Nitrate supply affects ammonium transport in canola roots

Abstract

Plants may suffer from ammonium (NH4 +) toxicity when NH4 + is the sole nitrogen source. Nitrate (NO3 –) is known to alleviate NH4 + toxicity, but the mechanisms are unknown. This study has evaluated possible mechanisms of NO3 – alleviation of NH4 + toxicity in canola (Brassica napus L.). Dynamics of net fluxes of NH4 +, H+, K+ and Ca2+ were assessed, using a non-invasive microelectrode (MIFE) technique, in plants having different NO3 – supplies, after single or several subsequent increases in external NH4Cl concentration. After an increase in external NH4Cl without NO3 –, NH4 + net fluxes demonstrated three distinct stages: release (t1), return to uptake (t2), and a decrease in uptake rate (t3). The presence of NO3 – in the bathing medium prevented the t1 release and also resulted in slower activation of the t3 stage. Net fluxes of Ca2+ were in the opposite direction to NH4 + net fluxes, regardless of NO3 – supply. In contrast, H+ and K+ net fluxes and change in external pH were not correlated with NH4 + net fluxes. It is concluded that (i) NO3 – primarily affects the NH4 + lowaffinity influx system; and (ii) NH4 + transport is inversely linked to Ca2+ net flux.