Influence of potassium substitution by rubidium and sodium on growth, ion accumulation, and ion partitioning in bean under high alkalinity

The effects of partial and complete substitution of potassium (K+) by rubidium (Rb+) and sodium (Na+) on plant growth and ion accumulation and partitioning was studied in bean young plants cultivated in nutrient solution with or without bicarbonate (HCO3-)-induced alkalinity. Plant growth was significantly decreased due to alkalinity and the substitution of K+, being leaves more affected than roots. Rubidium caused a severe toxicity reflected in a reduction in root dry mass and total chlorophyll concentration. Ion partitioning was markedly altered by alkalinity. Content of nitrogen (N), calcium (Ca), magnesium (Mg), iron (Fe), K, and Na were more accumulated in the roots in HCO3--treated plants, while decreased in the shoot. Iron (Fe) was accumulated at similar extent in plants with and without high alkalinity, except in plants grown in Rb+ solutions. However, Fe was more accumulated in the roots, suggesting that chlorophyll synthesis was impaired by reduced translocation or internal inactivation of Fe. Zinc total uptake was severely reduced under high alkalinity in plants grown in Na+ solutions, maybe due to decreased Zn activity. Calcium was translocated more actively to the leaves and Mg was accumulated more in the roots of plants in Na+solutions. Despite the severe decrease in plant dry mass caused by Rb+, there was a higher translocation of N, phosphorus (P), Ca, Mg, Fe, zinc (Zn), copper (Cu), and manganese (Mn) from the roots to the leaves.