NH4+ toxicity in higher plants: a critical review

Summary Ammonium (NH 4 + ) toxicity is an issue of global ecological and economic importance. In this review, we discuss the major themes of NH 4 + toxicity, including the occurrence of NH 4 + in the biosphere, response differences to NH 4 + nutrition among wild and domesticated species, symptoms and proposed mechanisms underlying toxicity, and means by which it can be alleviated. Where possible, nitrate (NO 3 − ) 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 NH 4 + transport. Throughout, we attempt to identify areas that are controversial, and areas that are in need of further examination.