Rapid ammonia gas transport accounts for futile transmembrane cycling under NH3/NH4 toxicity in plant roots

Abstract

Futile transmembrane NH3/NH4 + cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked to NH3/NH4 + toxicity. Surprisingly, the fundamental question of which species of the conjugate pair (NH3 or NH4 +) participates in such fluxes is unresolved. Using flux analyses with the short-lived radioisotope 13N and electrophysiological, respiratory, and histochemical measurements, we show that futile cycling in roots of barley (Hordeum vulgare) seedlings is predominately of the gaseous NH3 species, rather than the NH4 + ion. Influx of 13NH3/13NH4 +, which exceeded 200 μmol g-1 h-1, was not commensurate with membrane depolariz..