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

• 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 (gs) 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 es(T), which is the temperature-dependent saturation pressure of water vapour and the OTC effect on ea, which is the prevailing partial pressure of water vapour in the air (ΔVPD = Δes(T) − Δea). Both Δes(T) and Δea were positive during daylight hours. ΔVPD was small in comparison and sensitive to changes in Δes(T) or Δea. 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 Δea, probably caused by ceased transpiration, while the positive Δes(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 gs inside the OTCs compared to in the ambient air coincided with high ambient ozone concentrations ([O3]). Calculated wheat O3 uptake over an [O3] threshold of 40 nmol mol−1 was reduced by 8.7% in OTCs, assuming that VPD was the only factor limiting gs and that gs was the only resistance for O3 uptake. VPD is one factor of considerable importance for gs and the OTC impact on VPD may contribute to an underestimation of O3 effects expressed in relation to the external O3 exposure.

Subject headings

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Jordbruksvetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Agricultural Science (hsv//eng)

NATURVETENSKAP  -- Biologi -- Botanik (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Botany (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Meteorologi och atmosfärforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Meteorology and Atmospheric Sciences (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

Keyword

open-top chamber

ozone ozone index

ozone uptake

risk assessment

vapour pressure deficit

Open-top chamber

Publication and Content Type

ref (subject category)

art (subject category)