Jeanne Simon defended her thesis on 25 March 2021. Carried out within the framework of the APLIM flagship project, and funded by Agropolis Fondation, her subject was "A combined MRI, histology and modelling approach to study the distribution of resources in plant architecture - the case of tomato and the response to water deficit".
Jeanne was supervised by Nadia Bertin and Christophe Goze-Bac and hosted at INRAe.
In the face of climate change and the scarcity of a certain number of resources, biotic and abiotic stresses are increasingly frequent and new strategies must be implemented in the study of plant development and functioning. Water and carbon resources are major determinants of fruit growth, and quantifying them at variable temporal and spatial scales is a major challenge. Among the many methods used to quantify their flows, most are destructive or indirect. The democratization of a non-invasive technology based on Magnetic Resonance Imaging (MRI) giving access to precise measurements of flows, both in quality and quantity, in the plant, would allow to overcome the limits of current approaches in plant ecophysiology. This work focuses on the tomato, a model plant and the second most consumed fruit in the world. The main objective of this thesis work was to develop and apply MRI methods to measure water fluxes in the tomato plant. The second objective was to couple these MRI investigations to histological approaches in order to evaluate the structure-function links of conductive tissues. The third challenge was to use modeling as a tool for integrating the acquired knowledge in order to identify anatomical and functional traits potentially involved in the genetic and environmental plasticity of flow determinants. In response to these questions, we combined histological measurements at different plant scales (pedicel and stem), MRI experiments performed on a 9.4T MRI scanner and tomato ecophysiological modeling to integrate the results. We showed that the high genetic and water deficit-induced variability of the surface of the conductive tissues at the fruit entry partially explained the variability of fresh and dry mass of the fruits. A flow-MRI method based on the inflow principle was developed and then applied at different levels of the main stem and to study the plant’s response to water deficit. We demonstrated a reduction in xylem flow along the main stem due to a decrease in the active surface area from the bottom to the top of the stem, while the flow velocity was relatively stable. Finally, we used a structure function model to integrate the main results obtained, in particular concerning the theoretical conductivity deduced from MRI measurements. This integration has clearly shown the importance of measuring flows and water potentials in situ to better estimate the conductances in the xylem network. This exploration has allowed the development of a conceptual framework for the non-destructive study of water fluxes in tomato and other species. This work also opens perspectives to measure phloem flows as well as flows at the fruit pedicel level, which are important data to improve mechanistic models of resource allocation in the plant.