Permeability of Chloroplast Envelopes to Mg2+

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Abstract
When suspended in media lacking free Mg2+, chloroplasts from young pea plants (Pisum sativum CV Progress No. 9) lose 25 to 75% of their stromal Mg2+ content to the medium, without breakage. This effect amounts for the inhibition of protein synthesis in the dark by ATP in excess of the Mg2+ provided, since free ATP chelates Mg2+. The rate of loss is from 1 to 4.5 microgram-atoms Mg2+/milligram Chl/hour; and depleted chloroplasts take up Mg2+ from the medium at even faster rates, to a total amount not much more than that present originally (0.8 to 1.8 microgram-atoms/milligram Chl with an average of 1.33 ± 0.32 μg-atoms/mg Chl). Leakage is completely prevented by 0.25 to 0.40 millimolar external Mg2+. Addition of Mg2+ at a level sufficient to prevent leakage from intact chloroplasts results in approximately 20% stimulation in light-driven protein synthesis.