Studies on Chloroplast Degradation in vivo: III. THE EFFECT OF MAGNESIUM ON ELECTRON TRANSPORT AND PHOSPHORYLATION IN PLASTIDS ISOLATED FROM YOUNG AND AGEING COTYLEDONS

The magnesium dependence of NADP-reduction, oxygen evolution and concomitant ATP-formation was studied as a function of the age of the Cucurbita cotyledons from which the chloroplasts were isolated. Whereas NADP-reduction under alkaline assay conditions always exhibits a stimulation upon addition of Mg++, measurements in an acidic medium show an age-dependent reversal to inhibition. The data for oxygen evolution are more complex in nature, due to participation of consecutive reactions. The Mg++ concentration for optimal ATP formation decreases with the age of the cotyledonary material. The results are discussed from the viewpoint of a possible control of essential photosynthetic functions in vivo by magnesium.