The uptake of alkaloids by plants from the soil is determined by rhizosphere pH

• Published in: Rhizosphere Vol. 15; p. 100234
• Main Authors: Hijazin, Tahani, Radwan, Alzahraa, Lewerenz, Laura, Abouzeid, Sara, Selmar, Dirk
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2020
• Subjects: Alkaloidal contamination; Alkaloids; Horizontal natural product transfer; Membrane permeability; Uptake; Horizontal natural product transfer; Alkaloids; Alkaloidal contamination; Uptake; Membrane permeability
• ISSN: 2452-2198; 2452-2198
• DOI: 10.1016/j.rhisph.2020.100234

A tremendous high number of herbal products such as spices, herbal teas, or phytopharmaceuticals, contain significant amounts of toxic alkaloids. These widespread contaminations are mainly due to the uptake of alkaloids from the soil. Due to their alkalinity, it is obvious that the import of alkaloids is strongly affected by the pH of the rhizosphere. This so far unconsidered issue was analyzed in a model study by applying various alkaloids to hydroponically grown barley seedlings, adjusted to several pH values, i.e., 5, 6, 7, and 8. In most cases, the resulting concentration of the alkaloids imported into the barley leaves ranged from about 0.8 to 7.7 μmol/g d.w. However, caffeine was imported to a higher extent, resulting in concentration up to 19 μmol/g d.w. In contrast, the uptake of monocrotaline (0.04–0.13 μmol/g d.w.) and atropine (just traces) was quite lower. For sparteine, no uptake was recorded. Apart from caffeine, the uptake of alkaloids strongly depends on the pH in the medium. Accordingly, these experiments demonstrated the significance of the rhizosphere pH for the alkaloid uptake. Both parameters, the overall concentration of alkaloids as well as the share of the unprotonated alkaloids, are determined by the pH. Accordingly, for the first time, it became obvious that the alkaloidal contaminations of plant derived commodities significantly depend on the pH of the rhizosphere during the cultivation of the crop plants. Moreover, the extent of alkaloid accumulation in acceptor plants is also influenced by the rate of transpiration. In consequence, the uptake of the various alkaloids and its pH-dependency might be quite different and no general prediction on the extent of alkaloid uptake can be made. The uptake of alkaloids by plant roots, which is responsible for the widespread contaminations of plant-derived commodities, is strongly affected by the pH of the rhizosphere and the pka of the alkaloids. Both factors determine the ratio of protonated and unprotonated alkaloids, and thus their membrane permeability. [Display omitted] •Most alkaloids analyzed are taken up from the soil.•The extent of alkaloid import is strongly affected by the pH of the rhizosphere.•The alkaloid uptake is determined by the concentration of the unprotonated alkaloids.•In consequence, alkaloidal contaminations of plant products depend on the soil pH.