Intracellular pH Regulation during NO3− Assimilation in Shoot and Roots of Ricinus communis

Ricinus communis L. was used to test the Dijkshoorn-Ben Zioni hypothesis that NO(3) (-) uptake by roots is regulated by NO(3) (-) assimilation in the shoot. The fate of the electronegative charge arising from total assimilated NO(3) (-) (and SO(4) (2-)) was followed in its distribution between organic anion accumulation and HCO(3) (-) excretion into the nutrient solution. In plants adequately supplied with NO(3) (-), HCO(3) (-) excretion accounted for about 47% of the anion charge, reflecting an excess nutrient anion over cation uptake. In vivo nitrate reductase assays revealed that the roots represented the site of about 44% of the total NO(3) (-) reduction in the plants. To trace vascular transport of ionic and nitrogenous constituents within the plant, the composition of both xylem and phloem saps was thoroughly investigated. Detailed dry tissue and sap analyses revealed that only between 19 and 24% of the HCO(3) (-) excretion could be accounted for from oxidative decarboxylation of shoot-borne organic anions produced in the NO(3) (-) reduction process. The results obtained in this investigation may be interpreted as providing direct evidence for a minor importance of phloem transport of cation-organate for the regulation of intracellular pH and electroneutrality, thus practically eliminating the necessity for the Dijkshoorn-Ben Zioni recycling process.