Ozone-induced impairment of night-time stomatal closure in O3-sensitive poplar clone is affected by nitrogen but not by phosphorus enrichment

Nocturnal transpiration may be a key factor influencing water use in plants. Tropospheric ozone (O3) and availabilityof nutrients such as nitrogen (N) and phosphorus (P) in the soil can affect daytimewater use through stomata,but the combined effects of O3, N and P on night-time stomatal conductance (gs) are not known. Weinvestigated the effects of O3 and soil availability of N and P on nocturnal gs and the dynamics of stomatal responseafter leaf severing in an O3-sensitive poplar clone (Oxford) subjected to combined treatments over agrowing season in an O3 free air controlled exposure (FACE) facility. The treatments were two soil N levels (0and 80 kg N ha-1; N0 and N80), three soil P levels (0, 40 and 80 kg P ha-1; P0, P40 and P80) and three O3 levels(ambient concentration, AA [35.0 ppb as hourly mean]; 1.5 × AA; 2.0 × AA). The analysis of stomatal dynamicsafter leaf severing suggested that O3 impaired stomatal closure execution. As a result, nocturnal gs was increasedby 2.0 × AA O3 in August (+39%) and September (+108%). Night-time gswas correlatedwith POD0 (phytotoxicO3 dose) and increased exponentially after 40 mmol m-2 POD0. Such increase of nocturnal gswas attributed tothe emission of ethylene due to 2.0 × AA O3 exposure, while foliar abscisic acid (ABA) or indole-3-acetic acid(IAA) did not affect gs at night. Interestingly, the O3-induced stomatal opening at night was limited by N treatmentsin August, but not limited in September. Phosphorus decreased nocturnal gs, although P did not modifythe O3-induced stomatal dysfunction. The results suggest that the increased nocturnal gs may be associatedwith a need to improve N acquisition to cope with O3 stress.