Rubidium [Rb, 37]
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39.5 PLANTS
Rubidium is easily taken up by plants, similarly as other monovalent cations. The contents of available K control the Rb uptake. Increased bioavailability of Rb in acidic
soils (pH 3.6–5.0) is a secondary effect of leaching losses of K. There is a suggestion
that leaf Rb contents may be used as a method to assess early stage K deficiency in
plants on acidic soils (Drobner and Tyler vide Kabata-Pendias and Mukherjee 2007).
Rubidium may partly substitute for K sites in plants, but cannot substitute for K
metabolic roles. Thus, at higher concentrations, it is toxic to plants. However, in field
conditions, Rb toxicity is practically unknown.
Most of the higher plant species contain Rb within the range of 20–70 mg/kg.
Its contents in food plants vary highly from 3 to 15 mg/kg in cereal grains and lettuce
leaves, respectively. Higher Rb amounts are in fruits (mean in mg/kg): avocado (20)
and apple (50). Its highest content, at a mean value of 220  mg/kg, is reported for
soybean seeds. Plants from industrial regions contain always higher amounts of Rb.
Some fungi may accumulate increased amounts of Rb.
39.6 HUMANS
Rubidium is the most abundant element in the body (680 mg), which has no known
biological role (Emsley 2011). Concentrations of Rb in the blood (2.8  mg/L) and
serum (0.24  mg/L) are at the same range as concentrations of Zn and Cu, and it
shows a high Rb bioavailability from food (Barány et al. 2005). Rb mean level of
brain tissues is 5.49 mg/kg (Canavese et al. 2001). Its concentration in tissues reflects
Rb intake, and Rb depletion affects mineral (Na, K, P, Ca, Mg, Fe, Zn, and Cu) status (Yokoi et al. 1996). Rubidium is rapidly and highly absorbed and excreted by the
digestive tracts of mammals. It does not accumulate in any particular organ or tissue,
and normally is relatively low in bones.
Rubidium is not toxic and is removed relatively quickly in perspiration and urine.
However, taken in excess, it can be dangerous. The metabolism of Rb is closely
related to that of K, and they show interchangeability in a variety of biological systems, with little evidence for any toxicity. Homeostatic levels of Rb in the blood of
children (12 ± 3 mg/L) may suggest its essentiality (Lombeck et al. 1980). Studies in
animals have shown that Rb can be administered orally.
Biological interest in Rb has been stimulated by its close physicochemical relationship to K, and its presence in living tissues, at higher concentrations relatively to
those of K. It is higher than in the terrestrial environment. Relationships between K
and Rb have been found in a variety of physiological processes.
There is an evidence suggesting that Rb has a role involving some neurophysiological mechanism. In the heart, Rb is lower in conductive tissue than in adjacent
muscle tissue. Its content of brain differs significantly between defined functional
regions, and also decreases with age.
Some neurophysiological, neurochemical, and behavioral actions of Rb are opposite from those of Li. Rubidium has a disinhibiting action and has antidepressant