Stomatal response to VPD is not triggered by changes in soil-leaf hydraulic conductance in Arabidopsis or Callitris.

Stomatal closure under high VPDL (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψleaf) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψleaf appear to drive stomatal VPDL response, but many argue that dynamic changes in soil-to-leaf hydraulic conductance (Ks-l) make an important contribution to this response pathway, even in well-hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance (gc) in response to typical changes in daytime VPDL is influenced by dynamic changes in Ks-l. We use well-watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia-0) and the dry forest conifer Callitris rhomboidea. The dynamics of Ks-l and gc were continuously monitored by combining concurrent in situ measurements of Ψleaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPDL were imposed to induce stomatal closure and observe the impact on Ks-l. In both species, gc was observed to decline substantially as VPDL increased, while Ks-l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in Ks-l. Static Ks-l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.

Interspecific variation in the temperature response of mesophyll conductance is related to leaf anatomy.

Although mesophyll conductance (gm ) is known to be sensitive to temperature (T), the mechanisms underlying the temperature response of gm are not fully understood. In particular, it has yet to be established whether interspecific variation in gm -T relationships is associated with mesophyll anatomy and vein traits. In the present study, we measured the short-term response of gm in eight crop species, and leaf water potential (Ψleaf ) in five crop species over a temperature range of 15-35°C. The considered structural parameters are surface areas of mesophyll cells and chloroplasts facing intercellular airspaces per unit leaf area (Sm and Sc ), cell wall thickness (Tcw ), and vein length per area (VLA). We detected large interspecific variations in the temperature responses of gm and Ψleaf . The activation energy for gm (Ea,gm ) was found to be positively correlated with Sc , although it showed no correlation with Tcw . In contrast, VLA was positively correlated with the slope of the linear model of Ψleaf -T (a), whereas Ea,gm was marginally correlated with VLA and a. A two-component model was subsequently used to model gm -T relationships, and the mechanisms underlying the temperature response of gm are discussed. The data presented here indicate that leaf anatomy is a major determinant of the interspecific variation in gm -T relationships.

Phenotypic variation in leaf photosynthetic traits, leaf area index, and carbon discrimination of field-grown wheat genotypes and their relationship with yield performance in Mediterranean environments.

MAIN CONCLUSION: Leaf water potential, gas exchange, and chlorophyll fluorescence exhibited significant differences among genotypes, high environmental effects, but low heritability. The highest-yielding and drought-tolerant genotypes presented superior harvest index and grain weight, compared to drought-susceptible ones. Physiological phenotyping can help identify useful traits related to crop performance under water-limited conditions. A set of fourteen bread wheat genotypes with contrasting grain yield (GY) was studied in eight Mediterranean environments in Chile, resulting from the combination of two sites (Cauquenes and Santa Rosa), two water conditions (rainfed-WL and irrigated-WW), and four growing seasons (2015-2018). The objectives were to (i) evaluate the phenotypic variation of leaf photosynthetic traits after heading (anthesis and grain filling) in different environments; (ii) analyze the relationship between GY and leaf photosynthetic traits and carbon isotope discrimination (Δ13C); and (iii) identify those traits that could have a greater impact in the determination of tolerant genotypes under field conditions. Agronomic traits exhibited significant genotypic differences and genotype × environment (GxE) interaction. The average GY under the WW condition at Santa Rosa was 9.2 Mg ha-1 (range 8.2-9.9 Mg ha-1) and under the WL condition at Cauquenes was 6.2 Mg ha-1 (range 3.7-8.3 Mg ha-1). The GY was closely related to the harvest index (HI) in 14 out of 16 environments, a trait exhibiting a relatively high heritability. In general terms, the leaf photosynthetic traits presented low GxE interaction, but high environmental effects and low heritability, except for the chlorophyll content. The relationships between GY and leaf photosynthetic traits were weaker when performed across genotypes in each environment, indicating low genotypic effects, and stronger when performed across environments for each genotype. The leaf area index and Δ13C also presented high environmental effects and low heritability, and their correlations with GY were influenced by environmental effects. The highest-yielding and drought-tolerant genotypes presented superior HI and grain weight, but no clear differences in leaf photosynthetic traits or Δ13C, compared to drought-susceptible ones. It seems that the phenotypic plasticity of agronomic and leaf photosynthetic traits is very important for crop adaptation to Mediterranean environments.

Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night.

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (Gc ) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of Gc are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between Gc and the ability of stem tissues to rehydrate at night remains unclear. We investigated whether species-specific Gc responses aim to prevent branch embolisms, or enable night-time stem rehydration, which is critical for turgor-dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch-vulnerability curves of six common European tree species. Species-specific Gc reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P50 ). Instead, we found a stronger relationship with stem rehydration. Species with a stronger Gc control were less effective at refilling stem-water storage as the soil dries, which appeared related to their xylem architecture. Our findings highlight the importance of stem rehydration for water-use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety-efficiency stomatal control paradigm.

Soil-plant hydraulics explain stomatal efficiency-safety tradeoff.

The efficiency-safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency-safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax ) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50 ). However, the underlying mechanism of this gmax -ψgs50 tradeoff remains unknown. Here, we utilized a soil-plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil-plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50 . Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil-root interface, and therefore in the leaves. We demonstrated that the gmax -ψgs50 tradeoff can be predicted based on soil-plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions.

High vapour pressure deficit enhances turgor limitation of stem growth in an Asian tropical rainforest tree.

Tropical forests are experiencing increases in vapour pressure deficit (D), with possible negative impacts on tree growth. Tree-growth reduction due to rising D is commonly attributed to carbon limitation, thus overlooking the potentially important mechanism of D-induced impairment of wood formation due to an increase in turgor limitation. Here we calibrate a mechanistic tree-growth model to simulate turgor limitation of radial stem growth in mature Toona cilitata trees in an Asian tropical forest. Hourly sap flow and dendrometer measurements were collected to simulate turgor-driven growth during the growing season. Simulated seasonal patterns of radial stem growth matched well with growth observations. Growth mainly occurred at night and its pre-dawn build-up appeared to be limited under higher D. Across seasons, the night-time turgor pressure required for growth was negatively related to previous midday D, possibly due to a relatively high canopy conductance at high D, relative to stem rehydration. These findings provide the first evidence that tropical trees grow at night and that turgor pressure limits tree growth. We suggest including turgor limitation of tree stem growth in models also for tropical forest carbon dynamics, in particular, if these models simulate effects of warming and increased frequency of droughts.

The role of arbuscular mycorrhizal symbiosis in improving plant water status under drought.

Arbuscular mycorrhizal fungi (AMF) have been presumed to ameliorate crop tolerance to drought. Here, we review the role of AMF in maintaining water supply to plants from drying soils and the underlying biophysical mechanisms. We used a soil-plant hydraulic model to illustrate the impact of several AMF mechanisms on plant responses to edaphic drought. The AMF enhance the soil's capability to transport water and extend the effective root length, thereby attenuating the drop in matric potential at the root surface during soil drying. The synthesized evidence and the corresponding simulations demonstrate that symbiosis with AMF postpones the stress onset limit, which is defined as the disproportionality between transpiration rates and leaf water potentials, during soil drying. The symbiosis can thus help crops survive extended intervals of limited water availability. We also provide our perspective on future research needs and call for reconciling the dynamic changes in soil and root hydraulics in order to better understand the role of AMF in plant water relations in the face of climate changes.

The ecosystem wilting point defines drought response and recovery of a Quercus-Carya forest.

Soil and atmospheric droughts increasingly threaten plant survival and productivity around the world. Yet, conceptual gaps constrain our ability to predict ecosystem-scale drought impacts under climate change. Here, we introduce the ecosystem wilting point (ΨEWP ), a property that integrates the drought response of an ecosystem's plant community across the soil-plant-atmosphere continuum. Specifically, ΨEWP defines a threshold below which the capacity of the root system to extract soil water and the ability of the leaves to maintain stomatal function are strongly diminished. We combined ecosystem flux and leaf water potential measurements to derive the ΨEWP of a Quercus-Carya forest from an "ecosystem pressure-volume (PV) curve," which is analogous to the tissue-level technique. When community predawn leaf water potential (Ψpd ) was above ΨEWP (=-2.0 MPa), the forest was highly responsive to environmental dynamics. When Ψpd fell below ΨEWP , the forest became insensitive to environmental variation and was a net source of carbon dioxide for nearly 2 months. Thus, ΨEWP is a threshold defining marked shifts in ecosystem functional state. Though there was rainfall-induced recovery of ecosystem gas exchange following soaking rains, a legacy of structural and physiological damage inhibited canopy photosynthetic capacity. Although over 16 growing seasons, only 10% of Ψpd observations fell below ΨEWP , the forest is commonly only 2-4 weeks of intense drought away from reaching ΨEWP , and thus highly reliant on frequent rainfall to replenish the soil water supply. We propose, based on a bottom-up analysis of root density profiles and soil moisture characteristic curves, that soil water acquisition capacity is the major determinant of ΨEWP , and species in an ecosystem require compatible leaf-level traits such as turgor loss point so that leaf wilting is coordinated with the inability to extract further water from the soil.

Above and belowground traits impacting transpiration decline during soil drying in 48 maize (Zea mays) genotypes.

BACKGROUND AND AIMS: Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that the stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root-soil interface, which in turn depends on plant hydraulic traits. METHODS: We investigate the response of 48 contrasting maize (Zea mays) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. KEY RESULTS: Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. CONCLUSIONS: Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root-soil interface.

The role of root size and root efficiency in grain production, and water-and nitrogen-use efficiency in wheat.

BACKGROUND: The root system is the major plant organ involved in water and nutrient acquisition, influencing plant growth and productivity. However, the relative importance of root size and uptake efficiency remains undetermined. A pot experiment was conducted using two wheat varieties with different root sizes to evaluate their capacity for water and nitrogen (N) uptake and their effects on grain production, water-use efficiency (WUE), and N-use efficiency (NUE) under two water treatments combined with three N levels. RESULTS: The leaf water potential and root exudates of changhan58 (CH, small root variety) were higher or similar to those of changwu134 (CW, large root variety) under water/N treatment combinations, indicating that small roots can transport enough water to above the ground. The addition of N improved plant growth, photosynthetic traits, and WUE significantly. There were no significant differences in WUE or grain production between the two cultivars under well-watered conditions. However, they were significantly higher in CH than in CW under water deficit stress. Nitrogen uptake per unit root dry weight, glutaminase, and nitrate reductase activities were significantly higher in CH than in CW, regardless of moisture conditions. Root biomass was positively correlated with evapotranspiration, while the root/shoot ratio was negatively correlated with WUE (P < 0.05) but not with NUE. CONCLUSION: In a pot experiment, water and N uptake were more strongly associated with resource uptake availability than root size. This may provide guidance in wheat breeding programs for drought-prone regions. © 2023 Society of Chemical Industry.

Leaf water potential measurements using the pressure chamber: Synthetic testing of assumptions towards best practices for precision and accuracy.

Leaf water potential (ψleaf ), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of ψleaf (ψleafPC ) are most typical, yet, the practical complexity of the technique and of the underlying theory has led to ambiguous understanding of the conditions to optimize measurements. Consequently, specific techniques and precautions diversified across the global research community, raising questions of reliability and repeatability. Here, we surveyed specific methods of ψleafPC from multiple laboratories, and synthesized experiments testing common assumptions and practices in ψleafPC for diverse species: (i) the need for equilibration of previously transpiring leaves; (ii) leaf storage before measurement; (iii) the equilibration of ψleaf for leaves on bagged branches of a range of dehydration; (iv) the equilibration of ψleaf across the lamina for bagged leaves, and the accuracy of measuring leaves with artificially 'elongated petioles'; (v) the need in ψleaf measurements for bagging leaves and high humidity within the chamber; (vi) the need to avoid liquid water on leaf surfaces; (vii) the use of 'pulse' pressurization versus gradual pressurization; and (viii) variation among experimenters in ψleafPC determination. Based on our findings we provide a best practice protocol to maximise accuracy, and provide recommendations for ongoing species-specific tests of important assumptions in future studies.

The xylem of anisohydric Quercus alba L. is more vulnerable to embolism than isohydric codominants.

The coordination of plant leaf water potential (ΨL ) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem more conservatively regulate ΨL than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh. and Liriodendron tulipifera L., by synthesizing 1600 ΨL observations, 122 xylem embolism curves and xylem anatomical measurements across 10 forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated ΨL less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose ΨL regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future.

Root hydraulic phenotypes impacting water uptake in drying soils.

Soil drying is a limiting factor for crop production worldwide. Yet, it is not clear how soil drying impacts water uptake across different soils, species, and root phenotypes. Here we ask (1) what root phenotypes improve the water use from drying soils? and (2) what root hydraulic properties impact water flow across the soil-plant continuum? The main objective is to propose a hydraulic framework to investigate the interplay between soil and root hydraulic properties on water uptake. We collected highly resolved data on transpiration, leaf and soil water potential across 11 crops and 10 contrasting soil textures. In drying soils, the drop in water potential at the soil-root interface resulted in a rapid decrease in soil hydraulic conductance, especially at higher transpiration rates. The analysis reveals that water uptake was limited by soil within a wide range of soil water potential (-6 to -1000 kPa), depending on both soil textures and root hydraulic phenotypes. We propose that a root phenotype with low root hydraulic conductance, long roots and/or long and dense root hairs postpones soil limitation in drying soils. The consequence of these root phenotypes on crop water use is discussed.

Interannual variability of ecosystem iso/anisohydry is regulated by environmental dryness.

●Plants are characterized by the iso/anisohydry continuum depending on how they regulate leaf water potential (ΨL ). However, how iso/anisohydry changes over time in response to year-to-year variations in environmental dryness and how such responses vary across different regions remains poorly characterized. ●We investigated how dryness, represented by aridity index, affects the interannual variability of ecosystem iso/anisohydry at the regional scale, estimated using satellite microwave vegetation optical depth (VOD) observations. This ecosystem-level analysis was further complemented with published field observations of species-level ΨL . ●We found different behaviors in the directionality and sensitivity of isohydricity (σ) with respect to the interannual variation of dryness in different ecosystems. These behaviors can largely be differentiated by the average dryness of the ecosystem itself: in mesic ecosystems, σ decreases in drier years with a higher sensitivity to dryness; in xeric ecosystems, σ increases in drier years with a lower sensitivity to dryness. These results were supported by the species-level synthesis. ●Our study suggests that how plants adjust their water use across years - as revealed by their interannual variability in isohydricity - depends on the dryness of plants' living environment. This finding advances our understanding of plant responses to drought at regional scales.

Mesophyll photosynthetic sensitivity to leaf water potential in Eucalyptus: a new dimension of plant adaptation to native moisture supply.

Photosynthetic sensitivity to drought is a fundamental constraint on land-plant evolution and ecosystem function. However, little is known about how the sensitivity of photosynthesis to nonstomatal limitations varies among species in the context of phylogenetic relationships. Using saplings of 10 Eucalyptus species, we measured maximum CO2 -saturated photosynthesis using A-ci curves at several different leaf water potentials (ψleaf ) to quantify mesophyll photosynthetic sensitivity to ψleaf (MPS), a measure of how rapidly nonstomatal limitations to carbon uptake increase with declining ψleaf . MPS was compared to the macroclimatic moisture availability of the species' native habitats, while accounting for phylogenetic relationships. We found that species native to mesic habitats have greater MPS but higher maximum photosynthetic rates during non-water-stressed conditions, revealing a trade-off between maximum photosynthesis and drought sensitivity. Species with lower turgor loss points have lower MPS, indicating coordination among photosynthetic and water-relations traits. By accounting for phylogenetic relationships among closely related species, we provide the first compelling evidence that MPS in Eucalyptus evolved in an adaptive fashion with climatically determined moisture availability, opening the way for further study of this poorly explored dimension of plant adaptation to drought.

Stomatal closure of tomato under drought is driven by an increase in soil-root hydraulic resistance.

The fundamental question as to what triggers stomatal closure during soil drying remains contentious. Thus, we urgently need to improve our understanding of stomatal response to water deficits in soil and atmosphere. Here, we investigated the role of soil-plant hydraulic conductance (Ksp ) on transpiration (E) and stomatal regulation. We used a root pressure chamber to measure the relation between E, leaf xylem water potential (ψleaf-x ) and soil water potential (ψsoil ) in tomato. Additional measurements of ψleaf-x were performed with unpressurized plants. A soil-plant hydraulic model was used to simulate E(ψleaf-x ) for decreasing ψsoil . In wet soils, E(ψleaf-x ) had a constant slope, while in dry soils, the slope decreased, with ψleaf-x rapidly and nonlinearly decreasing for moderate increases in E. The ψleaf-x measured in pressurized and unpressurized plants matched well, which indicates that the shoot hydraulic conductance did not decrease during soil drying and that the decrease in Ksp is caused by a decrease in soil-root conductance. The decrease of E matched well the onset of hydraulic nonlinearity. Our findings demonstrate that stomatal closure prevents the drop in ψleaf-x caused by a decrease in Ksp and elucidate a strong correlation between stomatal regulation and belowground hydraulic limitation.

The role of leaf water potential in the temperature response of mesophyll conductance.

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψleaf ), which may influence mesophyll conductance (gm ). However, the effects of Ψleaf on gm have not yet been considered in models of the gm response to temperature. Temperature responses of gm and Ψleaf and the response of gm to Ψleaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψleaf was incorporated into an existing gm -T model. In wheat, Ψleaf was dramatically decreased with increasing T, whereas in rice Ψleaf was less sensitive or insensitive to T. Without taking Ψleaf into account, gm for wheat showed no response to T. However, at a given Ψleaf , gm was significantly higher at high temperature compared with low. After incorporating the function of Ψleaf into the gm -T model, we suggest that the gm -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψleaf gradient between temperatures, and the sensitivity of gm to Ψleaf , below a threshold (Ψleaf,0 ). The data presented here suggest that Ψleaf plays an important role in the gm -T relationship and should be considered in future studies related to the temperature response of gm and photosynthesis.

Leaf carbon and water status control stomatal and nonstomatal limitations of photosynthesis in trees.

Photosynthetic rate is concurrently limited by stomatal limitations and nonstomatal limitations (NSLs). However, the controls on NSLs to photosynthesis and their coordination with stomatal control on different timescales remain poorly understood. According to a recent optimization hypothesis, NSLs depend on leaf osmotic or water status and are coordinated with stomatal control so as to maximize leaf photosynthesis. Drought and notching experiments were conducted on Pinus sylvestris, Picea abies, Betula Pendula and Populus tremula seedlings in glasshouse conditions to study the dependence of NSLs on leaf osmotic and water status, and their coordination with stomatal control, on timescales of minutes and weeks, to test the assumptions and predictions of the optimization hypothesis. Both NSLs and stomatal conductance followed power-law functions of leaf osmotic concentration and leaf water potential. Moreover, stomatal conductance was proportional to the square root of soil-to-leaf hydraulic conductance, as predicted by the optimization hypothesis. Though the detailed mechanisms underlying the dependence of NSLs on leaf osmotic or water status lie outside the scope of this study, our results support the hypothesis that NSLs and stomatal control are coordinated to maximize leaf photosynthesis and allow the effect of NSLs to be included in models of tree gas-exchange.

Atmospheric CO2 and VPD alter the diel oscillation of leaf elongation in perennial ryegrass: compensation of hydraulic limitation by stored-growth.

We explored the effects of atmospheric CO2 concentration (Ca ) and vapor pressure deficit (VPD) on putative mechanisms controlling leaf elongation in perennial ryegrass. Plants were grown in stands at a Ca of 200, 400 or 800 µmol mol-1 combined with high (1.17 kPa) or low (0.59 kPa) VPD during the 16 h-day in well-watered conditions with reduced nitrogen supply. We measured day : night-variation of leaf elongation rate (LERday  : LERnight ), final leaf length and width, epidermal cell number and length, stomatal conductance, transpiration, leaf water potential and water-soluble carbohydrates and osmotic potential in the leaf growth-and-differentiation zone (LGDZ). Daily mean LER or morphometric parameters did not differ between treatments, but LERnight strongly exceeded LERday , particularly at low Ca and high VPD. Across treatments LERday was negatively related to transpiration (R2  = 0.75) and leaf water potential (R2  = 0.81), while LERnight was independent of leaf water potential or turgor. Enhancement of LERnight over LERday was proportional to the turgor-change between day and night (R2  = 0.93). LGDZ sugar concentration was high throughout diel cycles, providing no evidence of source limitation in any treatment. Our data indicate a mechanism of diel cycling between daytime hydraulic and night-time stored-growth controls of LER, buffering Ca and daytime VPD effects on leaf elongation.

Linking reliance on deep soil water to resource economy strategies and abundance among coexisting understorey shrub species in subtropical pine plantations.

Strategies for deep soil water acquisition (WAdeep ) are critical to a species' adaptation to drought. However, it is unknown how WAdeep determines the abundance and resource economy strategies of understorey shrub species. With data from 13 understorey shrub species in subtropical coniferous plantations, we investigated associations between the magnitude of WAdeep , the seasonal plasticity of WAdeep , midday leaf water potential (Ψmd ), species abundance and resource economic traits across organs. Higher capacity for WAdeep was associated with higher intrinsic water use efficiency, but was not necessary for maintaining higher Ψmd in the dry season nor was it an ubiquitous trait possessed by the most common shrub species. Species with higher seasonal plasticity of WAdeep had lower wood density, indicating that fast species had higher plasticity in deep soil resource acquisition. However, the magnitude and plasticity of WAdeep were not related to shallow fine root economy traits, suggesting independent dimensions of soil resource acquisition between deep and shallow soil. Our results provide new insights into the mechanisms through which the magnitude and plasticity of WAdeep interact with shallow soil and aboveground resource acquisition traits to integrate the whole-plant economic spectrum and, thus, community assembly processes.