Combined effects of light and water stress on chloroplast volume regulation.

A nuclear magnetic resonance technique was used to measure changes in the water content of Acer platanoides chloroplasts in leaf discs that had reached osmotic equilibrium with external solutions either in the dark or under exposure to light. Results showed that chloroplast volume regulation (CVR) maintained constant water content in the chloroplasts over a range of water potentials in the dark, but CVR failed when the water potential fell below a critical value. The critical potential was lower in the dark in sun leaves than in shade leaves. Upon exposure to intense light, CVR remained effective in sun leaves over the same range as in the dark, but it failed in shade leaves at all water potentials. Osmolytes are necessary for CVR, but KCl is relatively ineffective; increased concentrations of intracellular KCl did not fully support an increase in the range of CVR. The results indicate that leaves need reserve supplies of cytosolic osmolytes to maintain CVR at low water potentials, and a larger reserve supply is needed in leaves that are exposed to intense light.