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

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 mu mol/g d.w. However, caffeine was imported to a higher extent, resulting in concentration up to 19 mu mol/g d.w. In contrast, the uptake of monocrotaline (0.04-0.13 mu mol/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.