Nitrogen Nutrition and Ammonium Toxicity in Higher Plants

The wide range of plant responses to ammonium (NH4+) nutrition can be used to study the way NH4+ interferes with plant metabolism and to assess some characteristics related with NH4+ tolerance by plants. 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 (NO3ˉ) 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.