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

Abstract Nocturnal transpiration may be a key factor influencing water use in plants. Tropospheric ozone (O 3 ) and availability of nutrients such as nitrogen (N) and phosphorus (P) in the soil can affect daytime water use through stomata, but the combined effects of O 3 , N and P on night-time stomatal conductance ( g s ) are not known. We investigated the effects of O 3 and soil availability of N and P on nocturnal g s and the dynamics of stomatal response after leaf severing in an O 3 -sensitive poplar clone (Oxford) subjected to combined treatments over a growing season in an O 3 free air controlled exposure (FACE) facility. The treatments were two soil N levels (0 and 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 O 3 levels (ambient concentration, AA [35.0 ppb as hourly mean]; 1.5 × AA; 2.0 × AA). The analysis of stomatal dynamics after leaf severing suggested that O 3 impaired stomatal closure execution. As a result, nocturnal g s was increased by 2.0 × AA O 3 in August (+39%) and September (+108%). Night-time g s was correlated with POD 0 (phytotoxic O 3 dose) and increased exponentially after 40 mmol m −2 POD 0 . Such increase of nocturnal g s was attributed to the emission of ethylene due to 2.0 × AA O 3 exposure, while foliar abscisic acid (ABA) or indole-3-acetic acid (IAA) did not affect g s at night. Interestingly, the O 3 -induced stomatal opening at night was limited by N treatments in August, but not limited in September. Phosphorus decreased nocturnal g s , although P did not modify the O 3 -induced stomatal dysfunction. The results suggest that the increased nocturnal g s may be associated with a need to improve N acquisition to cope with O 3 stress.