Mode of action of photosynthesis inhibiting 4-hydroxy-benzonitriles containing nitro group

The effects of 3,5-disubstituted 4-hydroxy-benzonitriles - bromoxynil and compounds containing nitro group - on some photosynthetic parameters of spinach and wheat were investigated. There is considerable difference in the effectivity of the compounds even at the level of chloroplasts. On the basis of their effect on the development of CO2 fixation of greening wheat seedlings bromoxynil and 3,5-dinitro-4-hydroxy-derivative were ineffective, while the asymmetric - 3-nitro-5-halogenic-substituted - compounds showed a strong effectivity. The bromine and iodine containing asymmetric compounds reduced the intensity of the short wavelength fluorescence. All asymmetric compounds shifted the long wavelength fluorescence maximum with 5-8 nm to shorter wavelength. These results refer to inhibition of formation of chlorophyll forms absorbing at longer wavelengths and to the change in the conditions of energy migration. The ultrastructure of the chloroplasts in the treated seedlings became similar to the shade adapted chloroplasts.