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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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 O(2) 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 O(2) partial pressure was reduced from ambient (21% v/v) to 15%, whereas K(+) transport was not inhibited until O(2) concentrations were <5%. Since soil O(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.