Xylem vulnerability to cavitation can be accurately characterised in species with long vessels using a centrifuge method

Vulnerability to cavitation curves describe the decrease in xylem hydraulic conductivity as xylem pressure declines. Several techniques for constructing vulnerability curves use centrifugal force to induce negative xylem pressure in stem or root segments. Centrifuge vulnerability curves constructed for long‐vesselled species have been hypothesised to overestimate xylem vulnerability to cavitation due to increased vulnerability of vessels cut open at stem ends that extend to the middle or entirely through segments. We tested two key predictions of this hypothesis: (i) centrifugation induces greater embolism than dehydration in long‐vesselled species, and (ii) the proportion of open vessels changes centrifuge vulnerability curves. Centrifuge and dehydration vulnerability curves were compared for a long‐ and short‐vesselled species. The effect of open vessels was tested in four species by comparing centrifuge vulnerability curves for stems of two lengths. Centrifuge and dehydration vulnerability curves agreed well for the long‐ and short‐vesselled species. Centrifuge vulnerability curves constructed using two stem lengths were similar. Also, the distribution of embolism along the length of centrifuged stems matched the theoretical pressure profile induced by centrifugation. We conclude that vulnerability to cavitation can be accurately characterised with vulnerability curves constructed using a centrifuge technique, even in long‐vesselled species.     

An ignored anatomical variable: pore shape shows a nonrandom variation pattern in xylem cross sections

Anatomical characteristics of vessels have a profound impact on the hydraulic conductivity of the xylem. However, pore shape, the cross‐section shape of a vessel, has been ignored in previous hydraulic architecture studies. In this study, we examined whether pore shape tended to be circular, and whether variation in pore shape may be affected by water flow path‐length and cambial age. The circularity of pores in Betula platyphylla Roth and Quercus mongolica (only earlywood) were analyzed from the pith to the bark along the water flow path. It was found that although there were very few pores with perfectly circular shape, solitary pores did tend to be regularly circular. The pore shape of sequentially formed vessels in the xylem was influenced by cambial age and flow path‐length. Pore shape tended to be more circular as flow path‐length increased or cambial age decreased. Circular pore shape appears to be an important morphogenetic mechanism for hydraulic architecture development, and studies of it expand our understanding of the structure–function relationships of the xylem.     

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.     

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.     

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.     

Xylem function of arid-land shrubs from California, USA: an ecological and evolutionary analysis

Xylem traits were examined among 22 arid-land shrub species, including measures of vessel dimensions and pit area. These structural measures were compared with the xylem functional traits of transport efficiency and safety from cavitation. The influence of evolution on trait relationships was examined using phylogenetic independent contrasts (PICs). A trade-off between xylem safety and efficiency was supported by a negative correlation between vessel dimensions and cavitation resistance. Pit area was correlated with cavitation resistance when cross species data were examined, but PICs suggest that these traits have evolved independently of one another. Differences in cavitation resistance that are not explained by pit area may be related to differences in pit membrane properties or the prevalence of tracheids, the latter of which may alter pit area through the addition of vessel-to-tracheid pits or through changes in xylem conduit connectivity. Some trait relationships were robust regardless of species ecology or evolutionary history. These trait relationships are likely to be the most valuable in predictive models that seek to examine anatomical and functional trait relationships among extant and fossil woods and include the relationship among hydraulic conductivity and vessel diameter, between vessel diameter and vessel length, and between hydraulic conductivity and wood density.     

Structural determinants of increased susceptibility to dehydration‐induced cavitation in post‐fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem‐specific hydraulic conductivity (K_s) and resistance to cavitation (P₅₀) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co‐occurring post‐fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K_s. Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter‐vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.     

不同冲洗液对毛白杨和油松枝条水力导度和抵抗空穴化能力测定值的影响

植物通过木质部管道系统进行水分运输, 木质部的水分运输效率和抗空穴化能力等水力结构特征对于植物物种的分布、抗逆能力等方面起关键性作用。目前, 国内外学者一般采用“冲洗法”进行木质部水力结构研究, 然而在该方法中使用的不同冲洗溶质可能对植物木质部水力结构等产生较大影响, 因此该文研究了3种溶质的冲洗溶液对毛白杨(Populus tomentosa)和油松(Pinus tabulaeformis)枝条的水力导度和抵抗空穴化能力的影响。实验结果表明: 相对于去离子水, 用0.01 mol·L^-1的草酸和0.03 mol·L^-1KCl溶液作为冲洗溶液, 均导致毛白杨木质部导管和油松管胞的水力导度测定值的增大。KCl导致毛白杨和油松木质部抵抗空穴化能力测定值的提高, 草酸导致杨树抵抗空穴化能力测定值增强, 但导致油松抗空穴化能力显著(p<0.01)减弱。小枝水平上, 毛白杨和油松的水分运输效率和抗空穴化能力之间没有显著相关性。另外, 在截枝实验中发现, 毛白杨小枝木质部水力导度随长度增加变化不大, 而油松枝条的木质部水力导度有逐渐增大的趋势。以上的实验结果表明不同溶质下毛白杨和油松枝条的木质部水力导度和抵抗空穴化能力不同, 草酸和KCl可能对木质部管道系统及纹孔处的果胶等产生作用, 从而使毛白杨和油松的水力结构发生变化。毛白杨与油松水力结构在去离子水、草酸和KCl的作用下的不同结果及两物种截枝试验下水力导度的不同变化趋势表明, 导管运输系统和管胞运输系统可能具有不同的水分运输影响因素。     

EFFECTS OF TREE AGE ON SECONDARY XYLEM AND PHLOEM ANATOMY IN STEMS OF GREAT BASIN BRISTLECONE PINE (PINUS LONGAEVA)

Xylem and phloem tissue samples were collected from various-aged Great Basin bristlecone pine (Pinus longaeva D. K. Bailey) stems in southern Utah and southeastern California to determine whether the vascular cambia of older trees produce fewer xylem rays, shorter-lived xylem and phloem ray cells, fewer phloem sieve cells, and a thinner phloem. Increment cores were examined to determine whether ‘aged’ cambia produced narrower tracheids that might reduce water translocation. Sapwood thickness was measured and sapwood growth layers were counted on these cores. Regression and Classification and Regression Tree (CART) analyses of sample data found no age-related changes in cambial products. Phloem and xylem production appeared normal at all ages, with no evidence of cambial malfunction.     

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

Premise of the Study

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

Methods

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N‐1‐naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA‐treated and control plants.

Key Results

NPA‐treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA‐treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high‐pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA‐treated stems.

Conclusions

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem‐side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.     

Frost fatigue and spring recovery of xylem vessels in three diffuse‐porous trees in situ

Frost has been shown to cause frost fatigue (reduced cavitation resistance) in branch segments in the lab. Here, we studied the change in cavitation resistance and percent loss of conductivity (PLC) from fall to spring over 2 consecutive years in three diffuse‐porous species in situ. We used the cavitron technique to measure P₂₅, P₅₀ and P₉₀ (the xylem pressure causing a 25, 50 and 90% conductivity loss) and PLC and stained functioning vessels. Cavitation resistance was reduced by 64–87% (in terms of P₅₀), depending on the species and year. P₂₅ was impacted the most and P₉₀ the least, changing the vulnerability curves from s‐ to r‐shaped over the winter in all three species. The branches suffered an almost complete loss of conductivity, but frost fatigue did not necessarily occur concurrently with increases in PLC. In two species, there was a trade‐off between conduit size and vulnerability. Spring recovery occurred by growth of new vessels, and in two species by partial refilling of embolized conduits. Although newly grown and functioning conduits appeared more vulnerable to cavitation than year‐old vessels, cavitation resistance generally improved in spring, suggesting other mechanisms for partial frost fatigue repair.     

Water relations of a tropical vine-like bamboo (Rhipidocladum racemiflorum): root pressures, vulnerability to cavitation and seasonal changes in embolism

The occurrence of root pressure, the vulnerability of xylemvessels to drought-induced cavitation, and the seasonal changesin hydraulic conductivity due to embolism were studied in theculms of Rhipidocladum racemiflorum (Steud.) McClure, a tropicalvine-like bamboo from central Panama. Positive hydrostatic potentialsup to 120 kPa occurred only during the wet season when the transpirationrate of the plant was low, i.e. at night or during rain events.Although the xylem vessels were large and efficient for conductingwater, they were highly resistant to cavitation. Xylem waterpotentials lower than –4.5 MPa were required to induce50% loss of hydraulic conductivity in culms. The minimum waterpotential reached –3.75 MPa at the end of the 1993 dryseason, so loss of hydraulic conductivity due to embolism remained<10%. The species is adapted to drier habitats both by wayof a low vulnerability to xylem cavitation and by root pressuresin the wet season that could refill vessels that became embolizedduring a severe dry season. Key words: Rhipidocladum racemiflorum, root pressure, cavitation, embolism, water relations     

Moving beyond the cambium necrosis hypothesis of post-fire tree mortality: cavitation and deformation of xylem in forest fires

? It is widely assumed that post-fire tree mortality results from necrosis of phloem and vascular cambium in stems, despite strong evidence that reduced xylem conductivity also plays an important role. ? In this study, experiments with Populus balsamifera were used to demonstrate two mechanisms by which heat reduces the hydraulic conductivity of xylem: air seed cavitation and conduit wall deformation. Heat effects on air seed cavitation were quantified using air injection experiments that isolate potential temperature-dependent changes in sap surface tension and pit membrane pore diameters. Heat effects on conduit wall structure were demonstrated using air conductivity measurements and light microscopy. ? Heating increased vulnerability to cavitation because sap surface tension varies inversely with temperature. Heating did not affect cavitation via changes in pit membrane pore diameters, but did cause significant reductions in xylem air conductivity that were associated with deformation of conduit walls (probably resulting from thermal softening of viscoelastic cell wall polymers). ? Additional work is required to understand the relative roles of cavitation and deformation in the reduction of xylem conductivity, and how reduced xylem conductivity in roots, stems, and branches correlates and interacts with foliage and root necroses to cause tree mortality. Future research should also examine how heat necrosis of ray parenchyma cells affects refilling of embolisms that occur during and after the fire event.     

The physiological implications of primary xylem organization in two ferns

Xylem structure and function are well described in woody plants, but the implications of xylem organization in less‐derived plants such as ferns are poorly understood. Here, two ferns with contrasting phenology and xylem organization were selected to investigate how xylem dysfunction affects hydraulic conductivity and stomatal conductance (g_s). The drought‐deciduous pioneer species, Pteridium aquilinum, exhibits fronds composed of 25 to 37 highly integrated vascular bundles with many connections, high g_s and moderate cavitation resistance (P₅₀ = ?2.23 MPa). By contrast, the evergreen Woodwardia fimbriata exhibits sectored fronds with 3 to 5 vascular bundles and infrequent connections, low g_s and high resistance to cavitation (P₅₀ = ?5.21 MPa). Xylem‐specific conductivity was significantly higher in P. aqulinium in part due to its wide, efficient conduits that supply its rapidly transpiring pinnae. These trade‐offs imply that the contrasting xylem organization of these ferns mirrors their divergent life history strategies. Greater hydraulic connectivity and g_s promote rapid seasonal growth, but come with the risk of increased vulnerability to cavitation in P. aquilinum, while the conservative xylem organization of W. fimbriata leads to slower growth but greater drought tolerance and frond longevity.     

Light-dependent maintenance of hydraulic function in mangrove branches: do xylary chloroplasts play a role in embolism repair?

? To clarify the role of branch photosynthesis in tree functioning, the presence and function of chloroplasts in branch xylem tissue were studied in a diverse range of mangrove species growing in Australia. ? The presence of xylary chloroplasts was observed via chlorophyll fluorescence of transverse sections. Paired, attached branches were selected to study the effects of covering branches with aluminium foil on the gas exchange characteristics of leaves and the hydraulic conductivity of branches. ? Xylary chloroplasts occurred in all species, but were differently distributed among living cell types in the xylem. Covering stems altered the gas exchange characteristics of leaves, such that water-use efficiency was greater in exposed leaves of covered than of uncovered branches. ? Leaf-specific hydraulic conductivity of stems was lower in covered than in uncovered branches, implicating stem photosynthesis in the maintenance of hydraulic function. Given their proximity to xylem vessels, we suggest that xylary chloroplasts may play a role in light-dependent repair of embolized xylem vessels.     

Pit Membrane Degradation and Air-Embolism Formation in Ageing Xylem Vessels of Populus tremuloides Michx

Air-embolism formation in xylem vessels of Populus tremuloidesMichx. was quantified by its reduction of hydraulic conductivityin branch segments. Embolism was induced by increasing xylemtension in drying stems, or by inserting one end of a hydratedstem in a pressure bomb and increasing air pressure in the bomb.Both treatments produced the same response suggesting that embolismby water stress was caused by air entering water-filled vessels,presumably through inter-vessel pits. In rapidly-growing P.tremuloides branches, the vessels of the outer growth ring werefunctional whereas vessels in older xylem were mostly embolized.This selective embolizing of older vessels was associated witha marked increase in permeability of their inter-vessel pitsto air, relative to pits of younger vessels. Air-injection pressuresless than 1·0 MPa completely embolized older vesselsthat had been re-filled in the laboratory, whereas pressuresover 4·0 MPa were required to embolize young vessels.Greater permeability of old vessels was due to degradation oftheir pit membranes as seen in the scanning electron microscope;large openings were present that were not seen in pit membranesof young vessels. These holes would allow air to penetrate vesselends at low pressure differences causing embolism. Degradationof pit membranes causing the selective dysfunction of oldersapwood may be a general phenomenon initiating heartwood formationin many species. Key words: Xylem embolism, hydraulic conductivity, heartwood formation, cavitation, Populus tremuloides, Michx     

Development of successive cambia and formation of flat stems in Rhynchosia pyramidalis (Lam.) Urb. (Fabaceae)

Stem flattening in Rhynchosia pyramidalis (Fabaceae) is achieved by the development of crescent-shaped successive cambia on two opposite sides of the stem (referred hereafter as distal side). Other lateral sides of the stem (adjacent to supporting host and its opposite side, referred as proximal sides) usually possess single cambium. In the young stems, parenchymatous cells located outside to protophloem of distal side dedifferentiate and develop small segments of cambium. Concomitant to bidirectional differentiation of the secondary xylem and phloem, these newly developed cambial segments also extend in tangential directions. Differential activity of newly developed crescent-shaped cambial segments deposits more secondary xylem at median position as compared to their terminal ends of the stem on distal side; consequently, it pushes the cambial segment outside, thus resulting in crescent-shaped arcs of the cambia only on two opposite sides. After the production of 1–2 mm of secondary xylem, they cease to divide and new segments of cambial arc develop on the same side in a similar fashion. Such repeated behaviour of successive cambia development consequently leads to the formation of tangentially flat stems. The secondary xylem is diffusely porous with indistinct growth rings and is composed of vessels (wide and narrow), fibres, axial ray parenchyma cells, while phloem consisted of sieve elements, companion cells, axial and ray parenchyma. Rays in both xylem and phloem are uni- to multiseriate and heterocellular. The structure of secondary xylem and development of successive cambia is correlated with climbing habit.     

Xylem hydraulic safety and construction costs determine tropical tree growth

Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast‐growing trees have lower xylem safety margins than slow‐growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast‐growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth ‐ xylem vulnerability trade‐off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade‐off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth‐mortality trade‐off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.     

Lipids in xylem sap of woody plants across the angiosperm phylogeny

Lipids have been observed attached to lumen-facing surfaces of mature xylem conduits of several plant species, but there has been little research on their functions or effects on water transport, and only one lipidomic study of the xylem apoplast. Therefore, we conducted lipidomic analyses of xylem sap from woody stems of seven plants representing six major angiosperm clades, including basal magnoliids, monocots and eudicots, to characterize and quantify phospholipids, galactolipids and sulfolipids in sap using mass spectrometry. Locations of lipids in vessels of Laurus nobilis were imaged using transmission electron microscopy and confocal microscopy. Xylem sap contained the galactolipids di- and monogalactosyldiacylglycerol, as well as all common plant phospholipids, but only traces of sulfolipids, with total lipid concentrations in extracted sap ranging from 0.18 to 0.63 nmol ml⁻¹ across all seven species. Contamination of extracted sap from lipids in cut living cells was found to be negligible. Lipid composition of sap was compared with wood in two species and was largely similar, suggesting that sap lipids, including galactolipids, originate from cell content of living vessels. Seasonal changes in lipid composition of sap were observed for one species. Lipid layers coated all lumen-facing vessel surfaces of L. nobilis, and lipids were highly concentrated in inter-vessel pits. The findings suggest that apoplastic, amphiphilic xylem lipids are a universal feature of angiosperms. The findings require a reinterpretation of the cohesion-tension theory of water transport to account for the effects of apoplastic lipids on dynamic surface tension and hydraulic conductance in xylem.     

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.