Growth and development of root systems: Geometrical and structural aspects

The agronomist who wants to study the nutrient and water uptake of roots needs a quantitative three-dimensional dynamic model of the structure of root systems.The model presented takes into account current knowledge about the morphogenesis of root systems. It describes the root system as a set of root axes, characterised by their orders. The morphogenetic properties of root axes differ according to their order. The axes of order 1 are directly inserted on the stem, the axes of order 2 are inserted on axes of order 1, and so on. They tend to be more plagiotropec and to have less vascular bundles as the order increases.The evolution of the simulated structure is achieved by three processes: emission of new root axes from the shoot, growth and branching of existing root axes. The elongation of an axis depends on its order and on local growing conditions. Branches appear acropetally at a specified distance from the apex and from former branches, along ranks facing xylem poles, with a branching angle specific of their order.From the three-dimensional branched structures simulated by the model, various outputs, such as kinetics of growth and development, root profiles or cross-section maps can be computed, compared to observed data and used as inputs in uptake models. Some examples of such possible outputs are presented.

---

Résumé L'agronome qui veut étudier l'absorption hydrique et minérale des racines a besoin d'un modèle dynamique tri-dimensionnel de la structure du système racinaire.Le modèle présentd est fondé sur les connaissances actuelles sur la morphogenèse racinaire. Le système racinaire y est représenté comme un ensemble d'axes caractérisés par leur ordre. En effet, les propriétés morphogénétiques des racines varient d'un ordre d l'autre. Les axes d'ordre 1 sont insérés directement sur la tige, les axes d'ordre 2 sur les axes d'ordre 1, et ainsi de suite. Les axes tendent à être de plus en plus plagiotropes et à avoir de moins en moins de faisceaux vasculaires quand l'ordre augmente.Trois processus interviennent pour faire évoluer la structure racinaire simulée au cours du temps: l'émission de nouveaux axes racinaires à partir des tiges, la croissance, et la ramification des axes