Adaptive reorganization of protein and lipid components in chloroplast membranes as associated with herbicide binding

Cultivation of Spirodela oligorrhiza ( Kurtz ) Hegelm on a sublethal dose of atrazine results in a higher linolenic to linoleic acid ratio in the thylakoid membrane lipids, less starch, more osmiophilic globules, and a reduced stroma lamellar system. Also, the grana become randomly oriented and contain more numerous and elongated lamellae. These alterations in the lipid composition and ultrastructure of the chloroplast resemble those previously observed in triazine-resistant weed biotypes and in chloroplasts developed under low light. Thylakoid membranes from atrazine-adapted plants revealed an additional high-affinity binding constant for [14C]-diuron but the number of diuron binding sites actually decreased by 20 times compared to controls. The 32,000-dalton membrane protein of the chloroplast is synthesized actively, but its breakdown appears decreased compared to control plants. The adaptive reorganization of thylakoid components may be a compensatory mechanism for maintenance of a functional interaction of the proteins and lipids of the photosystem II complex.