Effects of ascorbate feeding on chlorophyll fluorescence and xanthophyll cycle components in the lichen Parmelia quercina (Willd.) Vainio exposed to atmospheric pollutants

The effects of environmental stresses on photosynthetic responses, ascorbate levels and pigment composition were investigated in samples of Parmelia quercina (Willd.) Vainio from control and polluted regions of the northern Castellon area (Valencia, Spain). In response to sustained pollutant stress in the field, lichen thalli had closed PSII traps and exhibited lower rates of electron transport and non-radiative energy dissipation. The xanthophyll concentration was not affected by exposure to atmospheric pollutants. The ascorbate concentration was lower in samples exposed to ambient air pollutants compared to control thalli. Ascorbate feeding of thalli from polluted sites stimulated electron flow, photochemical quenching and non-radiative energy dissipation. Additionally, ascorbate feeding enhances the de-epoxidation state of the xanthophyll pool in polluted thalli. The partial recovery for non-radiative energy dissipation was presumably due to the interaction between the increased thylakoid pH gradient and de-epoxidized xanthophylls. Furthermore, ascorbate feeding decreased photon excess in thalli from polluted sites owing to the stimulation of linear electron flow and non-radiative energy dissipation. The present study suggests that atmospheric pollutants, besides their intrinsic toxicity, put on an additional burden by hampering the operation of photoprotective mechanisms.