Ozone-induced ‘sluggish’ stomatal CO2 response depends on oxidative damage and pigment degradation in the Mediterranean shrub Viburnum lantana L.

Tropospheric ozone (O₃) is a pervasive air pollutant known to impair stomatal regulation in plants, i.e. stomatal sluggishness, in association with a reduction of photosynthesis. However, its impact on the dynamic responsiveness of stomata to carbon dioxide (CO₂) concentrations remains poorly understood. In this study, we investigated the effects of chronic O₃ exposure on both steady-state and dynamic leaf gas exchange response to low or high CO₂ concentration (50 or 1000 μmol mol􀀀 1) in Viburnum lantana L., a Mediterranean shrub species highly sensitive to oxidative stress. Ozone-exposed plants exhibited pronounced stomatal sluggishness in response to rapid CO₂ transitions, characterized by delayed closure, reduced opening amplitude, and prolonged response times. These impairments were associated with significant decreases in photosynthetic capacity, pigment degradation (chlorophylls and xanthophylls), and increased lipid peroxidation. Correlation analyses revealed strong links among pigment loss, oxidative membrane damage, and impaired stomatal kinetics, suggesting that both energetic and structural limitations contribute to O₃-induced stomatal dysfunction. These findings indicate that the CO₂ responsiveness of stomata under O₃ stress is not simply passive damage, but reflects a complex, multilevel breakdown of guard cell regulation. As atmospheric O₃ and CO₂ concentrations continue to rise, such impairment may critically constrain plant carbon–water balance, especially in sensitive woody species inhabiting Mediterranean montane environments.