A ROOT SHOOT PARTITIONING MODEL-BASED ON CARBON-NITROGEN WATER INTERACTIONS AND MUNCH PHLOEM FLOW

1. A root-shoot model of partitioning during vegetative plant growth is described, which simulates the transport of water, labile nitrogen (N) and labile carbon (C) through the xylem and phloem, and in which C, N and water interact to determine growth. 2. It is assumed that a fraction (lambda) of the N taken up by the root passes upwards in the xylem to the shoot where it is transferred to the phloem. The remaining fraction (1 - lambda) is transferred directly to the root phloem. Phloem translocation of labile C and N downwards from the shoot to the root is driven by a shoot-root gradient of labile C concentration, in accordance with the Munch flow hypothesis. 3. Shoot and root growth rates are assumed to be functions of local water potentials and of the local concentrations of labile C and N. 4. It is shown that, when the fraction of N that passes in the xylem to the shoot (lambda) is less than or equal to the fraction of plant biomass contained in shoot, the Munch flow mechanism of phloem translocation ensures that there is a higher labile N concentration in the root than in the shoot, opposite to the concentration gradient of labile C. This behaviour, in conjunction with the high to low root-shoot gradient of water potential, means that the model's root-shoot partitioning responses to changes in the availability of C, N and water are consistent with observations. 5. In particular, root-shoot responses to changes in C and N supply are more directly related to the existence of counter-gradients of C and N substrate concentration, than to the actual pathways of solute transport through the xylem and phloem that are involved. A more general analysis shows that, in balanced exponential growth, the establishment of counter-gradients of C and N depends only on the net transport of substrates between the shoot and root. 6. The model provides a mechanistic basis for interpreting observed root-shoot responses to deficits of other minerals such as phosphorus and magnesium.