The rapid light response of leaf hydraulic conductance: new evidence from two experimental methods

Previous studies have shown a rapid enhancement in leaf hydraulic conductance (K(leaf)) from low to high irradiance (from <10 to >1000 micromol photons m(-2) s(-1)), using the high-pressure flow meter (HPFM), for 7 of 14 tested woody species. However, theoretical suggestions have been made that this response might arise as an artifact of the HPFM. We tested the K(leaf) light response for six evergreen species using refined versions of the rehydration kinetics method (RKM) and the evaporative flux method (EFM). We found new evidence for a rapid, 60% to 100% increase in K(leaf) from low to high irradiance for three species. In the RKM, the leaf rehydration time constant declined by up to 70% under high irradiance relative to darkness. In the EFM, under higher irradiance, the flow rate increased disproportionately to the water potential gradient. Combining our data with those of previous studies, we found that heterobaric species, i.e. those with bundle sheath extensions (BSEs) showed a twofold greater K(leaf) light response on average than homobaric species, i.e. those without BSEs. We suggest further research to characterize this substantial dynamic at the nexus of plant light- and water-relations.     

Diurnal and seasonal changes of leaf lamina hydraulic conductance in bur oak (Quercus macrocarpa) and trembling aspen (Populus tremuloides)

The aim of this study was to examine the diurnal and seasonal variations in the sensitivity of leaf lamina (K _lam) hydraulic conductance to irradiance in bur oak (Quercus macrocarpa Michx.) and trembling aspen (Populus tremuloides Michx.), which vary in their responses of K _lam to irradiance. K _lam was determined using the high-pressure method and the measurements were carried out in June, August and September. The irradiance response of K _lam in bur oak was present throughout the day and declined in senescing leaves. In trembling aspen, K _lam declined from morning to late afternoon and drastically decreased before the onset of leaf senescence, but it was not sensitive to irradiance. In both tree species, the capacity of the petioles to supply water to leaf lamina changed during the day in accordance with the ability of the leaf lamina to transport water. Petiole hydraulic conductivity (K _pet) declined during the season in bur oak leaves, while it tended to increase in trembling aspen leaves. There was no correlation between the K _lam values and air temperature or light intensity at the time of leaf collection. For trembling aspen, K _pet was negatively correlated with the air temperature suggesting sensitivity to drought. We conclude that the water transport properties of petioles and leaf lamina in the two studied tree species reflect their ecological adaptations. Trembling aspen leaves have high hydraulic conductivity and high stomatal conductance regardless of the irradiance level, consistent with the rapid growth and high demand for water. In contrast, the increased lamina hydraulic conductivity and stomatal conductance under high irradiance in bur oak trees reflect a water conservation strategy.     

Turning over a new ‘leaf’: multiple functional significances of leaves versus phyllodes in Hawaiian Acacia koa

Hawaiian endemic tree Acacia koa is a model for heteroblasty with bipinnately compound leaves and phyllodes. Previous studies suggested three hypotheses for their functional differentiation: an advantage of leaves for early growth or shade tolerance, and an advantage of phyllodes for drought tolerance. We tested the ability of these hypotheses to explain differences between leaf types for potted plants in 104 physiological and morphological traits, including gas exchange, structure and composition, hydraulic conductance, and responses to varying light, intercellular CO₂, vapour pressure deficit (VPD) and drought. Leaf types were similar in numerous traits including stomatal pore area per leaf area, leaf area‐based gas exchange rates and cuticular conductance. Each hypothesis was directly supported by key differences in function. Leaves had higher mass‐based gas exchange rates, while the water storage tissue in phyllodes contributed to greater capacitance per area; phyllodes also showed stronger stomatal closure at high VPD, and higher maximum hydraulic conductance per area, with stronger decline during desiccation and recovery with rehydration. While no single hypothesis completely explained the differences between leaf types, together the three hypotheses explained 91% of differences. These findings indicate that the heteroblasty confers multiple benefits, realized across different developmental stages and environmental contexts.     

Systemic signalling in photosynthetic induction of Rumex K‐1 (Rumex patientia × Rumex tianschaious) leaves

The rapid induction of photosynthesis is critical for plants under light‐fleck environment. Most previous studies about photosynthetic induction focused upon single leaf, but they did not consider the systemic integrity of plant. Here, we verified whether systemic signalling is involved in photosynthetic induction. Rumex K‐1 (Rumex patientia × Rumex tianschaious) plants were grown under light‐fleck condition. After whole night dark adaptation, different numbers of leaves (system leaf or SL) were pre‐illuminated with light, and then the photosynthetic induction of other leaves (target leaf or TL) was investigated. This study showed that the pre‐illumination of SL promoted photosynthetic induction in TL. This promotion was independent of the number of SL, the light intensity on SL and the distance between SL and TL, indicating that this systemic signalling is non‐dose‐dependent. More interestingly, the photosynthetic induction was promoted by only the pre‐illumination of morphological upper leaf rather than the pre‐illumination of morphological lower leaf, indicating that the transfer of this signal is directional. The results showed that the transfer of this systemic signalling depends upon the phloem. This systemic signalling helps plants to use light energy more efficiently under light flecks.     

VvPIP2;4N aquaporin involvement in controlling leaf hydraulic capacitance and resistance in grapevine

Hydraulic capacitance (C) in a plant tissue buffers the xylem tension, storing and releasing water and has been highlighted in recent years as an important factor that affects water relations such as drought tolerance and embolism formation. Aquaporins (AQPs) are well known to control leaf hydraulic resistance (Rh) but their role in the control of C is unknown. Here, we assess Rh and C on detached grapevines wild‐type (WT) (cv. Brachetto) leaves and over‐expressing the aquaporin gene VvPIP2;4N (OE). For this purpose, we developed a new method inspired from the pressure‐volume curve technique and the rehydration‐kinetic‐method, which allowed us to monitor the dynamics of dehydration and rehydration in the same leaf. The recovery after dehydration was measured in dark, light non‐transpirative conditions, light‐transpirative conditions and light‐transpirative condition adding abscisic acid. Pressurizing to dehydrate leaves in the OE line, the recorded Rh and C were respectively lower and higher than those in the WT. The same results were obtained in the dark recovery by rehydration treatment. In the presence of light, either when leaves transpired or not (by depressing vapor pressure deficit), the described effects disappeared. The change in Rh and C did not affect the kinetics of desiccation of detached leaves in dark in air, in OE plants compared to WT ones. Our study highlighted that both Rh and C were influenced by the constitutive over‐expression of VvPIP2;4N. The effect of AQPs on C is reported here for the first time and may involve a modulation of cell reflection coefficient.     

SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO₂ Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.     

The dependence of leaf hydraulic conductance on irradiance during HPFM measurements: any role for stomatal response?

This paper examines the dependence of whole leaf hydraulic conductance to liquid water (K(L)) on irradiance when measured with a high pressure flowmeter (HPFM). During HPFM measurements, water is perfused into leaves faster than it evaporates hence water infiltrates leaf air spaces and must pass through stomates in the liquid state. Since stomates open and close under high versus low irradiance, respectively, the possibility exists that K(L) might change with irradiance if stomates close tightly enough to restrict water movement. However, the dependence of K(L) on irradiance could be due to a direct effect of irradiance on the hydraulic properties of other tissues in the leaf. In the present study, K(L) increased with irradiance for 6 of the 11 species tested. Whole leaf conductance to water vapour, g(L), was used as a proxy for stomatal aperture and the time-course of changes in K(L) and g(L) was studied during the transition from low to high irradiance and from high to low irradiance. Experiments showed that in some species K(L) changes were not paralleled by g(L) changes. Measurements were also done after perfusion of leaves with ABA which inhibited the g(L) response to irradiance. These leaves showed the same K(L) response to irradiance as control leaves. These experimental results and theoretical calculations suggest that the irradiance dependence of K(L) is more consistent with an effect on extravascular (and/or vascular) tissues rather than stomatal aperture. Irradiance-mediated stimulation of aquaporins or hydrogel effects in leaf tracheids may be involved.     

Plant hydraulic conductance measured by the high pressure flow meter in crop plants

A new high pressure flow meter (HPFM) method for measuring plant hydraulic conductances (K) was investigated to examine whether its results are comparable to those from a conventional evaporative flux (EF) method in crops. Hydraulic conductance (K) was measured by the two methods under quasi-steady-state conditions in six crops grown in pots: soybean (Glycine max L. Merr. cv. Tsurunoko daizu), sunflower (Helianthus annuus L. cv. Russian mammoth), kidney bean (Phaseolus vulgaris L. cv. Tsurunashi morocco), tomato (Lycopersicon esculentum Mill. cv. Sekai-ichi), green pepper (Capsicum annuum L. cv. shishitou), and eggplant (Solanum melongena L. cv. Seiguro chunaga nasu). There was a 1:1 agreement between K values measured by the two methods for K values of whole plant, root and stem, and leaf under quasi-steady-state conditions. Leaf water potential (psi leaf) and evaporative flux density (E) in sunflower was curvilinear, indicating whole plant K estimated by the EF method increased with increase of E. Predicted psi leaf (= E divided by whole plant K measured by the HPFM method) agreed with measured psi leaf. Diurnal changes were also found in K measured by the HPFM confirming that K changed in response to temperature and E. The HPFM revealed that variable conductance was located in all organs: roots, stems, petioles, and leaves. These observations indicated that the HPFM is valid for crops as well as for trees (as previously established by Tsuda and Tyree) and has advantages over the EF method because of the speed and ease of the HPFM method.     

Transient releases of acetaldehyde from tree leaves − products of a pyruvate overflow mechanism?

Emissions of acetaldehyde from tree leaves were investigated by proton‐transfer‐reaction mass spectrometry (PTR‐MS), a technique that allows simultaneous monitoring of different leaf volatiles, and confirmed by derivatization and high‐performance liquid chromatography analysis. Bursts of acetaldehyde were released by sycamore, aspen, cottonwood and maple leaves following light–dark transitions; isoprene emission served as a measure of chloroplastic processes. Acetaldehyde bursts were not accompanied by ethanol, but exposure of leaves to inhibitors of pyruvate transport or respiration, or anoxia, led to much larger releases of acetaldehyde, accompanied by ethanol under anoxic conditions. These same leaves have an oxidative pathway for ethanol present in the transpiration stream, resulting in acetaldehyde emissions that are inhibited in vivo by 4‐methylpyrazole, an alcohol dehydrogenase (Adh) inhibitor. Labelling of leaf volatiles with ¹³CO₂ suggested that the pools of cytosolic pyruvate, the proposed precursor of acetaldehyde bursts, were derived from both recent photosynthesis and cytosolic carbon sources. We hypothesize that releases of acetaldehyde during light–dark transitions result from a pyruvate overflow mechanism controlled by cytosolic pyruvate levels and pyruvate decarboxylase activity. These results suggest that leaves of woody plants contribute reactive acetaldehyde to the atmosphere under different conditions: (1) metabolic states that promote the accumulation of cytosolic pyruvate, triggering the pyruvate decarboxylase reaction; and (2) leaf ethanol oxidation resulting from ethanol transported from anoxic tissues.     

Oscillations in stomatal conductance of hybrid poplar leaves in the light and dark

Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s⁻¹ in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination.     

Low leaf hydraulic conductance associated with drought tolerance in soybean

Lack of water is the most serious environmental constraint on agricultural production. More efficient use of water resources is a key solution for increased plant productivity in water-deficit environments. We examined the hydraulic characteristics of a 'slow wilting' phenotype in soybean ( Glycine max Merr.), PI 416937, which has been shown to have relatively constant transpiration rates above a threshold atmospheric vapor pressure deficit (VPD). The VPD response of PI 416937 was confirmed. Three experiments are reported to examine the hypothesis that the VPD response was a result of low hydraulic conductance in leaves as compared to two other soybean genotypes. Results are reported from experiments to measure transpiration response to VPD when xylem water potential was maintained at zero, leaf rehydration response and leaf carbon assimilation response to petiole cutting. Major interspecific differences in leaf hydraulic properties were observed. The observed low leaf hydraulic conductance in PI 416937 is consistent with an increased water use efficiency, and an increased water conservation by limiting transpiration rates under high evaporative conditions but allowing normal gas exchange rates under more moderate evaporative conditions.     

Dynamics of leaf hydraulic conductance with water status: quantification and analysis of species differences under steady state

Leaf hydraulic conductance (K(leaf)) is a major determinant of photosynthetic rate in well-watered and drought-stressed plants. Previous work assessed the decline of K(leaf) with decreasing leaf water potential (Ψ(leaf)), most typically using rehydration kinetics methods, and found that species varied in the shape of their vulnerability curve, and that hydraulic vulnerability correlated with other leaf functional traits and with drought sensitivity. These findings were tested and extended, using a new steady-state evaporative flux method under high irradiance, and the function for the vulnerability curve of each species was determined individually using maximum likelihood for 10 species varying strongly in drought tolerance. Additionally, the ability of excised leaves to recover in K(leaf) with rehydration was assessed, and a new theoretical framework was developed to estimate how rehydration of measured leaves may affect estimation of hydraulic parameters. As hypothesized, species differed in their vulnerability function. Drought-tolerant species showed shallow linear declines and more negative Ψ(leaf) at 80% loss of K(leaf) (P(80)), whereas drought-sensitive species showed steeper, non-linear declines, and less negative P(80). Across species, the maximum K(leaf) was independent of hydraulic vulnerability. Recovery of K(leaf) after 1 h rehydration of leaves dehydrated below their turgor loss point occurred only for four of 10 species. Across species without recovery, a more negative P(80) correlated with the ability to maintain K(leaf) through both dehydration and rehydration. These findings indicate that resistance to K(leaf) decline is important not only in maintaining open stomata during the onset of drought, but also in enabling sustained function during drought recovery.     

Co-ordination of hydraulic and stomatal conductances across light qualities in cucumber leaves

Long-term effects of light quality on leaf hydraulic conductance (K(leaf)) and stomatal conductance (g(s)) were studied in cucumber, and their joint impact on leaf photosynthesis in response to osmotic-induced water stress was assessed. Plants were grown under low intensity monochromatic red (R, 640 nm), blue (B, 420 nm) or combined red and blue (R:B, 70:30) light. K(leaf) and g(s) were much lower in leaves that developed without blue light. Differences in g(s) were caused by differences in stomatal aperture and stomatal density, of which the latter was largely due to differences in epidermal cell size and hardly due to stomatal development. Net photosynthesis (A(N)) was lowest in R-, intermediate in B-, and highest in RB- grown leaves. The low A(N) in R-grown leaves correlated with a low leaf internal CO(2) concentration and reduced PSII operating efficiency. In response to osmotic stress, all leaves showed similar degrees of stomatal closure, but the reduction in A(N) was larger in R- than in B- and RB-grown leaves. This was probably due to damage of the photosynthetic apparatus, which only occurred in R-grown leaves. The present study shows the co-ordination of K(leaf) and g(s) across different light qualities, while the presence of blue in the light spectrum seems to drive both K(leaf) and g(s) towards high, sun-type leaf values, as was previously reported for maximal photosynthetic capacity and leaf morphology. The present results suggest the involvement of blue light receptors in the usually harmonized development of leaf characteristics related to water relations and photosynthesis under different light environments.     

Differential leaf expansion can enable hydraulic acclimation to sun and shade

Although leaf size is one of the most responsive plant traits to environmental change, the functional benefits of large versus small leaves remain unclear. We hypothesized that modification of leaf size within species resulting from differences in irradiance can allow leaves to acclimate to different photosynthetic or evaporative conditions while maintaining an efficient balance between hydraulic supply (vein density) and evaporative demand. To test this, we compared the function and anatomy of leaf hydraulic systems in the leaves of a woody angiosperm (Toona ciliata M. Roem.) grown under high and low irradiance in controlled conditions. Our results confirm that in this species, differential leaf expansion regulates the density of veins and stomata such that leaf hydraulic conductance and stomatal conductance remain proportional. A broader sample of field-grown tree species suggested that differences in leaf venation and stomatal traits induced by sun and shade were not regulated by leaf size in all cases. Our results, however, suggest that leaf size plasticity can provide an efficient way for plants to acclimate hydraulic and stomatal conductances to the contrasting evaporative conditions of sun and shade.     

Stem and leaf hydraulic properties are finely coordinated in three tropical rain forest tree species

Coordination of stem and leaf hydraulic traits allows terrestrial plants to maintain safe water status under limited water supply. Tropical rain forests, one of the world's most productive biomes, are vulnerable to drought and potentially threatened by increased aridity due to global climate change. However, the relationship of stem and leaf traits within the plant hydraulic continuum remains understudied, particularly in tropical species. We studied within‐plant hydraulic coordination between stems and leaves in three tropical lowland rain forest tree species by analyses of hydraulic vulnerability [hydraulic methods and ultrasonic emission (UE) analysis], pressure‐volume relations and in situ pre‐dawn and midday water potentials (Ψ). We found finely coordinated stem and leaf hydraulic features, with a strategy of sacrificing leaves in favour of stems. Fifty percent of hydraulic conductivity (P₅₀) was lost at ?2.1 to ?3.1 MPa in stems and at ?1.7 to ?2.2 MPa in leaves. UE analysis corresponded to hydraulic measurements. Safety margins (leaf P₅₀ – stem P₅₀) were very narrow at ?0.4 to ?1.4 MPa. Pressure‐volume analysis and in situ Ψ indicated safe water status in stems but risk of hydraulic failure in leaves. Our study shows that stem and leaf hydraulics were finely tuned to avoid embolism formation in the xylem.     

Potassium mediates coordination of leaf photosynthesis and hydraulic conductance by modifications of leaf anatomy

Typical symptoms of potassium deficiency, characterized as chlorosis or withered necrosis, occur concomitantly with downregulated photosynthesis and impaired leaf water transport. However, the prominent limitations and mechanisms underlying the concerted decreases of leaf photosynthesis and hydraulic conductance are poorly understood. Monocots and dicots were investigated based on responses of photosynthesis and hydraulic conductance and their components and the correlated anatomical determinants to potassium deficiency. We found a conserved pattern in which leaf photosynthesis and hydraulic conductance concurrently decreased under potassium starvation. However, monocots and dicots showed two different hydraulic‐redesign strategies: Dicots tended to show a decreased minor vein density, whereas monocots reduced the size of the bundle sheath and its extensions, rather than the minor vein density; both of these strategies may restrain xylem and outside‐xylem hydraulic conductance. Additionally, potassium‐deprived leaves developed with fewer mesophyll cell‐to‐cell connections, leading to a reduced area being available for liquid‐phase flow. Further quantitative analysis revealed that mesophyll conductance to CO₂ and outside‐xylem hydraulic resistance were the major contributors to photosynthetic limitation and increased hydraulic resistance, at more than 50% and 60%, respectively. These results emphasize the importance of potassium in the coordinated regulation of leaf photosynthesis and hydraulic conductance through modifications of leaf anatomy.     

Effect of genotype and graft type on the hydraulic characteristics and water relations of grafted melon

Abstract

From the measurements of the profiles of hydraulic conductance and water potential from soil through to the leaf system in fully established melon plants, the limits to water flow set by coupling of hydraulic conductance (k) with water relation parameters was evaluated in the laboratory using high pressure flow device (HPFM) and evaporative flux method (EF). The rootstock Arava was grafted onto self, and onto two genotypes (AR57 and AR82) using side and V graft types, and there was an ungrafted control. Hydraulic transport efficiency was estimated from measurements of evaporative flux (transpiration rate) and leaf water potential (ψL) measured between pre-dawn and sunset during the growth cycle. Measured parameters to characterize the hydraulic efficiency (architecture) of the vascular system of melon were normalized to areas of leaves and stem cross section; this enabled the examination of their physiological and ecological functions. The effects of rootstock genotype were more marked on graft union and scion water relations. Differences in the magnitudes of water relation parameters of hydraulic conductance, water potential (lwp) and evaporative water loss (EF) were detected. AR/RS82 side grafted exhibited high EF and Kh despite its lower leaf water potential compared to AR/RS57 V grafted. Self grafting (Arava/Arava grafts) in melon seems to improve water relations and xylem water transport efficiency. Parameters describing the hydraulic efficiency (architecture) of vascular system of melon plants were described in relation to plant attributes. The expression of hydraulic conductance of the root and shoot system relative to plant attributes did not eliminate differences in the magnitudes of conductance elements in tomato and melon. Differences obtained among the different melon grafts in whole plant leaf and stem area specific hydraulic conductance (Kl) indicate the carbon efficiency and hence the cost of resource allocation to areas of root surface and leaves. The role of plant water relations in root-shoot communications and whole plant regulation of water flux are inferred from this study.     

Evidence for xylem embolism as a primary factor in dehydration-induced declines in leaf hydraulic conductance

Hydraulic conductance of leaves (K(leaf)) typically decreases with increasing water stress and recent studies have proposed different mechanisms responsible for decreasing K(leaf) . We measured K(leaf) concurrently with ultrasonic acoustic emissions (UAEs) in dehydrating leaves of several species to determine whether declining K(leaf) was associated with xylem embolism. In addition, we performed experiments in which the surface tension of water in the leaf xylem was reduced by using a surfactant solution. Finally, we compared the hydraulic vulnerability of entire leaves with the leaf lamina in three species. Leaf hydraulic vulnerability based on rehydration kinetics and UAE was very similar, except in Quercus garryana. However, water potentials corresponding to the initial decline in K(leaf) and the onset of UAE in Q. garryana were similar. In all species tested, reducing the surface tension of water caused K(leaf) to decline at less negative water potentials compared with leaves supplied with water. Microscopy revealed that as the fraction of embolized xylem increased, K(leaf) declined sharply in Q. garryana. Measurements on leaf discs revealed that reductions in lamina hydraulic conductance with dehydration were not as great as those observed in intact leaves, suggesting that embolism was the primary mechanism for reductions in K(leaf) during dehydration.     

散孔材与环孔材树种枝干、叶水力学特性的比较研究

为揭示散孔材与环孔材树种树木水分生理特性的差异,选取了常见的3种散孔材落叶树种(毛白杨、法国梧桐和樱花)和3种环孔材落叶树种(刺槐、合欢和白蜡),研究了其枝干与叶水力学性质的差异及其协调性。结果表明:3种环孔材树种枝干横截面积基础上的最大比导水率(Ks-max)大于3种散孔材树种,但其木质部对空穴化的脆弱性(P50branch)高于散孔材树种,6种树木枝干的水分传输能力和抵抗空穴化能力之间存在一种相互制约的权衡关系。3种散孔材与3种环孔材树种的叶最大水力导度(Kl-max)和水力脆弱性(P50leaf)并无显著差异;对于3种散孔材树种,叶的水力脆弱性要高于枝干,但对3种环孔材树种而言,枝干的水力脆弱性要高于叶。6种树木枝干和叶的水力学性质(Kmax、P50)之间并无相关关系。这些结果表明:散孔材与环孔材树种的枝干水力学特性有明显差异,但叶水力学特性无差异;枝干与叶水力学性质之间是相互独立的。