Physiological parameters controlling plant-atmosphere ammonia exchange

Recent advances in characterizing the influence of different physiological and environmental parameters on NH3 exchange between plants and the atmosphere are presented. A central parameter in controlling the rate and direction of NH3 fluxes is the NH3 compensation point. It may vary from below 1 to over 20 nmol NH3 mol(-1) air. High compensation points seem to be a result of high tissue N status, rapid absorption of NH4+ from the root medium and/or low activity of glutamine synthetase, a key enzyme in NH4+ assimilation. These conditions cause the NH4+ concentration in leaf apoplast and leaf cells to increase. The NH3 compensation point also depends on plant developmental stage with peaks in NH3 emission related to leaf senescence and N remobilization. The leaf temperature has a profound influence on the NH3 compensation point: an increase in temperature from 15 to 30 degrees C may cause a plant to switch from being a strong sink for atmospheric NH3 to being a significant NH3 source. Stomatal conductance for NH3 relative to that of water vapour increases with tissue N status and with leaf senescence. At a given leaf temperature, the NH3 compensation point can be successfully predicted on basis of the pH and NH4+ concentration in the apoplast of the mesophyll cells. (C) 1998 Elsevier Science Ltd. All rights reserved.