Cation-induced inhibition of the 515-nm absorption change in chloroplasts: relation to structural changes

Divalent cations were found to inhibit the light-induced 515-nm absorption change in chloroplasts with half-maximal effects occurring between 0.3 and 0.7 mm. Monovalent cations were also effective but higher concentrations (~ 30–40 mm) were required for half-maximal effects. Divalent and monovalent cations also caused absorption changes of chloroplasts in the dark which superficially resemble 515-nm absorption changes. However, they can be correlated with volume changes and represent a combination of turbidity and pigment-absorption changes (flattening) which result from shrinkage. Half-maximal effects occurred at 0.8–1.2 mm for divalent cations and between 15 and 20 mm for monovalent cations. The relationship between salt-induced and osmotic-induced structural changes is also discussed.