Citric acid excretion and precipitation of calcium citrate in the rhizosphere of white lupin (<i>Lupinus albus</i> L.)

TLDR

Citric acid excretion in calcareous soils dissolves CaCO₃ and precipitates calcium citrate around proteoid roots. White lupin was cultivated for 13 weeks in a sterile, P‑deficient calcareous soil (20 % CaCO₃, pH 7.5) fertilized with nitrate. Proteoid roots, comprising about half the root dry weight, acidified the rhizosphere to pH 4.8, precipitating calcium citrate while reducing available P and increasing Fe, Mn, and Zn; the ~1 g plant⁻¹ citrate exudate (~23 % dry weight) indicates that white lupin releases citric acid, not citrate, to extract nutrients from a limited soil volume. Abstract.

Abstract

Abstract. White lupin ( Lupinus albus L.) was grown for 13 weeks in a phosphorus (P) deficient calcareous soil (20% CaCO 3 , pH(H 2 O)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 CaCO 3 and precipitation of calcium citrate in the zone of proteoid roots.