A Dynamic Model to Couple Carbon and Nitrogen Metabolism with Transport in Whole Plants.

Transport of metabolites in plants consists of movement at two scales of distance. The changes in concentration within compartments in each tissue are determined by short distance transport processes. Short distance movement is governed by diffusion or active transport across membranes separating compartments. This process is rapid and tends to a steady state distribution among the compartments: cytoplast, xylem, phloem, and apoplast. Each tissue or plant organ is divided into compartments with the same partial volumes for each compartment in each tissue. Long distance movement corresponds to flux of water and solutes in xylem and phloem between organs or tissues of a plant. This is driven by hydrostatic pressure, related to water potential in the xylem, and to turgor pressure in the phloem. The model plant is partitioned into root, stem, and leaf tissues, and a conductance is defined for movement in xylem and phloem between each tissue. Long-distance flux is calculated from the concentration in xylem or phloem times the flux of water, as defined by conductance and pressure difference between tissues. All these processes were programmed as finite difference equations using the VENSIM visual dynamic simulation modeling tool. The model was used to examine effects of diurnal variation in transpiration on movement in xylem and phloem in an idealized plant. The solute concentration of sugars in phloem had a diurnal variation, but flow rate did not change. There was a diurnal variation in both solute concentration and flow rate in the xylem. These variations from light to dark altered the availability of nitrate in leaves and sugars in roots.