Adaptation to rhizosphere acidification is a necessary prerequisite for wheat (Triticum aestivum L.) seedling resistance to ammonium stress

Abstract

Because soil acidification accompanies ammonium (NH4+) stress, the tolerance of higher plants to ammonium is associated with their adaptation to root medium acidification. However, the underlying mechanisms of this adaptation have not been fully elucidated. The objective of this study was thus to elucidate the effect of rhizosphere pH on NH4+ tolerance in different winter wheat cultivars (Triticum aestivum L.). Hydroponic experiments were carried out on two wheat cultivars: AK58 (an NH4+-sensitive cultivar) and XM25 (an NH4+-tolerant cultivar). Four pH levels resembling acidified (4.0, 5.0, 6.0 and 7.0) were tested and 5 mM NH4+ nitrogen (AN) was used as a stress treatment, with 5 mM nitrate nitrogen used as a control. The addition of AN led to a severe reduction in biomass and an increase in free NH4+, amino acids, and the activities of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in the shoots and roots of the two wheat cultivars. Further decreases in growth medium pH led to further increases in free NH4+, but decreases in total amino acids and the activities of GS and NADH-dependent glutamate synthase (NADH-GDH). However, there was less of an increase in free NH4+ and less of a reduction in the activities of GS and NADH-GDH in the cultivar XM25 compared with AK58. In addition, total soluble sugar content and the root-to-shoot soluble sugar ratio were also decreased by AN treatment, except in the shoots of XM25. Decreasing pH resulted in lower root-to-shoot soluble sugar ratios with greater reductions in the AK58 cultivar. These results indicate that wheat growth was inhibited significantly by the addition of NH4+ combined with low pH. Low medium pH reduced the capacity for nitrogen assimilation and interrupted carbohydrate transport between the shoot and root. The NH4+-tolerant cultivar XM25 was better adapted to low rhizosphere pH due to its increased capacity for assimilating NH4+ efficiently and thereby avoiding toxic levels of intracellular NH4+ at low medium pH.