Abstract
Sodium displaces Ca2+ from membranes (GR Cramer, A Läuchli, VS Polito Plant Physiol 1985 79: 207-211) and this can be related to the (Ca2+)/(Na+)2 activity ratio in the external solution (GR Cramer, A Läuchli 1986 J Exp Bot 37: 321-330). Supplemental Ca2+ is known to mitigate the adverse effects of salinity on plant growth. In this report we investigated the effects of NaCl (0-250 millimolar) and Ca2+ (0.4 and 10 millimolar) on the ion activities in solution and on root growth of cotton (Gossypium hirsutum L.). Ion activities were analyzed using the computer program, GEOCHEM. Most ion activities in a 0.1 modified Hoagland solution were significantly reduced by both NaCl and supplemental Ca2+. Ion-pair formation and precipitation were significant for some ions, especially phosphate. Root growth of 6-day-old seedlings was stimulated by low NaCl concentrations (25 millimolar). At higher NaCl concentrations, root growth was inhibited; the concentration at which this occurred depended on the Ca2+ concentration and the growth index used. Supplemental Ca2+ mitigated the inhibition of root growth caused by NaCl. There was a curvilinear relationship between root growth and the (Ca2+)/(Na+)2 ratio in the nutrient solution. The mechanisms by which Na+ and Ca2+ may affect root growth are discussed.
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