Physiological responses of Quercus ilex leaves to water stress and acute ozone exposure under controlled conditions

The combined effect of water stress and ozone (O-3) on stomatal O-3 flux, damage to photosynthesis, and detoxification by biogenic volatile organic compounds (BVOC) in Quercus ilex leaves was studied. A 4-weeks O-3 exposure (250 ppb, 4 h per day) caused a reduction of photosynthesis and stomatal conductance, which was fully recovered 1 week after the end of the treatment, in well-watered and water-stressed plants. Measurements of stomatal O-3 flux revealed a low stomatal flux of the pollutant, which became minimal after stomatal closure caused by water stress. An induction of volatile monoterpenes, important compounds in the O-3 scavenging system in Q. ilex, and a burst of lipoxygenase compounds (LOX), which are released as gaseous by-products of membrane peroxidation, was observed after 2-3 weeks of O-3 fumigation. However, these compounds were also released in control leaves that were exposed to ozone only briefly, to determine stomatal O-3 flux. The low stomatal flux that occurred in water stress conditions helped avoiding permanent damage to Q. ilex leaves, although during the O-3 treatment photosynthesis was severely limited by stomatal closure. In well-watered plants, O-3 fumigation caused a noticeable increase of nocturnal stomatal conductance. If confirmed on adult plants under field conditions, this effect can imply larger flux of O-3 at night and possible detrimental effects of O-3 on leaf functions in plants exposed to high nocturnal O-3 levels.