Recently, it has been suggested (Horton et al. 1992) that aggregation of the light-harvesting a-b complex (LHC II) in vitro reflects the processes which occur in vivo during fluorescence induction and related to the major non-photochemical quenching (qE). Therefore the requirement of this chlorophyll a-b containing protein complex to produce qN was investigated by comparison of two barley mutants either lacking (chlorina f2) or depressed (chlorina 1ยฐ4) in LHC II to the wild-type and pea leaves submitted to intermittent light (IL) and during their greening in continuous light.
It was observed that qN was photoinduced in the absence of LHC II, i.e. in IL grown pea leaves and the barley mutants. Nevertheless, in these leaves qN had no (IL, peas) or little (barley mutants) inhibitory effect on the photochemical efficiency of QA reduction measured by flash dosage response curves of the chlorophyll fluorescence yield increase induced by a single turn-over flash.
During greening in continuous light of IL pea leaves, an inhibitory effect on QA photoreduction associated to qN developed as Photosystem II antenna size increased with LHC II synthesis. Utilizing data from the literature on connectivity between PS II units versus antenna size, the following hypothesis is put forward to explain the results summarized above, qN can occur in the core antenna or Reaction Center of a fraction of PS II units and these units will not exhibit variable fluorescence. Other PS II units are quenched indirectly through PS II-PS II exciton transfer which develops as the proportion of connected PS II units increases through LHC II synthesis.
Abbreviations: F -chlorophyll fluorescence levels, subscripts o and m are minimal and maximal levels of darkadapted leaves; o', m' and s-minimal, maximal and steady state levels in the presence of actinic light; Fv and Fv' -(Fm-Fo) and (Fm'-Fo ~) respectively; AF -(Fm'-Fs); tSF -single turn-over flash-induced fluorescence yield increase; 6Fmax -single turn-over saturating flash-induced fluorescence yield increase; IL-intermittent light; LHC II -light-harvesting chl a-b complex associated with PS II; PFD -photon flux density; PS II -Photosystem II; QA -primary quinonic electron acceptor of PS II; qP-photochemical quenching; qN -non-photochemical quenching; tr -relative efficiency of PS II photoreduction of QA; ed -~r of dark-adapted leaves