Ammonia flux between oilseed rape plants and the atmosphere in response to changes in leaf temperature, light intensity, and air humidity - Interactions with leaf conductance and apoplastic NH4+ and H+ concentrations

NH3 exchange between oilseed rape (Brassica napus) plants and the atmosphere was examined at realistic ambient NH3 levels under controlled environmental conditions. Different leaf conductances to NH3 diffusion were obtained by changing leaf temperature (10 to 40 degrees C), light intensity (0 to 600 mu mol m(-2) s(-1)), and air humidity (20 to 80%), respectively. NH3 adsorption to the cuticle with subsequent NH3 transport through the epidermis had no significant effect on the uptake of atmospheric NH3, even at 80% relative air humidity. NH3 fluxes increased linearly with leaf conductance when light intensities were increased from 0 to 600 mu mol m(-2) s(-1). Increasing leaf temperatures from 10 to 35 degrees C caused an exponential increase in NH3 emission from plants exposed to low ambient NH3 concentrations, indicating that leaf conductance was not the only factor responding to the temperature increase. The exponential relationship between NH3 emission and temperature was closely matched by the temperature dependence of the mole fraction of gaseous NH3 above the leaf apoplast (NH3 compensation point), as calculated on the basis of NH4+ and H+ concentrations in the leaf apoplast at the different leaf temperatures. NH3 fumigation experiments showed that an increase in leaf temperature may cause a plant to switch from being a strong sink for atmospheric NH3 to being a significant NH3 source. In addition to leaf temperature, the size of the NH3 compensation point depended on plant N status and was related to plant ontogeny.