Studies on the mode of action of the herbicidal effect of 2,4,5-trichlorophenoxyacetic acid on germinating Norway spruce

Abstract

A dramatic effect of 2,4,5-trichlorophenoxyacetic acid on the growth of germinating plants was observed. This effect was expressed as abnormal growth. A pronounced increase in ethylene formation (10-fold) was observed in dark-grown plants treated with 200 μM 2,4,5-trichlorophenoxyacetic acid. In a crude peroxisomal fraction prepared from cotyledons of seedlings treated with 200 μM trichlorophenoxyacetic acid, large increases in fatty acyl-CoA oxidase (6.9-fold) and catalase (2.7-fold) activities were observed. A reduced activity of photosystem 11 was observed in chloroplasts from cotyledons of seedlings treated with 2,4,5-trichlorophenoxyacetic acid. In the dark-grown seedlings, this activity decreased to 35% of control. An inhibitory effect of 2,4,5-trichlorophenoxyacetic acid on photosystem II was also found in in vitro experiments with chloroplasts isolated from cotyledons of untreated plants. An increase in lipid peroxidation in chloroplasts treated with 200 μM 2,4,5-trichlorophenoxyacetic acid was also observed. Electron microscope studies on germinating Norway spruce seedlings revealed a pronounced degenerative effect of 2,4,5-trichlorophenoxyacetic acid on chloroplasts, expressed as swollen chloroplats and disorganized orientation of thylakoid membranes. Vesicles were often present in the stroma and the thylakoid. In some chloroplasts, a complete disintegration of compartments was observed. The present study suggests that the principal mode of action of 2,4,5-trichlorophenoxyacetic acid on Norway spruce seedlings is via damage of the thylakoid membrane and inhibition of photosystem II. Additionally, our results support a possible role of increased oxidative stress as an additional mode of action of 2,45-trichlorophenoxyacetic acid.