K is required by plants for a wide range of metabolic processes and growth. When cells are limited for K, other monovalent cations, such as Rb+, may substitute for some of the biochemical and biophysical functions of K in the plant, thereby further reducing the organism's requirement for K. Studies have shown that the growth of sugar beets increased when Rb was supplemented to a K-deficient medium, although Rb can be toxic in excess (El-Sheikh et al., 1967). 

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K is also important for the growth of some archaeal halophiles. At low concentrations (8 mM) of K, the intracellular K level is reduced to ~1.4 M, and if Rb or Cs are available, these cations accumulate to levels of ~0.6 and 0.3 M, respectively, with a concomitant reduction in K (Jensen et al., 2015). This seems to present strong evidence for a conditionally beneficial role for both Rb and Cs, at least in this specific condition. 

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The effects of Rb have also been characterized in mammals. At the cellular level, Rb can displace a fraction of intracellular K, but the sum of the two ions remains relatively constant (Kirk et al., 1984). Replacement of K by Rb, and even Cs in rats, is dramatic in muscle tissue, where Rb or Cs can replace more than half of intracellular K (Relman et al., 1957).