Altered stomatal dynamics of two Euramerican poplar genotypes submitted to successive ozone exposure and water deficit.

The impact of ozone (O) pollution events on the plant drought response needs special attention because spring O episodes are often followed by summer drought. By causing stomatal sluggishness, O could affect the stomatal dynamic during a subsequent drought event. In this context, we studied the impact of O exposure and water deficit (in the presence or in the absence of O episode) on the stomatal closure/opening mechanisms relative to irradiance or vapour pressure deficit (VPD) variation. Two genotypes of Populus nigra x deltoides were exposed to various treatments for 21 days. Saplings were exposed to 80 ppb/day O for 13 days, and then to moderate drought for 7 days. The curves of the stomatal response to irradiance and VPD changes were determined after 13 days of O exposure, and after 21 days in the case of subsequent water deficit, and then fitted using a sigmoidal model. The main responses under O exposure were stomatal closure and sluggishness, but the two genotypes showed contrasting responses. During stomatal closure induced by a change in irradiance, closure was slower for both genotypes. Nonetheless, the genotypes differed in stomatal opening under light. Carpaccio stomata opened more slowly than control stomata, whereas Robusta stomata tended to open faster. These effects could be of particular interest, as stomatal impairment was still present after O exposure and could result from imperfect recovery. Under water deficit alone, we observed slower stomatal closure in response to VPD and irradiance, but faster stomatal opening in response to irradiance, more marked in Carpaccio. Under the combined treatment, most of the parameters showed antagonistic responses. Our results highlight that it is important to take genotype-specific responses and interactive stress cross-talk into account to improve the prediction of stomatal conductance in response to various environmental modifications.