Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (Lupinus albus L.) |
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Authors: | B DINKELAKER V RÖMHELD H MARSCHNER |
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Institution: | Institut für Pflanzenernährung, Universität Hohenheim, D-7000 Stuttgart 70, F.R.G. |
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Abstract: | Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO3, pH(H2O)7.5) which had been sterilized prior to planting and fertilized with nitrate as source of nitrogen. In response to P deficiency, proteoid roots developed which accounted for about 50% of the root dry weight. In the rhizosphere soil of the proteoid root zones, the pH dropped to 4.8 and abundant white precipitates became visible. X-ray spectroscopy and chemical analysis showed that these precipitates consisted of calcium citrate. The amount of citrate released as root exudate by 13-week-old plants was about 1 g plant?1, representing about 23% of the total plant dry weight at harvest. In the rhizosphere soil of the proteoid root zones the concentrations of available P decreased and of available Fe, Mn and Zn increased. The strong acidification of the rhizosphere and the cation/anion uptake ratio of the plants strongly suggests that proteoid roots of white lupin excrete citric acid, rather than citrate, into the rhizosphere leading to intensive chemical extraction of a limited soil volume. In a calcareous soil, citric acid excretion leads to dissolution of CaCO3 and precipitation of calcium citrate in the zone of proteoid roots. |
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Keywords: | Lupinus albus Fabaceae white lupin root exudates citric acid excretion calcium citrate precipitation phosphorus deficiency proteoid roots rhizosphere |
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