Analysis of dark drops, dark-induced changes in chlorophyll fluorescence during the recording of the OJIP transient

Abstract

In a search for a powerful and easy-to-handle instrument for investigation of photosynthetic energy and electron transfer reactions, the mPEA (Hansatech Instrument Ltd., King’s Lynn, Norfolk, PE30 4NE, U.K.) was developed. The instrument can record simultaneously, in vivo, the photo-induced changes in prompt (PF) and delayed (DF) chlorophyll fluorescence and modulated infrared light reflection at 820 nm (MR820) using a protocol of alternating periods of illumination and darkness. In this way the standard OJIP induction transient of PF is modified by the dark intervals causing a decrease of the PF intensity during the dark periods (so called dark fluorescence drops, DD). The dependence of the relative DD on the redox level of QA was analyzed for different initial states of bean leaves. A strong linear correlation was found between the relative dark drops and the fraction of oxidized QA for the phase starting before the J level and going up to the P level of the PF induction transient. We propose that the experimentally measurable DDs offer a tool for in vivo quantification of the redox reactions of QA and QB during the fluorescence rise from F0 to FM.