Antioxidative responses of three oak species under ozone and water stress conditions

Plants are frequently exposed to adverse environmental conditions such as drought and ozone (O₃). Under these conditions, plants can survive due to their ability to adjust their metabolism. The aim of the present study was to compare the detoxification mechanisms of three oak species showing different O₃ sensitivity and water use strategy. Two-year-old seedlings of Quercus ilex, Q. pubescens and Q. robur were grown under the combination of three levels of O₃ (1.0, 1.2 and 1.4 times the ambient O₃ concentration) and three levels of water availability (on average 100, 80 and 42% of field capacity i.e. well-watered, moderate drought and severe drought, respectively) in an O₃ Free Air Controlled Exposure facility. Ozone and drought induced the accumulation of reactive oxygen species (ROS) and this phenomenon was species-specific. Sometimes, ROS accumulation was not associated with membrane injury suggesting that several antioxidative defence mechanisms inhibited or alleviated the oxidative damage. Both O₃ and drought increased total carotenoids that were able to prevent the peroxidation action by free radicals in Q. ilex, as confirmed by unchanged malondialdehyde by-product values. The concomitant decrease of total flavonoids may be related to the consumption of these compounds by the cell to inhibit the accumulation of hydrogen peroxide. Unchanged total phenols confirmed that Q. ilex has a superior ability to counteract oxidative conditions. Similar responses were found in Q. pubescens, although the negative impact of both factors was less efficiently faced than in the sympatric Q. ilex. In Q. robur, high O₃ concentrations and severe drought induced a partial rearrangement of the phenylpropanoid pathways. These antioxidative mechanisms were not able to protect the cell structure (as confirmed by ROS accumulation) suggesting that Q. robur showed a lower degree of tolerance than the other two species.