In Silico Evaluation of the Water and Carbon Flows in the Soil-Rhizosphere-Plant Continuum

Background and Motivation: Changes in water and carbon balances both impact and are impacted by the processes of soil microbial communities and plants, creating complex feedback loops. Climate change is disrupting these cycles, leading to extreme weather events like droughts and floods that complicate agricultural resource management. This creates an urgent need to develop optimal genotype-environment-management (GxExM) strategies to enhance crop resilience. Setting up multi-scale, multi-domains, multi-physics models can yield new and relevant insights into the interrelated processes driving the resilience of environments to new climates and sustainable agricultural practices. Functional Structural Plant Models (FSPMs) can be an important part of such frameworks by simulating how structure and function interact at the scale of individual plants and their surrounding local environment. Material and Methods: To better represent the water and carbon balance in the soil-rhizosphere-plant continuum, the existing CPlantBox model was extended by adding or adapting several modules related to water and carbon flow from roots to leaves while at the same time adjusting and improving the numerical schemes. As a first implementation, the plant model was embedded into an updated multiscale soil-rhizosphere model. In a parallel implementation, the plant model was linked to an auxin flow module and a hormonal regulation network to evaluate the effects of sucrose and auxin on branching patterns. Scientific results: The implementation of the plant model enabled us to observe the effects of dry spells on water and carbon flow within the plant. The developmental stage of the plant at the onset of a dry spell was a crucial factor influencing these effects. Initially, dry atmospheric conditions tended to trigger increased transpiration and respiration. However, the reduced plant growth resulting from water scarcity could lead to lower leaf area (and thus transpiration) and respiration over the long term, when the dry spell occurred earlier in the plant's development. Using the soil-rhizosphere-plant model, we could examine how plant-mediated dry spells impact soil carbon and water balance. Increased exudation from older plants during dry spells benefited active microbial communities limited by carbon, resulting in relatively higher microbial CO2 emissions. Conversely, such conditions hindered carbon utilization in less active communities, as they were more constrained by water availability. In another line of research, we investigated auxin-mediated plant growt. We found that, by implementing a simplified auxin flow model within the CPlantBox FSPM, we could replicate key branching patterns observed in experimental studies and literature. These results underline the key role of carbon-dependent phytohormones for the development of plant shoot. Conclusion: The newly developed soil-rhizosphere-plant model offers a novel integrated representation of water and carbon flows across domains and processes that are typically simulated in isolation. Through this integration we gained valuable insights into complex interaction processes. Furthermore, the multiscale implementation enabled us to precisely evaluate water and carbon flow, as well as reactions at the soil-plant interface, while limiting the increase of the computation time. The results from the branching model indicate that the representation of auxin and sucrose flow is sufficient to simulate bud release and branching patterns at the plant scale, provided the plant is not experiencing stress. However, these results represent only the first examples of model implementation. Later studies could focus more strongly on model calibration, validation and implementation for new research questions. Moreover, several key functional aspects could be incorporated to enhance the model's accuracy and robustness, particularly under limiting conditions such as nitrogen scarcity.