Chloride in Soils and its Uptake and Movement within the Plant: A Review

Natural inputs of chlorine (Cl) to soils come mainly from rainwater, sea spray, dust and air pollution. In addition, human practices, such as irrigation and fertilization, contribute significantly to Cl deposition. In the soil solution, Cl occurs predominantly as the chloride anion (Cl~). The Cl~ anion does not form complexes readily, and shows little affinity (or specificity) in its adsorption to soil components. Thus, Cl~ movement within the soil is largely determined by water flows. Chlorine is an essential micronutrient for higher plants. It is present mainly as Cl~. Chloride is a major osmotically active solute in the vacuole and is involved in both turgorand osmoregulation. In the cytoplasm it may regulate the activities of key enzymes. In addition, Cl~ also acts as a counter anion, and Cl~ fluxes are implicated in the stabilization of membrane potential, regulation of intracellular pH gradients and electrical excitability. Chloride enters plants through the roots, and there is some concern over the uptake of the long-lived radionuclide 'CI, which enters into the food chain through plants. Chloride is thought to traverse the root by a symplastic pathway, and Cl~ fluxes across the plasma membrane and tonoplast of root cells have been estimated. These fluxes are regulated by the Cl" content of the root. Chloride is mobile within the plant. The Cl~ concentrations of xylem and phloem saps have been determined and Cl~ fluxes through the xylem and phloem have been modelled. Measurements of transmembrane voltages and Cl~ activities in cellular compartments suggest (1) that active Cl~ transport across the plasma membrane dominates Cl~ influx to root cells at low Cl" concentrations in the soil solution and that passive Cl~ influx to root cells occurs under more saline conditions, and (2) that both active and passive Cl~ transport occurs at the tonoplast. Electrophysiological studies have demonstrated the presence of an electrogenic C1~/2H symporter in the plasma membrane of root-hair cells and Cl~ channels mediating either Cl~ influx or Cl~ efflux across the plasma membrane. Similarly, there is both biochemical and electrophysiological evidence that Cl~ channels mediate Cl" fluxes in either direction across the tonoplast and that a Cl~/nH+ antiport mediates Cl~ influx to the vacuole. This article reviews the availability of Cl~ in the soil, the roles and distribution of Cl~ within the plant, the magnitude of Cl" fluxes across membranes and between tissues, the mechanisms of Cl~ transport across membranes and the electrical characteristics and molecular biology of Cl~ channels. © 2001 Annals of Botany Company