Flash-induced Mn 2+ oxidation observed by ESR spectrometry in lettuce chloroplasts

Abstract

The requirement of manganese for the process of photosynthetic oxygen evolution has been recognized for many years [I]. It has been suggested that manganese acts as an electron carrier between water and photosystem If [2]. Moreover it has been proposed that Mn is part of the positive charge accumulator [3] which is involved in the formation of the S-states postulated by the model for oxygen evolution [4]. In [5] we used electron spin resonance (ESR) spectroscopy to study manganese, in lettuce chloroplasts. We showed an ESR signal, originating from bound manganese, which decreased in amplitude upon continuous illumination of the chloroplasts. The on-off kinetics of the signal of control chloroplasts was compared to that of chloroplasts incubated with Tris, treated with heat, or to which specific inhibitors (DCMU, FCCP) or artifical electron donors (NH*OH, phenylene d&mine) were added. We have concluded from these studies that the decrease in the ~p~tude of the Mn” ESR signal reflects oxidation of Mn2+ by photosystem II, and the dark restoration is rereduction to Mr?‘. Comparing our results with those of others showed some differences: the manganese ESR signal from active spinach chloroplasts was too weak [6] to be able to work on, but still it was distinguishable (cf. fig.1 in [6]). The signal increased tremendously upon Tris treatment [6,7] as well as upon washing with chaotropic agents [7]. In this case a lint-sensitive decrease of the Mn ESR signal was demonstrated although oxygen evolution was orbited [7]. On the