MODELING 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 involved in the abstraction process.