NH₄ ⁺ toxicity in higher plants: A critical review

Ammonium (NH₄ ⁺) toxicity is an issue of global ecological and economic importance. In this review, we discuss the major themes of NH₄ ⁺ toxicity, including the occurrence of NH₄ ⁺in the biosphere, response differences to NH₄ ⁺ nutrition among wild and domesticated species, symptoms and proposed mechanisms underlying toxicity, and means by which it can be alleviated. Where possible, nitrate (NO₃ ^-) nutrition is used as point of comparison. Particular emphasis is placed on issues of cellular pH, ionic balance, relationships with carbon biochemistry, and bioenergetics of primary NH₄ ⁺ transport. Throughout, we attempt to identify areas that are controversial, and areas that are in need of further examination.