Effects of pH on ammonium uptake byTypha latifoliaL.

Abstract

ABSTRACTThe effects of solution pH on NH4+uptake kinetics and net H+extrusion byTypha latifoliaL. were studied during short‐term (days) and long‐term (weeks) exposure to pH in the range of pH 3.5–8.0. The NH4+uptake kinetics were estimated from depletion curves using a modified Michaelis‐Menten model.T. latifoliawas able to grow in solution culture with NH4+as the sole N source and to withstand a low medium pH for short periods (days). With prolonged exposure (weeks) to pH 3.5, however, the plants showed severe symptoms of stress and stopped growing. The solution pH affected NH4+uptake kinetics. The affinity for NH4+, as quantified by the half saturation constant (K1/2) andCmin(the NH4+concentration at which uptake ceases), decreased with pH.K1/2was increased from 7.1 to 19.2 mmol m−3andCminfrom 2.0 to 5.7 mmol m−3by lowering the pH in steps from 8.0 to 3.5.Vmaxwas, however, largely unaffected by pH (∼22 μmol h−1g−1root dry weight). Under prolonged exposure to constant pH, growth rates were highest at PH 5.0 and 6.5. At pH 8.0 growth was slightly depressed and at pH 3.5 growth completely stopped. NH4+uptake kinetics were similar at pH 5.0, 6.5 and 8.0 whereas at pH 3.5 NH4+uptake almost completely stopped. The ratio between net H+extrusion and NH4+uptake decreased significantly at low pH. The adverse effects of low pH on NH4+uptake kinetics are probably a consequence of a reduced H+‐ATPase activity and/or an increased re‐entry of H+at low pH, and the associated decrease in the electrochemical gradient across the plasma membranes of the root cells.