A dynamic whole-plant model of integrated metabolism of nitrogen and carbon. 2. Balanced growth driven by C fluxes and regulated by signals from C and N substrate

A whole-plant model of C and N metabolism is presented for the juvenile sta ge. It is aimed at comparing the growth performance of (wild) plant species in a range of environments with respect to irradiance and availability of nitrate (NO3-) and ammonium (NH4+). State variables are the structural mass es of leaves, stem and root, NO3- concentrations in root and shoot, non-str uctural carbohydrate (C) densities in leaves, stem and root and non-structu ral organic N concentration in the whole plant. Explicit expressions for NO 3- influx, efflux, translocation and assimilation, and for NH4+ uptake and assimilation have been formulated in an accompanying paper. Photosynthetic rate is derived from electron-transport rate which depends on irradiance an d chlorophyll concentration on a leaf-area basis. The latter is proportiona l to non-structural organic N concentration. Photosynthetic N is considered non-structural. Unique features of the model are the use of metabolite sig nals and the treatment of C allocation and balanced growth. Metabolite sign als are dimensionless functions of non-structural compounds (NO3-, C, organ ic N) and modify rate variables involved in N uptake and assimilation, C al location and growth. Carbon allocation is driven by concentration differenc es of the cytosolic C pools in stem and root and is modified by the N statu s of the plant such that a high N status increases the apparent size of the shoot. Photosynthate is unloaded into C buffers which degrade at a constan t specific rate. The sugar fluxes which arise from these buffers drive the growth rate of stem and root. No parameters are included for maximum specif ic growth or for activity or strength of sinks. Primary stem growth is prop ortional to growth of the leaf compartment: leaves arise from stems in a mo dular fashion. Leaves are autonomous with respect to their C balance. The m odel is presented as a system of differential equations which is integrated numerically. Parameter values, e.g., for uptake and assimilation capacitie s and costs of uptake, assimilation, maintenance and growth, are estimated for a grass species, Dactylis glomerata. Juvenile growth is simulated under optimal conditions with respect to irradiance and NO3- availability and co mpared with literature data. Diurnal and daily patterns of C utilisation an d respiration, expressed as percentages of gross photosynthetic rate, are d iscussed. The model satisfactorily simulates typical responses to nutrient and light limitation and pruning, such as redirected C allocation, adjusted root and leaf weight ratios and compensatory growth. A sensitivity analysi s is included for selected parameters.