Simulating the partitioning of biomass and nitrogen between roots and shoot in crop and grass plants

Quantification of the assimilate partitioning between roots and shoot has been one of the components that need improvement in crop growth models. In this study we derived two equations for root-shoot partitioning of biomass and nitrogen (N] that hold for crops grown under steady-state conditions. The equations are based on the concept of the functional balance between N uptake by roots and carbon fixation by shoots, and incorporate the assumption that plants control their root-shoot partitioning in order to maximize relative growth rate. The equations do not have their own parameters but use several variables as inputs that can be calculated from sub-models for root N uptake and shoot carbon fixation in a general plant growth model. Given reports from the literature that the partitioning models — if expressed as a function of plant-N status — might be suitable for steady-state as well as non-steady-state conditions, our equations were deliberately applied to non-steady-state conditions. The predicted crop root-shoot partitioning and its responses to radiation, water and N agreed qualitatively with the expected trends. The predicted response to elevated carbon dioxide varied and depended on the timing and amount of N applied. Quantitative tests with data from root and shoot pruning experiments with grass plants carried out by others showed that model predictions also agreed with observed root-shoot ratios, suggesting that our equations provided a valuable semi-mechanistic approach to the prediction of root-shoot relationships under any growth conditions.