24-235 Scaling Up a Model for Plant Growth From
il y a 4 semaines
Doctorat, 36 mois
- Temps plein
- Aucune expérience exigée
- Maitrise, IEP, IUP, Bac+4
- Life Sciences in Space
**Mission**:
Long-term space exploration, like the exploration of the Moon or Mars requires the development of an efficient and robust life-support system (LSS) to recycle atmosphere, water, and waste for the crew survival. According to some NASA’s plans at least 15% of the food should be produced on board for long trips. The European Space Agency (ESA) has been developing the MELiSSA (Micro-Ecological Life-Support System Alternative) project for more than 35 years. It concerns the growth of higher plants, which is strongly influenced by the environmental conditions (g, p, T, RH, air flow, partial pressure of O2 or CO2). Optimal growth conditions require a high level of control and management, and thus a thorough understanding of the key phenomena and of their interactions with the environment (like transpiration or mass transfer).
The goal of this study is to improve and validate the mechanistic physical model under development at the Institut Pascal (GePEB department) to predict the effect of microgravity or of a reduced gravity environment on plant growth at its morphological, physicochemical, and biochemical levels. Indeed, during growth the limiting phenomena are primarily the (i) evapo-transpiration transfer at the leaf associated with surface temperature regulation of the leaf, (ii) migration of water in the plant (coming from the ground or hydroponic medium after absorption by the roots) and (iii) CO2/O2 exchange produced from the process of photosynthesis. Thus, a mechanistic, multi-layer, and multi-scale (space and time) approach is being investigated. Today the MELiSSA model for higher plants is based on mass and energy balances at a single leaf level, with the objective of estimating the transpiration gas exchange coefficients KLa for the environmental conditions of a space habitat.
The influence of gravity was introduced by L. Poulet (CNES-CNRS PhD, 2018), considering the altered gas exchanges due to the low or lack of free convection in reduced gravity. As a result, reduced biomass production is expected. The global trend of the model proved to fit with some experimental data obtained in parabolic flights by Kitaya et al. (2001, 2006) but wider validation is needed. J. Kuzma (CNES-ESA PhD ongoing) developed a specific leaf replica system for the VP175 parabolic campaign (CNES, October 2023) to characterize the evaporative flux in transient stages and to validate the model. Several air velocity intensities are investigated and the inclination of the replica with the mean stream. Leaf replicas are investigated to perform purely physical evaporative experiments to avoid all biochemical/biological concomitant issues or genetic variations that develop during growth.
The objective of this thesis is to scale up the model from leaf to canopy levels to develop knowledge-based description of mass, heat, and energy exchange instead of empirical models (not developed for closed or space environments). Mass (carbon, gas) and heat exchange are markedly dependent on leaf and plant canopy boundary layer thickness, which depends on the airflow movements, and thus on forced and free convection—a result of buoyancy forces and therefore on gravity. For the scaling up it will be fundamental to determine the interactions with the surroundings, as it is usually not a simple sum of individual leaf interactions but a nonlinear integration. For example, each leaf significantly affects its surroundings by modifying locally wind speed, irradiance, or local relative humidity.
The focus will be on the scale up of one order of magnitude in size of our replica system, from a 5 cm square leaf to a 50 cm square canopy in a controlled environment. Several leaf shapes should be investigated: square, triangle or circle. Two “layers of leaves” could be considered to investigate the interaction between the understory and the canopy, and different angles (-30 to +30 degrees, cf Kuzma 2023). The flow stream should be varied from 0 to 1.5 m/s in standard conditions: 20°C ambient temperature and 70% average humidity level.
The initial goal will be to develop a CFD model to simulate the coupled heat and mass transfer, based on the resolution of the 3D Navier-Stokes and energy equations with the Phoenics software. The humidity level could be represented by the addition of a conservation equation of water concentration, to represent the modification of local RH level by an individual leaf and the RH profile in the canopy. The evaporative flux will be simulated as a source term on the lower side of leaves, as stomata (50 micrometers pores for plant transpiration) are mainly located underneath. The focus for the simulation will be on specific and limit cases, like for (i) gravity 0-g (or 10-2), g/6 (Moon), 1-g; (ii) streamflow intensity; (iii) canopy geometrical configuration.
The second goal will be to develop a
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