Ozone uptake in the sun and shade crown of spruce: quantifying the physiological effects of ozone exposure

Summary The uptake of air pollutants depends both on pollutant concentration and on stomatal conductance. This paper deals with the uptake of ozone (O₃) from the air into the needles of Norway spruce [Picea abies (L.) Karst.] under ambient climatic conditions. Regulation of O₃ uptake by the stomata is shown and also the difference between the physiologically active O₃ concentration and the O₃ concentration of the ambient air. Data from the sun and shade crown of spruce trees at 1000 m a.s.l. are presented. Analysis of data from three vegetation periods has shown that at low ambient O₃ concentrations the O₃ uptake is largely regulated by stomatal conductance. Water vapour pressure deficit (VPD) of the atmosphere is the climatic factor which showed the highest positive correlation with O₃ concentration. However, a high leaf-air VDP led to stomatal closure, thus reducing the O₃ uptake in the needles despite high O₃ concentrations in the ambient air. The potential O₃ stress caused by high O₃ concentrations can be strongly mitigated by this natural closing of the stomata and the simultaneous occurrence of moderate drought stress.