Defense mechanisms in Salvia officinalis under chronic ozone exposure

Tropospheric ozone (O3) is a major air pollutant causing negative effects on health of living organisms. As O3 commonly has adverse effects on plant tissue at both physiological and biochemical levels, the present study investigated the mechanisms of response of sage, Salvia officinalis – an aromatic herb largely grown in the Mediterranean area – to a chronic O3 exposure (120 ppb of O3 for 36 consecutive days, 5 h day-1), specifically focusing on the relationships between the oxidative burst and the regulation of phytohormones and their interaction as an integrated defense mechanism. Starting from 14 days from the beginning of exposure (FBE), O3 induced a leaf yellowing and a reduction of photosynthetic activity (-50%, in comparison with controls) due to both stomatal and mesophyll limitations (stomatal conductance: -52%; intercellular CO2 concentration: +11%). Membrane permeability was also impaired by O3-induced oxidative pressure (malondialdehyde: +27%). Plants tried to cope with such oxidative stress by (i) activating the antioxidant systems (e.g., ascorbic acid, glutathione), and (ii) regulating the phytohormone response. Despite an increased lipoxygenase activity occurred at 29 FBE (+61%), levels of jasmonic acid never changed, probably due to an antagonistic role played by ethylene (ET). Indeed, ET increased (+1.6-fold) from 22 days FBE until the end of exposure, also promoting the accumulation of salicylic acid (+73%). Abscisic acid increased throughout the whole experiment likely inducing stomatal closure. These outcomes suggest that the crosstalk between phytohormones played a central role in S. officinalis defense against O3.