The open-top chamber impact on vapour pressure deficit and its consequences for stomatal ozone uptake

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The vapour pressure deficit (VPD) in open-top chambers (OTCs) was analysed in relation to time of day and ambient meteorology. Effects of observed VPD differences (ΔVPD) between OTCs and the ambient air (AA) on stomatal conductance ( g s) were simulated using 10 model functions from the literature. The dataset originated from 17 OTC crop experiments performed in Belgium, Germany and Sweden. ΔVPD is the resulting difference between the OTC effect on e s( T), which is the temperature-dependent saturation pressure of water vapour and the OTC effect on e a, which is the prevailing partial pressure of water vapour in the air (ΔVPD = Δ e s( T) − Δ e a). Both Δ e s( T) and Δ e a were positive during daylight hours. ΔVPD was small in comparison and sensitive to changes in Δ e s( T) or Δ e a. ΔVPD was negative between 07:30 and 10:30 and positive thereafter with a maximum at 20:30 (local time). The positive afternoon ΔVPD was due to an early decrease in Δ e a, probably caused by ceased transpiration, while the positive Δ e s( T) persisted throughout the evening, most likely because of restrained cooling in the OTCs. Both the negative morning ΔVPD and the positive evening ΔVPD were more pronounced during clear, warm and dry weather. Circumstances when VPD had a stronger limiting effect on g s inside the OTCs compared to in the ambient air coincided with high ambient ozone concentrations ([O 3]). Calculated wheat O 3 uptake over an [O 3] threshold of 40 nmol mol −1 was reduced by 8.7% in OTCs, assuming that VPD was the only factor limiting g s and that g s was the only resistance for O 3 uptake. VPD is one factor of considerable importance for g s and the OTC impact on VPD may contribute to an underestimation of O 3 effects expressed in relation to the external O 3 exposure.