Because of highly intensive farming practices,crop plants could suffer relatively long terms of ammonium (NH4+) excess stress introduced by overdose application of nitrogen fertilizers.However,the lack of sufficient understanding of plant responses to NH4+ excess stress impairs the detection of effective solutions to this problem.The present work examined the biological influences of over-supplied NH4+ in Arabidopsis thaliana using two mutant lines each with an ammonium transporter (AMT) gene (AtAMT1;1 or AtAMT1;3) knocked out.Our results indicated that lacking one of the major components of root NH4+-absorbing systems significantly alleviated the toxicity effects on Arabidopsis plants by reducing the accumulation of free NH4+,suggesting that persistent absorption of NH4+ through AMT was the main cause of excessive accumulation of free NH4+ in the plants.Shading treatment led to a reduced transpirational driving force and thereby constrained the accumulation of toxic NH4+ in the plants,finally resulting in higher NH4+-promoted growth in the wild type (WT).Under the shading treatment,the amt1;1 and amt1;3 mutant plants acquired insufficient NH4+ and showed reduced growth when compared with the WT.Furthermore,the foliar application of sucrose notably alleviated the inhibitory effects on plant growth in the WT but had no effect on either the amt1;1 or amt1;3 mutant plants,indicating that carbon scarcity associated with NH4+ excess is probably a major cause of NH4+ toxicity in plants.Accordingly,increasing carbon source could be a potentially effective approach that alleviates the inhibition caused by NH4+ excess and increases nitrogen use efficiency under NH4+ over-supply.