Transport of Methylammonium and Ammonium Ions byElodea densa

Abstract

Excised leaves of Elodea densa rapidly absorb methylamine1 from dilute solutions (up to 2-0 mM). The influx isotherm is hyperbolic, with a of approximately 160 /?M. Influx is reduced following transfer of leaves from light to darkness, and at low temperature. Low concentrations of ammonia reduce the influx greatly, apparently by competition between NH} and CH3NH}, but K+ and Na+ have little effect, nor has removal of Cl~. Influx is very insensitive to external pH over the range 5-0 to 9-0, with usually a small increase between pH 9-0 and 10-0. When leaves are pretreated in solutions containing nitrogenous compounds subsequent influx can be decreased (by ammonia), unchanged (by methylamine) or even increased (by arginine, proline and imidazol). Influx of methylamine and ammonia lowers influx of K+ (Rb+) and of CI and increases efflux of K+ into solutions initially free of K+. Fluxes of Ca++ are not affected and there is net efflux of H+ into unbuffered solutions. The results show that uptake of methylamine and ammonia under these conditions is primarily by transport (uniport) of CH3NH} and NH{ and that diffusion of CH3NH2 and NH3 is insignificant. In Elodea, unlike some of the plants that have been previously studied, maintenance of charge-balance during transport of CH3NH} and NH} appears to involve accumulation of organic acid anions.