The role of the xanthophyll cycle in regulating the energy flow to the PS II reaction centers and therefore in photoprotection was studied by measurements of light-induced absorbance changes, Chl fluorescence, and photosynthetic O2 evolution in sun and shade leaves of Hedera canariensis. The light-induced absorbance change at 510 nm (ΔA510) was used for continuous monitoring of zeaxanthin formation by de-epoxidation of violaxanthin. Non-radiative energy dissipation (NRD) was estimated from non-photochemical fluorescence quenching (NPQ).
High capacity for zeaxanthin formation in sun leaves was accompanied by large NRD in the pigment bed at high PFDs as indicated by a very strong NPQ both when all PS II centers are closed (F'm) and when all centers are open (F'o). Such Fo quenching, although present, was less pronounced in shade leaves which have a much smaller xanthophyll cycle pool.
Dithiothreitol (DTT) provided through the cut petiole completely blocked zeaxanthin formation. DTT had no detectable effect on photosynthetic O2 evolution or the photochemical yield of PS II in the short term but fully inhibited the quenching of Fo and 75% of the quenching of Fm, indicating that NRD in the antenna was largely blocked. This inhibition of quenching was accompanied by an increased closure of the PS II reaction centers.
In the presence of DTT a photoinhibitory treatment at a PFD of 200 μmol m-2 s-1, followed by a 45 min recovery period at a low PFD, caused a 35% decrease in the photon yield of O2 evolution, compared to a decrease of less than 5% in the absence of DTT. The Fv/Fm ratio, measured in darkness showed a much greater decrease in the presence than in the absence of DTT. In the presence of DTT Fo rose by 15–20% whereas no change was detected in control leaves.
The results support the conclusion that the xanthophyll cycle has a central role in regulating the energy flow to the PS II reaction centers and also provide direct evidence that zeaxanthin protects against photoinhibitory injury to the photosynthetic system.
Energy dissipation photoinhibition xanthophyll cycle zeaxanthin
F, Fm, Fo, Fv
Fluorescence yield at actual degree of PS II center closure, when all centers are closed, when all centers are open, variable fluorescence