The Inhibition of Phloem Translocation by Ammonia

Bean plants (Phaseolus vulgaris L. cv. Fardenlosa Shiny) were labelled with carbon-11 via their first trifoliate leaves when 3-weeks-old and the transient inhibitions of translocation caused by the application of ammonium chloride solutions (10 mol m−3) to a peeled region of stem were studied. At pH 6·5 the ammonium was without effect. At pH 11·0 even a brief application inhibited translocation for many minutes, while longer applications inhibited translocation for considerably longer. Solutions of 10 mol m−3 sodium chloride were without effect at either pH. At pH 6·5 ammonium chloride solution contains predominantly ammonium ions (NH4+) and at pH 11·0 predominantly dissolved ammonia gas (NH3). Hence we conclude that phloem transport within bean stems is inhibited by dissolved ammonia gas but not ammonium ions.