Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks.

Mesophyll conductance (g mCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O 3 ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O 3 and drought on g mCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O 3 (ambient [35.2 nmol mol -1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO 2 inside a leaf. The combination of O 3 and WD significantly decreased net photosynthetic rate (P N ) regardless of the species. The reduction of photosynthesis was associated with a decrease in g mCO2 and stomatal conductance (g sCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (V cmax ) and maximum electron transport rate (J max ) with decreased diffusive conductance parameters. The reduction of g mCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between g mCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between g sCO2 and ABA under high O 3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of P N was related to g mCO2 rather than g sCO2 . The results suggest that g mCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O 3 pollution.