Oxygen-Dependent Exclusion of Sodium Ions from Shoots by Roots of Zea mays (cv Pioneer 3906) in Relation to Salinity Damage

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Abstract
Using radio-tracers, we measured Na+ and K+ accumulation in roots and transport to shoots in Zea mays (cv Pioneer 3906) as a function of NaCl concentration and O2 partial pressure in the nutrient solution. Under fully aerobic conditions, roots partially excluded Na+ from the shoots over a wide range of NaCl concentration (0.2-200 millimolar). With root anoxia, the exclusion mechanism broke down so that much greater amounts of Na+ reached the shoots, with simultaneous inhibition of K+ transport. The ratio Na+/K+ entering the shoot consequently increased 90 to 200 times. Increases in Na+ transport were first detected when the O2 partial pressure was reduced from ambient (21% v/v) to 15%, whereas K+ transport was not inhibited until O2 concentrations were 2 deficiency can often accompany high salinity in irrigation agriculture, failure of the Na+ exclusion mechanism may be a contributory factor in salinity damage of salt-sensitive glycophytes.