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Ricinus communis was used to test the Ben Zioni-Dijkshoorn hypothesis that NO(3) uptake by roots can be regulated by NO(3) assimilation in the shoot. The rate of the anion charge from assimilated NO(3) (-) (and SO(4) (2-)) was followed in its distribution between organic acid anion accumulation and HCO(3) (-) efflux into the nutrient solution. In plants adequately supplied with NO(3) (-), HCO(3) (-) efflux accounted for between 56 and 63% of the anion charge. When the plants were subjected to a low NO(3) regime HCO(3) (-) excretion accounted for only 23% of the charge. A comparison of mature plants growing for a 10-day period at the two levels of NO(3) nutrition revealed that the uptake of NO(3) (-) at the higher level was increased 3-fold, whereas K uptake was unaltered. To trace ion movement within the plant, the ionic constituents of xylem and phloem sap were determined. In xylem sap these constituents were found to be predominantly K(+), Ca(2+), and NO(3) (-), whereas in the phloem sap they were mainly K(+) and organic acid anions. Results have been obtained which may be interpreted as providing direct evidence of NO(3) uptake by roots regulated by NO(3) reduction in the tops, the process being facilitated by the recirculation of K(+) in the plant.

Nitrate Uptake by Roots as Regulated by Nitrate Assimilation in the Shoot of Castor Oil Plants