Cavitation fatigue and its reversal in sunflower (Helianthus annuus L.)

"Cavitation fatigue" is the increased susceptibility of a xylem conduit to cavitation as a result of its prior cavitation. It was investigated whether cavitation fatigue induced in vivo could be repaired in intact plants. Sunflowers (Helianthus annuus L.) were subjected to soil drought in the greenhouse. Native embolism and vulnerability to cavitation was measured in well-watered controls and after 5 d and 10 d of controlled drought. A dramatic cavitation fatigue was observed where droughted xylem that was refilled in the laboratory developed up to 60 PLC (percentage loss of hydraulic conductivity) at -1 MPa versus only 5.2 PLC in non-droughted controls. Rewatered plants showed the complete reversal of cavitation fatigue over 4 d. Reversal of fatigue was correlated with the refilling of embolized vessels in the intact plants (r(2)=0.91, P<0.01), suggesting that xylem transport to fatigued vessels was required for their repair. The in vivo reversal of fatigue was partially duplicated in excised stem segments by perfusing them with root exudates from droughted (DR) and well-watered (WW) plants. The DR exudate had a greater effect, and this was associated with a greater pH in the DR versus WW saps, but there was no difference in total cation concentration. Perfusions with 2 mM CaCl(2) and KCl solutions also partially reversed cavitation fatigue as opposed to no effect with deionized water, suggesting a role of ions in addition to a pH effect. It is suspected that fatigue is caused by stretching and partial disruption of linkages between cellulose microfibrils in inter-conduit pit membranes during air seeding, and that the reversal of fatigue involves restoring these linkages by ingredients in xylem sap.     

Cavitation resistance of peduncle,petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P₅₀) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P₅₀ was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.     

Seasonality of cavitation and frost fatigue in Acer mono Maxim.

Although cavitation is common in plants, it is unknown whether the cavitation resistance of xylem is seasonally constant or variable. We tested the changes in cavitation resistance of Acer mono before and after a controlled cavitation–refilling and freeze–thaw cycles for a whole year. Cavitation resistance was determined from ‘vulnerability curves’ showing the percent loss of conductivity versus xylem tension. Cavitation fatigue was defined as a reduction of cavitation resistance following a cavitation–refilling cycle, whereas frost fatigue was caused by a freeze–thaw cycle. A. mono developed seasonal changes in native embolisms; values were relatively high during winter but relatively low and constant throughout the growing season. Cavitation fatigue occurred and changed seasonally during the 12‐month cycle; the greatest fatigue response occurred during summer and the weakest during winter, and the transitions occurred during spring and autumn. A. mono was highly resistant to frost damage during the relatively mild winter months; however, a quite different situation occurred during the growing season, as the seasonal trend of frost fatigue was strikingly similar to that of cavitation fatigue. Seasonality changes in cavitation resistance may be caused by seasonal changes in the mechanical properties of the pit membranes.     

Frost fatigue response to simulated frost drought using a centrifuge in Acer mono Maxim

In the coldest part of winter, water uptake is blocked by the frozen soil and frozen stems known as ‘frost drought’ causing severe embolisms in woody plants. Frost drought in stems was simulated in a centrifuge by a synergy between freeze–thaw cycles and the different tensions induced by changing the rotation speed. Frost fatigue was defined as a reduction of embolism resistance after a freeze–thaw cycle and determined from ‘vulnerability curves’, which showed percent losses of conductivity vs tension (positive value) or xylem pressure (negative value). Different tensions combined with a controlled freeze–thaw cycle were induced to investigate the effects on frost resistance over the course of year. During the growing season, Acer mono Maxim. developed significant frost fatigue, and a significant positive correlation was found between frost fatigue response and exogenous tension. During the dormant season, A. mono showed very high embolism resistance to frost drought, even under a tension of 2 MPa. When the exogenous tension was increased to 3 MPa while the stem was frozen, significant frost fatigue occurred. Longer freezing times had more serious effects on frost fatigue in A. mono. A hypothesis was raised that at the same low temperature, the severer the drought (higher tension) when stems were frozen, the higher frost fatigue response would be induced.     

Xylem embolism and drought-induced stomatal closure in maize

Water relations during drought and xylem vulnerability to embolism were studied on four maize ( Zea mays L.) genotypes having contrasting grain yields under drought conditions. Drought provoked a drop in xylem pressure, leaf water potential and whole-plant transpiration. Transpiration was reduced to a minimum value when xylem pressures reached ca. -1.6 MPa. This value corresponded to the threshold xylem pressure below which xylem embolism developed to a substantial degree in leaf midribs. Therefore, xylem embolism always remained low in leaf veins, even when plants exhibited clear water-stress symptoms. This suggests that stomatal closure during drought contains xylem embolism to a minimum value. Cavitation resistance was not related to grain yield under drought conditions for the four genotypes evaluated. However, it can be speculated that an increase in cavitation resistance by cultural practices or genetic selection may increase drought survival in maize.     

Cavitation studies on whole Ricinus plants by acoustic detection

Summary Acoustic detection has been used to investigate the incidence of cavitation in whole potted Ricinus plants subjected to water stress by withholding water. Cavitation proceeded rather slowly and was detectable before and during wilting. Techniques which restricted water uptake more drastically such as root cooling or overlapping cuts induced more rapid click production and wilting; a response already described for excised leaves. When water stress was removed by rewatering, or rewarming a cooled root system, cavitation soon ceased. This response was more sluggish of over-delayed.Cavitation in aging leaves on well watered plants has also been examined. Despite the onset of senescence over many days there was no evidence that dry patches, which often develop extensively, are a consequence of water shortage induced by xylem blockage. Leaves, falling naturally by abscission in still air, were often remarkably turgid with water potentials similar to those of healthy attached leaves. Only after losing water was cavitation apparent, as usual for excised mature leaves. Sometimes more persistent leaves did cavitate in situ, just before abscission, showing that in normal leaves xylem blockage can occasionally precede leaf fall by several hours.     

Functional and ecological xylem anatomy

Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure (P_x), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.     

Stomatal factors and vulnerability of stem xylem to cavitation in poplars

The relationships between the vulnerability of stem xylem to cavitation, stomatal conductance, stomatal density, and leaf and stem water potential were examined in six hybrid poplar (P38P38, Walker, Okanese, Northwest, Assiniboine and Berlin) and balsam poplar (Populus balsamifera) clones. Stem xylem cavitation resistance was examined with the Cavitron technique in well-watered plants grown in the greenhouse. To investigate stomatal responses to drought, plants were subjected to drought stress by withholding watering for 5 (mild drought) and 7 (severe drought) days and to stress recovery by rewatering severely stressed plants for 30 min and 2 days. The clones varied in stomatal sensitivity to drought and vulnerability to stem xylem cavitation. P38P38 reduced stomatal conductance in response to mild stress while the balsam poplar clone maintained high leaf stomatal conductance under more severe drought stress conditions. Differences between the severely stressed clones were also observed in leaf water potentials with no or relatively small decreases in Assiniboine, P38P38, Okanese and Walker. Vulnerability to drought-induced stem xylem embolism revealed that balsam poplar and Northwest clones reached loss of conductivity at lower stem water potentials compared with the remaining clones. There was a strong link between stem xylem resistance to cavitation and stomatal responsiveness to drought stress in balsam poplar and P38P38. However, the differences in stomatal responsiveness to mild drought suggest that other drought-resistant strategies may also play a key role in some clones of poplars exposed to drought stress.     

Focus on Water: Cavitation and Its Discontents: Opportunities for Resolving Current Controversies

Cavitation has long been recognized as a key constraint on the structure and functional integrity of the xylem. Yet, recent results call into question how well we understand cavitation in plants. Here, we consider embolism formation in angiosperms at two scales. The first focuses on how air-seeding occurs at the level of pit membranes, raising the question of whether capillary failure is an appropriate physical model. The second addresses methodological uncertainties that affect our ability to infer the formation of embolism and its reversal in plant stems. Overall, our goal is to open up fresh perspectives on the structure-function relationships of xylem.A central question in the biology of vascular plants is under what conditions the continuity of the liquid phase, essential for the transport of water from soil to leaves, is lost (Tyree and Zimmerman, 2002). Without the high-conductance pathway for water movement through the xylem, vascular plants could not sustain the water loss associated with the diffusional uptake of CO₂ from a subsaturated atmosphere. As a result, substantial effort in the field of xylem transport focuses on quantifying vulnerability to cavitation and its impact. Yet, a number of recent studies raise questions regarding how well we understand cavitation in plant stems, pointing to apparently anomalous or inconsistent experimental results that suggest methodological artifacts (Choat et al., 2010; Ennajeh et al., 2011; Wheeler et al., 2013). Such results provide the motivation for this Update. We recognize that there is currently no consensus on the extent to which any particular experimental approach is subject to a problem; in addition, we note that the implications of such potential artifacts for the estimation of leaf xylem vulnerability involves further methodological considerations beyond our current scope. Our intention with this Update is to clarify the physical basis for a number of potential experimental artifacts relating to angiosperm stem xylem, in the hope that this will be useful for designing experiments that can resolve these issues.In this spirit, we begin with a discussion of how cavitation occurs, sketch an alternative model to meniscal failure for how air seeding across homogenous pit membranes could occur in the absence of discrete pores, and discuss the implications of these two models for the relative importance of probabilistic versus deterministic constraints on air-seeding resistance. We then turn to recent evidence that xylem vulnerability to cavitation in some species may have been overestimated and consider possible physical effects that could lead to biases sensitive to conduit size in the three principal methods of vulnerability estimation (dehydration, air injection, and centrifugation); to the extent that such biases are quantitatively important, these methods cannot be considered independent for long-vesseled species. Experimental approaches that may prove helpful in reconciling some of the divergent results between methods or protocols are proposed. The potential for measurement artifacts to impact our understanding of embolism repair is also discussed.     

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。     

Xylem cavitation caused by drought and freezing stress in four co-occurring Juniperus species

Previous studies indicate that conifers are vulnerable to cavitation induced by drought but in many cases, not by freezing. Rarely have vulnerability to drought and freezing stress been studied together, even though both influence plant physiology and the abundance and distribution of plants in many regions of the world. We studied vulnerability to drought- and freezing-induced cavitation, along with wood density, conduit reinforcement, tracheid diameter, and hydraulic conductivity, in four Juniperus species that typically occupy different habitats, but uniquely co-occur at the same site in Arizona, AZ. We combined drought with a freeze-thaw cycle to create freezing-induced vulnerability curves. All four species demonstrated greater vulnerability to drought + freezing- than to drought-induced cavitation alone ( P  < 0.0001). Mean tracheid diameter was correlated with vulnerability to drought + freezing-induced cavitation (r = 0.512, P  = 0.01). The vulnerability to cavitation of each species followed expected rankings based on relative moisture within each species' natural distribution. Species with naturally drier distributions showed greater resistance to both drought- and drought + freezing-induced cavitation. Even conifer species with relatively small tracheid diameters can experience xylem embolism after a single freeze-thaw cycle when under drought stress.     

Intra-specific variation in xylem cavitation in interior live oak (Quercus wislizenii A. DC.).

Xylem cavitation induced by water stress reduces plant hydraulic conductance and can indicate the habitat a species evolved in and its phylogenetic background. Species differ widely in cavitation resistance, but less is known about intra-specific variation. Cavitation resistance was assessed for field-collected adult and sapling size classes from three populations of interior live oak (Quercus wislizenii A. DC.) in California, USA. Root and stem cavitation resistance of two-year old seedlings from a greenhouse experiment was also measured. Cavitation resistance curves were determined by injecting air into the vascular system to induce cavitation and measuring the subsequent decline in hydraulic conductance. Based on the air-seeding hypothesis, the absolute value of the air pressures should be equivalent to the tensions that cause cavitation under dehydrating conditions. Conductance declined exponentially with applied pressure for both roots and stems. Comparisons between populations did not reveal significant differences despite good statistical power. The 50% loss in conductance point occurred between 1.0-1.6 MPa; conductance declined more slowly thereafter. Conductance was 21-30% of maximum at 4.0 MPa and 7-14% at 8.0 MPa. Saplings exhibited a nearly identical pattern compared with adults except at 4.0 MPa, where saplings exhibited slightly less cavitation (7%). Greenhouse seedling stems were more resistant compared with both field-collected adults and with seedling roots. The 50% loss in conductance point occurred at 0.83 and 2.6 MPa for seedling roots and stems, respectively. Seedling stems maintained conductance of 20.9% at 8.0 MPa while most roots were fully cavitated between 5.0-8.0 MPa.     

Relationships among xylem transport, biomechanics and storage in stems and roots of nine Rhamnaceae species of the California chaparral

Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species. Traits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs). Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively. Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.     

植物干旱胁迫下水分代谢、碳饥饿与死亡机理

植物在生长发育过程中受众多环境因子共同作用。随着全球气候变化,气温升高、降水量下降等问题频繁出现。目前气象学家一致预测未来环境变暖会使干旱更加频繁剧烈,这一环境改变使植物死亡更加严重。植物在水分胁迫、特别是干旱胁迫条件下,体内水分代谢与碳代谢会发生失衡现象:光合速率降低、蒸腾速率降低,带来生长降低;为维持植物新陈代谢,植物呼吸作用必然下调。在长期干旱胁迫条件下植物体内碳水化合物储存发生失衡现象,这种失衡使植物陷入碳饥饿现象。另外,由于水分失衡而出现的木质部栓塞和空穴会进一步加剧水分运输障碍,而修复空穴则需要大量非结构性碳水化合物(NSC),这使植物陷入两难选择。总结了植物干旱胁迫下,碳饥饿与水分代谢、植物死亡关系的相关研究,对未来的研究方向和重点提出建议,以期对未来的植物死亡研究提供帮助。     

The relationship between xylem conduit diameter and cavitation caused by freezing

The centrifuge method for measuring the resistance of xylem to cavitation by water stress was modified to also account for any additional cavitation that might occur from a freeze-thaw cycle. A strong correlation was found between cavitation by freezing and mean conduit diameter. On the one extreme, a tracheid-bearing conifer and diffuse-porous angiosperms with small-diameter vessels (mean diameter <30 μm) showed no freezing-induced cavitation under modest water stress (xylem pressure = −0.5 MPa), whereas species with larger diameter vessels (mean >40 μm) were nearly completely cavitated under the same conditions. Species with intermediate mean diameters (30–40 μm) showed partial cavitation by freezing. These results are consistent with a critical diameter of 44 μm at or above which cavitation would occur by a freeze–thaw cycle at −0.5 MPa. As expected, vulnerability to cavitation by freezing was correlated with the hydraulic conductivity per stem transverse area. The results confirm and extend previous reports that small-diameter conduits are relatively resistant to cavitation by freezing. It appears that the centrifuge method, modified to include freeze–thaw cycles, may be useful in separating the interactive effects of xylem pressure and freezing on cavitation.     

Reproductive water supply is prioritized during drought in tomato

Reproductive success largely defines the fitness of plant species. Understanding how heat and drought affect plant reproduction is thus key to predicting future plant fitness under rising global temperatures. Recent work suggests reproductive tissues are highly vulnerable to water stress in perennial plants where reproductive sacrifice could preserve plant survival. However, most crop species are annuals where such a strategy would theoretically reduce fitness. We examined the reproductive strategy of tomato (Solanum lycopersicum var. Rheinlands Ruhm) to determine whether water supply to fruits is prioritized above vegetative tissues during drought. Using optical methods, we mapped xylem cavitation and tissue shrinkage in vegetative and reproductive organs during dehydration to determine the priority of water flow under acute water stress. Stems and peduncles of tomato showed significantly greater xylem cavitation resistance than leaves. This maintenance of intact water supply enabled tomato fruit to continue to expand during acute water stress, utilizing xylem water made available by tissue collapse and early cavitation of leaves. Here, tomato plants prioritize water supply to reproductive tissues, maintaining fruit development under drought conditions. These results emphasize the critical role of water transport in shaping life history and suggest a broad relevance of hydraulic prioritization in plant ecology.     

Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration

1. An air-injection method was used to study loss of water transport capacity caused by xylem cavitation in roots and branches of Pinus edulis (Colorado Pinyon) and Juniperus osteosperma (Utah Juniper). These two species characterize the Pinyon–Juniper communities of the high deserts of the western United States. Juniperus osteosperma can grow in drier sites than P. edulis and is considered the more drought tolerant.
2. Juniperus osteosperma was more resistant to xylem cavitation than P. edulis in both branches and roots. Within a species, branches were more resistant to cavitation than roots for P. edulis but no difference was seen between the two organs for J. osteosperma . There was also no difference between juveniles and adults in J. osteosperma ; this comparison was not made for P. edulis .
3. Tracheid diameter was positively correlated with xylem cavitation pressure across roots and stems of both species. This relation suggests a trade-off between xylem conductance and resistance to xylem cavitation in these species.
4. During summer drought, P. edulis maintained higher predawn xylem pressures and showed much greater stomatal restriction of transpiration, consistent with its greater vulnerability to cavitation, than J. osteosperma .
5. These results suggest that the relative drought tolerance of P. edulis and J. osteosperma results in part from difference in their vulnerability to xylem cavitation.     

荒漠河岸林植物木质部导水与栓塞特征及其对干旱胁迫的响应

 以同处于干旱区的塔里木河下游(铁干里克)和黑河下游(乌兰图格)断面为研究区, 比较了荒漠河岸林主要建群种胡杨(Populus euphratica)、柽柳(Tamarix spp.)、疏叶骆驼刺(Alhagi sparsifolia)和花花柴(Karelinia caspia)在长期遭受不同干旱胁迫下的根、枝条木质部导水力和栓塞化程度的变化特征, 并分析了木质部导水对干旱胁迫的响应及适应策略。结果表明: 1) 黑河下游荒漠河岸林植物的导水能力显著高于塔里木河下游, 其中柽柳、胡杨、疏叶骆驼刺和花花柴根木质部的初始比导率(K_s0)分别高11.97、6.74、7.10和3.73倍, 枝条的K_s0分别高9.48、3.65、2.07和1.88倍, 地下水埋深导致的干旱胁迫程度不同是诱发荒漠植物导水能力差异的根本原因; 2)柽柳耐干旱能力最强, 适应范围较宽, 而花花柴、疏叶骆驼刺的耐旱性相对较弱, 适生范围较窄, 这可能与植物的根系分布有关; 3)干旱胁迫较轻时, 枝条木质部是荒漠河岸林植物水分传输的主要阻力部位, 干旱胁迫严重时, 根木质部是限制植株水流的最大阻碍部位; 4)荒漠河岸林植物主要通过调节枝条木质部的水流阻力来适应干旱胁迫, 且其适应策略与干旱胁迫程度有关, 干旱胁迫轻时, 植物通过限制枝条木质部水流来协调整株植物的均匀生长; 干旱胁迫严重时, 植物通过牺牲劣势枝条、增强优势枝条水流来提高植株整体生存的机会。     

Long-term acclimatization of hydraulic properties, xylem conduit size, wall strength and cavitation resistance in Phaseolus vulgaris in response to different environmental effects

Phaseolus vulgaris grown under various environmental conditions was used to assess long-term acclimatization of xylem structural characteristics and hydraulic properties. Conduit diameter tended to be reduced and 'wood' density (of 'woody' stems) increased under low moisture ('dry'), increased soil porosity ('porous soil') and low phosphorus ('low P') treatments. Dry and low P had the largest percentage of small vessels. Dry, low light ('shade') and porous soil treatments decreased P50 (50% loss in conductivity) by 0.15-0.25 MPa (greater cavitation resistance) compared with 'controls'. By contrast, low P increased P50 by 0.30 MPa (less cavitation resistance) compared with porous soil (the control for low P). Changes in cavitation resistance were independent of conduit diameter. By contrast, changes in cavitation resistance were correlated with wood density for the control, dry and porous soil treatments, but did not appear to be a function of wood density for the shade and low P treatments. In a separate experiment comparing control and porous soil plants, stem hydraulic conductivity (kh), specific conductivity (ks), leaf specific conductivity (LSC), total pot water loss, plant biomass and leaf area were all greater for control plants compared to porous soil plants. Porous soil plants, however, demonstrated higher midday stomatal conductance to water vapour (gs), apparently because they experienced proportionally less midday xylem cavitation.     

Use of centrifugal force in the study of xylem cavitation

Two methods were evaluated for using centrifugal force to measurethe occurrence of cavitation as a function of negative pressuresin xylem. The general protocol was to measure the hydraulicconductivity of xylem segments (stem or root pieces) beforeand after centring them on a centrifuge rotor and spinning themabout their long axis to generate negative xylem pressure. Thepercentage decrease in conductivity from the initial to finalmeasurement was used to quantify the embolism resulting fromcavitation during spinning. In one approach, segments were spunwith their ends exposed to air. This method could only be usedwhen xylem conduits were much shorter than the segment. Resultsfrom an angiosperm (Betula occidentalis) and a gymnosperm (AblesIasiocarpa) corresponded to previous observations of embolismcaused by air dehydration where negative pressure was measuredwith the pressure chamber. Results also agreed with embolismcaused by injection of air into the xylem, in support of theair-seeding hypothesis for cavitation. In a second approach,segments were spun in a rotor designed to keep the segment endsimmersed in water during spinning. This gave the same resultsas for non-immersed segments. Immersing the segment ends allowedmeasurements on any material, regardless of conduit length,as demonstrated for roots of B. occidentalis. The chief advantageof the centrifugal force method is the rapidity and precisionwith which any desired xylem pressure can be imposed. Key words: Cavitation, embolism, drought stress, water relations, water transport