Salt Uptake in Plants

Ion Absorption and Retention by Chlorella pyrenoidosa. III. Selective Accumulation of Rubidium, Potassium and Sodium

“…The selective order of the alkali metal cations found in the Chlorella system, Rb<K< <Na ( fig 2,3) is similar to the ranking observed in most animals, plants, and microorganisms (3,7,28). This ranking which follows the order of monovalent cations in Group I of the periodic table is not the only possible one and has been found to be partially or completely reversed in some organisms (2,28,29).…”

“…This ranking which follows the order of monovalent cations in Group I of the periodic table is not the only possible one and has been found to be partially or completely reversed in some organisms (2,28,29). A similar reversal of the selective order of monoovalent cations has also been observed in non-living systems and has been shown to depend on the nature of the anionic charge intensity, the geometry of the matrix holding together the fixed negative charges, and the hvdration of the svstem (6,10,16).…”

“…The separation factors for K -Na and Rb -Na exceed those found in better defined organic nonliving systems by a factor of 100 to 1000 and it is therefore not possible at the present to explain selectivity in many organisms without ad hoc assumptions (6,16,28,29). Regardless of the molecular nature of the ion discrimination system, one could state that for a cell system which accumulates ion X in preference to Y, the cell is in a lower energy state when containing X.…”

“…The predominant direction of the interchange process can be considered to be the reflection of the selective preference of the cell (6,7,13,15), since in systems permeable to the ions involved, such asymmetric movements are a prerequisite for the maintenance of a characteristic steady state cell composition (2,7,28,29).…”

“…Alternative explanations include sieve effects at the membrane involving ion size and charge (11,28,29), retention in the electrical field created by high concentrations of negatively charged cell constituents (6,13,15,16) as well as differential solubility of organometallic salts in the cytoplasm (21).…”