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JXB Advance Access published online on December 14, 2006
Journal of Experimental Botany, doi:10.1093/jxb/erl238
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© The Author [2006]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

Nitrate supply affects ammonium transport in canola roots

Olga Babourina1, Konstantin Voltchanskii1, Bart McGann1, Ian Newman2 and Zed Rengel1

1School of Earth and Geographical Sciences, M087, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009
2School of Mathematics and Physics, University of Tasmania, GPO Box 252-21, Hobart, Tasmania 7001

* To whom correspondence should be addressed. E-mail: olgab{at}cyllene.uwa.edu.au

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 ({tau}1), return to uptake ({tau}2), and a decrease in uptake rate ({tau}3). The presence of NO3– in the bathing medium prevented the {tau}1 release and also resulted in slower activation of the {tau}3 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.

Key words: Ammonium toxicity, Brassica napus, Ca2+, H+, ion fluxes, nitrate


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