The alleviating effect of ammonium on aluminum toxicity in Lespedeza bicolor results in decreased aluminum-induced malate secretion from roots compared with nitrate

Abstract

NH 4 + and NO 3 − supply often affect aluminum (Al) toxicity in plants growing in acidic soils. Lespedeza bicolor is a leguminous shrub well adapted to acid infertile soils. Secretion of organic acid anions has been demonstrated to be the main mechanism responsible for its Al resistance. The objective of this study was to investigate how NH 4 + and NO 3 − supply affect Al toxicity in L. bicolor and whether Al-induced organic acid anion secretion is involved in this effect. Long-term (36 d) Al treatment increased L. bicolor root and shoot growth in a NH 4 + medium. NH 4 + treatment also decreased Al, Ca, Mg, Fe, and Mn contents in the roots of L. bicolor compared with NO 3 − treatment. After short-term (24 h) Al treatment, L. bicolor seedlings had higher absolute root elongation and relative root elongation, and lower Al contents in root tips with NH 4 + treatment than with NO 3 − treatment. In vitro Al adsorption kinetics experiment with isolated root cell wall showed that the accumulative amount of Al adsorbed in root cell walls was lower with NH 4 + treatment than with NO 3 − treatment. The roots of L. bicolor seedlings subjected to Al stress secreted less malate with NH 4 + treatment than with NO 3 − treatment, whereas there was no significant difference in the malate content of L. bicolor roots between the treatments after exposure to Al. Taken together, these results show that NH 4 + alleviated the toxicity of Al in L. bicolor in solution culture compared with NO 3 − , and this alleviating effect further resulted in decreased Al-induced malate secretion from L. bicolor roots.