Physiological regulation of plant-atmosphere ammonia exchange

Abstract

Plants have a compensation point for NH3 which ranges from 0.1 to 20 nmol mol-1, and may be several-fold higher or lower than naturally occurring atmospheric NH3 concentrations. This implies that NH3 fluxes over vegetated surfaces are bi-directional and that ammonia exchange with the atmosphere in many cases contributes significantly to the nitrogen economy of vegetation. Physiological regulation of plant–atmosphere NH3 fluxes is mediated via processes involved in nitrogen uptake, transport and metabolism. A rapid turnover of NH3 + in plant leaves leads to the establishment of a finite NH3 + concentration in the leaf apoplastic solution. This concentration determines, together with that of H+, the size of the NH3 compensation point. Barley and oilseed rape plants with access to NH3 + in the root medium have higher apoplastic NH3 + concentrations than plants absorbing NO3 -. Furthermore, the apoplastic NH3 + concentration increases with the external NH3 + concentration. Inhibition of GS leads to a rapid and substantial increase in apoplastic NH3 + and barley mutants with reduced GS activity have higher apoplastic NH3 + than wild-type plants. Increasing rates of photorespiration do not affect the steady-state NH3 + or H+ concentration in tissue or apoplast of oilseed rape, indicating that the NH3 + produced is assimilated efficiently. Nevertheless, NH3 emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH3 in the apoplast. Sugarbeet plants grown with NO3 - seem to be temporarily C-limited in the light due to a repression of respiration. As a consequence, the activity of chloroplastic GS declines during the day causing a major part of NH3 + liberated in photorespiration to be assimilated during darkness when 2-oxoglutarate is supplied in high rates by respiration.