Resolution of components of non-photochemical chlorophyll fluorescence quenching in barley leaves

Non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves has been analysed by monitoring its relaxation in the dark, by applying saturating pulses of light. At least three kinetically distinct phases to qN recovery are observed, which have previously been identified (Quick and Stitt 1989) as being due to high-energy state quenching ('fast'), excitation energy redistribution due to a state transition ('medium') and photoinhibition ('slow'). However, measurements of chlorophyll fluorescence at 77 K from leaf extracts show that state transitions only occur in low light conditions, whereas the 'medium' component of qN is very large in high light. The source of that part of the 'medium' component not accounted for by a state transition is discussed.