Evolution of ozone and drought stresses, singly or combined, in Quercus cerris

The objective of this study was to evaluate the impact of ozone (O3) and drought, either singularly or jointly applied, on the physiological and biochemical traits of the Mediterranean tree species Quercus cerris. Three-year-old saplings were subjected to moderate drought (daily irrigated with 30% of effective evapotranspiration) and/or fumigated with 100 ppb of O3 (5 h d-1) for 77 days, in order to simulate a 2050 environmental scenario. After 28 days and at the end of the stresses exposure the following parameters were analysed: i) leaf water status and membrane integrity; ii) diurnal courses of photosynthetic process; iii) osmolytes and accessories pigments content. Under drought (alone or in combination with O3), 42 days from the beginning of the treatment, plants showed lateral and tip yellow-brown necrosis in the youngest fully expanded leaves. At the end of the fumigation, no O3 leaf injury was observed, but all plants (with exception of controls) showed a marked phylloptosis. Drought stress (single and combined) impaired plants performances already after 28 days. Predawn leaf water potential (PDΨw), malondialdehyde (MDA) and proline levels were more affected by drought applied individually than in combination [more than 5-fold vs 3-fold lower in comparison to controls, +11 vs +4%, +112 vs +45%, respectively]. The circadian patterns of photosynthetic process were impaired similarly at the quali-quantitative level. Leaf carbon dioxide (CO2) assimilation rate (A) decreased (-65% compared to control, as daily mean) due to both stomatal [-67% of stomatal conductance (gs)] and biochemical limitations [intercellular CO2 concentrations (Ci) showed unchanged or higher values during the whole day], same as the maximum (Fv/Fm) and effective (Y) quantum yields of photosystem II (-9 and -41%, respectively). On the other hand, in O3-stressed plants, similar responses were observed only after 77 days of the exposure: -80% of PDΨw values (compared to controls, as daily mean); +17% of MDA concentrations; proline content from lower values (observed after 28 days) increased to constitutive levels; -68% of A and -70% of gs; Ci and Fv/Fm, unchanged; -13% of Y values, (only photoinhibiton was observed). Moreover, at the end of the treatment, the effects of coupled stresses were stronger than that of drought applied individually in terms of water status, lipid peroxidation and proline content while the circadian patterns of photosynthesis continued to be affected similarly at the quali-quantitative levels. Differential abscisic acid (ABA) evolutions, strictly twinned with its precursors (neo- and violaxanthin), were observed during the summer. After 28 days of treatment, the content of this phytohormone rose only in plants subjected to drought (+143%), while an increase of neoxanthin was observed in O3 and combined ones (+15 and +31%, respectively). At the end of the exposure, ABA concentration significantly increased in O3 (single and in combination) stressed plants (+250 and +64%, respectively), while violaxanthin content decreased (-32 and -47%, respectively). In conclusion, in Q. cerris: i) drought stress (single and combined) should be considered more harmful than O3, although it should be mitigated by this pollutant for a limited time; ii) drought and combined stresses impaired similarly the diurnal pattern of photosynthetic processes at the quali-quantitative levels, and iii) the osmolytes (in particular ABA) could play a key role in the responses to environmental constrains