Reliability of Photosystem II thermoluminescence measurements after sample freezing: Few artifacts with Photosystem II membranes but gross distortions with certain leaves

Abstract

Changes of thermoluminescence (TL) properties reported for Photosystem II (PS II) membranes after removal of functional Cl- recently have been attributed to an exposure of the experimental material to freezing temperatures in the absence of a cryoprotectant like glycerol [Krieger et al. (1998) Biochim Biophys Acta 1364: 46]. In the present study, freezing-induced modifications of the TL emissions of PS II membranes were confirmed to be a problem in TL studies, but glycerol was not always a reliable antidote. The TL measurements of this investigation lead to the conclusion that effects of sample freezing do not invalidate previous studies which have reported that Cl- depletion shifts the TL emission to higher temperatures. Nevertheless, in agreement with the study of Krieger et al. (1998), it is shown that at pH 6.2 and in the absence of DCMU, Cl- removal causes only a very small displacement of the TL emission peak. While the TL was affected mainly quantitatively by freezing when PS II membranes were the experimental material, substantial qualitative changes of the TL were observed with certain leaves. These are attributed tentatively to redox potential changes of the primary acceptors of PS II which allowed a reduction of QA by reduced QB via reverse electron flow. Experiments aimed at mimicking the altered TL emission in PS II membranes in vitro suggest actions of secondary metabolites and acids. Thylakoids in the leaf tissue may have become exposed to such compounds because of damage to cellular membranes during freezing. On the basis of the results of this investigation, it is recommended that sample freezing be avoided in TL studies whenever possible, regardless of the type of experimental material.