Drought stress has contrasting effects on antioxidant enzymes activity and phenylpropanoid biosynthesis in Fraxinus ornus leaves: an excess light stress affair?

The experiment was conducted using Fraxinus ornus plants grown outside under full sunlight irradiance, and supplied with 100% (well-watered, WW), 40% (mild drought, MD), or 20% (severe drought, SD) of the daily evapotranspiration demand, with the main objective of exploring the effect of excess light stress on the activity of antioxidant enzymes and phenylpropanoid biosynthesis. Net CO₂ assimilation rate at saturating light and daily assimilated CO₂ were significantly smaller in SD than in WW and MD plants. Xanthophyll-cycle pigments supported nonphotochemical quenching to a significantly greater extent in SD than in MD and WW leaves. As a consequence, the actual efficiency of PSII (Φ(PSII)) was smaller, while the excess excitation-energy in the photosynthetic apparatus was greater in SD than in WW or MD plants. The concentrations of violaxanthin-cycle pigments relative to total chlorophyll (Chl(tot)) exceeded 200 mmol mol⁻¹ Chl(tot) in SD leaves at the end of the experiment. This leads to hypothesize for zeaxanthin a role not only as nonphotochemical quencher, but also as chloroplast antioxidant. Reductions in ascorbate peroxidase and catalase activities, as drought-stress progressed, were paralleled by greater accumulations of esculetin and quercetin 3-O-glycosides, both phenylpropanoids having effective capacity to scavenge H₂O₂. The drought-induced accumulation of esculetin and quercetin 3-O-glycosides in the vacuoles of mesophyll cells is consistent with their putative functions as reducing agents for H₂O₂ in excess light-stressed leaves. Nonetheless, the concentration of H₂O₂ and the lipid peroxidation were significantly greater in SD than in MD and WW leaves. It is speculated that vacuolar phenylpropanoids may constitute a secondary antioxidant system, even on a temporal basis, activated upon the depletion of primary antioxidant defences, and aimed at keeping whole-cell H₂O₂ within a sub-lethal concentration range.