Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed to describe quantitatively the uptake, flow, storage and utilization of NO3-N over a 9 d period in mid-vegetative growth of sand cultured castor bean (Ricinus communis L.) fed 12 mol m−3 nitrate and exposed to a mean salinity stress of 128 mol m−3 NaCl. Model construction used information on increments or losses of NO3-N or total reduced N in plant parts over the study period and concentration data for NO3-N and reduced (amino acid) N in phloem sap and pressure-induced xylem exudates obtained from stem, petiole and leaf lamina tissue at various levels up a shoot. The resulting models indicated that the bulk (87%) of incoming nitrate was reduced, 51% of this in the root, the remainder principally in the laminae of leaves. The shoot was 60% autotrophic for N through its own nitrate assimilation, but was oversupplied with surplus reduced N generated by the root and fed to the shoot through the xylem. The equivalent of over half (53%) of this N returned to the root as phloem translocate and, mostly, then cycled back to the shoot via xylem. Nitrate comprised almost half of the N of most xylem samples, but less than 1% of phloem sap N. Laminae of leaves of different age varied greatly in N balance. The fully grown lower three leaves generated a surplus of reduced N by nitrate assimilation and this, accompanied by reduced N cycling by xylem to phloem exchange, was exported from the leaf. Leaf 4 was gauged to be just self-sufficient in terms of nitrate reduction, while also cycling reduced N. The three upper leaves (5–7) met their N balance to varying extents by xylem import, phloem import (leaves 6 and 7 only) and assimilation of nitrate. Petioles and stem tissue generally showed low reductase activities, but obtained most of their N by abstraction from xylem and phloem streams. The models predicted that nodal tissue of lower parts of the stem abstracted reduced N from the departing leaf traces and transferred this, but not nitrate, to xylem streams passing further up the shoot. As a result, xylem sap was predicted to become more concentrated in N as it passed up the shoot, and to decrease the ratio of NO3-N to reduced N from 0·45 to 0·21 from the base to the top of the shoot. These changes were reflected in the measured N values for pressure-induced xylem exudates from different sites on the shoot. Transfer cells, observed in the xylem of leaf traces exiting from nodal tissue, were suggested to be involve