A Unified Physically Based Model to Simulate Water and Carbohydrates Allocation Along the Soil‐Fruit Axis

The primary objective of modern crop management is to enhance fruit sweetness and size while maximizing water and fertilizer efficiency. This requires optimizing irrigation to regulate soil-plant-atmosphere interactions and water fluxes. While dry soil conditions can reduce fruit yield, controlled root water stress can improve fruit carbohydrate and nutrient concentration. Understanding the relationship between soil water status and fruit quality necessitates models that predict water and carbohydrate distribution within the soil-fruit system. Existing models, such as the SUGAR model, describe the biochemical processes influencing fruit composition, but simplify soil-plant interactions. To enhance mechanistic linkages between soil processes and fruit development, this study integrates, for the first time, the HYDRUS hydrological model with the SUGAR model. By incorporating a root water uptake approach based on root hydraulic architecture, the model calculates stem water potential from soil hydraulic state and atmospheric conditions, providing a more comprehensive depiction of soil-plant interactions. The model is calibrated and validated using experimental data on tomato crops under varied irrigation conditions. Results show that model predictions closely matched observations, with relative errors generally low across scenarios: 5%–6% for fruit water mass, 2%–18% for dry mass, 10%–11% for soluble sugars, and 7%–8% for starch. Finally, a breakdown of simulated fluxes indicates that phloem flux is the main driver of fruit growth and that the active uptake of carbohydrates is a key mechanism for sugar accumulation in fruits. The results offer insights into water and carbohydrate allocation under different watering regimes, advancing predictive capabilities for sustainable crop management strategies.