Millisecond Delayed Light as an Indicator of Electrical Gradients across Chloroplast Thylakoids

Abstract

DELAYED light emission from photosynthetic organisms was discovered by Strehler and Arnold¹. The emitted light has a spectrum similar to that of chlorophyll a fluorescence and can often persist for minutes after terminating the illumination. In recent years it has been found that the intensity of emission during the first few milliseconds of the decay is sensitive to the high energy state of the chloroplasts². Wraight and Crofts³ have suggested that this sensitivity is due to the establishment of electrical and pH gradients across the thylakoids during the illumination stage. The linking of the high energy state with these gradients is an essential feature of Mitchell's chemiosmotic hypothesis⁴. Wraight and Crofts³ have argued that the light-induced pH and electrical gradients act in such a way as to decrease the activation energy necessary to lift electrons from the metastable state, created during the preillumination, to the first singlet of chlorophyll. If this hypothesis is correct, the establishment of an electrical gradient across the thylakoid membranes by some means other than light-induced electron transport should change the intensity of millisecond delayed light emission. One possible way to create membrane potentials is to subject chloroplasts to salt gradients. The magnitude of the potentials developed will be a function of the concentration gradients and the relative rates of penetration of the ions across the thylakoid membranes^5,6.