title: 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 (tau1), return to uptake (tau2), and a decrease in uptake rate (tau3). The presence of NO3- in the bathing medium prevented the tau1 release and also resulted in slower activation of the tau3 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.
authors.affiliation: School of Earth and Geographical Sciences, M087, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. olgab@cyllene.uwa.edu.au
authors.forename: Olga
authors.initials: O
authors.lastname: Babourina
authors.forename: Konstantin
authors.initials: K
authors.lastname: Voltchanskii
authors.forename: Bart
authors.initials: B
authors.lastname: McGann
authors.forename: Ian
authors.initials: I
authors.lastname: Newman
authors.forename: Zed
authors.initials: Z
authors.lastname: Rengel
mesh_terms: D029688:Brassica napus / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D017136:Ion Transport / Q000502:physiology; D009566:Nitrates / Q000378:metabolism* / Q000502:physiology; D018517:Plant Roots / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D011522:Protons; D000644:Quaternary Ammonium Compounds / Q000378:metabolism* / Q000633:toxicity
chemical_list: D009566:Nitrates; D011522:Protons; D000644:Quaternary Ammonium Compounds; D011188:Potassium; D002118:Calcium