Physiological effects of sublethal atrazine on barley chloroplast thylakoid membranes

Abstract

This study was conducted to more clearly define the physiological effects of PS II herbicides on chloroplast thylakoid membrane activity and composition. Barley (Hordeum vulgare L. cv Boone) was grown in hydroponic culture at 20°C in a growth chamber with a light intensity of 500 μmole photons m(-2) s(-1). Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), a Photosystem II herbicide, was supplied continuously via the roots to 7-day-old plants. Atrazine concentrations greater than 0.07 ppm (0.32 μM) were associated with decreased leaf chlorophyll (chl), lowered chl a/b ratio, inhibition of chloroplast electron transport, and plant death within 1 to 2 weeks. Atrazine at 0.07 ppm was defined as sublethal because no toxic effects were observed. Sublethal atrazine induced a decrease in chl a/b ratio with no effect on leaf chl content. Photosynthetic electron transport was either unaffected in fully expanded leaves or slightly stimulated in expanding leaves by treatment of intact plants with 0.07 ppm atrazine. The major effect of sublethal atrazine was on the chl-protein complex composition. Sublethal atrazine increased the level of the Photosystem II light-harvesting complex (LHC-II) and lowered the level of the CP1a Photosystem I complex relative to controls. The numbers of Photosystem II and Photosystem I reaction centers and cytochrome b 6/f complexes per unit chl were not affected by sublethal atrazine. The overall result was an atrazine-induced redistribution of light-harvesting chl from Photosystem I to Photosystem II with no effect on the number of thylakoid membrane-protein complexes associated with electron transport.