Spatial distribution of solutes, K, Na, Ca and their deposition rates in the growth zone of primary cotton roots: Effects of NaCl and CaCl₂

Abstract

Cotton (Gossypium hirsutum L. cv. Acala SJ-2) seedlings were grown in modified Hoagland nutrient solution with or without 150 mM NaCl and supplemental 10 mM CaCl₂. The spatial distribution of bulk-tissue osmotic potential (Ψ_S) and total osmotica, K, Na and Ca contents were determined in the growth zone of the primary root. This information was combined with the growth-velocity data from an earlier study (Zhong and Läuchli 1993) to estimate net deposition rates of osmoticum, water, K, Na and Ca by using the continuity equation. The Ψ_S was essentially uniform along the growth region for all treatments and considerably lowered by 150 mM NaCl in the medium. Total osmotica deposition was well synchronized with growth and deposition rates were enhanced by 150 mM NaCl. Osmoregulation in the treatments with 150 mM NaCl was indicated by an apparent solute accumulation which appeared to be due to the enhancement of osmoticum deposition rates. The presence of 150 mM NaCl greatly reduced the deposition rates of K and Ca throughout the growth zone; 10 mM Ca mitigated this effect only on K deposition in the apical 2.5-mm region. The deposition rate of Na was increased greatly by 150 mM NaCl; the increase was reduced by 10 mM Ca. At 150 mM NaCl, selectivity of K versus Na of the root was enhanced greatly in the apical 2mm region by the presence of 10 mM Ca; this mitigating effect by Ca declined rapidly with distance from the root tip. We conclude that one possible mechanism by which supplemental Ca alleviates the inhibitory effects of NaCl on cotton root growth is by maintaining plasma-membrane selectivity of K over Na.