Physiological regulation of plant-atmosphere ammonia exchange

Plants have a compensation point for NH(3) which ranges from 0.1 to 20 nmol mol(-1), and may be several-fold higher or lower than naturally occurring atmospheric NH(3) concentrations. This implies that NH(3) 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 NH(3) fluxes is mediated via processes involved in nitrogen uptake, transport and metabolism. A rapid turnover of NH(4)(+) in plant leaves leads to the establishment of a finite NH(4)(+) concentration in the leaf apoplastic solution. This concentration determines, together with that of H(+), the size of the NH(3) compensation point. Barley and oilseed rape plants with access to NH(4)(+) in the root medium have higher apoplastic NH(4)(+) concentrations than plants absorbing NO(3)(-). Furthermore, the apoplastic NH(4)(+) concentration increases with the external NH(4)(+) concentration. Inhibition of GS leads to a rapid and substantial increase in apoplastic NH(4)(+) and barley mutants with reduced GS activity have higher apoplastic NH(4)(+) than wild-type plants. Increasing rates of photorespiration do not affect the steady-state NH(4)(+) or H(+) concentration in tissue or apoplast of oilseed rape, indicating that the NH(4)(+) produced is assimilated efficiently. Nevertheless, NH(3) emission increases due to a temperature-mediated displacement of the chemical equilibrium between gaseous and aqueous NH(3) in the apoplast. Sugarbeet plants grown with NO(3)(-) 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 NH(4)(+) liberated in photorespiration to be assimilated during darkness when 2-oxoglutarate is supplied in high rates by respiration.