Herbicides which Inhibit Electron Transport or Produce Chlorosis and Their Effect on Chloroplast Development in Radish Seedlings II. Pigment Excitation, Chlorophyll Fluorescence and Pigment-Protein Complexes

Abstract

Abstract DCMU, bentazon, amitrole and SAN 6706 affected the formation of the pigment-protein complexes and caused drastic alterations in the absorption of light and in the transfer of the absorbed energy in the antennae systems. Bentazon and DCMU, photosystem II inhibitors, did not change the pigment absorption and fluorescence excitation spectra. After application of both herbicides the long wavelength fluorescence emission band at 740 nm was reduced similar as in young developing leaves. Although DCMU and bentazon inhibit the photosynthetic electron transport at the same site, bentazon mainly suppressed the formation of the photosystem I complexes CP 1 a and CP 1 while DCMU mainly reduced the photosystem II complex CPa. Bentazon specifically enhanced the formation of LHCP3. This may be important for the increased grana stacking in plastids from bentazon treated plants. The bleaching herbicides amitrole and SAN 6706 inhibited the formation of carotenoids leading to an accumulation of lycopene, phytofluene and phytoene, while the accumulation of chlorophylls was suppressed. This bleaching effect was most pronounced during growth under higher intensities of light. In weak light (100 lux) amitrole reduced the long wavelength fluorescence maximum but the fluorescence excitation was not affected. With amitrole at 2000 lux and SAN 6706 at 100 lux the long wavelength emission band was further decreased and the fluorescence excitation spectra point to a less efficient energy transfer to chlorophyll a. The fluorescence spectra changed due to herbicide treatment resembled those of not yet fully developed leaves. In contrast to the photosystem II herbicides the bleaching herbicides amitrole and SAN 6706 had a similar effect on the formation of pigment-protein complexes. After growth at 2000 lux both herbicides suppressed the formation of the photosystem I complex CP 1 a and the photosystem II complex CPa. At 100 lux only the formation of CPla was affected. Except for DCMU all herbicides assayed primarily changed the formation of photosystem I.