STUDIES ON THE MODE OF ACTION OF THE HERBICIDAL EFFECT OF 2,4,5-TRICHLOROPHENOXYACETIC ACID ON GERMINATING NORWAY SPRUCE

A dramatic effect of 2,4,5-trichlorophenoxyacetic acid on the growth o f germinating plants was observed. This effect was expressed as abnorm al growth. A pronounced increase in ethylene formation (10-fold) was o bserved in dark-gown plants treated with 200 mu M 2,4,5-trichloropheno xyacetic acid. In a crude peroxisomal fraction prepared from cotyledon s of seedlings treated with 200 mu M trichlorophenoxyacetic acid, larg e increases in fatty acyl-CoA oxidase (6.9-fold) and catalase (2.7-fol d) 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 acti vity decreased to 35% of control. An inhibitory effect of 2,4,5-trichl orophenoxyacetic acid on photosystem II was also found in in vitro exp eriments with chloroplasts isolated from cotyledons of untreated plant s. An increase in lipid peroxidation in chloroplasts treated with 200 mu M 2,4,5-trichlorophenoxyacetic acid was also observed. Electron mic roscope studies on germinating Norway spruce seedlings revealed a pron ounced degenerative effect of 2,4,5-trichlorophenoxyacetic acid on chl oroplasts, expressed as swollen chloroplats and disorganized orientati on of thylakoid membranes. Vesicles were often present in the stroma a nd the thylakoid. In some chloroplasts, a complete disintegration of c ompartments was observed. The present study suggests that the principa l mode of action of 2,4,5-trichlorophenoxyacetic acid on Norway spruce seedlings is via damage of the thylakoid membrane and inhibition of p hotosystem II. Additionally, our results support a possible role of in creased oxidative stress as an additional mode of action of 2,45-trich lorophenoxyacetic acid.