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 volum
e regulation (CVR) maintained constant water content in the chloroplas
ts over a range of water potentials in the dark, but CVR failed when t
he water potential fell below a critical value. The critical potential
was lower in the dark in sun leaves than in shade leaves. Upon exposu
re to intense light, CVR remained effective in sun leaves over the sam
e range as in the dark, but it failed in shade leaves at all water pot
entials. Osmolytes are necessary for CVR, but KCl is relatively ineffe
ctive; increased concentrations of intracellular KCl did not fully sup
port an increase in the range of CVR. The results indicate that leaves
need reserve supplies of cytosolic osmolytes to maintain CVR at low w
ater potentials, and a larger reserve supply is needed in leaves that
are exposed to intense light.