Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O-3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity-WS and WW treatments, respectively) and O(3)exposure (1.0, 1.2, and 1.4 times the ambient concentration-AA, 1.2AA, and 1.4AA, respectively) were carried out onQuercus roburL.,Quercus ilexL., andQuercus pubescensWilld. seedlings, to study physiological traits (1. isotope signature [delta C-13, delta O-18 and delta N-15], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, A(sat), and stomatal conductance, g(s)]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased A(sat)inQ. roburandQ. pubescenswhile water stress decreased it in all three oak species. Ozone did not affect delta C-13, whereas delta O-18 was influenced by O(3)especially inQ. robur. This may reflect a reduction of g(s)with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O(3)on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by delta C-13 and by delta O-18 in all three oak species, while delta N-15 did not define plant response to stress conditions in any species. The delta C-13 signal was correlated to leaf water content (LWC) inQ. roburandQ. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O(3)exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.