• abiotic stress
• drought
• global climate change
• ozone
• salinity
• vegetation spectroscopy

In this era of global warming it is necessary to understand the mechanisms that allow native plant species to tolerate these environmental constraints and the way such mechanisms interact. The main scope of this research work was to investigate the responses of oak species to single and combined abiotic stresses that are common in the Mediterranean urban environments (drought, O3 and salinity).
Firstly, three Mediterranean oak species (Querucs ilex, Q. pubescens and Q. cerris) with different features (drought tolerance, evergreen or deciduous species) were selected in order to assess their responses under a long period of moderate drought (30% of the effective daily evapotranspiration) and/or O3 stress (80 ppb of O3 for 5 h d-1 for 77 consecutive days). The chronic O3 treatment had a minor impact compared to drought, highlighting that the plasticity of the species is dependent on the environment in which they live. Species that inhabit environments characterized by long periods of water deficiency (i.e. Q. ilex and Q. pubescens) are usually more plastic under the same stress compared with those that rarely face the same environmental constraint (Q. cerris). Furthermore, this dataset shows that biochemical and physiological adjustments may reduce the impact of O3 when combined with the effect of drought.
Following the results came out from this first experiment, and since the mitigative effect of drought against O3 seems more consistent when plant is exposed to short and harsh period of severe drought rather than to longer O3 exposure but under moderate drought, other two sets of experiments were performed. Firstly, the interactions of severe drought (20% of the effective daily evapotranspiration) and O3 (80 ppb, 5 h d-1, for 28 consecutive days) were investigated in Q. cerris focusing on its hydric relations, synthesis/production of compatible solutes and lipophilic antioxidant compounds. Although, leaf-intrinsic adjustments occurred (stomatal limitations) and the synthesis of stress-associated metabolites was altered, plants were not able to delay or prevent the negative impact of drought. Furthermore, it was evident that drought alone induced fairly higher effects in comparison to O3, whereas when O3 was applied together with drought it showed some “mitigating effects” against limited water availability. In fact, plants of Q. cerris exposed to both drought and O3 seemed able to partially adjust and optimize their photosynthetic activity. This response could be interpreted as a photosynthetic acclimation leading to a premature senescence of fully-developed leaves as a strategy to respond to multiple stress conditions, likely addressed to alert younger leaves.
Later, physiological and biochemical responses of Q. ilex (evergreen, isohydric species) were compared to those of the sympatric Q. pubescens (deciduous, anisohydric species) under the same environmental constrains of the previous experiment. This study confirms the higher ability of evergreen species to counteract the effect of drought and O3 when compared to deciduous species in Mediterranean environment. This ability of evergreens seems to be correlated with the stronger necessity of these species, which inhabits usually unfavorable environments, to protect their long-lived leaves from several negative environmental factors. This peculiarity seems less relevant for highly-demanding, fast-growing deciduous species characterized by shorter leaf lifespan, which have superior fitness than evergreens in non-limiting environment.
Then, the response of Q. ilex (the most studied oak species, as well as the species that has been shown the higher tolerance and plasticity in the first experiment) to drought (20% of the effective daily evapotranspiration) and/or an acute O3-exposure (200 ppb, 5 h) was also investigated. This study, which focused on the interaction between reactive oxygen species (ROS), phytohormones and signalling molecules to evaluate if the response of Q. ilex resembles the biotic defense reactions, shows that: (i) in well-watered conditions, O3 induced a signalling pathway similar to that triggered by a pathogen only in terms of ROS pattern (showing an O3-sensitive behavior); (ii) different trends and consequently different roles of phytohormones and signalling molecules were observed in relation to the leaf hydric status and O3 (applied both singularly and consequently), and (iii) these differences were ascribable to the fact that in drought conditions most defense processes induced by O3 were compromised/altered.
Furthermore, the response of Q. ilex was also investigated also under salinity and O3. Q. ilex saplings were firstly exposed to salinity (150 mM NaCl, 15 days), and the effect on photosynthesis, hydric relations and ion partitioning were evaluated (Experiment I). Then, salt-treated plants were exposed to 80 ppb of O3 for 5 h (Experiment II). The results of the first experiment indicate that evergreen sclerophylls such as Q. ilex, with their long-lived leaves have a low photosynthetic efficiency on a mass basis because these species invest preferentially in vascular and cell wall formation. This induces these species to decrease intercellular spaces and increase cell wall thickness, increasing CO2 drawdown but also maintaining high foliar RWC and osmotic stress tolerance. These mechanisms are consistent with a conservative strategy adopted by Q. ilex to preserve its long-lived leaves against different abiotic stresses. Furthermore, the dataset of the second experiment shows that O3 did not exacerbate the oxidative stress observed in salt-treated plants although a further relevant enhancement of the Halliwell-Asada cycle was necessary to counteract the O3-induced damage when the leaf status was already negatively affected by a previous salt exposure. This harmonic response is an extra burden for plants, and growth can suffer as a result in the long-term, if these single O3 episodes take place repeatedly.
Assessing the impact of climate change and air pollution on ecosystems is still a challenging task and the development of adequate monitoring techniques is necessary for assessing vegetation status. Thus, a final study was performed showing that reflectance spectroscopy can be an alternative method for monitoring leaf water potential and also making a posteriori measurements of pre-dawn leaf water potential (PDΨW) on tropical live oak Q. oleoides sampled in four Central American populations (Belize, Costa Rica, Honduras and Mexico) and grown under differential water availability. This study confirms that spectroscopic approaches are quick and non-destructive, providing the possibility to screen more samples in the field and over multiple time periods. In addition, this dataset demonstrates that spectroscopic retrievals of PDΨW in response to environmental variation (e.g. water availability) can be used as a surrogate for standard approaches.