Abstract: | Drew, M. C. and Lauchli, A. 1986. The role of the mesocotylin sodium exclusion from the shoot of Zea mays L. (cv. Pioneer3906).J. exp. Bot. 38: 409418. The mesocotyl, located between the root and shoot, can stronglyaccumulate Na+ from the ascending transpiration stream, therebypotentially acting as a sink to protect the shoot from excessNa+. To determine the quantitative importance of the mesocotylas a Na+ sink, we grew plants with either short (9·0mm) or long(21 mm) mesocotyls, the latter resembling the sizefound in field-grown plants. At 13 d, plants were transferredfrom Na + -free nutrient solution to a 22Na+ labelled solutionin which the concentration of NaCl was (mol m3) 1·0,10 or 100. The concentration of Na+ accumulated in the mesocotylin 24 h (g1 fr. wt.) exceeded that in the roots thatwere directly exposed to the nutrient solution. The amountsof 22Na+ retained in the long mesocotyl were about double thatin the short ones and increased with time of exposure and NaClconcentration. At 1·0 and 10 mol m3 NaCl, theamounts of 22Na+ retained in the mesocotyl were 619%of those reaching the shoot in 24 h, but with 100 mol m3NaCl, a damaging concentration for maize, this declined to 38%.The mesocotyl, even as a fully elongated structure is, therefore,unlikely to provide an appreciable alternative sink for Na+when NaCl reaches injurious concentrations. Key words: Ion transport, potassium, roots, salinity |