Chloroplasts isolated from a plant species, Dimorphotheca pluvialis Moench (Asteraceae), grown under visible light without supplementary UV-B radiation, have poor grana development. The dominant components in the membrane organizations are doublets and triplets of stroma lamellae. However, such chloroplasts possess the capacity for electron transfer through the whole photosynthetic chain, based on the measurement of the reduction of ferredoxin. The chloroplasts isolated from D. pluvialis plants grown under visible light in combination with supplementary UV-B radiation have much greater membrane stacking in the thylakoid organization when compared with those grown without supplementary UV-B. The changes in the membrane organization induced by UV-B are also reflected in photochemical activity: oxygen-evolving phoptosystem II (PSII) activity is increased in UV-B chloroplasts compared with chloroplasts grown without UV-B. On this basis, it is concluded that the dynamic relation between the granal stacks and the stroma lamellae in the chloroplasts is closely related to the strategy by which plants are able to cope with a variety of environments and to optimize the photosynthetic machinery for the environmental conditions. The increase in membrane stacking in D. pluvialis, induced by supplementary UV-B radiation during plant growth, suggests that UV-B may be an essential factor for the development of membrane stacking in chloroplasts of higher plants. The mechanism of action of UV-B in this process is discussed.