Sodium Does Not Compete with Calcium in Saturating Plasma Membrane Sites Regulating ²²Na Influx in Salinized Maize Roots

Abstract

Half maximal inhibition of sodium ('Na ) influx into maize (Zea mays L.) root segments incubated in solutions containing from 0.25 to 100 millimolar NaCI was consistently attained with external calcium activity at 0.26 ± 0.10 millimolar. Sodium ions do not appear to compete with calcium during initial binding to sites on the plasma membrane that participate in the regulation of sodium influx under saline conditions.Increasing the activity of calcium ions in the saline root media of glycophytic crop plants can reduce sodium accumulation and partially reverse the growth inhibition induced by excess salinity in the rhizosphere (2,4,5,8,15 (2) suggested that this could be the primary root response to salt stress. The displacement ofthese essential calcium ions by sodium would disrupt membrane stability thus accelerating efflux of intracellular solutes and influx of toxic sodium from the rhizosphere; additional calcium ions in the rhizosphere would reverse these processes by competing more effectively with sodium ions for calcium binding sites on the plasma membrane (2, 8, 9). Direct support for this hypothesis was initially provided by observations of NaCl induced displacement of surface Ca2" from intact cotton root hairs and maize root protoplasts, as visualized using microfluorometric techniques (2, 9). However, these techniques may be prone to interpretation errors and a later report suggested that the predominant effect of external salinization on calcium associated fluorescence in maize protoplasts was due to the release of calcium from intracellular membranes rather than its displacement from the plasma membrane surface (10). .N.). 331 An alternative method for investigating salinity effects on sodium influx into roots and its relation to calcium binding at the plasma membrane is described here. We assume that if sodium ions are able to competitively displace calcium from plasma membrane binding sites associated with the regulation of sodium influx, then the external calcium activity required to inhibit sodium influx should increase as external salinity is increased. If, alternatively, external salinity does not competitively displace calcium, then the calcium activity required to inhibit sodium influx should remain constant as external salinity is increased. To determine which mechanism is operative, we investigated the effects of a range of external concentrations of NaCl (0.25-100 mM) on rates on sodium (22Na) influx into washed maize root segments. Sodium influx at each NaCl concentration was assayed at several different calcium concentrations from 0 to 10 mM. Salinization of solutions increases their ionic strength. Such increases in ionic strength can progressively reduce the effective concentration and the physiologically effective chemical activity of calcium ions, through ion to ion interactions (1,3,6). Thus, instead of relating sodium influx to total calcium concentrations, we investigated the relationship between sodium influx and the chemical activity of the calcium ions in each of the...

Salinity–mineral nutrient relations in horticultural crops

“…Increasing the external concentration of calcium largely counteracted this displacement (Lynch et al, 1987;Lynch and La Èuchli, 1988). Maize has proved to be a suitable test plant for investigating the effects of salinity on the whole plant, as well as on the organ and cellular levels of organisation, and for defining plant response to the competitive interactions between Na and Ca 2 (Zidan et al, 1991). In high Na /Ca 2 substrates, maize is particularly sensitive to growth inhibition and exhibits significant changes in morphology and anatomy (Maas and Grieve, 1987;Evlagon et al, 1990;Cramer, 1992).…”

“…In high Na /Ca 2 substrates, maize is particularly sensitive to growth inhibition and exhibits significant changes in morphology and anatomy (Maas and Grieve, 1987;Evlagon et al, 1990;Cramer, 1992). Inhibition of maize root cell production and elongation appear to underlie the regulation of root growth by NaCl-salinity and these processes may be partially restored by addition of substrate-calcium (Cramer et al, 1988;Zidan et al, 1991). Water transport properties of maize primary roots are adversely affected by NaCl-induced morphological and anatomical changes (Evlagon et al, 1990;Neumann et al, 1994).…”

Inhibition of root growth by salinity stress: Toxicity or an adaptive biophysical response?

“…The fact that raising the calcium concentration (and activity) in dilute nutrient solutions used for hydroponic culture of salinized plants can increase root (and leaf) growth, is well established (e.g. Lehaye and Epstein, 1969;Maas and Grieve, 1987;Zidan et al, 1991). Salinity induced changes in root length, mature cell length and root diameter could be caused by a) toxic effects of salt on the metabolism of expanding cells and diameter are means, ±SD, n = 10 after4d growth in aerated nutrient solutions (NS) ± 100 mol JIl-3 NaCI ± 10 mol m-3 CaCI2 (Adapted from Evlagon et al, 1990).…”

Salt-imposed restrictions on the uptake of macroelements by roots of Arabidopsis thaliana

“…Calcium addition to a medium alleviates salt stress symptoms in many species (Maathuis and Amtmann 1999). In salt-affected plants, Ca 2+ supply restored root elongation (Zidan et al 1990) and shoot growth (Yeo 1998) and reduced total Na + accumulation (Cramer et al 1989;Zidan et al 1991). We previously found that growth inhibition in salt-treated Arabidopsis plants could be attributed to the limitations in K + , Ca 2+ and/or N supply to the leaf (Attia et al 2008).…”