Whole shoots of Phaseolus vulgaris L. and other species were exposed to a range of partial pressures of gaseous ammonia in air and the resulting fluxes were measured. Net uptake is linear with partial pressure in the range 5 to 50 nanobars and is zero at a finite partial pressure, termed the ammonia compensation point. Below the compensation point, ammonia (or possibly other volatile amines) is evolved by the leaves. The compensation points in several species are near the low partial pressures found in unpolluted air and approximate to the Km of glutamine synthetase in vitro. In P. vulgaris L., the compensation point increases with temperature.Plants are regarded as sinks for atmospheric anunonia; Aneja (2) has reviewed this literature. This view stems largely from a number of experiments carried out at ammonia partial pressures in excess of those found naturally. However, Farquhar et al. (8) found that, at realistically low partial pressures (5 ± 3 nbar), no fluxes into or out of healthy leaves of Zea mays could be detected. An evolution of 0.6 nmol m-2 s-' was observed from leaves showing senescence. They inferred that, even in healthy leaves, a finite partial pressure of ammonia must exist in the substomatal cavities. This is consistent with the large number of reactions involving NH3/NH4+ in plant cells (6). An equation may be written for the molar flux density, J (nmol m-2 s-'), of ammonia into the leaf through stomata:where g (mol m-2 s-') is the conductance to diffusion of ammonia through stomata and the boundary layer surrounding the leaf, na (nbar) is the ambient partial pressure of ammonia (typically 1 to 8 nbar in unpolluted areas) (11), and P (bar) is the atmospheric pressure. Farquhar (6) The aerial portion of the plant was enclosed in a glass chamber, with a stainless steel base, measuring 64 x 64.5 x 49.5 cm. The chamber was sealed at the base of the stem using Terostat (Terosomwerke, GmbH, Heidelberg) to exclude the soil as a source of or sink for ammonia. The chamber was two-thirds surrounded by a bank of vertical fluorescent lights, and both the chamber and the lights were air-cooled with a 30-cm high-speed fan. The temperatures of two leaves were monitored using thermocouples constructed from 0.1-nmm copper and 0.13-mm constantan wires.Air temperature was varied using an electric heater in the chamber, and the air was mixed by two small fans. The lights were surrounded by reflective aluminum sheeting, and the irradiance on one side of a vertical plane in the chamber was 250 ,uE m-2 s-1.Air was drawn from outside the building through the chamber and into a 30-cm long, 1.8-cm diameter ammonia collector, similar to that of Denmead et al. (3), containing 38 cm3 of 3-mm diameter glass beads and 5 ml 0.1 N H2S04 with plugs of glass wool in the middle and at either end. Air was also drawn through a similar system in parallel but with no plant chamber interposed. The former collector tube is called the sample tube and the latter, the control tube. The flow rates into the collection tubes we...
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