Root-to-shoot surface ratio adaptation to soil hydraulic constraints: linking experiments to a soil-plant hydraulics model

Climate change is associated with rising temperatures and an increased frequency of drought events. Plants growing in water-limited environments must develop strategies to adapt to soil water availability. In the short term, stomatal regulation enables the control of transpiration and maintenance of plant water status during drought. Under prolonged water deficit, plants are expected to adjust their shoot-root allocation to sustain growth and survival. Although these adaptive responses are conceptually intuitive, the underlying processes and controlling factors remain poorly understood. Understanding short- and long-term plant responses to drought is crucial for investigating plant adaptation to climate changes.

In this work, we hypothesize that soil properties and climatic demand are key factors affecting plant stomatal conductance in the short term and root-to-shoot surface ratio (RSSR) over the longer term. Indeed, results of a simplified soil-plant hydraulic model demonstrated that the regulation of stomatal conductance and of RSSR should be texture dependent. We investigate these relationships through experiments conducted under controlled environmental conditions. Specifically, we assess how soil water content and soil type influence the RSSR of an isohydric species (maize) and an anisohydric species (sunflower). The experimental findings are subsequently analysed using a simplified soil-plant hydraulic model.

The experiment was conducted in a growth chamber controlling photoperiod, temperature, relative humidity, PAR, and VPD. Maize and sunflower were grown in pots using two contrasting substrates, sand and loam, whose hydraulic properties were characterized using the Hyprop system. Two irrigation regimes were imposed to maintain soil water content within predefined target ranges. Each of the 8 species × substrate × treatment combinations included 10 replicates.

Root and shoot biomass and surface were measured at 3 collects to capture plant growth dynamics. Soil water content was monitored by gravimetric measurements before and after each irrigation, with irrigation volumes adjusted to maintain the target moisture range. In addition, stomatal conductance and leaf water potential were punctually measured to characterize plant functioning.

We used a simplified soil-plant hydraulic model representing the system as three resistances in series (soil, roots, xylem), driven by soil-to-leaf water potential gradients (Carminati & Javaux, 2020). This model was employed to predict the optimal RSSR maximizing carbon assimilation while minimizing the risk of embolism.

Our results show that, despite differences in leaf and root surfaces, RSSRs remain within a similar range for both species. RSSR adaptation to soil texture is lower in maize (isohydric) than in sunflower (anisohydric). In addition, RSSR strongly depends on soil water potential (ψ_soil), with a stronger response in sunflower. This relationship is further constrained by soil texture through its hydraulic conductivity. For a given RSSR, plants grown in loam are able to sustain at lower ψ_soil compared with those grown in sand. To survive at similar ψ_soil in a sandy soil, plants would require a substantial increase in RSSR. However, root active surface depends on soil types and modulates the RSSR-ψ_soil relationship. Model predictive potential could be further improved by including additional information on active root surface.