Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO(3)(−) or NH(4)(+), the solutions being maintained at pH 5.5. In NO(3)(−)-fed plants excess nutrient anion over cation uptake was equivalent to net OH(−) efflux, and the total charge from NO(3)(−) and SO(4)(2−) reduction equated to the sum of organic anion accumulation plus net OH(−) efflux. In NH(4)(+)-fed plants a large H(+) efflux was recorded in close agreement with excess cation over anion uptake. This H(+) efflux equated to the sum of net cation (NH(4)(+) minus SO(4)(2−)) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO(3)(−) that is taken up and reduced in NO(3)(−)-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH(4)(+)-fed plants absorbed NH(4)(+) was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO(3)(−)-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients

Intracellular pH Regulation during NO(3)(−) 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

Characterization of Phloem Iron and Its Possible Role in the Regulation of Fe-Efficiency Reactions

`Fe-efficiency reactions' are induced in the roots of dicotyledonous plants as a response to Fe deficiency. The role of phloem Fe in the regulation of these reactions was investigated. Iron travels in the phloem of Ricinus communis L. as a complex with an estimated molecular weight of 2400, as determined by gel exclusion chromatography. The complex is predominantly in the ferric form, but because of the presence of reducing compounds in the phloem sap, there must be a fast turnover in situ between ferric and ferrous (k ≈ 1 min(−1)). Iron concentrations in R. communis phloem were determined colorimetrically or after addition of (59)Fe to the nutrient solution. The iron content of the phloem in Fe-deficient plants was lower (7 micromolar) than in Fe-sufficient plants (20 micromolar). Administration of Fe-EDTA to leaves of Phaseolus vulgaris L. increased the iron content of the roots within 2 days, and decreased proton extrusion and ferric chelate reduction. The increase in iron content of the roots was about the same as the difference between iron contents of roots grown on two iron levels with a concomitantly different expression of Fe-efficiency reactions. We conclude that the iron content of the leaves is reflected by the iron content of the phloem sap, and that the capacity of the phloem to carry iron to the roots is sufficient to influence the development of Fe-efficiency reactions. This does not preclude other ways for the shoot to influence these reactions