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Schobert, C; Komor, E: Transport of nitrate and ammonium into the phloem and the xylem of ricinus-communis seedlings, Journal of Plant Physiology, 140, 306-309 (1992)
Abstract:
Ricinus communis seedlings were used to study the transfer of exogenously supplied nitrate and ammonium into the phloem and the xylem. Uptake into the phloem was tested after removal of the endosperm by incubating the cotyledons in nitrate and/or ammonium. Analysis of phloem sap proved that there is only restricted loading of nitrate into the phloem and ammonium was not taken up at all, even when offered at 10 mM to the cotyledons.In contrast, when the roots were incubated in nitrate, rapid transfer into the xylem occurred. Both significantly higher exudation rates and nitrate concentrations in the root pressure exudate were detected when roots were incubated in KNO3 compared with NH4NO3. Ammonium when offered to the roots at 1 mM was not transported into the xylem, but when the roots were incubated in 10 mM ammonium, it was delivered into the xylem. Again, exudation rate and concentration of ammonium in the root pressure exudate were affected by the accompanying ion. When NH4NO3 was offered, the exudation rate was stimulated two-fold and ammonium concentration was as much as three times higher than when roots were incubated in NH4Cl.These complex mutually interdependent effects on water and nitrogen ion fluxes are discussed with respect to permeability and metabolism of the ions and also with respect to balanced charge flow into the xylem.

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