Le rôle des dynamiques transitoires dans les changements ontogénétiques du root-shoot ratio

International audienceSimple models of herbaceous plant growth based on optimal partitioning theory predict, at steady state, an isometric relationship between shoot and root biomass during plant ontogeny: i.e. a constant root-shoot ratio. This prediction has received mixed empirical support, suggesting either that optimal partitioning is too coarse an assumption to model plant biomass allocation, or that additional processes need to be modelled to account for empirical findings within the optimal partitioning framework. Here we use simulations to test two potential explanations for observed non-isometric relationships, namely root senescence and transient growth dynamics. We build a simple plant growth model based on optimal partitioning theory combined with empirically measured plant functional traits. We assess its ability to reproduce various experimental plant growth patterns, namely relative growth rate, root weight ratio and root nitrogen content. We then test whether this realistically parameterized minimal model can reproduce empirically observed non-isometric relationships during plant ontogeny, and whether additional model ingredients may produce non-isometric trajectories. We finally compare our model predictions regarding the transient dynamics of allometric relationships with experimental findings.We find that our simple model based on optimal partitioning theory is able to accurately reproduce overall plant growth patterns, but fails at explaining non-isometric growth trajectories. Among the additional model ingredients tested, only transient dynamics produces non-isometric growth trajectories. The role of transient dynamics in the shape of root-shoot relationships is further supported by our re-analysis of a plant growth experiment. Our model with the inclusion of transient dynamics enables to reconcile theoretical predictions based on optimal partitioning with empirically measured ontogenetic root-shoot allometries