Photochemical and thermal phases of chlorophyll a fluorescence

The measurement of variable chlorophyll (Chl) a fluorescence is widely used as a convenient and versatile tool in photosynthesis research. In many app lications empirical correlations and simplified models of Chi a fluorescenc e are used with success, Nevertheless, variable Chi a fluorescence provides only indirect and complex image of processes occurring within photosynthet ic membranes and such simplifications have only limited validity, In this r eview we elucidate some controversial and still unresolved questions about the origin and interpretation of the variable Chi a fluorescence induction and the proper use of variable Chi a fluorescence for studies of photochemi cal events in photosystem 2 (PS2). Although the major part of variable Chi a fluorescence reflects the photochemical closure of the PS2 reaction cente rs (RCs) and can be considered as a function of the redox state of the prim ary acceptor Q(A), up to 50 % of the change in the Chl a fluorescence yield can be of secondary, nonphotochemical origin. We review the possible sourc es of the inherent heterogeneity in the origin of variable Chi a fluorescen ce. We also comment on the practical implications this bears for the use of variable Chi a fluorescence.