplasts, although a higher concentration of the former is required for the same effect.4. The chlorophyll fluorescence emission spectra at 77 °K show a slight decrease in the bands at 685 and 735 nm, and no or only a very slight decrease at 695 nm upon addition of high con centrations of either D CM U or DNOC before the onset of illumination.5. The degree of polarization of chlorophyll a fluorescence is lower after addition of DCM U or DNOC upon excitation by 460 or 660 nm light.It is concluded that, although the chemical structure of DNOC is completely different from that of DCM U, its site and mechanism of inhibition is similar to that of DCMU. Both herbicides inhibit electron transport between the primary electron acceptor of photosystem II and the plastoquinone pool. This causes a closing of the reaction centers of photosystem II. However, the interaction with the inhibited molecule is different for the two herbicides.The herbicide 4,6-dinitro-o-cresol (DNOC) is a potent inhibitor of photosynthesis. Van Rensen et al. [1] showed that the uncoupled Hill reaction (with ferricyanide as an electron acceptor) is in hibited 50 percent with 1 j u m DNOC. This herbicide has no influence on the silicomolybdate-mediated Hill reaction in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCM U), but inhibits the phenylenediamine-mediated photoreduction of ferri cyanide in the presence of the plastoquinone anta * On leave from the

Electronic excitation energy transfer was studied for chlorophyll a in a solid solution of poly­styrene by measuring the concentration quenching of quantum yield, polarization, and lifetime of fluorescence. The concentration quenching of the experimental fluorescence quantum yield is adequately described by Kelly and Porter’s empirical formula (Proc. Roy. Soc., Lond. A 315, 149, 1970), and of polarization of fluorescence by the Jablonski theory (Acta Phys. Pol., 14, 295, 1955). With increasing concentration of chlorophyll a, the fluorescence peak at 672 nm (mainly monomer) is red-shifted, the intensity of the emission peak at ∼730 nm (mainly aggregate) relative to that at the shorter wavelength is increased. The R̅0 values, calculated by using total concentrations, for the emission at 672 nm and 730 nm are 73 ± 2 Å and 45 ±1 Å, respectively. This may suggest that the chlorophyll monomers have a greater efficiency of energy transfer than the aggregates, which fluoresce at ∼ 730 nm.