Uptake and Transport of Radiochloride and Tritiated Water by Various Zones of Onion Roots of Different Chloride Status

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Abstract
An apparatus is described for studying the simultaneous absorption and transport of radiochloride (Cl36) and tritiated water (THO) by separate zones of individual exuding Allium cepa roots. The net rate of Cl36accumulation by all root zones (0-3, 3-6, 6-9 cm) initially decreased with time, whereas the rate of Cl36 transport to the exudate remained constant over a 12-hour period. High chloride roots accumulated and transported to the exudate less Cl36 than low chloride roots. The concentration of Cl36 in the exudation stream was found to exceed the concentration of Cl36 in the absorbing root tissue (3-6 cm) of high chloride roots even when 2/3 of the root (0-3 and 6-9 cm) was absorbing water and thus diluting Cl36 in the xylem stream. This indicates that ion transport into the xylem is an energy requiring process. It was suggested that a system of transport sites exist throughout the cytoplasm which actively absorb and transfer chloride ions into both vacuoles and xylem at a constant rate. The reoccupation of transport sites by chloride ions moving out of vacuoles would reduce the number of sites available for absorption of external ions which would explain the decreased rate of Cl36 accumulation with time and the decreased rate of Cl36 absorption and transport in high chloride roots. The uptake and movement of THO to root exudates was unaffected by the chloride status of the tissue. The 6-9 cm root zone appeared to absorb THO more rapidly than the acropetal zones.