Canopy stomatal uptake of NOX (NO, NO2), SO2 and O-3 by three mature urban plantations (of Schima superba, Eucalyptus citriodora and Acacia auriculaeformis) were studied using the sap flow-based approach under free atmospheric conditions. The annual mean concentration for NO, NO2, SO2 and O-3 were 18.2, 58.1, 12.8 and 42.4 mu g m(-3), respectively. The atmospheric concentration exhibited a spring or winter maximum for NO, NO2 and SO2, whereas the concentration maximum for O-3 occurred in the autumn. Despite the daytime mean canopy stomatal conductance (G(C)) being positively related with the photo-synthetically active radiation (PAR) and negatively with the vapour pressure deficit (VPD), the maximal daytime mean G(C) did not appear when the PAR was at its highest level or the VPD was at its lowest level because a positive correlation was noted between the daytime mean PAR and VPD (P < 0.001) under field conditions. The G(C) value was regulated by the cooperation of the PAR and VPD. When analysing the respective effect of the PAR or VPD on G(C) separately, a positive logarithmical correlation was noted between the daytime mean G(C) and PAR as the following equation: G(C) = a x lnPAR - b (P < 0.01), and the daytime mean G(C) was negatively logarithmically correlated with the VPD: G(C) = G(sref) - m x lnVPD (P < 0.001). The daytime mean G(C) declined with decreases in the soil water content (SWC) under similar meteorological condition. Differences in the seasonal pattern of the canopy stomatal conductance and atmospheric concentrations led to a differentiated peak flux. The flux for NO, NO2 and SO2 exhibited a spring maximum, whereas the flux maximum for O-3 appeared in the autumn or summer. The annual cumulative stomatal flux for NO, NO2, O-3 and SO2 was 100.19 +/- 3.76, 510.68 +/- 24.78, 748.59 +/- 52.81 and 151.98 +/- 9.33 mg m(-2) a(-1), respectively. When we focus on the foliar uptake of trace gases, the effect of these gases on the vegetation in turn should be considered, particularly for regions with serious air pollution problems. These trace gases had not yet reached injury levels, except for NO2. Flux-based measurements were better suited for evaluating the risk of O-3 damage to vegetation than the exposure-based method.

Sap flow; Canopy stomatal conductance; Environmental factors; Stomatal uptake

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