Nitrate supply affects ammonium transport in canola roots

Plants may suffer from ammonium (NH4 +) toxicity when NH4 + is the sole nitrogen source. Nitrate (NO 3 -) 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 (τ1), return to uptake (τ2), and a decrease in uptake rate (τ3). The presence of NO3 - in the bathing medium prevented the τ1 release and also resulted in slower activation of the τ3 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 + low-affinity influx system; and (ii) NH4 + transport is inversely linked to Ca2+ net flux. © The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.