Sodium Exclusion from the Shoots by Roots ofZea mays(cv. LG 11) and its Breakdown with Oxygen Deficiency

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Abstract
The effects of sodium chloride salinity and root oxygen deficiency (anoxia) were studied in 11-12d old maize plants (Zea mays L. cv. LG 11) in nutrient solution culture. Transport of 22Na by the roots to the shoot in 24 h was markedly increased by anoxia when the external concentration of NaCl was in the range 0·1-10·9 mol m−3. Anoxia severely inhibited uptake of 42K by roots and its transport to the shoot, so that the ratio of Na+/K+ moving into the shoot was increased by a factor of approximately 10. When the external concentration of NaCl was increased to 2.4 mol m−3, the roots showed much less ability to exclude Na+ under aerobic conditions, and anoxia caused no further increase in the movement of Na+ to the shoot. It is concluded that at the higher concentration the ability of the roots to exclude Na+, presumably through an active mechanism in the xylem parenchyma cells or in the root cortex and transporting Na+ to the outer solution, is saturated by excessive inward diffusion of Na+. The ratio of Na+/K+ transported to the shoot increased by a factor of 600 when the concentration of NaCl was increased from 2·4 mol m−3 to 40 mol m−3 and roots were made anoxic. Such imbalances in the supply of cations to the shoot, particularly when roots are oxygen-deficient, may contribute to salinity damage.