Foliar water uptake (FWU) has been identified as a mechanism commonly used by trees and other plants originating from various biomes. However, many questions regarding the pathways and the ...implications of FWU remain, including its ability to mitigate climate change-driven drought. Therefore, answering these questions is of primary importance to adequately address and comprehend drought stress responses and associated growth. In this review, we discuss the occurrence, pathways, and consequences of FWU, with a focus predominantly on tree species. Subsequently, we highlight the tight coupling between FWU and foliar fertilizer applications, discuss FWU in a changing climate, and conclude with the importance of including FWU in mechanistic vegetation models.
Foliar water uptake (FWU) has been identified as a mechanism commonly used by plants originating from a range of biomes.FWU can rehydrate tissues and result in turgor-driven growth.FWU and the absorption of foliar fertilizers are interlinked, making FWU research important for both natural and agricultural ecosystems.As the number of climate change-induced drought events increases, so too will the relative importance of FWU.Future models should include FWU to correctly assess the impact of climate change on tree growth.
Summary
The long‐standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an ...isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source–stem water isotopic offsets is still poorly understood.
Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction. By contrast, water in nonconductive xylem tissues is always depleted in deuterium compared with sap water, irrespective of wood anatomy.
Previous studies have shown that isotopic heterogeneity also exists in soils at the pore scale in which water adsorbed onto soil particles is more depleted in deuterium than unbound water. Data collected at a riparian forest indicated that sap water matches best unbound soil water from depth below −70 cm, while bulk stem and soil water differ markedly.
We conclude that source–stem isotopic offsets can be explained by micrometre‐scale heterogeneity in the isotope ratios of water within woody stems and soil micro‐pores.
• The significance of shoot surface water uptake (SSWU) has been debated, and it would depend on the range of conditions under which it occurs. We hypothesized that the decline of leaf hydraulic ...conductance (K
leaf) in response to dehydration may be recovered through SSWU, and that the hydraulic conductance to SSWU (K
surf) declines with dehydration.
• We quantified effects of leaf dehydration on K
surf and effects of SSWU on recovery of K
leaf in dehydrated leaves of Avicennia marina.
• SSWU led to overnight recovery of K
leaf, with recovery retracing the same path as loss of K
leaf in response to dehydration. SSWU declined with dehydration. By contrast, K
surf declined with rehydration time but not with dehydration.
• Our results showed a role of SSWU in the recovery of leaf hydraulic conductance and revealed that SSWU is sensitive to leaf hydration status. The prevalence of SSWU in vegetation suggests an important role for atmospheric water sources in maintenance of leaf hydraulic function, with implications for plant responses to changing environments.
• A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We ...explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica.
• We measured δ²H and δ18O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes.
• Regardless of substrate, soil and stem water δ²H were similar only near permanent wilting point. Under moister conditions, stem water δ²H was 11 ± 3‰ more negative than soil water δ²H, coherent with field studies. Under drier conditions, stem water δ²H became progressively more enriched than soil water δ²H. Although stem water δ18O broadly reflected that of soil water, soil–stem δ²H and δ18O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies.
• Soil–stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil–plant continuum.
Summary
Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake ...depth, leaf‐level water‐use efficiency (WUEi) and water potential in drought‐prone plant communities is not well understood.
We conducted a 135‐d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co‐occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season.
Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller‐diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought‐induced canopy defoliation and/or mortality. By contrast, larger‐diameter species with deeper water uptake, higher leaf‐level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation.
Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.
Moderate soil drying can cause a strong decrease in the soil‐root system conductance. The resulting impact on root water uptake depends on the spatial distribution of the altered conductance ...relatively to remaining soil water resources, which is largely unknown. Here, we analyzed the vertical distribution of conductance across root systems using a novel, noninvasive sensor technology on pot‐grown faba bean and maize plants. Withholding water for 4 days strongly enhanced the vertical gradient in soil water potential. Therefore, roots in upper and deeper soil layers were affected differently: In drier, upper layers, root conductance decreased by 66%–72%, causing an amplification of the drop in leaf water potential. In wetter, deeper layers, root conductance increased in maize but not in faba bean. The consequently facilitated deep‐water uptake in maize contributed up to 21% of total water uptake at the end of the measurement. Analysis of root length distributions with MRI indicated that the locally increased conductance was mainly caused by an increased intrinsic conductivity and not by additional root growth. Our findings show that plants can partly compensate for a reduced root conductance in upper, drier soil layers by locally increasing root conductivity in wetter layers, thereby improving deep‐water uptake.
Summary statement
Soil drying leads to a strong reduction of root conductance in shallow soil layers, which maize partly compensates by facilitating deep water uptake via local increases in root conductivity.
Schroeder's paradox discovered by Schroeder in 1905 refers to the phenomenon that polymers have different maximum water uptake in the liquid and saturated vapor phases. For more than a hundred years, ...people have often debated whether this phenomenon conforms to thermodynamics. As proton exchange membrane fuel cell (PEMFC) gradually becomes a promising renewable energy utilization device, its impact on the physical properties of the proton exchange membrane has been studied widely. This paper reviews the theory and experiments on Schroeder's paradox over more than 100 years, especially the exploration of perfluorosulfonic acid membranes and PEMFCs in recent decades. Since membrane water content determines the operational performance of the PEMFC, this paper discusses and analyzes the effect of Schroeder's paradox on the PEMFC performance, including mechanical properties, electrical conductivity, and water transport mechanism. The effect of this phenomenon on the non-equilibrium operation of PEMFC has been highlighted, such as cold start-up, because of the different properties of membranes in contact with liquid water and air. This review gives an introduction to critical aspects of Schroeder's paradox to serve governments and organizations to promote the application of PEMFC in different regions.
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•Schroeder's paradox in proton exchange membrane fuel cells is reviewed.•Mechanisms and experimental methods of Schroeder's paradox are critically analyzed.•Evaluated the effect of Schroeder's paradox on membrane performance.•The existence of Schroeder's paradox lower freezing point is discussed.•Future research directions of Schroeder's paradox are presented.
Version 5 of the Community Land Model (CLM5) introduces the plant hydraulic stress (PHS) configuration of vegetation water use, which is described and compared with the corresponding parameterization ...from CLM4.5. PHS updates vegetation water stress and root water uptake to better reflect plant hydraulic theory, advancing the physical basis of the model. The new configuration introduces prognostic vegetation water potential, modeled at the root, stem, and leaf levels. Leaf water potential replaces soil potential as the basis for stomatal conductance water stress, and root water potential is used to implement hydraulic root water uptake, replacing a transpiration partitioning function. Point simulations of a tropical forest site (Caxiuanã, Brazil) under ambient conditions and partial precipitation exclusion highlight the differences between PHS and the previous CLM implementation. Model description and simulation results are contextualized with a list of benefits and limitations of the new model formulation, including hypotheses that were not testable in previous versions of the model. Key results include reductions in transpiration and soil moisture biases relative to a control model under both ambient and exclusion conditions, correcting excessive dry season soil moisture stress in the control model. PHS implements hydraulic gradient root water uptake, which allows hydraulic redistribution and compensatory root water uptake and results in PHS utilizing a larger portion of the soil column to buffer shortfalls in precipitation. The new model structure, which bases water stress on leaf water potential, could have significant implications for vegetation‐climate feedbacks, including increased sensitivity of photosynthesis to atmospheric vapor pressure deficit.
Key Points
An updated soil‐plant‐atmosphere continuum model based on hydraulic theory is implemented in the Community Land Model (version 5)
Prognostic leaf water potential replaces soil matric potential as the basis for stomatal conductance water stress
Prognostic root water potential is used to implement hydraulic root water uptake, replacing a “soil wilting point” approach
A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress ...conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.
We tested for isotope exchange between bound (immobile) and mobile soil water, and whether there is isotope fractionation during plant water uptake. These are critical assumptions to the formulation ...of the ‘two water worlds’ hypothesis based on isotope profiles of soil water.
In two different soil types, soil-bound water in two sets of 19-l pots, each with a 2-yr-old avocado plant (Persea americana), were identically labeled with tap water. After which, one set received isotopically enriched water whereas the other set received tap water as the mobile phase water. After a dry down period, we analyzed plant stem water as a proxy for soil-bound water as well as total soil water by cryogenic distillation.
Seventy-five to 95% of the bound water isotopically exchanged with the mobile water phase. In addition, plants discriminated against 18O and 2H during water uptake, and this discrimination is a function of the soil water loss and soil type.
The present experiment shows that the assumptions for the ‘two water worlds’ hypothesis are not supported. We propose a novel explanation for the discrepancy between isotope ratios of the soil water profile and other water compartments in the hydrological cycle.
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