Amitrole effects on chloroplasts of barley plants grown at different temperature.

The effects of amitrole, a bleaching herbicide affecting carotenogenesis, were studied in barley plants grown in light at 20 degrees C and at 30 degrees C. At the lower temperature the herbicide caused dramatic damage to the leaf chloroplasts, compared with control plants. The synthesis of protective carotenoids was drastically impaired, with some accumulation of lycopene and a very low production of beta-carotene and xanthophylls. In chloroplasts this effect of amitrole was related to a fall in chlorophyll content, reduction of the thylakoid system and destruction of most 70S ribosomes. Though the damaged organelles maintained a certain ability to synthesize proteins, chlorophyll-binding polypeptides encoded by plastid DNA were not found in the altered thylakoids which, however, contained chlorophyll-binding proteins and other polypeptides encoded by the nuclear DNA. Amitrole appeared to be less effective in plants grown at 30 degrees C. The synthesis of the protective carotenoids was greatly increased and the chloroplasts exhibited structure and composition almost similar to those of control organelles. The possible existence in the carotenoid biosynthetic pathway of alternative thermo-modulated steps, with different amitrole sensitivity, is suggested.