The Role of the Mesocotyl in Sodium Exclusion from the Shoot ofZea maysL. (cv. Pioneer 3906)

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Abstract
Drew, M. C. and Lauchli, A. 1986. The role of the mesocotyl in sodium exclusion from the shoot of Zea mays L. (cv. Pioneer 3906).—J. exp. Bot. 38: 409–418. The mesocotyl, located between the root and shoot, can strongly accumulate Na+ from the ascending transpiration stream, thereby potentially acting as a sink to protect the shoot from excess Na+. To determine the quantitative importance of the mesocotyl as a Na+ sink, we grew plants with either short (9·0 mm) or long(21 mm) mesocotyls, the latter resembling the size found in field-grown plants. At 13 d, plants were transferred from Na + -free nutrient solution to a 22Na+ labelled solution in which the concentration of NaCl was (mol m–3) 1·0, 10 or 100. The concentration of Na+ accumulated in the mesocotyl in 24 h (g–1 fr. wt.) exceeded that in the roots that were directly exposed to the nutrient solution. The amounts of 22Na+ retained in the long mesocotyl were about double that in the short ones and increased with time of exposure and NaCl concentration. At 1·0 and 10 mol m–3 NaCl, the amounts of 22Na+ retained in the mesocotyl were 6–19% of those reaching the shoot in 24 h, but with 100 mol m–3 NaCl, a damaging concentration for maize, this declined to 3–8%. The mesocotyl, even as a fully elongated structure is, therefore, unlikely to provide an appreciable alternative sink for Na+ when NaCl reaches injurious concentrations.