PHOTODYNAMIC HERBICIDES .8. MANDATORY REQUIREMENT OF LIGHT FOR THE INDUCTION OF PROTOPORPHYRIN-IX ACCUMULATION IN ACIFLUORFEN-TREATED CUCUMBER

It is shown that continuous illumination is mandatory for the induction of tetrapyrrole accumulation in acifluorfen-sodium-treated plants and for the photosensitization of tetrapyrrole-dependent photodynamic damage. At low concentrations of acifluorfen-sodium (up to 20-mu-M), protoporphyrin IX appears to be the major light-induced tetrapyrrole that accumulates in the treated plants. At higher concentrations of acifluorfen-sodium, monovinyl chlorophyllide a accumulates in addition to protoporphyrin IX. In the light, the development of photodynamic injury appears to be directly related to the accumulation of the light-induced tetrapyrroles. For example when acifluorfen-sodium-treated plants are returned to darkness, or are treated with tetrapyrrole biosynthesis inhibitors, tetrapyrrole accumulation and photodynamic injury come to a halt. In-vivo and in-organello studies failed, however, to support the commonly held hypothesis that the induction of tetrapyrrole accumulation in the light, in acifluorfen-sodium-treated plants, is only dependent on the inhibition of protoporphyrinogen oxidase. Indeed, when plastids capable of very high rates of tetrapyrrole biosynthesis and accumulation were incubated with delta-aminolevulinic acid and acifluorfen-sodium, either in darkness or in the light, a severe inhibition of protoporphyrin IX and total tetrapyrrole formation was observed. Although these results are compatible with the inhibition of tetrapyrrole formation by acifluorfen-sodium at the level of protoporphyrinogen oxidase, they indicate that, in addition to that inhibition, other cellular processes are probably involved in the light-dependent accumulation of protoporphyrin IX in acifluorfen-sodium-treated plants.