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Reversible Abolition of Enhancement in Isolated Spinach Chloroplasts

^a Department of Biology, Carleton University, Ottawa, Ontario

The photosynthetic evolution of oxygen by isolated chloroplasts of Spinacia oleracea L. was studied using a modulated oxygen electrode. The enhancement effect, measured as the increase in the relative quantum yield of 650-nanometer light due to the presence of 710-nanometer light, was profoundly influenced by the concentration of inorganic cations in the bathing medium. Chloroplast fragments immersed in a solution containing a very low concentration of MgCl₂ or KCl, did not display enhancement but could be made to do so by raising the concentration of MgCl₂ to 3 mM, or that of KCl to 35 mM. This change in the enhancement properties was completely reversible. The maximum value of enhancement in a MgCl₂ solution appeared to occur at a concentration between 15 and 30 mM, while in KCl, the enhancement effect increased almost linearly up to a concentration of 100 mM.

The appearance of the initial oxygen transient, the burst, was also controlled by the concentration of inorganic cations. Increasing concentrations of MgCl₂ or KCl caused the burst to grow in size in a way which was linearly related to the corresponding increase in enhancement.