PLANT GROWTH REGULATION BY PHOTOCHEMICAL REACTION

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The two major pigments involved in photosynthesis are chlorophylls and carotenoids. It has been suggested that inhibitors of the pigments biosynthesis cause growth retardation of plants by inducing accumulation of phototoxic intermediate of chlorophyll or deterioration of anti-oxidative activity to active oxygen species. Light is required for the inhibitor-induced plant growth retardation. This study aimed to investigate a mechanism of light involvement for the regulation of the pigment biosynthesis and the action of the pigment synthesis inhibitors. The results of the study are summarized as follows.1.Chlorophyll synthesis inhibitors such as diphenyl ether-type herbicides caused rapid and abnormal accumulation of protoporphyrin IX in treated cells. Protoporphyrin IX is known to an efficient photosensitizer generating singlet oxygen under light. This acitive oxygen induced peroxidation of membrane lipids leading to cell death.2.delta-Aminolevulinic acid (ALA), precursors of chlorophyll synthesis, showed phytotoxicity and caused huge accumulation of protoporphyrin IX.Therefore, it is considered that chemicals accumulating this intermediate may utilize as plant growth regulators.3.ALA synthesis was stimulated under light irradiation. In plant cells light induces not only singlet oxygen generation but stimulation of chlorophyll synthesis. However, some plant species induce ALA and protoporphyrin IX accumulation in the dark. This suggest that plant has species-dependent regulation mechanism of chlorophyll synthesis.4.No phytotoxic compound was found from intermediate of carotenoids synthesis and related chemicals.