OZONE DEPOSITION TO A COTTON (GOSSYPIUM-HIRSUTUM L) FIELD - STOMATAL AND SURFACE WETNESS EFFECTS DURING THE CALIFORNIA OZONE DEPOSITION EXPERIMENT

Removal of tropospheric ozone from polluted airbasins by deposition to vegetation may be an important determinant of regional air quality. T he physiological and physical processes that determine the magnitude o f deposition, and the relative contributions of uptake by photosynthet ic tissues versus ozone destruction on plant and soil surfaces, are no t well understood. The California Ozone Deposition Experiment (CODE) d emonstrated substantial deposition of ozone to several vegetated surfa ces in the San Joaquin Valley. Using data collected during CODE, we ex amine the roles of stomatal conductance and leaf wetness from dew in m ediating ozone deposition to an extensive field of irrigated cotton. S tomatal conductance, photon flux density, leaf area index (L), leaf we tness, ozone deposition, and canopy photon extinction coefficient (K) were measured. Single leaf measurements were scaled to canopy values o f stomatal conductance to ozone (g(c)). Deposition velocity (V-d) and surface conductance (g(surf)) were strongly positively correlated with g(c). Under dry canopy conditions g(c) < g(surf) indicating a signifi cant residual conductance of a non-stomatal pathway for ozone depositi on (g(r)), possibly reflecting reaction of O-3 with nitric oxide emitt ed from fertilized soil. Dewfall reduced ozone deposition and eliminat ed g(r). However, g(r) may actually have been increased by dew in amph istomatous cotton, as it was in hypostomatous grape during CODE. If so , canopy wetness reduced the stomatal pathway (g(c)) by occlusion of a daxial pores sufficiently to offset both the nitric oxide titration an d the true wetness enhancement of g(r). We conclude that ozone deposit ion to cotton is largely controlled by stomatal responses, Stomatal re sponses may readily be modelled, potentially providing sufficient info rmation to infer ozone deposition, In contrast to the case for hyposto matous grape, ozone uptake (particularly by the stomatal pathway) is r educed by leaf surface wetness in amphistomatous cotton. Alternative m odels of single leaf stomatal conductance and expected errors of +/- 2 0% in model parameters did not affect these conclusions. (C) 1997 Else vier Science B.V.