• The differential accumulation of various polyphenols, particularly of flavonoids and hydroxycinnamates, was studied in leaves of Ligustrum vulgare exposed to increasing sunlight under well watered or drought-stress conditions. • Light- and drought-induced changes in leaf polyphenol concentrations were normalized to the CO2 assimilation rate. The functional roles of flavonoids and hydroxycinnamates were analysed through tissue localization using multispectral fluorescence microimaging, and through efficiencies to scavenge superoxide radicals (O2 - ) and to screen UV wavelengths. • Clear effects of light and water treatments on leaf polyphenol concentrations were not observed, as the CO2 assimilation rate varied according to sunlight and water availability. However, biosynthesis of quercetin 3-O-rutinoside, luteolin 7-O-glucoside and echinacoside, which were efficient O2 - scavengers, increased sharply in response to solar radiation. By contrast, carbon for the synthesis of p-coumaric acid and monohydroxyflavones, efficient UV screeners but poor O2 - scavengers, did not vary depending on light treatments. Flavonoids accumulated in both the adaxial epidermis and the palisade tissue because of sunlight irradiance, whereas echinacoside occurred largely in abaxial tissues. • We hypothesize that flavonoids may serve antioxidant functions in response to excess light and drought stress, and that a coordinate control system between hydroxycinnamate and flavonoid pathways operated in L. vulgare exposed to excess light.
Keywords: apigenin glycosides; drought stress; echinacoside; excess light; fluorescence microimaging; luteolin glycosides; quercetin 3-O-rutinoside; superoxide radical.