MassFlowDyn I: a carbon transport and partitioning model for root system architecture

Carbon partitioning is important for understanding root development but little is known about its regulation. Existing models suggest that partitioning is controlled by the potential sink strength. They cannot, however, simulate hierarchical uptake other than by using absolute priorities. Moreover, they cannot explain that the changes in photoassimilate partitioning result from changes in photosynthesis. In this paper we present a model of phloem sieve circulation, based on the model of Minchin et al. (Journal of Experimental Botany44: 947–955, 1993). The root system was represented by a network of segments to which meristems were connected. The properties of the segments were determined by the differentiation stage. Photoassimilate import from each organ was assumed to be limited by a metabolic process and driven by Michaelis–Menten kinetics. The axial growth was proportional to meristem respiration, which drives the flux of new cells required for root elongation. We used the model to look at trophic apical dominance, determinate and indeterminate root growth, the effect of the activity of a root on competition with its neighbours, and the effect of photoassimilate availability on changes in partitioning. The simulated phloem mass flow yielded results of the same order of magnitude as those generally reported in the literature. For the main well vascularized axis, the model predicted that one single apical meristem larger than its neighbouring laterals, was enough to generate a taprooted system. Conversely, when the meristem of laterals close to the collar had a volume similar to that of the taproot, the predicted network became fibrous. The model predicted a hierarchical priority for organ photoassimilate uptake, similar to that described in the literature, during the decline in photosynthetic activity. Our model suggests that determinate growth of the first laterals resulted from a local shortage of photoassimilate at their meristem, as a result of the limited transport properties of the developed roots.