Photoabatement by anthocyanin shields photosynthetic systems from light stress

Leaves and other chlorophyllous tissues of plants often show transient or permanent anthocyanin coloration. The question of whether anthocyanins can function as effective light screens to modulate photosynthesis in plants was addressed by comparing photosynthetic responses in reddish-purple pods with those in green pods of the ornamental leguminous tree Bauhinia variegata. For these comparisons the actinic radiation employed was either red radiation (RR) which was poorly absorbed by anthocyanin or blue-green radiation (BGR) which was strongly absorbed by anthocyanin. Photon yields of photosystem 2 (PS2) photochemistry and photochemical chlorophyll fluorescence quenching coefficients (qp), measured over a range of photon flux densities (PFD) up to 1200 μmol m s at 23 °C and at five temperatures from 8 to 28 °C at a PFD of 260 μmol m s, were almost identical in green pods irradiated with either RR or BGR and in purple pods irradiated with RR. However, qp values remained much higher in purple pods irradiated with BGR, e.g., 0.80 in BGR versus 0.29 in RR at a PFD of 1200 μmol m s at 23 °C, and 0.67 in BGR versus 0.28 in RR at a PFD of 260 μmol m s at 8 °C. The higher values of qp in BGR compared to RR indicated that photoabatement by anthocyanin allowed the first stable acceptor of PS2, Q(A), to be kept in a more oxidized state, thus decreasing the likelihood of photoinhibition. This was confirmed by demonstrating a lower susceptibility to photoinhibition in purple pods than in green pods in the sunlight, either naturally in pods on trees or in detached pods exposed to photoinhibitory conditions. We conclude that photoabatement by anthocyanin is a mechanism for allowing maintenance of higher oxidative levels of PS2 acceptor during episodes of high radiation stress, thereby minimizing photodamage to photosynthetic tissues