Net (apparent) photosynthesis rate (Pn) of jointed goatgrass (Aegilops cylindrica Host # AEGCY) leaves in the greenhouse became light saturated at a photosynthetic photon flux density (PPFD) of about 1000 μE·m–1-2·s–1 with a maximum Pn of 27 mg CO2 ·dm–2 ·h–1. Diffusive resistance to water vapor (rl) of adaxial leaf surfaces was 43% that of abaxial surfaces, in part, because stomatal density was 50% greater on adaxial leaf surfaces than on abaxial surfaces. Dark respiration rate (Rd) was 1.6 mg CO2 ·dm−2·h−1. Light compensation point (CPl) was 21 μE·m−2·s−1 and CO2 compensation point (CPc) was 32 ppmv. In the field, where light intensity and temperature were greater than in the greenhouse, leaves became light saturated for Pn at a higher intensity, and Rd and CPl were three times greater than in the greenhouse. Pn and Rd of spikes at anthesis were at least 30% less and 200% greater, respectively, than the values for leaves.

Photosynthetic productivity parameters were determined for mayweed chamomile, a troublesome annual weed of the cropping systems in the Pacific Northwest. At a photosynthetic photon flux density of 1800 μE m−2 s–1, maximum net photosynthetic rate of greenhouse-grown plants was 35 mg CO2 dm−2 h–1 and maximum transpiration rate was 6.7 μg H2O cm−2 s–1. Dark respiration rate was 1.4 mg CO2 dm−2 h–1 and the light compensation point was 17.5 μE m−2 s–1. Carbon dioxide compensation point increased from 25 ppm at 15 C to 43 ppm at 30 C. At saturating photosynthetic photon flux densities, optimum leaf temperature for net photosynthesis was about 25 C. Maximum net photosynthesis of leaves of field-grown plants averaged 15.8 mg CO2 dm−2 h–1. After a 24-h exposure to 0.075 kg ha–1 metribuzin, maximum net photosynthesis and transpiration were reduced 85 and 40%, respectively. Soil water deficits reduced maximum net photosynthesis about 50%.

Gas exchange of jointed goatgrass leaves was affected by temperature, irradiance level, and soil matric potential. Net photosynthesis of leaves under saturating irradiance (PPFD3 = 1850 (μE·m–2·s−1) was optimum at about 20 C. At 25 C, net photosynthesis was nearly 90% of maximum at a PPFD of 800 μE·m–2·−1. Transpiration, and presumably water use, increased steadily with temperature from 10 to 40 C. Dark respiration rate and compensation points for light and for CO2 increased exponentially, or nearly so, from 10 to 40 C. Soil moisture deficits of −130 kPa reduced net photosynthesis and transpiration by about 30 and 55%, respectively, compared to well-watered plants.

The dynamic-chamber technique was used to investigate the correlation between NH3 and NO2 fluxes and different climatic and physiological parameters: air temperature; relative air humidity; photosynthetic photon fluence rate; NH3 and NO2 concentrations; transpiration rate; leaf conductance for water vapour; and photosynthetic activity. The experiments were performed with twigs from the sun crown of mature beech trees (Fagus sylvatica) at a field site (Höglwald, Germany), and with 12-wk-old beech seedlings under controlled conditions. Both sets of experiments showed that NO2 and NH3 fluxes depended linearly on NO2 and NH3 concentration, respectively, in the concentration ranges representative for the field site studied, and on water-vapour conductance as a measure for stomatal aperture. The NO2 compensation point determined in the field studies (the atmospheric NO2 concentration with no net NO2 flux) was 1.8–1.9 nmol mol−1. The NH3 compensation point varied between 3.3 and 3.5 nmol mol−1 in the field experiments, and was 3.0 nmol mol−1 in the experiments under controlled conditions. The climatic factors T and PPFR were found to influence both NO2 and NH3 fluxes indirectly, by changing stomatal conductance. Whilst NO2 flux showed a response to changing relative humidity that could be explained by altered stomatal conductance, increased NH3 flux with increasing relative humidity (>50%) depended on other factors. The exchange of NO2 between above-ground parts of beech trees and the atmosphere could be explained exclusively by uptake or emission of NO2 through the stomata, as indicated by the quotient between measured and predicted NO2 conductance of approx. 1 under all environmental conditions examined. Neither internal mesophyll resistances nor additional sinks could be observed for adult trees or for beech seedlings. By contrast, the patterns of NH3 flux could not be explained by an exclusive exchange of NH3 through the stomata. Deposition into additional sinks on the leaf surface, as indicated by an increase in the quotient between measured and predicted NH3 conductance, gained importance in high air humidity, when the stomata were closed or nearly closed and/or when atmospheric NH3 concentrations were high. Although patterns of NH3 gas exchange did not differ between different months or years at high NH3 concentrations (c. 140 nmol mol−1), it must be assumed that emission or deposition fluxes at low ambient NH3 concentration (0.8 and 4.5 nmol mol−1) might vary significantly with time because of variation in the NH3 compensation point.

The global emission of ammonia (NH3) is about 54 Mt N. The major global sources are excreta from domestic animals and fertilizers, but oceans, biomass burning and crops are also important. About 60% of the global NH3 emission is estimated to come from anthropogenic sources. NH3-N emissions are of the same order as the NOx-N emissions on both global and European scales. Emitted NH3 returns to the surface mainly in the form of dry deposition of NH3 and wet deposition of ammonium (NH4+). In countries with high NH3 emission densities, dry deposition of NH3 from local sources and wet deposition of NH4+ from remote sources dominate the deposition. In countries with low NH3 emission densities only wet deposition of NH4+ from remote sources dominates the deposition. Surface exchange of NH3 is essentially bi-directional, depending on the NH3 compensation point concentration of the vegetation and the airborne concentration. In general, the compensation point is larger for agricultural than semi-natural plants, and varies with plant growth stage. According to basic thermodynamics the leaf tissue or stomatal compensation point of NH3 doubles for each increase of 5°C. However, exchange of NH3 does not only occur through the stomata, but it can also be deposited to leaf surfaces, as well as emitted back to the atmosphere from drying leaf surfaces. Atmospheric transport and deposition models can be used to interpolate NH3 concentrations and depositions in space and time, to calculate import/export balances and to estimate past or future situations. Adverse effects on sensitive ecosystems caused by high N deposition can be reduced by lowering the emissions and, to a limited extent, also by removing sources close to the ecosystem to be protected.