The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L.: IV. Flow and metabolism of inorganic nitrogen and malate depending on nitrogen nutrition and salt treatment

Ricinus plants were supplied with nutrient solutions containing different N-sources or different nitrate concentrations and were also exposed to mild salinity. Between 41 and 51 d after sowing, the ratio of inorganic to total nitrogen in xylem and phloem saps, the content of inorganic nitrogen and malate in tissues, and nitrate reductase activities were determined. The flows of nitrate, ammonium, and malate between root and shoot were modelled to identify the site(s) of inorganic nitrogen assimilation and to show the possible role of malate in a pH-stat mechanism. Only in the xylem of nitrate-fed plants did inorganic nitrogen, in the form of nitrate, play a role as the transport solute. The nitrate percentage of total nitrogen in the xylem sap generally increased in parallel with the external nitrate concentration. The contribution of the shoot to nitrate reduction increased with higher nitrate supply. Under salt treatment relatively more nitrate was reduced in the root as compared with non-treated plants. Ammonium was almost totally assimilated in the root, with only a minor recycling via the phloem. Nitrate reductase activities measured in vitro roughly matched, or were somewhat lower than, calculated rates of nitrate reduction. From the rates of nitrate reduction (OH--production) and rates of malate synthesis (2H+-production) it was calculated that malate accumulation contributed 76, 45, or 39% to the pH-stat system during nitrate reduction in plants fed with 0.2, 1.0 or 4.0 mM nitrate, malate flow in the phloem played no role. In tissues of ammonium-fed plants no malate accumulation was found and malate flows in xylem and phloem were also relative low.