DARK REACTIONS OF CHLOROPLAST SUSPENSIONS

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Abstract
Chloroplast suspensions from the majority of plant species produce titratable acidity in darkness when mixed with ferricyanide – ferric oxalate solution. The rate of production and yield of acid increases between pH 6.0 and 7.5. This oxidation–reduction requires the presence of ferricyanide or ferric oxalate and does not involve enzymes. The nonsugar reducing substances in chloroplast suspensions which participate are water-soluble, and may include ascorbic acid, glutathione, and tannins. By applying a correction for dark acid production, photochemical activity in chloroplast suspensions showing this reaction may be measured by the Holt and French titration method. Boiled leaf sap of Broad Windsor bean produces carbon dioxide rapidly when mixed with ferricyanide – ferric oxalate solution, apparently by oxidative decarboxylation. Crude chloroplast suspensions from this species also exhibit oxygen absorption and dark acid production. Simultaneous measurement of these reactions shows no evidence of quantitative relations. The precipitation of oxalate causes an alkaline drift in pH when crude chloroplast suspensions of Lycopodium or Sedum are mixed with Hill solution in darkness. With crude spinach chloroplast suspensions, interference by dark reactions in photochemical activity measurements is generally negligible.