Responses to ozone pollution of alfalfa exposed to increasing salinity levels.

Stomatal closure and biosynthesis of antioxidant molecules are two fundamental components of the physiological machinery that leads to stress adaptation during plant exposure to salinity. Since high stomatal resistance may also contribute in counteracting O3 damages, we hypothesized that soil salinization may increase O3 tolerance of crops. An experiment was performed with alfalfa grown in Filtered (AOT40 = 0 in both years) and Not-Filtered (AOT40 = 9.7 in 2005 and 6.9 ppm h in 2006) Open Top Chambers. Alfalfa yield was reduced by O3 (-33%) only in plants irrigated with salt-free water, while the increasing levels of soil salinity until 1.06 dS m-1 reduced both stomatal conductance and plant O3 uptake, thus linearly reducing O3 effects on yield. Therefore a reliable flux-based model for assessing the effects of O3 on crop yield should take into account soil salinity.