| Abstract: | Long distance transport of sodium in bean plants In 8–10 days old bean plants Na+ (22Na) has been applied to either a certain root zone, the stem, or the base and tip respectively of a primary leaf and the long distance transport of Na+ was studied in the following 12–48 h. The long distance transport of Na+ applied to the root zone 9–12 cm behind the tip was strongly restricted towards the shoot and hardly detectable towards the root tip (phloem transport). Presence of K+ in the surrounding solution strongly increased the Na+ efflux from the roots. After leaf application within 48 h 30–40% of the absorbed Na+ had been translocated out of the leaf in direction of the root where, from the basal root zones, intensive Na+ efflux took place. This Na+ efflux was hardly affected by presence of K+ in the external medium and was usually more than 10% of the Na+ taken up by the leaves. From the Na+ taken up by the hypocotyl within 12 h more than 25% had been released from the basal root zones into the nutrient solution. Less than 1% of the Na+ applied either to the leaf or the stem was translocated towards the shoot apex. Separation of the hypocotyl into cortex and stele at the end of the experiment demonstrated the high capacity of the stele for Na+ accumulation. Within the hypocotyl the transfer of Na+ from the stele to the cortex and the phloem seems to be a rapid process whereas the release of Na+ from the phloem into the stele is obviously very restricted. The long distance transport of Na+ within the phloem of the shoot is strictly basipetal and very efficient. Low Na+ contents of bean leaves are therefore the result of several regulating mechanisms: K+ stimulated Na+ efflux in the roots, restricted long distance transport in the xylem due to high Na+ accumulation in the stele, Na+ pumps at the phloem in stem and leaves for phloem loading of Na+, and finally strictly basipetal retranslocation of Na+ in the phloem into the roots and efflux into the solution from basal root zones. |
