The essential role of calcium in selective cation transport by plant cells

Abstract

The mechanisms whereby plant cells absorb inorganic ions from the external media are selective: they discriminate among different ions. Particularly intriguing is the discrimination between potassium and sodium ions. Most plant cells accumulate much higher concentrations of potassium than of sodium ions, even from media in which the sodium concentration greatly exceeds that of potassium.Perhaps the best way to investigate these specificities is to study the manner in which different ions affect each other's absorption. In a previous paper (4) experiments were described demonstrating that in excised barley roots potassium and rubidium compete with each other in an identical absorption mechanism. At moderate concentrations, neither sodium nor lithium ions competed for the potassium-rubidium transport mechanism. The deduction was that there are specific ion binding sites on carrier molecules which effect the transport of the ions across cellular membranes. Potassium and rubidium ions are bound and transported by sites which fail to discriminate between them, hence these ions compete with each other for identical carrier sites. Sodium and lithium are not bound by the potassium-rubidium sites, hence do not compete with potassium and rubidium but are transported by separate sites.In this paper experiments with barley roots are presented which show that calcium ions are essential for the integrity of the selective absorption mechanism referred to, and information is given about some quantitative features of the calcium effect. MATERIALS & METHODSThe distilled water piped into the laboratory was passed through a mixed ion exchange column2 to free it from heavy metal and other impurities. This purified water was stored in pyrex carboys. Glassware was rinsed with about 0.5 M HCl, followed by exhaustive rinsing with distilled water. Salts used 'Received January 9, 1961. This assembly was placed in the dark, at room temperature. The solution was aerated continuously. Three days after the seeds were planted on the cheesecloth, the stainless steel screen with the seedlings was removed from the beaker, the roots were rinsed in two changes of water, and the assembly put on a beaker with fresh solution. The watchglass was not replaced. Two days after this, the roots were rinsed repeatedly in water, excised just below the screen, rinsed several more times, and suspended in about four liters of water.ABSORPTION EXPERIMENTS. For the experiments proper, roots were gently blotted on dry cheesecloth which had been washed and rinsed with distilled water. One-gram samples were weighed out on a torsion balance and transferred to 100 ml water in 30 X 300 mm culture tubes. The tubes were placed in a waterbath maintained at 30°C, and the water in the tubes was aerated. To start the absorption period proper, each tube in turn was removed from the bath, the water was decanted, the roots were rinsed twice with about 100 ml water each time, and 100 ml of prepared experimental solution were then poured in. The tubes were replaced in the bath, an...

Deferral of Leaf Senescence with Calcium

“…Its importance in the regulation of ion transport is well known (6,26). Its effects on the maintenance of RNA and of protein levels in Lemna have been described by Trewavas (24,25), and these components are considered central indices of senescence.…”

Sodium as nutrient and toxicant

“…Na + has been shown to suppress K + influx in both its high-and low-affinity ranges, particularly at millimolar concentrations (Cheeseman 1982;Jeschke 1982;Schachtman and Schroeder 1994;Rubio et al 1995;Gassmann et al 1996;Maathuis et al 1996;Santa-María et al 1997;Martínez-Cordero et al 2005;Kronzucker et al 2006Kronzucker et al , 2008Nieves-Cordones et al 2007). Some studies have reported only weak Na + effects (Epstein 1961;Epstein et al 1963; see also Seemann and Critchley 1985), or even stimulations of K + influx by Na + (Rubio et al 1995;Spalding et al 1999), but such studies are in the minority. As pointed out earlier in this review, and powerfully by others , some conflicting data on this front most probably arise from differences in experimental approaches, in particular between heterologous expression systems, excised roots, and intact plants (Haro et al 2010).…”

Interaction Between Potassium and Calcium in Their Absorption by Intact Barley Plants. II. Effects of Calcium and Potassium Concentration on Potassium Absorption

“…In addition to the competitive effect of Ca on K absorption (19), other stimulatory (3,4,7,11,21) and inhibitory (3, 7,1 1) effects of Ca have also been reported for short-term, excised-root experiments. The stimulatory effects of Ca have been attributed to the need to maintain the integritv of cellular memibranes (14,20) and of selective ion transport mechanisms by which K is actively absorbed (4).…”