Simulating transpiration and leaf water relations in response to heterogeneous soil moisture and different stomatal control mechanisms

Aims Stomata can close to avoid cavitation under decreased soil water availability. This closure can be triggered by hydraulic (‘H’) and/or chemical signals (‘C’,‘H + C’). By combining plant hydraulic relations with a model for stomatal conductance, including chemical signalling, our aim was to derive direct relations that link soil water availability, expressed as fraction of roots in dry soil (fdry), to transpiration reduction. Methods We used the mechanistic soil-root water flow model R-SWMS to verify this relation. Virtual split root experiments were simulated, comparing horizontal and vertical splits with varying fdry and different strengths of stomatal regulation by chemical and hydraulic signals. Results Transpiration reduction predicted by the direct relations was in good agreement with numerical simulations. For small enough potential transpiration and large enough root hydraulic conductivity and stomatal sensitivity to chemical signalling isohydric plant behaviour originates from H + C control whereas anisohydric behaviour emerges from C control. For C control the relation between transpiration reduction and fdry becomes independent of transpiration rate whereas H + C control results in stronger reduction for higher transpiration rates. Conclusion Direct relations that link effective soil water potential and leaf water potential can describe different stomatal control resulting in contrasting behaviour