Magali Nuixe (Thesis defended March 7, 2023)

Forest and grassland are the main perennial terrestrial ecosystems with high potential to mitigate global warming by the sequestration of large amounts of carbon in both the wood and the soil. This carbon storage from vegetation depends on a tight coupling between the processes of photosynthetic carbon fixation and transpiration. Root systems perform the crucial task of absorbing water from the soil to meet the climatic demand by the transpiring canopy. In the plant, the water is transported from roots to leaves by the xylem flow, sustaining essential plant functions such as growth or photosynthetic activity. On the reverse way, the carbon flow transports photoassimilates resulting from photosynthesis from leaves to the carbon sinks thanks to the phloem flow. The carbon assimilation and then the carbon sequestration potential depend on transports from both the xylem and phloem flows. With the significant increase of extreme climatic events associated with climate change, it is of the prime importance to understand and thus to measure plant water uptake and carbon flow during and after drought stress periods directly in field conditions. Unfortunately, a sensor able to measure selectively these flows or quantities directly in the fields does not exist. The objective of this PhD work is to develop such sensor.

Magnetic resonance imaging (MRI) is the method of choice to locally and non-invasively measure flow rates. These measurements have been performed on high-field MRI on model systems. To move the magnet, the magnetic field must be decreased, decreasing also the measurement sensibility. However, information on water (relaxation, diffusion) can still be measured. The team owns such device, especially designed to match the need of this project. The aim of the PhD work is to (1) demonstrate the versatility of the in-situ MRI spectrometer by performing measurements on two contrasted plant functional types: tree and perennial grass species and (2) validate the in-situ MRI method by (i) comparing MRI measurements to the existing analytical methods and (ii) inducing an edaphic drought stress to determine the method sensitivity.

This thesis is co-directed by Amidou Traore (QuaPA, AgroResonance) and Catherine Picon-Cochard (UREP).