Ammonium Nutrition inRicinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps

Abstract

Allen, S. and Smith, J. A. C. 1986. Ammonium nutrition in Ricinus communis: its effect on plant growth 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 NO3-N were compared with plants raised on a range of NH4-N concentrations. At NH^-N concentrations between 0-5 and 4 0 mol m 3, freshand dry-weight yields of 62-d-old plants were not significantly different from those of the NOJ-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 NH? -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 NOJ-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 NO and carboxylate were replaced by Cl~ 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 m3 Cl in plants on 8 0 mol m3 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 CI 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. Key words—Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, phloem, amino acids, carboxylate. Correspondence to: Department of Biological Sciences, University of Dundee, Dundee DDI 4HN, U.K.