In vivo study of chloroplast volume regulation.

This paper describes a new technique that can be used to study chloroplast volume regulation in vivo. Nuclear magnetic resonance spectroscopy was used to measure relative amounts of chloroplast water in Acer platanoides leaves as they dried in air, and also in leaf disks exposed to aqueous polyethylene glycol, sucrose, or glycerol. The chloroplasts retained a constant quantity of water as leaf water potentials varied between -0.05 and -1.90 MPa, indicating that volume regulation was effective throughout this range. The chloroplasts lost water when the water potential fell below -1.90 MPa, except when leaf disks were exposed to glycerol, suggesting that the lower limit of effective volume regulation is determined by physiological levels of osmotic solutes and that glycerol can be used for chloroplast osmoregulation.