Ozone-induced impairment of night-time stomatal closure in O-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 (O) 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, N and P on night-time stomatal conductance (g) are not known. We investigated the effects of O and soil availability of N and P on nocturnal g and the dynamics of stomatal response after leaf severing in an O-sensitive poplar clone (Oxford) subjected to combined treatments over a growing season in an O free air controlled exposure (FACE) facility. The treatments were two soil N levels (0 and 80 kg N ha; N0 and N80), three soil P levels (0, 40 and 80 kg P ha; P0, P40 and P80) and three O 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 impaired stomatal closure execution. As a result, nocturnal g was increased by 2.0 × AA O in August (+39%) and September (+108%). Night-time g was correlated with POD (phytotoxic O dose) and increased exponentially after 40 mmol m POD. Such increase of nocturnal g was attributed to the emission of ethylene due to 2.0 × AA O exposure, while foliar abscisic acid (ABA) or indole-3-acetic acid (IAA) did not affect g at night. Interestingly, the O-induced stomatal opening at night was limited by N treatments in August, but not limited in September. Phosphorus decreased nocturnal g, although P did not modify the O-induced stomatal dysfunction. The results suggest that the increased nocturnal g may be associated with a need to improve N acquisition to cope with O stress.