Dynamic aspects of soil water availability for plant transpiration: Emphasize on the plant hydraulic resistance

Plant root system can be conceptualized as a network of water saturated porous pipes, at the interface between plant aerial system, where transpiration takes place, and another porous medium: the soil. For more than sixty years, the idea of modeling plant transpiration by using the electric analogy has been quite common. It was however not clear how to quantify a representative soil water potential (SWP) and a single plant hydraulic resistance in a system where SWP is a distributed non-uniform variable and multiple pathways of water co-exist in the network of plant hydraulic resistances. Recently, Couvreur et al. (2012) demonstrated that simple plant-scale equations and hydraulic parameters, respecting physics-based principles of water flow in complex hydraulic networks, can be used to model plant transpiration and root water uptake distribution. The first objective of this study is to revisit the concept of soil water availability for plant transpiration as impacted by plant hydraulic properties. The resulting mechanistic plant water stress function is compared to its empirical equivalent. Similarities suggest that the slope of empirical water stress functions might correspond to the ratio of the plant hydraulic conductance (Krs) to the plant potential transpiration rate. The second objective is to investigate with a detailed modeling approach (using the model R-SWMS, Javaux et al. 2008) how such water availability for plant transpiration may be affected by processes locally affecting root hydraulic properties (e.g., formation of air gaps at soil-root interface, regulation of aquaporin activity). This study demonstrates that considering root radial conductances as functions of local SWP results in strategic behaviors regarding water availability, which can be identified at the whole plant scale. This study offers a first insight on the effect of dynamic local root hydraulic properties on soil water availability for plant transpiration. By better understanding complex interactions between hydraulic processes involved in soil-plant hydrodynamics, better prospects on how root hydraulic traits affect plant water stress might be achieved