Andrea Carminati

This paper investigates the development of azimuthal root asymmetry in riparian plant cuttings as a response to hydrodynamic forces, a novel tropism response that we refer to as “dynamotropism.” Laboratory experiments were performed within an experimental flume using Salix fragilis cuttings growing in rhizoboxes filled with quartz sand and undergoing different treatment conditions. These aimed to isolate and differentiate the effects that hydrodynamic forces acting on the stem exert on the root vertical biomass development from that of hyporheic flow. Results reveal that in response to an applied external (drag) force acting on the stem, root biomass increases significantly in the upstream direction, opposite the drag force, resulting in approximately 30% greater total biomass upstream than downstream. This suggests a structural adaptation of the below‐ground biomass to resist mechanical loading of the aboveground part of the cutting. While the effect of force seems to influence overall root production, the effect of hyporheic flow does not affect the total biomass in either directions. Hyporheic flow, in particular, tends to enhance root growth toward the downstream side at higher depth than in the top soil layers. Hence, flow and force appear to promote opposing patterns of root development along the sediment depth, whose evidence is maximized when both stimuli are applied simultaneously. When acting together, the relative magnitudes of such stimuli become crucial in determining the final root architecture where mechanical force dominated over flow stimulus. The results of this work path the way to novel plant nursing strategies for bioengineering applications.

Plants are often assumed to optimize carbon assimilation against water loss through stomatal regulation, responding instantaneously to varying atmospheric demand and soil water supply. However, a growing body of work highlights the importance of nighttime recovery processes for understanding plant water use, yet nighttime processes are not considered in existing supply–demand models. Here, we extend an existing supply–demand model by including flux imbalances between atmospheric demand and soil supply buffered by plant water storage. The flux imbalances accumulate during the day and are compensated overnight. Applying the conceptual model to field data from Quercus pubescens Willd., we show that progressive soil drying constrains the rate of nighttime rehydration. Precisely, below a critical soil water potential threshold (ψsoil,crit), nighttime rehydration begins to decline because rehydration rates become too low relative to the duration of the night (‘the night is not long enough’). This threshold marks a transition beyond which maintaining high transpiration rates becomes increasingly risky. The observations show that stomatal regulation begins near this threshold, pointing to the concomitance between daytime stomatal regulation and limited nighttime rehydration. These results suggest that progressive soil drying constrains nighttime rehydration and reveal a critical hydraulic threshold that coincides with the onset of stomatal regulation and highlight that plant water use is constrained not only by daytime fluxes but also by nighttime recovery.

Soil water content underpins hydrological, ecosystem, and land–atmosphere processes, and estimates based on SMAP and ERA5-Land support climate modelling and land management. Yet these products often disagree with in situ observations in magnitude and dynamic range, and it remains unclear whether this reflects poor representation of wetting–drying dynamics or the absolute water-content range (or both). A Long Short-Term Memory (LSTM) model was developed to represent relative wetting–drying dynamics from horizontally polarised SMAP brightness temperature and ERA5-Land precipitation and potential evaporation. LSTM-model predictions were converted into absolute soil water content using site-specific minimum and maximum water contents (hydrological bounds) deduced from observations, SMAP soil moisture retrieval (SMAP-SM), or ERA5-Land soil moisture predictions (ERA5-SM). The model was trained at three COSMOS-UK sites and evaluated at 16 independent COSMOS-UK sites using field-scale cosmic-ray neutron sensing (CRNS) and point-scale time-domain transmissometry (TDT), with SMAP-SM and ERA5-SM as benchmarks. Despite the small number of input variables, the LSTM-model was equally good or better than the other products. Using observed bounds, the LSTM-model achieved median RMSEs of 4.4 (CRNS) and 6.7 percentage points (TDT), with near-zero bias; SMAP-SM was dry-biased and ERA5-SM wet-biased. Product-derived bounds increased absolute errors. With respect to anomaly dynamics, ERA5-SM slightly outperformed the LSTM-model, while SMAP-SM was weakest. The results indicate that SMAP-SM and ERA5-SM track relative dynamics more reliably than absolute soil water content. Future research must invest in both (i) estimating relative soil moisture dynamics and (ii) assessing the hydrological bounds to translate those dynamics into absolute values.

Background and Aims: The hydraulic conductance of the soil is expected to become limiting for plant transpiration in drying soils. Specifically, coarse textured soils are expected to limit transpiration at less negative soil water potential (Ψ _soil ). Yet, there is still limited experimental evidence on Ψ _soil and relative transpiration rate at which the decline in soil conductance impacts plant water status and the whole soil to plant hydraulic conductance. This gap is related to the lack of accurate and temporally resolved measurements of soil and plant water potential and hydraulic conductance. Methods: We combine rehydration techniques and water potential time-series measurements in wheat plants in two contrasting soil textures to measure the relative importance of soil and plant hydraulic conductance on stem water potential and transpiration rate. Results: We found that during imposed water stress, a 50% reduction in transpiration rate was concomitant with the soil hydraulic conductance (K _soil ) falling below the plant hydraulic conductance (K _plant ): K _soil declined to 18–56% of K _plant in loam and to 7–39% in sand. Conclusions: The relative importance of soil and plant hydraulics was texture-specific, with coarse-textured soils imposing greater resistance on soil to leaf water flow at less negative Ψ _soil than fine-textured soils. Upon rewetting, the soil–plant hydraulic conductance rapidly reached values it had before the plant was exposed to drying. These results highlight the soil-texture dependency of soil hydraulic properties on plant response to soil drying. The high temporal resolution and non-invasive experimental method is capable to resolve the effect of declining soil hydraulic conductance on the overall soil–plant conductance. Highlight: Leaf rehydration kinetics show that soil, not plant, limits water transport during drought—especially in coarse soils where hydraulic conductance drops at less negative water potential.

The global rise in vapor pressure deficit (VPD) threatens crop production. Plants mitigate high VPD by regulating their transpiration rate (TR). Yield is related to TR, but whether genetic yield gains reflect shifts in TR-VPD responses remains debated. Over the last century, U.S. spring wheat yields have tripled in the Midwest. We aimed to determine (1) whether these gains relate to changes in water-use patterns under rising VPD and (2) to identify underlying traits. We examined whole-plant TR responses to rising VPD under non-limiting soil moisture in 14 wheat cultivars released over 105 years, representing a gradient in yield potential. Yield was quantified in a common trial, and morphological and hydraulic properties shaping TR-VPD responses were characterized under controlled conditions. Yield gains temporarily coincided with shifts in water-use patterns under rising VPD. Breeding has maintained a restricted water loss under high VPD in all cultivars. However, the tripling of yield potential around the Green Revolution coincided with the linearization of the TR-VPD response and a reduction in evaporative surface area in modern cultivars. This suggests a more effective water use rather than a reduced total water loss. Daily water-use regulation was associated with traits controlling water demand and supply. TR was restricted at lower VPD (VPDBP), but less effectively (higher slope after VPDBP, more linear TR-VPD response) in plants with a high water demand at low VPD, relative to their water channeling ability. These findings highlight opportunities to improve yields by fine-tuning hydraulic traits.

Summary: The regulation of stomata in response to environmental changes is one of the most dynamic and important behaviours in vascular plants, playing a central role in determining gas exchange and vulnerability to drought. Yet whether this regulation is determined by intrinsic species physiology or by soil hydraulic properties or a combination of both remains unsolved. Here, we use a long‐term dataset of needle water potential (Ψ _needle ) measurements to compare stomatal regulation in a hardy conifer, Callitris rhomboidea , growing at two sites with contrasting soil textures: sand and clay‐loam. Inferred stomatal sensitivity to water potential differed between soil types when expressed against soil water potential but exhibited a universal relationship to Ψ _needle . This observation aligned with the predictions from an optimal soil water extraction model, indicating that distinct optimal water extraction in the two soil types could be achieved through a common relationship between stomatal conductance and Ψ _needle . Our results suggest that the primary driver of stomatal response to soil and atmospheric dryness lies in a common response to leaf water potential. This work reconciles competing ideas of stomatal control, indicating how the appearance of soil‐specific behaviour is in fact the expected consequence of a single, optimal physiological behaviour across soil types. Characterising the variation in species‐specific stomatal response to leaf water potential will enable comparisons of water extraction behaviour between species in different soils.

Summary: Plant water potential is a central integrator of plant water status, linking hydraulic function with physiological performance and ecosystem water dynamics across species and systems. This review is motivated by the need to capture these dynamics under rapidly changing environmental conditions, which are often missed by discrete measurements. We evaluate the main approaches for continuous monitoring of plant water potential, including direct in situ sensors, indirect methods based on plant water content, and remote‐sensing proxies. We discuss the principles, measurement mechanisms, practical constraints, and environmental sensitivities of each approach. Relative to traditional methods, such as pressure chambers, continuous measurements offer major advantages by resolving rapid variation in water status and strengthening inference on plant–soil–atmosphere interactions. These approaches are especially valuable under dynamic field conditions, where temporal variability in vapor pressure deficit, soil moisture, temperature, and radiation strongly shapes hydraulic behavior. We conclude that continuous monitoring has substantial potential to advance plant and ecosystem science, but wider application will depend on careful interpretation and greater harmonization across comparable methodologies. By synthesizing core principles, methodological challenges and best practices, this review provides a practical framework for researchers and practitioners applying continuous water potential measurements.

Summary: Plant water storage contributes to transpiration, but it is unclear how its relevance in supporting transpiration depends on the stringency of stomatal regulation. Here, we show the compounding effect of stomatal regulation and hydraulic capacitance on plant water use, by means of a soil–plant hydraulic model and measurements of leaf water potential, sap flow, stomatal conductance and capacitance in beech and spruce in the field. We found that large capacitance led to a large buffering effect on leaf water potential, explained by increasing amounts of transpiration sourced from internal plant water storage. However, the extent to which capacitance allows plants to sustain transpiration depends on the stringency of stomatal regulation. For stomata that limit leaf water potential at a fixed threshold (as observed in spruce), large capacitance increased transpiration throughout all soil water conditions. By contrast, for flexible stomatal regulation mechanisms optimizing transpiration over leaf water potential (as observed in beech), large capacitance caused stomata to close earlier in the day under wet soil conditions. Our findings suggest a trade‐off between developing tissues that can store large water volumes and stomatal regulation mechanisms that allow leaf water potential to reach more negative values during periods of high transpiration demand.