Stomatal control of xylem embolism

Abstract. The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short-term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.     

A method for measuring hydraulic conductivity and embolism in xylem

Abstract Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m^?3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.     

Intra- and inter-plant variation in xylem cavitation in Betula occidentalis

A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.     

木本植物木质部栓塞脆弱性研究新进展

木质部空穴化和栓塞是木本植物在干旱等条件下遭受水分胁迫时产生的木质部输水功能障碍, 在全球气候变化的大背景下, 栓塞脆弱性对干旱响应的研究已成为热点和重要内容。近年来有关木质部栓塞脆弱性与植物输水结构和耐旱性的关系已有大量研究并取得一定成果, 但是, 不同学者在不同地区对不同材料的研究结果存在很大不同。该文就近年来这一研究领域取得的成果及争议问题进行了概括和总结, 主要涉及木质部栓塞脆弱性(P₅₀)及脆弱曲线的建立方法、木质部栓塞脆弱性与木质部结构(导管直径、导管长度、纹孔膜、木质部密度、纤维及纤维管胞)间的关系和木质部栓塞脆弱性与耐旱性的关系, 并对未来工作进行展望, 提出在未来的工作中应对同一树种使用Cochard Cavitron离心机法、Sperry离心机技术与传统方法建立的脆弱曲线进行比较验证、计算P₅₀值、分析植物个体器官水平差异(根、茎、叶)、测定树种生理生态指标, 探索植物栓塞脆弱性与输水结构和耐旱性的关系, 从而评估不同类型植物在未来气候变化下的耐旱能力。     

7种木本植物根和小枝木质部栓塞的脆弱性

用脆弱曲线表示的植物木质部栓塞脆弱性反映了植物木质部栓塞程度与其水势间的关系。众多学者的研究结果表明,脆弱曲线能够提供有关植物的许多生理生态信息,与植物的木质部结构、部位、分布、抗寒、抗旱性等存在一定关系,但各国学者利用不同材料研究得出的结果各异,为了研究木质部栓塞的这种差异是否由于树木对环境适应性不同引起,选取西北农林科技大学西林校区内自然状况下生长良好的5个耐旱树种:刺槐(RobiniapseudoacaciaL.)、元宝枫(AcertruncatumBge.)(低水势忍耐脱水耐旱树种)、白榆(UlmuspumilaL.)(亚低水势忍耐脱水耐旱树种)、油松(PinustabulaeformisCarr.)、白皮松(PinusbungeanaZucc.ex.Endl.)(高水势延迟脱水耐旱树种),及中生的女贞(LigustrumlucidumAit.)和柳树(SalixmatsudanaKoidz.f.pendulaSchneid.)为研究对象,绘制了它们根和小枝的木质部栓塞脆弱曲线,探讨了中生树种和不同耐旱类型树种根和小枝木质部栓塞脆弱性的差异。结果表明:根和小枝的栓塞脆弱性主要由木质部结构决定,栓塞脆弱性顺序基本一致,小枝容易发生木质部栓塞的,其根也较容易发生栓塞;同一树种根和小枝的木质部栓塞脆弱性与植物的耐旱性有关,与树种的耐旱策略无关;一般是中生树种的栓塞脆弱性:小枝>根;耐旱树种的栓塞脆弱性:根>小枝。     

Limitation of plant water use by rhizosphere and xylem conductance: results from a model

Hydraulic conductivity ( K ) in the soil and xylem declines as water potential ( Ψ ) declines. This results in a maximum rate of steady-state transpiration ( E _crit) and corresponding minimum leaf Ψ ( Ψ _crit) at which K has approached zero somewhere in the soil–leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area ( A _R: A _L), the loss of rhizosphere K limited E _crit and Ψ _crit. Above the threshold, loss of xylem K from cavitation was limiting. The A _R: A _L threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0·20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their A _R: A _L threshold. Corollary predictions are (1) within a soil type the A _R: A _L should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the A _R: A _L should increase and maximum cavitation resistance should decrease.     

Climate-related trends in sapwood biophysical properties in two conifers: avoidance of hydraulic dysfunction through coordinated adjustments in xylem efficiency, safety and capacitance

In the Pacific north‐west, the Cascade Mountain Range blocks much of the precipitation and maritime influence of the Pacific Ocean, resulting in distinct climates east and west of the mountains. The current study aimed to investigate relationships between water storage and transport properties in populations of Douglas‐fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) adapted to both climates. Sapwood thickness, capacitance, vulnerability to embolism, and axial and radial conductivity were measured on samples collected from trunks of mature trees. The sapwood of ponderosa pine was three to four times thicker than Douglas‐fir. Radial conductivity was higher in west‐side populations of both species, but axial conductivity was higher in the east‐side populations and in Douglas‐fir. Eastern populations of both species had sapwood that was more vulnerable to embolism than west‐side populations. Sapwood capacitance was similar between species, but was about twice as great in east‐side populations (580 kg m^?3 MPa^?1) as in west‐side populations (274 kg m^?3 MPa^?1). Capacitance was positively correlated with both mean embolism pressure and axial conductivity across species and populations, suggesting that coordinated adjustments in xylem efficiency, safety and water storage capacity may serve to avoid embolism along a gradient of increasing aridity.     

Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.     

Anisohydric but isohydrodynamic: seasonally constant plant water potential gradient explained by a stomatal control mechanism incorporating variable plant hydraulic conductance

Isohydric and anisohydric regulations of plant water status have been observed over several decades of field, glasshouse and laboratory studies, yet the functional significance and mechanism of both remain obscure. We studied the seasonal trends in plant water status and hydraulic properties in a natural stand of Eucalyptus gomphocephala through cycles of varying environmental moisture (rainfall, groundwater depth, evaporative demand) in order to test for isohydry and to provide physiological information for the mechanistic interpretation of seasonal trends in plant water status. Over a 16 month period of monitoring, spanning two summers, midday leaf water potential (psi(leaf)) correlated with predawn psi(leaf), which was correlated with water table depth below ground level, which in turn was correlated with total monthly rainfall. Eucalyptus gomphocephala was therefore not seasonally isohydric. Despite strong stomatal down-regulation of transpiration rate in response to increasing evaporative demand, this was insufficient to prevent midday psi(leaf) from falling to levels below -2 MPa in the driest month, well into the region likely to induce xylem air embolisms, based on xylem vulnerability curves obtained in the study. However, even though midday psi(leaf) varied by over 1.2 MPa across seasons, the hydrodynamic (transpiration-induced) water potential gradient from roots to shoots (delta psi(plant)), measured as the difference between predawn and midday psi(leaf), was relatively constant across seasons, averaging 0.67 MPa. This unusual pattern of hydraulic regulation, referred to here as isohydrodynamic, is explained by a hydromechanical stomatal control model where plant hydraulic conductance is dependent on transpiration rate.     

Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.     

The effects of acclimation to sunlight on the xylem vulnerability to embolism in Fagus sylvatica L.

We assessed the effects of irradiance received during growth on the vulnerability of Fagus sylvatica L. xylem vessels to water-stress-induced embolism. The measurements were conducted on (1) potted saplings acclimated for 2 years under 100% and 12% incident global radiation and (2) branches collected from sun-exposed and shaded sides of adult trees. Both experiments yielded similar results. Light-acclimated shoots were less vulnerable to embolism. Xylem water potential levels producing 50% loss of hydraulic conductivity were lower in sun-exposed branches and seedlings than in shade-grown ones (–3·0 versus –2·3 MPa on average). The differences in vulnerability were not correlated with differences in xylem hydraulic conductivity nor vessel diameter. Resistance to cavitation was correlated with transpiration rates, midday xylem and leaf water potentials in adult trees. We concluded that vulnerability to cavitation in Fagus sylvatica may acclimate to contrasting ambient light conditions.     

Water-stress-induced xylem embolism in three species of conifers

Abstract. The mechanism of water-stress-induced xylem embolism was studied in three species of conifers: Abies balsamea (L.) Mill., Picca rubens Sarg, and Juniperus virginiana L. Each species showed a characteristic relationship between xylem tension and the loss of hydraulic conductivity by air embolism. Abics balsamea and Picca rubens began to embolize at tensions between 2 and 3 MPa and were completely non-conducting between 3 and 4 MPa. Juniperus virginiana was least vulnerable, beginning to embolize at 4 and still retaining approximately 10% conductivity at 10 MPa. As with a previous study of the vessel-bearing Accr saccharum Marsh., a brief perfusion of branch segments with an oxalic acid and calcium solution (10 and 0.1 mol m⁻³. respectively) increased the vulnerability of the xylem to embolism; this was especially pronounced in Abies balsamea . In order to test whether embolism was caused by aspiration of air into functional tracheids from neighbouring embolized, ones (the 'air-seeding'hypothesis), hydrated branch segments were injected with air at various pressures and measured for embolism. Results supported the air-seeding hypothesis because the relationship between injection pressure and embolism for both native and oxalic-calcium-treated segments was essentially the same as for embolism induced by xylem tension. Structural and experimental evidence suggested the air seeding occurred through inter-tracheid pit membranes when the thickened torus region of the membrane became displaced from its normal sealing position over the pit aperture. Thus, the embolism-inducing tension may be a function of pit membrane flexibility. This tension is of ecological significance because it reflects to some extent the range of xylem tensions to which a species is adapted.     

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current‐year growth

Xylem vessel structure changes as trees grow and mature. Age‐ and development‐related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water‐stress‐induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current‐year growth of young (1–4 years), intermediate (2–7 years), and older (3–10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple‐aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.     

Grapevine species from varied native habitats exhibit differences in embolism formation/repair associated with leaf gas exchange and root pressure

Drought induces xylem embolism formation, but grapevines can refill non‐functional vessels to restore transport capacity. It is unknown whether vulnerability to embolism formation and ability to repair differ among grapevine species. We analysed in vivo embolism formation and repair using x‐ray computed microtomography in three wild grapevine species from varied native habitats (Vitis riparia, V. arizonica, V. champinii) and related responses to measurements of leaf gas exchange and root pressure. Vulnerability to embolism formation was greatest in V. riparia, intermediate in V. arizonica and lowest in V. champinii. After re‐watering, embolism repair was rapid and pronounced in V. riparia and V. arizonica, but limited or negligible in V. champinii even after numerous days. Similarly, root pressure measured after re‐watering was positively correlated with drought stress severity for V. riparia and V. arizonica (species exhibiting embolism repair) but not for V. champinii. Drought‐induced reductions in transpiration were greatest for V. riparia and least in V. champinii. Recovery of transpiration after re‐watering was delayed for all species, but was greatest for V. champinii and most rapid in V. arizonica. These species exhibit varied responses to drought stress that involve maintenance/recovery of xylem transport capacity coordinated with root pressure and gas exchange responses.     

6个耐旱树种木质部结构与栓塞脆弱性的关系

木质部栓塞脆弱性对干旱响应的研究已成为全球气候变化背景下的热点和重要内容。该文以6个耐旱树种刺槐(Robinia pseudoacacia)、沙棘(Hippophae rhamnoides)、榆树(Ulmus pumila)、元宝枫(Acer truncatum)、旱柳(Salix matsudana)、榛(Corylus heterophylla)为研究对象, 采用Cochard Cavitron离心机技术建立木质部栓塞脆弱曲线, 计算木质部栓塞脆弱性, 利用染色法、硅胶注射法等测定木质部导管直径、导管内径跨度、导管连接度、导管密度、导管长度和木质部密度, 探究木质部结构与栓塞脆弱性的关系, 区分6个耐旱树种木质部结构在抗栓塞性上的差异, 以期建立6个耐旱树种在木质部结构方面的抗栓塞性指标。结果表明: 6个耐旱树种木质部栓塞脆弱性大小为刺槐>榆树>沙棘>旱柳>元宝枫>榛, 其中, 刺槐、沙棘和榆树的栓塞脆弱曲线为“r”形, 而元宝枫、旱柳和榛的栓塞脆弱曲线为“s”形, 脆弱曲线为“r”形的树种与脆弱曲线为“s”形的树种栓塞脆弱性差异极显著(p < 0.01)。线性分析表明: 木质部结构影响各树种的栓塞脆弱性, 其中, 木质部密度影响最大(t = 0.702), 导管直径次之(t = 0.532), 导管长度影响最小(t = 0.010)。     

Ion-mediated increase in the hydraulic conductivity of Laurel stems: role of pits and consequences for the impact of cavitation on water transport

Changes in hydraulic conductivity (K(h)) were measured in stems of Laurus nobilis L. during perfusion with KCl, NaCl or sucrose solutions. Ionic solutes induced marked increase of K(h) with respect to deionized water but sucrose had no effect. The kinetics of KCl-induced K(h) increase was measured together with changes in [K(+)] of the perfused solution. K(h) increases were paralleled by increases in the [K(+)](out)/[K(+)](in) ratio. Samples of different lengths or with increasing percentage loss of conductivity (PLC) due to xylem cavitation were tested, with the aim of increasing radial flow through intervessel pits. KCl solutions enhanced the K(h) of 12-cm-long samples with a concentration-dependent effect up to 100 mm KCl. DeltaK(h) increased from 3 to 30% in 1.5- and 12-cm-long samples, respectively and remained constant for longer samples. Increasing PLC induced an exponential increase in DeltaK(h). PLC measured with KCl solutions was significantly less than that measured with deionized water, suggesting that measurements of PLC can be affected by the composition of the perfused solution. Experiments support the hypothesis that the 'ionic effect' is mediated by physico-chemical changes of pectins of the pit membranes and raise the possibility that plants might alter the ionic composition of the xylem sap to alleviate the hydraulic impact of cavitation.     

Xylem hydraulic characteristics of subtropical trees from contrasting habitats grown under identical environmental conditions

Five evergreen subtropical tree species growing under identical environmental conditions were investigated to establish which hydraulic properties are genotypically rigid and which show phenotypic plasticity. Maximum xylem-specific conductivity ( k _s) correlated well with the anatomical characteristics (conduit diameter and density) for the four angiosperms Tecomaria capensis , Trichilia dregeana , Cinnamomum camphora and Barringtonia racemosa ; the anatomy of the gymnosperm Podocarpus latifolius was not assessed. Huber values (functional xylem cross-sectional area : leaf area) varied inversely with k _s among species. Maximum leaf-specific conductivity was similar in the five unrelated species. Vulnerability of xylem to cavitation differed between species, as did the relationship between transpiration and water potential. Models of these parameters and isolated midday readings confirm that these trees operate at similar maximum leaf-specific conductivity ( k _l) values. The data are consistent with the hypothesis that conductivity characteristics ( k _l, k _s) are influenced by environment, whereas vulnerability to cavitation is genetically determined.