Isoprenoids and phenylpropanoids are part of the antioxidant defense orchestrated daily by drought stressed Platanus x acerifolia plants during Mediterranean summers

The hypothesis was tested that isoprenoids and phenylpropanoids play a prominent role in countering photooxidative stress, following the depletion of antioxidant enzymes activity in plants exposed to severe drought stress under high solar irradiance and high temperatures, a recurrent condition in Mediterranean summers, exacerbated by current and future climate warming. Plants of Platanus × acerifolia, a high isoprene emitting species and an ornamental tree widely used in urban planning, were drought stressed for two weeks during summer (WS) and compared to unstressed controls (WW). Water relations and photosynthetic parameters were measured at mild, moderate and severe drought stress level. Volatile and non-volatile isoprenoids, antioxidant enzymes and phenylpropanoids were measured with the same time course, but in four different periods of the day. Photosynthesis was severely inhibited by drought stress, due to early stomatal closure. When the stress was severe, biochemical components started to limit photosynthesis, whereas the photochemical machinery was not affected. Isoprene emission and zeaxanthin concentration were higher in WS than in WW leaves, particularly at mild and moderate stress levels, and during the hottest hours of the day. The activities of catalase and ascorbate peroxidase steeply declined during the day, while the activity of guaiacol peroxidase and the mesophyll concentration of quercetin increased during the day, peaking in the hottest hours in both WW and WS plants. Our experiment revealed a sequence of antioxidants that were daily used by the plant to orchestrate defense against oxidative stress induced by drought and associated high light and high temperature conditions. Secondary metabolites and non chloroplastic enzymes seem important to complement chloroplast (and peroxisomal) antioxidant enzymes to counter oxidative stress during the hottest hours of the day.