Phenotype Assessment and Putative Mechanisms of Ammonium Toxicity to Plants

Ammonium (NH₄⁺) and nitrate (NO₃^-) are the primary inorganic nitrogen (N) sources that exert influence on plant growth and development. Nevertheless, when NH₄⁺ constitutes the sole or dominant N source, it can inhibit plant growth, a process also known as ammonium toxicity. Over multiple decades, researchers have shown increasing interest in the primary causes, mechanisms, and detoxification strategies of ammonium toxicity. Despite this progress, the current investigations into the mechanisms of ammonium toxicity remain equivocal. This review initially presents a comprehensive assessment of phenotypes induced by ammonium toxicity. Additionally, this review also recapitulates the existing mechanisms of ammonium toxicity, such as ion imbalance, disruption of the phytohormones homeostasis, ROS (reactive oxygen species) burst, energy expenditure, and rhizosphere acidification. We conclude that alterations in carbon-nitrogen (C-N) metabolism induced by high NH₄⁺ may be one of the main reasons for ammonium toxicity and that SnRK1 (Sucrose non-fermenting 1-related kinase) might be involved in this process. The insights proffered in this review will facilitate the exploration of NH₄⁺ tolerance mechanisms and the development of NH₄⁺-tolerant crops in agricultural industries.