Effects of dimethazone (FMC 57020) on chloroplast development: II. Pigment synthesis and photosynthetic function in cowpea ( Vigna unguiculata L.) primary leaves

Abstract

The herbicidal action of dimethazone [FMC 57020; 2-(2-chlorophenyl) methyl-4,4-dimethyl-3-isoxalidinone] on cowpea ( Vigna unguiculata L.) primary leaves was studied. Seeds were imbibed in 0.5 m M herbicide for 1 day and then seedlings were grown in darkness. In 6-day-old, etiolated seedlings, there was no effect of the herbicide on protochlorophyllide accumulation or on phototransformation of protochlorophyllide to chlorophyllide, however, the Shibata shift was greatly slowed. Accompanying this was a delay in phytylation of chlorophyllide. Protochlorophyllide resynthesis in a dark period after phototransformation of existing protochlorophyllide in etiolated tissue was also slowed by dimethazone. In the light, carotenoid accumulation and chlorophyll accumulation were slowed by the herbicide, resulting a pale green appearance of the leaves. The capacity for CO 2-dependent oxygen evolution or FeCN-dependent oxygen evolution did not develop in dimethazone-treated tissue during 24 hr of light exposure. In situ measurement of variable fluorescence and cytochrome f photooxidation/dark reduction indicated that some cyclic electron transport developed very slowly in dimethazone-treated plants. No effect of the herbicide was found on either FeCN-dependent oxygen evolution or variable fluorescence in fully greened tissues. Dimethazone was concluded to have an effect on chloroplast development rather than a direct effect on photosynthesis.