The mechanism of NH3 and SO2 uptake by leaves and its physiological effects = [Het mechanisme van de NH3 en SO2 opname door bladeren en hierdoor veroorzaakte fysiologische effecten]

The relation between uptake of atmospheric ammonia (NH _{<font size="-2">3</font>} ) and sulphur dioxide (SO _{<font size="-2">2</font>} ) by individual leaves, photosynthesis and stomatal conductance was examined. The experiments were carried out with bean plants ( <em>Phaseolus vulgaris L.</em> ) and poplar shoots ( <em>Populus euramericana L.</em> ). The method of analysis was derived from methods used in photosynthetic research. The uptake of NH _{<font size="-2">3</font>} or SO _{<font size="-2">2</font>} was experimentally determined by using a leaf chamber specially developed for this research. Simultaneously, transpiration and carbondioxide (CO _{<font size="-2">2</font>} ) assimilation of leaves were measured.The adsorption of NH _{<font size="-2">3</font>} and SO _{<font size="-2">2</font>} strongly increased with increasing air humidity, indicating a major role of water in the adsorption process. A descriptive model for the adsorption in the cuticle-watersystem is proposed. The affinity of SO _{<font size="-2">2</font>} for the leaf surface was found to be approximately twice that of NH _{<font size="-2">3</font>} . A mixture of these gases in the air mutually stimulated their adsorption on the leaf surface. No significant desorption or transport of these gases through the cuticle could be detected.The uptake of NH _{<font size="-2">3</font>} into leaves appeared to be dependent on the leaf boundary layer and stomatal resistance and NH _{<font size="-2">3</font>} concentration at the leaf surface. In contrast, a less clear relation between SO _{<font size="-2">2</font>} uptake and stomatal resistance was found, in particular at a low vapor pressure deficit (VPD). The measured flux was larger than can be calculated from the boundary layer and stomatal resistance for H _{<font size="-2">2</font>} O, suggesting a lower resistance of the diffusion pathway. The same was observed for NH _{<font size="-2">3</font>} at a low temperature and VPD. It is postulated that this discrepancy is due to a difference in path length.Under the conditions of the present research the physiological effects caused by a prolonged exposure to NH _{<font size="-2">3</font>} or SO _{<font size="-2">2</font>} became notable at concentrations of about 100 μg.m <sup><font size="-2">-3</font></SUP>. The NH _{<font size="-2">3</font>} exposure had a positive effect on photosynthesis, stomatal conductance and NH _{<font size="-2">3</font>} uptake, whereas a small irreversible inhibition of photosynthesis and stomatal conductance was induced by the SO _{<font size="-2">2</font>} exposure.The relations assessed in this study can be used to construct a descriptive model for NH _{<font size="-2">3</font>} and SO _{<font size="-2">2</font>} transfer into leaves as a function of wind velocity, light intensity, air temperature and humidity.