Review NH4 toxicity in higher plants: a critical review

Review NH4 toxicity in higher plants: a critical review

18 Pages · 2002 · 181 KB · English Review. NH4. + toxicity in higher plants: a critical review. Dev T. Britto, Herbert J. Kronzucker*. Division of Life Sciences, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada. Received December 14, 2001 · Accepted February 2

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J Plant Physiol 159 567 – 584 (2002) Urban & Fischer Verlag http://wwwurbanfischerde/journals/jpp Review NH 4+ toxicity in higher plants: a critical review Dev T Britto, Herbert J Kronzucker* Division of Life Sciences, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A 4, Canada Received December 14, 2001 · Accepted February 22, 2002 Abstract 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 pro posed 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 I Introduction Ammonium (NH 4+) is a paradoxical nutrient ion in that, al though it is a major nitrogen (N) source whose oxidation state eliminates the need for its reduction in the plant cell (Salsac et al 1987), and although it is an intermediate in many meta bolic reactions (Joy 1988), it can result in toxicity symptoms in many, if not all, plants when cultured on NH 4+ as the ex clusive N source (Vines and Wedding 1960, Givan 1979, van der Eerden 1982, Fangmeier et al 1994, Gerendas et al 1997) Observations of NH 4+ toxicity to plants were made at least as early as 1882, when Charles Darwin described NH 4+ induced growth inhibition inEuphorbia peplus(cited in Schenk and Wehrmann 1979) Sensitivity to NH 4+ may be a * Email corresponding author: [email protected] universal biological phenomenon, as it has also been ob served in many animal systems (Petit et al 1990, Kosenko et al 1991, 1995, Tremblay and Bradley 1992, Gardner et al 1994), including humans, where it has been implicated in par ticular in neurological disorders (Marcaida et al 1992, Mira bet et al 1997, Butterworth 1998, Haghighat et al 2000, Mur thy et al 2000), and also in insulin disorders (Sener and Ma laisse 1980) Many research efforts have been directed to ward unraveling the causes and mechanisms of NH 4+ toxicity in plants, and while present knowledge is far from complete, a more comprehensive understanding of this phenomenon is beginning to emerge This review will present key findings from this extensive body of work, with special focus on more recent developments in the field, and on nitrate (NO 3–) nutri tion as a point of comparison In addition, we offer clarifica tion of central issues that have been clouded by speculation in the past, and identify several critical areas for further re search 0176 1617/ 0 2 /159/06567 $ 1500/0 568 Dev T Britto, Herbert J Kronzucker II Ecology of NH 4+ toxicity 1 N H 4+ in the biosphere Nitrogen concentrations in soil solution can range over sev eral orders of magnitude (Jackson and Caldwell 1993, Nes doly and Van Rees 1998) In many natural and agricultural ecosystems, NH 4+ is the predominant N source (Vitousek et al 1982, Blew and Parkinson 1993, Pearson and Stewart 1993, van Cleve et al 1993, Bijlsma et al 2000), and is almost always present to some extent in the majority of ecosystems For instance, a survey of boreal and temperate forest ecosys tems shows forestfloor soil solution [NH 4+] values ranging from approximately 04 to 4 mmol/L [NH 4+], with a mean value of 2 mmol/L (based on Vitousek et al 1982, see also Bijlsma et al 2000) In agricultural soils, [NH 4+] can be even higher, often ranging from 2 to 20 mmol/L (Wolt 1994) The relative abundance of NH 4+ compared to NO 3– in soil solution is determined by a number of factors, of which the accumula tion of organic matter, soil pH, soil temperature, the presence of allelopathic chemicals, and soil oxygenation status are the most important (Rice and Pancholy 1972, Haynes and Goh 1978, Lodhi 1978, Dijk and Eck 1995) Typically, low pH, low temperature, accumulation of phenolicbased allelopathic compounds, and poor oxygen supply

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