Structure and activity of chloroplasts of sunflower leaves having various water potentials.

Changes in membrane integrity, conformation and configuration, and in photosystem II (PS II) activity (measured as dichloroindophenol photoreduction) of sunflower (Helianthus annuus L.) chloroplasts were studied after leaf tissue had been desiccated to various water potentials (ψ w ). Fixatives for electron microscopy were adjusted osmotically to within 1 bar of the ψ w of the tissue to prevent rehydration during fixation. PS II activity decreased to 50% of the control activity at a ψ w of-26 bar. At this ψ w , leaf viability was being lost but there was virtually no loss of integrity of the thylakoid lamellar system. Even at extreme ψ w (below-100 bar), thylakoids retained much structural detail but were less stained. At-26 bar, intrathylakoid spacing (configuration) and lamellar thickness (conformation) were decreased in vivo. Upon isolation of the plastids, the differences in configuration disappeared but the differences in conformation remained. The decreases in membrane conformation and PS II activity both, in vivo and in vitro suggest that alterations in conformation may cause decreases in chloroplast activity at ψ w as low as-26 bar. Since structural detail was maintained, however, previous observations of altered membrane integrity, which involved tissue fixed without osmotic support, may have been affected by tissue rehydration during fixation.