Abstract
Grana, prepared by differential centrifugation of sonic-treated chloroplasts, undergo light-induced shrinkage as determined by packed volume, light-scattering, and ultrastructural studies. Evidence for light-induced H + ion uptake was also obtained. Two mechanisms for light-induced shrinkage are described: (a) In one mechanism both the rate and extent of shrinkage is increased upon addition of salts of weak acids. This mechanism for shrinkage appears to occur osmotically as a result of extrusion of weak acids from grana driven by light-induced H + ion uptake, (b) In an alternative mechanism, shrinkage is enhanced in the presence of salts of certain divalent cations. This shrinkage was not reversed in the dark. This mechanism appears to involve a light-dependent nonosmotic uptake or binding of divalent cations.
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