Herbicides which Inhibit Electron Transport or Produce Chlorosis and Their Effect on Chloroplast Development in Radish Seedlings. III. Plastid Pigment and Quinone Composition

Abstract

Abstract The effect of DCMU, bentazon, amitrole and SAN 6706 on the formation of chloroplast pigments and quinones was investigated using plants that were grown in total darkness or continuous white, red or far-red light. All herbicides assayed affected the formation of chlorophylls, carotenoids and quinones but DCMU had only minor effects. Like for chlorophylls and carotenoids the formation of quinones was most suppressed in plants grown in the presence of the herbicide in continuous white or red light, but the effect on the formation of quinones was much lower as compared to the pigments. The observation that the biosynthesis of quinones is still maintained in SAN 6706 treated bleached plastids which are lacking chlorophylls and carotenoids indicates that quinones are synthesized at the plastitd envelope and stored in the osmiophilic plastoglobuli. Amitrole and SAN 6706 induced a strong chlorosis. It was of particular interest that chlorosis was also induced by the photosystem II inhibitor bentazon. DCMU was not effective. The inhibitor concentration for 50% inhibition in the chlorophyll and carotenoid content was 5 × 10-6 ᴍ for SAN 6706,3 × 10-4 ᴍ for amitrole and 10-3 ᴍ for bentazon. As already reported by others SAN 6706 treated plants accumulated phytoene in large amounts. The highest phytoene content was observed in plants that were grown in the dark. Amitrole treated plants accumulated lycopene. But in addition other carotenoid precursors like phytoene and phytofluene were also accumulated. In contrast to phytoene lycopene was only accumulated in plants that were grown in the light. Particularly for SAN 6706 and amitrole the expression of the bleaching effect was depending on the light intensity and light quality that was used during plant growth. The herbicide effect ewas predominantly expressed at higher light intensities and after irradiation with red light. The observation that the induction of chlorosis is very sensitive to red light as compared to white or blue light is suggesting that phytochrome is involved in the development of the herbicide toxicity. It also supports that in SAN treated plants chlorophylls are photodecomposed directly by light because of the lack of photoprotecting carotenoids but mainly β-carotene in these plastids. Further support for this was given by the demonstration that SAN treated plants which were grown at very low light intensities turned green and were photosynthetically active.