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1. Chloroplasts isolated from pea seedlings were incubated in sucrose-tris medium reinforced with salts of calcium, magnesium, manganese or iron, at concentrations up to 10mm. 2. Measurements of chloroplast-pellet volume and water content showed that the bivalent cations brought about a contraction in chloroplast volume and a loss of chloroplast water. This was further substantiated by density-gradient centrifugations. 3. Measurements of the light-scattering and apparent fluorescence of chloroplast suspensions confirmed this conclusion and eliminated the possibility of contraction being caused by centrifugal forces. 4. The uptake of (45)Ca(2+) was measured and shown to be competitive with diluent Ca(2+), Mg(2+) or Mn(2+) ions, indicating a mechanism of low specificity. 5. The chloroplast contraction was insensitive to light but could be made sensitive by the addition of ferric EDTA. This light-sensitivity was inhibited by added 3-(p-chlorophenyl)-1,1-dimethylurea and so probably involves the Hill reaction. 6. On the basis of these observations it is suggested that the process of contraction does not consume much energy, but that in light-activated contraction a previous step occurs that is conducive to contraction and that is energy-transducing. It is postulated that this step results in a local increase in concentration of bivalent ions, which promotes contraction.

Volume contraction of isolated pea chloroplasts promoted by bivalent cations