Abstract
Abstract. Stomatal regulation and whole plant hydraulic signaling affect water fluxes and stress in plants. Land surface models and crop models use a coupled photosynthesis–stomatal conductance modeling approach. Those models estimate the effect of soil water stress on stomatal conductance directly from soil water content or soil hydraulic potential without explicit representation of hydraulic signals between the soil and stomata. In order to explicitly represent stomatal regulation by soil water status as a function of the hydraulic signal and its relation to the whole plant hydraulic conductance, we coupled the crop model LINTULCC2 and the root growth model SLIMROOT with Couvreur's root water uptake model (RWU) and the HILLFLOW soil water balance model. Since plant hydraulic conductance depends on the plant development, this model coupling represents a two-way coupling between growth and plant hydraulics. To evaluate the advantage of considering plant hydraulic conductance and hydraulic signaling, we compared the performance of this newly coupled model with another commonly used approach that relates root water uptake and plant stress directly to the root zone water hydraulic potential (HILLFLOW with Feddes' RWU model). Simulations were compared with gas flux measurements and crop growth data from a wheat crop grown under three water supply regimes (sheltered, rainfed, and irrigated) and two soil types (stony and silty) in western Germany in 2016. The two models showed a relatively similar performance in the simulation of dry matter, leaf area index (LAI), root growth, RWU, gross assimilation rate, and soil water content. The Feddes model predicts more stress and less growth in the silty soil than in the stony soil, which is opposite to the observed growth. The Couvreur model better represents the difference in growth between the two soils and the different treatments. The newly coupled model (HILLFLOW–Couvreur's RWU–SLIMROOT–LINTULCC2) was also able to simulate the dynamics and magnitude of whole plant hydraulic conductance over the growing season. This demonstrates the importance of two-way feedbacks between growth and root water uptake for predicting the crop response to different soil water conditions in different soils. Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, particularly in heterogeneous fields. The newly coupled soil–plant model marks a promising approach but requires further testing for other scenarios regarding crops, soil, and climate.
Highlights
Soil water status is amongst the key factors that influence photosynthesis, evapotranspiration, and growth processes (Hsiao, 1973)
Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, in heterogeneous fields
It uses a stomatal regulation model that assumes that stomatal conductance is not influenced by the leaf water hydraulic head as long as the leaf hydraulic head is above a critical leaf hydraulic threshold
Summary
Soil water status is amongst the key factors that influence photosynthesis, evapotranspiration, and growth processes (Hsiao, 1973). The water stress factor calculated by the Feddes model (Feddes et al, 1978) based on the soil water pressure heads involves indirect linkages between the root zone water pressure head and the hydraulic head in the shoot in the sense that the water stress factors are adapted when the potential transpiration rate changes Such models like the Feddes approach represent the role of the root and plant hydraulic conductance indirectly and require calibration for different crop types and growing seasons (Cai et al, 2018; Vandoorne et al, 2012; Wesseling et al, 1991). The Couvreur model allows the different stomatal regulations to be presented (i.e., isohydric and anisohydric in Tardieu and Simonneau, 1998) (Couvreur et al, 2014, 2012)
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