Light-induced movement of magnesium ions in intact chloroplasts. Spectroscopic determination with Eriochrome Blue SE

The metallochromic indicator Eriochrome Blue SE was used to measure light-induced internal movement of Mg²⁺ in intact chloroplasts. By dual-wavelength spectroscopy (measuring wavelength 554 nm, reference 592 nm) a light-induced, dark-reversible absorbance increase of Eriochrome Blue in samples of isolated intact chloroplasts was observed. The light/dark difference spectrum of Eriochrome Blue between 550 and 590 nm (reference wavelength 562 nm) indicated that this absorbance increase was caused by an increased concentration of free Mg²⁺ in a neutral or slightly alkaline chloroplast compartment.

The signal was seen only with intact, but not with broken, envelope-free chloroplasts, which had lost most of their divalent cations. This is interpreted to show that the indicator responds to an increase of Mg²⁺ concentration in the chloroplast stroma, which represents an efflux of Mg²⁺ from the intra-thylakoid space caused by light-dependent proton pumping.

As calculated from corrected values of the absorbance increase of Eriochrome Blue, the light-induced internal release of Mg²⁺ was close to 100 nequiv per mg chlorophyll at pH 7.6 and 250 nequiv at pH 7.1. This corresponds to a light-dependent increase in the concentration of free Mg²⁺ in the stroma of about 2 and 5 mM, respectively.