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Articles

Oxygen-Dependent Exclusion of Sodium Ions from Shoots by Roots of Zea mays (cv Pioneer 3906) in Relation to Salinity Damage ¹

Department of Land, Air and Water Resources, University of California, Davis, CA 95616, Agricultural and Food Research Council Letcombe Laboratory, Wantage, OX12 9JT, United Kingdom

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 <5%. Since soil O₂ 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.

¹ Supported by a grant from the Kearney Foundation of Soil Science and a Fulbright Senior Research Scholarship (M. C. D.).

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