Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.     

八个树种叶水力性状对水分条件的响应及其驱动因素

树木叶片的水力效率和安全性会对水分条件的改变做出一定的响应, 进而影响树木的生长和分布, 然而叶导水率(K_leaf)和叶水力脆弱性(P₅₀)对不同水分条件的响应模式及其影响因素尚不清楚。该研究选取了晋西北关帝山和黑茶山两种水分条件下的8种树种, 测量其水力性状、叶片导管和形态性状, 比较两地不同树种的K_leaf和P₅₀的变化, 分析叶片水力效率和安全性之间的权衡关系, 并探讨叶片水力性状在不同树种及水分条件下的响应模式及其驱动因素。结果表明: 对同一树种而言, 湿润的关帝山叶最大导水率(K_max)和P₅₀均高于干旱的黑茶山; 对同一地区而言, 从在高水分条件下生长的树种到在易干旱环境生长的树种, K_max和P₅₀均逐渐下降。K_max、P₅₀、膨压丧失点水势(TLP)之间均存在显著相关关系。两地叶片P₅₀与导管密度、导管塌陷预测值((t/b)³)、叶片厚度、比叶质量显著正相关, 与导管直径、叶面积显著负相关, 不同树种的K_leaf和P₅₀与叶导管性状的关系大于叶形态性状。同一树种的关帝山到黑茶山P₅₀变化量(δP₅₀)与比叶质量和叶干物质含量在两地的变化量显著正相关, 同一树种δP₅₀与叶形态性状变化量的关系大于与叶导管性状的。以上结果表明: 随着水分条件变差, 叶片水力效率降低, 水力安全性提高, 不同树种叶片水力效率与安全性之间存在一定的权衡关系, 不同树种叶水力性状的差别受叶导管性状影响的程度大于受叶形态性状的影响, 同一树种叶水力安全性对水分条件变化的响应主要依靠叶形态性状的驱动, 树木在提高自身叶水力安全的同时增加了叶构建的碳投资。     

The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species

The hydraulic conductance of the leaf lamina (K_lamina) substantially constrains whole‐plant water transport, but little is known of its association with leaf structure and function. K_lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K_lamina varied from 7.40 × 10^?5 kg m^?2 s^?1 MPa^?1 for Acer saccharum shade leaves to 2.89 × 10^?4 kg m^?2 s^?1 MPa^?1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher K_lamina than shade leaves. K_lamina was co‐ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K_lamina was also co‐ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K_lamina was independent of inter‐related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K_lamina was thus co‐ordinated with structural and functional traits relating to liquid‐phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.     

Co-ordination between leaf biomechanical resistance and hydraulic safety across 30 sub-tropical woody species

Background and AimsLeaf biomechanical resistance protects leaves from biotic and abiotic damage. Previous studies have revealed that enhancing leaf biomechanical resistance is costly for plant species and leads to an increase in leaf drought tolerance. We thus predicted that there is a functional correlation between leaf hydraulic safety and biomechanical characteristics.MethodsWe measured leaf morphological and anatomical traits, pressure–volume parameters, maximum leaf hydraulic conductance (K_leaf-max), leaf water potential at 50 % loss of hydraulic conductance (P50_leaf), leaf hydraulic safety margin (SM_leaf), and leaf force to tear (F_t) and punch (F_p) of 30 co-occurring woody species in a sub-tropical evergreen broadleaved forest. Linear regression analysis was performed to examine the relationships between biomechanical resistance and other leaf hydraulic traits.Key ResultsWe found that higher F_t and F_p values were significantly associated with a lower (more negative) P50_leaf and a larger SM_leaf, thereby confirming the correlation between leaf biomechanical resistance and hydraulic safety. However, leaf biomechanical resistance showed no correlation with K_leaf-max, although it was significantly and negatively correlated with leaf outside-xylem hydraulic conductance. In addition, we also found that there was a significant correlation between biomechanical resistance and the modulus of elasticity by excluding an outlier.ConclusionsThe findings of this study reveal leaf biomechanical–hydraulic safety correlation in sub-tropical woody species.     

Properties of Fly Ash as Hydraulic Barrier

This study examines the suitability of pozzolanic fly ash as a hydraulic barrier and the use of bentonite to enhance geotechnical properties of fly ash. The behavior of fly ash is studied not only with water but also with different pore fluids, such as acid, alkali, salts, and neutral organic fluid to assess its chemical compatibility. While some geotechnical properties of fly ash meet the requirements of liner material, the disadvantage of using of fly ash alone is that it has a low cation exchange capacity and high hydraulic conductivity. The compressibility of fly ash reduces with alkaline solution but increases with acidic solutions. While alkaline or neutral inorganic solutions do not affect the hydraulic conductivity of fly ash, the addition of dilute acid increases the hydraulic conductivity. Addition of bentonite improves the geotechnical properties of fly ash such as cation exchange capacity, shrinkage and volume change behavior, etc. Fly ash-bentonite mixtures possess low shrinkage and hence do not crack. Compacted fly ash-bentonite mixtures undergo very little volume changes under various stress conditions. The hydraulic conductivity of fly ash is reduced after amendment with bentonite. Though the unconfined compressive strength of the mixture is lower than that of fly ash alone, the fly ash-bentonite mixture still possesses good strength. The compressibility of fly ash bentonite mixtures are lower with different pore fluids studied than with water. The hydraulic conductivity of fly ash-bentonite mixtures are slightly higher in different pore fluids studied than with water.     

Role of aquaporins in root water transport of ectomycorrhizal jack pine (Pinus banksiana) seedlings exposed to NaCl and fluoride

Effects of ectomycorrhizal (ECM) fungus Suillus tomentosus on water transport properties were studied in jack pine (Pinus banksiana) seedlings. The hydraulic conductivity of root cortical cells (L_pc) and of the whole root system (L_pr) in ECM plants was higher by twofold to fourfold compared with the non‐ECM seedlings. HgCl₂ had a greater inhibitory effect on L_pc in ECM compared with non‐ECM seedlings, suggesting that the mercury‐sensitive, aquaporin (AQP)‐mediated water transport was largely responsible for the differences in L_pc between the two groups of plants. L_pc was rapidly and drastically reduced by the 50 mm NaCl treatment. However, in ECM plants, the initial decline in L_pc was followed by a quick recovery to the pre‐treatment level, while the reduction of L_pc in non‐ECM seedlings progressed over time. Treatments with fluoride reduced L_pc by about twofold in non‐ECM seedlings and caused smaller reductions of L_pc in ECM plants. When either 2 mm KF or 2 mm NaF were added to the 50 mm NaCl treatment solution, the inhibitory effect of NaCl on L_pc was rapidly reversed in both groups of plants. The results suggest that AQP‐mediated water transport may be linked to the enhancement of salt stress resistance reported for ECM plants.     

The stem xylem of Patagonian shrubs operates far from the point of catastrophic dysfunction and is additionally protected from drought‐induced embolism by leaves and roots

Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Ψ) and the Ψ at which stem or leaf hydraulic function was reduced by 50% (Ψ – Ψ₅₀), and the difference between leaf and stem Ψ₅₀. Leaf Ψ₅₀ – stem Ψ₅₀ increased with decreasing rooting depth. Large diurnal decreases in root‐specific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Ψ₅₀ became more negative with decreasing species‐specific Ψ_soil and minimum stem Ψ, leaf Ψ₅₀ was independent of Ψ and minimum leaf Ψ. Species with embolism‐resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers.     

The hydraulic architecture of Juniperus communis L. ssp. communis: shrubs and trees compared

Juniperus communis ssp. communis can grow like a shrub or it can develop a tree-like habit. In this study, the hydraulic architecture of these contrasting growth forms was compared. We analysed the hydraulic efficiency (leaf-specific conductivity, k(l); specific conductivity, k(s); Huber value, HV) and the vulnerability to cavitation (the water potential corresponding to a 50% loss of conductivity, Psi(50)), as well as anatomical parameters [mean tracheid diameter, d; mean hydraulic diameter, d(h); cell wall reinforcement (t/b)(h)(2)] of shrub shoots, tree stems and tree branches. Shrub shoots were similar to tree branches (especially to lower branches) in growth form and conductivity (k(l) = 1.93 +/- 0.11 m(2) s(-1) MPa(-1) 10(-7), k(s) = 5.71 +/- 0.19 m(2) s(-1) MPa(-1) 10(-4)), but were similar to tree stems in their vulnerability to cavitation (Psi(50) = -5.81 +/- 0.08 MPa). Tree stems showed extraordinarily high k(l) and k(s) values, and HV increased from the base up. Stem xylem was more vulnerable to cavitation than branch xylem, where Psi(50) increased from lower (Psi(50) = -6.44 +/- 0.19 MPa) to upper branches (Psi(50) = -5.98 +/- 0.13 MPa). Conduit diameters were correlated with k(l) and k(s). Data indicate that differences in hydraulic architecture correspond to changes in growth form. In some aspects, the xylem hydraulics of tree-like Juniperus communis differs from that of other coniferous tree species.     

毛白杨根系液流与水力再分配特征

为确定毛白杨(Populus tomentosa)根系是否存在水力再分配现象,并探究其发生特征与影响因子,该研究以四年生毛白杨为研究对象,利用热比率法对3株样树的共计7条侧根(R1–R7)进行长期液流监测,并对土壤水分以及气象因子进行同步测定。结果显示:毛白杨存在两种水力再分配模式,分别为干旱驱动的水力提升和降雨驱动的水力下降,水力再分配的发生模式与特征受侧根分布深度与直径大小的影响。在整个生长季尺度上,毛白杨根系再分配的水量较低;但在极端干旱条件下,部分侧根再分配的水量可达其日总液流量的64.6%,表明水力再分配会为干旱侧根提供大量水分。根系吸水与气象-土壤的耦合因子(太阳辐射(R_s)×土壤含水率(SWC)、水汽压亏缺(VPD)×SWC、参考蒸散发(ET_o)×SWC)间存在显著相关关系,但水力再分配与所选因子基本不相关。此外,毛白杨浅层根中存在特殊的日间逆向液流现象,其液流量最高可占日液流总量的79.2%(R1)到90.7%(R2),该现象可能对浅层根系抗旱起到重要作用。     

木本植物木质部的冻融栓塞应对研究进展

冻融栓塞在中高纬度地区木本植物中普遍存在。抗冻融栓塞能力对在寒冷环境中木本植物的生长和安全越冬十分关键, 这直接决定植物分布范围。冻融栓塞是由于冰中气体溶解度低, 木质部水分在低温下冷冻, 使之前水中溶解的气体逸出到导管中, 随后木质部中的冰融化又使气泡扩张而引发的栓塞现象。木质部解剖结构的差异会影响植物的抗冻融栓塞能力, 植物还可以通过调节木质部正压、代谢耗能等方式主动修复冻融栓塞, 也可通过增加树液溶质含量等逃避冷冻, 以减少低温损伤。然而, 与干旱栓塞相比, 目前对木质部冻融栓塞的形成以及植物响应和调节机制的理解不足。为此, 该文首先综述了木质部冻融栓塞的形成机制和植物的逃避、忍耐、修复等3种冻融栓塞的应对策略, 然后总结了木质部抗低温胁迫能力的生理表现、影响因子和评价指标, 并在此基础上讨论了低温抗性、干旱抗性和水力效率之间的多元权衡关系, 最后提出今后该领域中的5个优先研究问题: (1)不同植物冰冻的最低温度阈值; (2)是否存在应对低温胁迫的水力脆弱性分割机制; (3)冻融栓塞修复与代谢消耗的关系; (4)低温抗性、干旱抗性和水力效率之间的权衡关系; (5)抗冻融栓塞性状是否能够纳入经济性状谱系。     

Transverse hydraulic redistribution by a grapevine

Root hydraulic redistribution has been shown to occur in numerous plant species under both field and laboratory conditions. To date, such water redistribution has been demonstrated in two fundamental ways, either lifting water from deep edaphic sources to dry surface soils or redistributing water downward (reverse flow) when inverted soil Ψ_s gradients exist. The importance of hydraulic redistribution is not well documented in agricultural ecosystems under field conditions, and would be important because water availability can be temporally and spatially constrained. Herein we report that a North American grapevine hybrid (Vitis riparia × V. berlandieri cv 420 A) growing in an agricultural ecosystem can redistribute water from a restricted zone of available water under a drip irrigation emitter, laterally across the high resistance pathways of the trunk and into roots and soils on the non-irrigated side. Deuterium-labelled water was used to demonstrate lateral movement across the vine's trunk and reverse flow into roots. Water redistribution from the zone of available water and into roots distant from the source occurred within a relatively short time frame of 36 h, although overnight deposition into rhizosphere soils around the roots was not detected. Deuterium was eventually detected in rhizosphere soils adjacent to roots on the non-irrigated side after 7 d. Application of identical amounts of water with the same deuterium enrichment level (2%) to soils without grapevine roots showed that physical transport of water through the vapour phase could not account for either downward or transverse movement of the label. These results confirmed that root presence facilitated the transport of label into soils distant from the wetted zone. When deuterium-labelled water was allowed to flow directly into the trunk above the root–trunk interface, reverse flow occurred and lateral movement across the trunk and into roots originating around the collar region did not encounter large disproportionate resistances. Rapid redistribution of water into the entire root system may have important implications for woody perennial cultivars growing where water availability is spatially heterogeneous. Under the predominantly dry soil conditions studied in this investigation, water redistributed into roots may extend root longevity and increase the vines water capacitance during periods of high transpiration demand. These benefits would be enhanced by diminished water loss from roots, and could be equally important to other cited benefits of hydraulic redistribution into soils such as enhancement of nutrient acquisition.     

Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire

Hydraulic redistribution (HR) of water from moist to drier soils, through plant roots, occurs world‐wide in seasonally dry ecosystems. Although the influence of HR on landscape hydrology and plant water use has been amply demonstrated, HR's effects on microbe‐controlled processes sensitive to soil moisture, including carbon and nutrient cycling at ecosystem scales, remain difficult to observe in the field and have not been integrated into a predictive framework. We incorporated a representation of HR into the Community Land Model (CLM4.5) and found the new model improved predictions of water, energy, and system‐scale carbon fluxes observed by eddy covariance at four seasonally dry yet ecologically diverse temperate and tropical AmeriFlux sites. Modeled plant productivity and microbial activities were differentially stimulated by upward HR, resulting at times in increased plant demand outstripping increased nutrient supply. Modeled plant productivity and microbial activities were diminished by downward HR. Overall, inclusion of HR tended to increase modeled annual ecosystem uptake of CO₂ (or reduce annual CO₂ release to the atmosphere). Moreover, engagement of CLM4.5′s ground‐truthed fire module indicated that though HR increased modeled fuel load at all four sites, upward HR also moistened surface soil and hydrated vegetation sufficiently to limit the modeled spread of dry season fire and concomitant very large CO₂ emissions to the atmosphere. Historically, fire has been a dominant ecological force in many seasonally dry ecosystems, and intensification of soil drought and altered precipitation regimes are expected for seasonally dry ecosystems in the future. HR may play an increasingly important role mitigating development of extreme soil water potential gradients and associated limitations on plant and soil microbial activities, and may inhibit the spread of fire in seasonally dry ecosystems.     

Water flow through junctions in Douglas-fir roots

Roots are important conduits for the redistribution of water within the rooting zone. Root systems are often highly branched, and water flow between regions undoubtedly involves passage through junctions between individual roots. This study considered junctions in the roots of Douglas-fir with regard to the resistances encountered by water flow through the xylem. Flow into the root branch distally along the main root encountered much greater resistance than flow into the branch and proximally along the main root (toward the plant stem). When the main root proximal to the junction was gradually shortened, the resistance to flow in the branch root and distally along the main root increased dramatically. Thus, flow in this manner appears to depend on lateral flow within the root over many centimetres proximal to the junction and not just within the direct connection at the junction. These results suggest that the hydraulic nature of junctions is an important aspect of hydraulic redistribution of water within the soil utilizing flow through roots.     

Xylem hydraulics strongly influence the niche differentiation of tree species along the slope of a river valley in a water-limited area

Xylem hydraulic characteristics govern plant water transport, affecting both drought resistance and photosynthetic gas exchange. Therefore, they play critical roles in determining the adaptation of different species to environments with various water regimes. Here, we tested the hypothesis that variation in xylem traits associated with a trade-off between hydraulic efficiency and safety against drought-induced embolism contributes to niche differentiation of tree species along a sharp water availability gradient on the slope of a unique river valley located in a semi-humid area. We found that tree species showed clear niche differentiation with decreasing water availability from the bottom towards the top of the valley. Tree species occupying different positions, in terms of vertical distribution distance from the bottom of the valley, showed a strong trade-off between xylem water transport efficiency and safety, as evidenced by variations in xylem structural traits at both the tissue and pit levels. This optimized their xylem hydraulics in their respective water regimes. Thus, the trade-off between hydraulic efficiency and safety contributes to clear niche differentiation and, thereby, to the coexistence of tree species in the valley with heterogeneous water availability.     

Static and dynamic bending has minor effects on xylem hydraulics of conifer branches (Picea abies,Pinus sylvestris)

The xylem hydraulic efficiency and safety is usually measured on mechanically unstressed samples, although trees may be exposed to combined hydraulic and mechanical stress in the field. We analysed changes in hydraulic conductivity and vulnerability to drought‐induced embolism during static bending of Picea abies and Pinus sylvestris branches as well as the effect of dynamic bending on the vulnerability. We hypothesized this mechanical stress to substantially impair xylem hydraulics. Intense static bending caused an only small decrease in hydraulic conductance (?19.5 ± 2.4% in P. abies) but no shift in vulnerability thresholds. Dynamic bending caused a 0.4 and 0.8 MPa decrease of the water potential at 50 and 88% loss of conductivity in P. sylvestris, but did not affect vulnerability thresholds in P. abies. With respect to applied extreme bending radii, effects on plant hydraulics were surprisingly small and are thus probably of minor eco‐physiological importance. More importantly, results indicate that available xylem hydraulic analyses (of conifers) sufficiently reflect plant hydraulics under field conditions.     

非饱和土壤水力参数的模型及确定方法

土壤水力参数的选取和确定是土壤中水分运动和污染物迁移预测的基础,本文在国内外研究成果的基础上,综述了土壤水力参数的模型(土壤水分特征曲线模型、土壤导水率模型)及其直接测定法和间接推求法,并比较了土壤水力参数的模型及确定方法的适用性与局限性,以便为生态环境建设和农业可持续发展研究中土壤水力参数的选取提供依据。     

Water and ion fluxes of abscisic acid-treated root systems of pear, Pyrus communis

Water and ion fluxes of intact root systems of Pyrus communis L. cv. Old Home × Farmingdale 97 immersed in a nutrient solution were determined at various pressures and temperatures. Water flux (J_v was normalized on the basis of initial flow rates of a root system after 30 min at 0.50 MPa and 25°C, expressed as the ratio Q_v. Q_v responded linearly to pressures between 0.20 and 0.62 MPa, implying a constant root hydraulic conductivity (L_p) within this range. Similarly Q_v was linearly related to temperatures between 7 and 35°C; however, large, rapid temperature changes resulted in a break of the Arrhenius plot of Q_v versus the reciprocal of temperature, Abscisic acid (ABA) from 2 × 10⁻⁶ to 10⁻⁴ M , applied to intact root systems, increased Q_v within 10–20 min, with the effect leveling off after 1.5 h. At a pressure of 0.50 MPa, ABA at 10_-4 M enhanced Q_v by 28%. The stimulation of Q_v was not due to the ethanol solvent since 0.13 or 1.33% ethanol decreased Q_v-, The osmotic potential of the xylem fluid was determined and was used to calculate total normalized solute flux. The results suggest that ABA-induced or ethano1-induced changes in Q_v were mainly due to changes in L_p and not to changes in ion transport to the xylem.     

Impact of light quality on leaf and shoot hydraulic properties: a case study in silver birch (Betula pendula)

Responses of leaf and shoot hydraulic conductance to light quality were examined on shoots of silver birch (Betula pendula), cut from lower (‘shade position’) and upper thirds of the crowns (‘sun position’) of trees growing in a natural temperate forest stand. Hydraulic conductances of leaf blades (K_lb), petioles (K_P) and branches (i.e. leafless stem; K_B) were determined using a high pressure flow meter in steady state mode. The shoots were exposed to photosynthetic photon flux density of 200–250 µmol m^?2 s^?1 using white, blue or red light. K_lb depended significantly on both light quality and canopy position (P < 0.001), K_B on canopy position (P < 0.001) and exposure time (P = 0.014), and none of the three factors had effect on K_P. The highest values of K_lb were recorded under the blue light (3.63 and 3.13 × 10^?4 kg m^?2 MPa^?1 s^?1 for the sun and shade leaves, respectively), intermediate values under white light (3.37 and 2.46 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively) and lowest values under red light (2.83 and 2.02 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively). Light quality has an important impact on leaf hydraulic properties, independently of light intensity or of total light energy, and the specific light receptors involved in this response require identification. Given that natural canopy shade depletes blue and red light, K_lb may be decreased both by reduced fluence and shifts in light spectra, indicating the need for studies of the natural heterogeneity of K_lb within and under canopies, and its impacts on gas exchange.