Comparison of calculated and measured foliar O3 flux in crop and forest species

We designed a new gas exchange system that concurrently measures foliar H2O, O-3, and CO2 flux (HOC flux system) while delivering known O-3 concentrations. Stomatal responses of three species were tested: snapbean, and seedlings of California black oak (deciduous broadleaf) and blue oak (evergreen broadleaf). Acute O-3 exposure (120-250 ppb over an hour) was applied under moderate light and low vapor pressure deficits during near steady state conditions. The rate of stomatal closure was measured when the whole plant was placed in the dark. An adjacent leaf on each plant was also concurrently measured in an O-3-free cuvette. Under some conditions, direct measurements and calculated foliar 03 flux were within the same order of magnitude; however, endogenously low gs or O-3 exposure-induced depression of gs resulted in an overestimation of calculated O-3 fluxes compared with measured O-3 fluxes. Sluggish stomata in response to light extinction with concurrent O-3 exposure, and incomplete stomatal closure likewise underestimated measured O-3 flux. (c) 2006 Elsevier Ltd. All rights reserved.