Differential Effects of K⁺ and Na⁺ on Oxygen Evolution Activity of Photosynthetic Membranes from Two Halophytes and Spinach

Abstract

KCl reduced oxygen evolution by Sarcocornia quinqueflora thylakoids to well below the level of activity in the presence of NaCI. This inhibition occurred at both high and low pH. On the other hand, both KCl and NaCl induced almost indistinguishable effects on oxygen evolution in spinach thylakoids. Photosystem (PS) Il membranes from Avicennia marina showed a similar inhibition of oxygen evolution by KCl to S. quinqueflora thylakoids, but spinach PS Il particles were not inhibited by KCl. The KCl inhibition of oxygen evolution of halophytic thylakoids was localised to the oxygen-evolving complex and required little KCl, as significant inhibition occurred with only 25 mM. K₂SO₄ was also inhibitory, but Na₂SO₄ was not, indicating that K⁺ was the inhibitory ion. Na₂SO₄ was partly able to reverse this inhibition caused by KCl, suggesting the inhibition to be a function of the K⁺ /Na⁺ ratio. These results have implications concerning the ionic relations of the leaf cells of halophytes.