Osmotic adjustments support growth of poplar cultured cells under high concentrations of carbohydrates.

KEY MESSAGE: Poplar callus maintained a specific difference in osmotic potential with respect to media when supplemented with different carbohydrate concentrations. This balance in osmotic potential guaranteed the growth capacity. Osmotic stress is caused by several abiotic factors such as drought, salinity, or freezing. However, the threshold of osmotic potential that allows the growth under stress conditions has not been thoroughly studied. In this study, different levels of osmotic stress in Populus alba (L.) callus have been induced with the addition of mannitol or sorbitol in the medium (from 0 to 500 mM). The key factor for preserving the growth was observed to be the restoration of a constant difference in osmotic potential between callus and medium for all the tested conditions. The osmotic adjustments were primarily achieved with the uptake of mannitol or sorbitol from the media considering their chemical properties instead of their biological functions. The decrease in water content (from - 1 to - 10% after 21 days) and mineral elements, such as potassium, calcium, and magnesium, together with the alterations in cell morphology, did not show negative effects on growth. The activity of sorbitol dehydrogenase was detected for the first time in poplar (+ 4.7 U l-1 in callus treated with sorbitol compared to control callus). This finding suggested the importance of choosing carefully the molecules used to exert osmotic stress for separating the dual function of carbohydrates in osmotic adjustments and cell metabolism.

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.