Putting Two Water Transport Models to the Test under Wet and Dry Conditions

In order to improve fruit quality and quantity, accurate monitoring of the water status is necessary. The water status can be continuously predicted by using a mechanistic water transport and storage model (e. g., Steppe et al., 2006, 2008). This model typically links measurements of sap flow rate (SF) and stem diameter variations (D) to simulate stem water potential (Psi(stem)), which is recognised as one of the best indicators for evaluating plant water status. Despite good model performance under sufficient water availability, the model fails under dry conditions. However, a proper simulation of water transport under drought is essential for many applications. For example, grapevines are often subjected to some level of drought stress during the growing season in order to improve the quality of the grapes. Therefore, we aim at adjusting the existing model to improve its performance in simulating water transport during drought conditions. First, a dynamic function describing changes in hydraulic xylem resistance is used to replace the former constant parameter, and represents the resistances encountered in the soil, root and stem (R-X). Second, also the former constant radial flow resistance between xylem and storage tissues has been replaced by an equation (R-S). For the first time, equations for R-X and R-S instead of parameters were used in the model, and simulations were compared to the original ones. Both models functioned well under wet conditions, but where the original model failed under dry events, the adapted model could still accurately simulate D and Psi(stem) under these conditions. The adapted model is thus capable of describing the grapevine's hydraulic response to both wet and (severe) drought conditions and seems very promising within the context of an automatic plant-based system for water status monitoring.