Ozone and soil salinity interactions in Mediterranean woody species

Tropospheric ozone is an important oxidative agent that may cause reduced production in plants. Its concentrations depend mainly on NOx emission and are hence higher close to urbanized areas. Mediterranean evergreen species characterize coastal vegetation, but are also used to separate agricultural fields: both these areas often have a high soil salinity. In this experiment the effect of ozone, soil salinity and the interaction of the two was tested in Open Top Chambers on Quercus ilex and Arbutus unedo. The hypothesis was that the effects of the two oxidative stresses combined would reduce the stomatal closure due to soil salinity and that the activation of protective pigments would have been higher in A. unedo. The experiment lasted 65 days during which plants were watered with tap water or with a salt solution of 75 mM, following a split plot design plants were also exposed to two different ozone levels filtered air and ozone enriched air (-50% and +30% of ambient ozone concentrations). Results show that ozone alone had little effects on stomatal control and photosynthesis, while soil salinity caused a reduction of the same. This reduction was much more pronounced in Q. ilex (65%) than in A. unedo (30%). Notably, Q. ilex accumulated the ions preferentially in the roots while they were homogeneously distributed in all compartments in A. unedo. This difference partially explains the difference in pigment concentration which increased strongly in A. unedo, but not in Q. ilex. In the interaction, ozone reduced the stomatal closure in Q. ilex also allowing for a higher photosynthesis, but this did not occur in A. unedo where the effect was slightly synergic and caused a further increase of pigment concentrations in order to avoid photodamages. Results suggests that both species are resistant to oxidative stresses, but due to two different strategies, these differences imply a potential shift in the ranking of species performance especially in coastal areas in case of increased ozone concentrations and higher soil salinity caused by the lowering of the water table.