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Articles

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

Department of Plant Sciences, The University of Leeds, Leeds LS2 9JT England

Ricinus communis was used to test the Ben Zioni-Dijkshoorn hypothesis that NO₃ uptake by roots can be regulated by NO₃ assimilation in the shoot. The rate of the anion charge from assimilated NO₃^– (and SO₄^2–) was followed in its distribution between organic acid anion accumulation and HCO₃^– efflux into the nutrient solution. In plants adequately supplied with NO₃^–, HCO₃^– efflux accounted for between 56 and 63% of the anion charge. When the plants were subjected to a low NO₃ regime HCO₃^– excretion accounted for only 23% of the charge. A comparison of mature plants growing for a 10-day period at the two levels of NO₃ nutrition revealed that the uptake of NO₃^– 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⁺, Ca²⁺, and NO₃^–, 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 NO₃ uptake by roots regulated by NO₃ reduction in the tops, the process being facilitated by the recirculation of K⁺ in the plant.

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