Relations between transpiration, soil water potential, and leaf water potential at the crop stand scale.

• Applicant: Professor Dr. Jan Vanderborght
• Subject Area: Soil Sciences
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 538902328

In sufficiently wet soils and when transpiration rates are not too high, the relationship between soil water potential, Ψsoil, leaf water potential, Ψleaf, and transpiration rate is linear since the conductances of the different parts of the water flow path from the soil, the roots, the shoot, and the leaves do not depend on the water potentials. However, when water potentials drop, the conductances of certain parts decrease and the relation becomes non-linear leading to large changes in Ψleaf when E increases. The overall hypothesis of the research unit is that plants try to avoid these drastic, potentially lethal, Ψleaf decreases by closing stomata and other adaptation mechanisms. In this subproject, we will investigate how the relation between transpiration rate, E, the 'effective' soil water potential in the root zone, Ψsoil,eff, and the 'effective' leaf water potential, Ψleaf,eff evolve in crop stands and grasslands under different pedoclimatic conditions. Since soil water potentials may vary considerably with depth, we will investigate how Ψsoil should be averaged across soil profiles to obtain Ψsoil,eff and how this averaging depends on root system properties and on the soil hydraulic properties. For heterogeneous plant communities, such as grasslands, we will investigate how Ψsoil can be averaged as a function of the root distributions and properties of the individual plants and how Ψleaf can be averaged to obtain Ψleaf,eff. We hypothesize that hydraulic adaptation to water limitations will result in a linear (E, Ψsoil,eff, Ψleaf,eff) relation, of which the slope for a constant Ψsoil,eff corresponds with the effective soil-root-system conductance, Ksrs,eff. In order to stay in the linear (E, Ψsoil,eff, Ψleaf,eff) range in drier pedoclimatic conditions, we hypothesize that plants will adapt by reducing Ksrs,eff by reducing growth and reducing root tissue conductance. These adaptations are a response to a longer-term average of plant water status. Therefore, we will also investigate how stomatal closure in response to chemical vs. hydraulic signaling can avoid short-term excursions of plant water status in the non-linear regime of (E, Ψsoil,eff, Ψleaf). In annual crop systems, hydraulic adaptation will affect growth and the ratio of shoot:root conductances, while in perennial grasslands, the root distributions and combination of species will adapt to the pedoclimatic conditions.

• DFG Programme: Research Units
• Subproject of: FOR 5820: Soil-plant hydraulics impacting transpiration and plant growth in response to drought [SOPHY]
• Co-Investigator: Professor Mathieu Javaux, Ph.D.