Ammonium nutrition in Ricinus communis: Its effect on plant growth and the chemical composition of the whole plant, xylem and phloem saps

Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinus communis: its effect on plantgrowth and the chemical composition of the whole plant, xylem and phloem saps.-J. exp. Bot. 37: 1599-1610.The growth and chemical composition of Ricinus communis cultivated hydroponically on 12 mol m - 3 NO3--N were compared with plants raised on a range of NH4+-N concentrations. At NH4+-N concentrations between 0·5 and 4·0 mol m-3, fresh- and dry-weight yields of 62-d-old plants were not significantly different from those of the NO3--N controls. Growth was reduced at 0·2 mol m-3 NH4+-N and was associated with increased root. shoot and C: organic N ratios, suggesting that the plants were N-limited. At 8·0 mol m-3 NH4+-N, growth was greatly restricted and the plants exhibited symptoms of severe 'NH4+ toxicity'. Plants growing on NH4+-N showed marked acidification of the rooting medium, this effect being greatest on media supporting the highest growth rates.Shoot carboxylate content per unit dry weight was lower at most NH4+-N concentrations than in the NO3--N controls, although it increased at the lowest NH4+-N levels. Root carboxylate content was comparable on the two N sources, but also increased substantially at the lowest NH4+-N levels. N source had little effect on inorganic-cation content at the whole-plant level, while NO3- and carboxylate were replaced by Cl{multimap} as the dominant anion in the NH4+-N plants. This was reflected in the ionic composition of the xylem and leaf-cell saps, the latter containing about 100 mol m-3 Cl- in plants on 8·0 mol m-3 NH4+. Xylem-sap organic-N concentration increased more than threefold with NH4+-N (with glutamine being the dominant compound irrespective of N source) while in leaf-cell sap it increased more than 12-fold on NH4+-N media (with arginine becoming the dominant species). In the phloem, N source had little or no effect on inorganic-cation, sucrose or organic-N concentrations or sap pH, but sap from NH4+-N plants contained high levels of Cl- and serine.Collectively, the results suggested that the toxic effects of high NH4+ concentrations were not the result of medium acidification, reduced inorganic-cation or carboxylate levels, or restricted carbohydrate availability, as is commonly supposed. Rather, NH4+ toxicity in R. communis is probably the result of changes in protein N turnover and impairment of the photorespiratory N cycle. © 1986 Oxford University Press.