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 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 O3, N and P on night-time stomatal conductance (gs) are not known. We investigated the effects of O3 and soil availability of N and P on nocturnal gs and the dynamics of stomatal response after leaf severing in an O3-sensitive poplar clone (Oxford) subjected to combined treatments over a growing season in an O3 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 O3 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 O3 impaired stomatal closure execution. As a result, nocturnal gs was increased by 2.0 × AA O3 in August (+39%) and September (+108%). Night-time gs was correlated with POD0 (phytotoxic O3 dose) and increased exponentially after 40 mmol m-2 POD0. Such increase of nocturnal gs was attributed to the 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 treatments in August, but not limited in September. Phosphorus decreased nocturnal gs, although P did not modify the O3-induced stomatal dysfunction. The results suggest that the increased nocturnal gs may be associated with a need to improve N acquisition to cope with O3 stress.