Dihydroxamic acid complexes of iron (III): ligand pKa and coordinated water hydrolysis constants

A series of dihydroxamic acid ligands of the formula [RN(OH)C(O)]₂(CH₂)_n, (n = 2, 4, 6, 7, 8; R = CH₃, H) has been studied in 2.0 M aqueous sodium perchlorate at 25.0 °C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (pK_a) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The pK_a1 and pK_a2 values are: n = 2, R = CH₃ (8.72, 9.37); N = 4, R = CH₃ (8.79, 9.37); N = 6, R = CH₃; N = 7, R = CH₃ (8.95, 9.47); N = 8, R = CH₃ (8.93, 9.45); N = 8, R = H (9.05, 9.58). Equilibrium constants for the hydrolysis of coordinated water (log K) have been estimated for the 1:1 feeric complexes of the ligands n = 2, 4, 8; R = CH₃. The N = 8 ligand forms a monomeric complex with Fe(III) while the n = 2 and 4 ligands form dimeric complexes. For hydrolysis of the n = 8 monomeric complex, log K₁ = −6.36 and log K₂ = −9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K_1–4, for the dimeric complexes are as follows: n = 2 (−6.37, −5.77, −10.73, −11.8); n = 4 (−5.54, −5.07, −11.57, −10.17). The log K₂ values for the dimers are unexpectedly high, higher in fact than log K₁, inconsistent with the formation of simple ternary hydroxo complexes. A scheme is proposed for the hydrolysis of the ferric dihydroxamate dimers, which includes the possible formation of μ-hydroxo and μ-oxo bridges.     

Equilibrium and structure of thallium(III)–ethylenediamine complexes in pyridine solution and in solid

The formation of three [Tl(en)_n]³⁺ complexes (n=1–3) in a pyridine solvent has been established by means of ²⁰⁵Tl and ¹H NMR. Their stepwise stability constants based on concentrations, K_n=[Tl(en)_n ³⁺]/{[Tl(en)_n−1 ³⁺]·[en]}, at 298 K in 0.5 M NaClO₄ ionic medium in pyridine, were calculated from ²⁰⁵Tl NMR integrals: log K₁=7.6±0.7; log K₂=5.2±0.5 and log K₃=2.64±0.05. Linear correlation between both the ²⁰⁵Tl NMR shifts and spin–spin coupling ²⁰⁵Tl–¹H versus the stability constants has been found and discussed. A single crystal with the composition [Tl(en)₃](ClO₄)₃ was synthesized and its structure determined by X-ray diffraction. The Tl³⁺ ion is coordinated by three ethylenediamine ligands via six N-donor atoms in a distorted octahedral fashion.     

Kinetic evidence for isomerization of the dopamine receptor-raclopride complex

The binding kinetics of the specific dopamine D₂ antagonist [³H]raclopride to dopamine D₂ receptors in rat neostriatum were studied. The pseudo-first-order rate constants of [³H]raclopride binding with these membranes revealed a hyperbolic dependence upon the antagonist concentration, indicating that the reaction had at least two consecutive and kinetically distinguished steps. The first step was fast binding equilibrium, characterized by the dissociation constant K_A = 12 ± 2 nM. The following step corresponded to a slow isomerization of the receptor-antagonist complex, characterized by the isomerization equilibrium constant K_i = 0.11. The dissociation constant K_d = 1.3 nM, calculated from these kinetic data, was similar to K_d = 2.4 nM, determined from equilibrium binding isotherm for the radioligand. Implications of the complex reaction mechanism on dopamine D₂ receptor assay by [³H]raclopride were discussed.     

Characterization of the B2 receptor and activity of bradykinin analogs in SHP-77 cell line by Cytosensor microphysiometer

The Cytosensor microphysiometer device (Molecular Devices, Sunnyvale, CA) is capable of measuring the rate at which cells acidify their environment in response to ligand–receptor binding. By measuring the extracellular acidification response (ECAR) we characterized some aspects of ligand–B₂ receptor interaction in SHP-77 cell line. SHP-77 cells maximally acidified their environment within 30 s after the exposure to bradykinin (BK) or the BK agonist, B9972, with the maximum effect seen at a ligands concentration of 1 μM. Fetal bovine serum (FBS) modulated the binding of BK or B9972, showing that B9972 is a partial agonist. In addition, the binding of BK agonist or antagonist to the B₂ receptor showed different ECAR and different interaction with other intracellular and plasma membrane proteins. Our microphysiometrical results showed that two parameters, antagonist binding affinity (pD₂) and antagonist potency (pIC₅₀) are required to characterize BK antagonist activity for the B₂ receptor in the SHP-77 cell line. The previously used parameter of B₂ antagonist activity, pA₂, had high variation and poor correlation with the inhibition of SHP-77 cell growth in vitro and suppression of tumor growth when SHP-77 cells were injected to mice. Our results permit us to conclude that BK agonists and antagonists differ in their interactions with the B₂ receptor and consequently elicit different cell responses. Based on our results, we have developed a new microphysiometrical assay for analyzing the activity of BK agonists and antagonist in SHP-77 cells, which may facilitate the discovery of new potent anticancer drugs.     

Two distinct types of cell surface folic acid-binding proteins in Dictyostelium discoideum

Folic acid is degraded too fast by Dictyostelium discoideum to study binding of this ligand to cell surface binding proteins. Folate deaminase activity was inhibited in the presence of 3.3 × 10⁻⁴ M 8-azaguanine. This inhibitor enabled us to detect two folate binding proteins. One type bound folic acid and deamino-folic acid with the same affinity (K_0.5 = 3–6 × 10⁻⁷ M) and apparently negative cooperativity. Binding to only this type was observed if 8-azaguanine was omitted. The second type bound folic acid noncooperatively with K_d = 7 × 10⁻⁷ M. Deamino-folic acid did not compete even at a 1000-fold excess. This type may correspond to the chemotactic receptor.     

Mode of binding of [3H]dibenzocycloalkenimine (MK-801) to the N-methyl-D-aspartate (NMDA) receptor and its therapeutic implication

Binding of the labeled anticonvulsant drug [³H]dibenzocycloalkenimine (³H]MK-801) to the N-methyl-D-aspartate (NMDA) receptor and its dissociation from the receptor at 25°C are slow processes, both of which follow first order kinetics (t_1/270 and 180 min, respectively). Both reactions are markedly accelerated by glutamate and glycine (t_1/22-8 and 4 min, respectively), which allow bimolecular association kinetics of the labeled drug with the receptors whereas equilibrium binding of [³H]MK-801 (K_d 2–4 nM) is hardly affected by glutamate and glycine. The data suggest that MK-801 acts as a steric blocker of the NMDA receptor channel. The competitive antagonist D-(−)-2-amino-5-phosphovaleric acid (AP-5) freezes the receptor in a state which precludes either binding of [³H]MK-801 to the receptor channel or its dissociation from it. These findings have therapeutic implications.     

New herbicide-binding site in the photosynthetic electron-transport chain: Competitive herbicide binding at the photosystem I phylloquinone-(vitamin K1)-binding site

The binding of herbicides to the phylloquinone-(primary electron acceptor A₁)-binding site in green plant photosystem (PS) I reaction centers is shown. Dissociation constants (K_d) of various herbicides to the phylloquinone-binding site were estimated by analyzing their competitive inhibition of the reconstitution of the phylloquinone analogue, menadione (vitamin K₃), to the phylloquinone-extracted spinach PS I particles. The phylloquinone-binding site was found to bind o-phenanthroline (K_d = 1.2 × 10⁻⁴ M), but only weak binding was observed with atrazine (K_d > 10⁻² M), although both are known to bind specifically to the quinone-(Q_B)-binding site in reaction centers of purple photosynthetic bacteria or PS II. The inhibitors of the cytochrome b/c₁(ƒ) complex, myxothiazol (K_d=9.5 × 10⁻⁶ M) or antimycin A (K_d = 2.8 × 10⁻⁶ M), also strongly bound to the phylloquinone site. This is the first report showing that the PS I reaction center complex also has a herbicide-binding site, although the site is probably not sensitive in vivo to these herbicides due to its higher affinity for phylloquinone than herbicides. The inhibitor specificity of the PS I phylloquinone site is different from that of the other quinone-functioning sites in the photosynthetic or respiratory electron-transfer chain, suggesting it to have a unique structure.     

Acid dissociation constants of diphytanylglycerolphosphorylglycerol-methylphosphate, and diphytanylglycerolphosphorylglycerophosphate and its deoxy analog

Acid dissociation constants of 2,3-diphytanyl-sn-glycero-1-phosphoryl-sn-3′-glycero-1′-methylphosphate (PGP-Me), the major phospholipid in extreme halophiles (Halobacteriaceae), and of the demethylated 2,3-diphytanyl-sn-glycero-1-phosphoryl-sn-3′-glycero-1′-phosphate (PGP) and its deoxy analog 2,3-diphytanyl-sn-glycero-1-phosphoryl-1′-(1′,3′-propanediol-3′-phosphate) (dPGP), were calculated by an original mathematical procedure from potentiometric titration data in methanol/water (1:1, v/v) and found to be as follows: for PGP-Me (and presumably PGP), pK₁=3.00 and pK₂=3.61; for PGP, pK₃=11.12; and for dPGP, pK₁=2.72, pK₂=4.09, and pK₃=8.43. High-resolution ³¹P NMR spectra of intact PGP-Me in benzene/methanol or in aqueous dispersion showed two resonances corresponding to the two P-OH groups, each of which exhibited a chemical shift change in the pH range 2.0–4.5, corresponding to acid dissociation constants pK₁=pK₂=3.2; no further ionization (pK₃) was detected at higher pH values in the range 5–12. The present results show that PGP-Me titrates as a dibasic acid in the pH range 2–8, but above pH 8, it titrates as a tribasic acid, presumably PGP, formed by hydrolysis of the methyl group during the titration with KOH. Calculation of the concentrations of the ionic molecular species of PGP-Me, PGP and dPGP as a function of pH showed that the dianionic species predominate in the pH range 5–9, covering the optimal pH for growth of Halobacteriaceae. The results are consistent with the concept that the hydroxyl group of the central glycerol moiety in PGP-Me and PGP is involved in the formation of an intramolecular hydrogen-bonded cyclic structure of the polar headgroup, which imparts greater stability to the dianionic form of PGP-Me and PGP in the pH range 5–9 and facilitates lateral proton conduction by a process of diffusion along the membrane surface of halobacterial cells.     

Biological characterization of A-ring steroids

Hydroxylated 2,19-methylene-bridged androstenediones were designed as potential mimics of enzyme oxidized intermediates of androstenedione. These compounds exhibited competitive inhibition with low micromolar affinities for aromatase. These inhibitory constants (K_i values) were 10 times greater than the 2,19-methylene-bridged androstenedione constant (K_i = 35–70 nM). However, expansion of the 2,19-carbon bridge to ethylene increased aromatase affinity by 10-fold (K_i = 2 nM). Substitution pf a methylene group with oxygen and sulfur in this expanded bridge resulted in K_i values of 7 and 20 nM, respectively. When the substituent was an NH group, the apparent inhibitory kinetics changed from competitive to uncompetitive. All of these analogs exhibited time-dependent inhibition of aromatase activity following preincubation of the inhibitor with human placental microsomes prior to measuring residual enzyme activity. Part of this inhibition was NADPH cofactor-dependent for the 2,19-methyleneoxy- but not for the 2,19-ethylene-bridged androstenedione. The time-dependent inhibition for these four analogs was very rapid since they exhibited τ₅₀ values, the t_1/2 for enzyme inhibition at infinite inhibitor concentration, of 1 to 3 min. These A-ring-bridged androstenedione analogs represent a novel series of potent steroidal aromatase inhibitors. The restrained A-ring bridge containing CH₂, O, S, or NH could effectively coordinate with the heme of the P450 aromatase to allow the tight-binding affinities reflected by their nanomolar K_i values.     

Solubilization of rat liver inositol 1,4,5-trisphosphate receptor

High affinity Ins(1,4,5)P₃-binding sites of permeabilized hepatocytes are probably the ligand recognition sites of the receptors that mediate the effects of Ins91,4,5)P₃ on intracellular Ca²⁺ mobilization. We have now solubilized these sites from rat liver membranes in the zwitterionic detergent, CHAPS, and shown that the solubilized bind Ins(1,4,5)P₃ with an affinity (K_d = 7.26 ± 0.52 nM, Hill coefficient H = 1.05 ± 0.06) similar to that of the sites in native membranes (K_d = 6.02 ± 0.02). ATP and a range of inositol phosphates (Ins(2,4,5)P₃ Ins(4,5)P₂, and inositol 1,4,5-trisphosphorothioate) also bound with similar affinities to the native and solubilized sites. Solubilization of the liver InsP₃ receptor will allow its further characterization, purification, and comparison of its properties with those of InsP₃ receptors already purified from cerebellum and smooth muscle.     

Modulation of ouabain-insensitive Na(+)-ATPase activity in the renal proximal tubule by Mg(2+), MgATP and furosemide

In addition to the (Na⁺+K⁺)ATPase another P-ATPase, the ouabain-insensitive Na⁺-ATPase has been observed in several tissues. In the present paper, the effects of ligands, such as Mg²⁺, MgATP and furosemide on the Na⁺-ATPase and its modulation by pH were studied in the proximal renal tubule of pig. The principal kinetics parameters of the Na⁺-ATPase at pH 7.0 are: (a) K_0.5 for Na⁺=8.9±2.2 mM; (b) K_0.5 for MgATP=1.8±0.4 mM; (c) two sites for free Mg²⁺: one stimulatory (K_0.5=0.20±0.06 mM) and other inhibitory (I_0.5=1.1±0.4 mM); and (d) I_0.5 for furosemide=1.1±0.2 mM. Acidification of the reaction medium to pH 6.2 decreases the apparent affinity for Na⁺ (K_0.5=19.5±0.4) and MgATP (K_0.5=3.4±0.3 mM) but increases the apparent affinity for furosemide (0.18±0.02 mM) and Mg²⁺ (0.05±0.02 mM). Alkalization of the reaction medium to pH 7.8 decreases the apparent affinity for Na⁺ (K_0.5=18.7±1.5 mM) and furosemide (I_0.5=3.04±0.57 mM) but does not change the apparent affinity to MgATP and Mg²⁺. The data presented in this paper indicate that the modulation of the Na⁺-ATPase by pH is the result of different modifications in several steps of its catalytical cycle. Furthermore, they suggest that changes in the concentration of natural ligands such as Mg²⁺ and MgATP complex may play an important role in the Na⁺-ATPase physiological regulatory mechanisms.     

Kinetics and equilibria of Ga(III)---thiocyanate complex formation. Mechanism of ligand substitution reactions of Ga(III) in aqueous solution

The kinetics and equilibria of complex formation by Ga(III) with NCS⁻ in aqueous solution have been measured over a range of acidities and temperatures, the contributing paths to the reaction resolved, and their rate constants and activation parameters determined. The hydrolysis equilibria required to carry out this resolution of kinetic behaviour have also been measured.

Unlike the other reported complexation reactions of Ga(III) in aqueous solution, the separate reaction pathways can be assigned with no ambiguity. At 25 °C and ionic strength 0.5 M, the observed forward rate constant for the complex formation is described by {k₁ + k₂K_1h/[H⁺] + k₃K_1hK_2h/[H⁺]²} M⁻¹ s⁻¹. For these conditions, the first and second successive hydrolysis constants of Ga(H₂O)₆³⁺ are given by pK_1h = 3.69 ± 0.01 and pK_2h = 3.74 ± 0.04. The rate constants corresponding to the reactions of the species Ga(H₂O)₆³⁺, Ga(H₂O)₅(OH)²⁺ and Ga(H₂O)₄(OH)₂⁺ with NCS⁻ are k₁ = 57 ± 4 M^{−1 −1}, k₂ = (1.08 ± 0.01) × 10⁵ M⁻¹ s⁻¹ and k₃ = 3 × 10⁶ M⁻¹ s⁻¹ respectively. The complexation equilibrium quotient [GaNCS²⁺]/([Ga³⁺][NCS⁻]) has been independently determined by spectrophotometric titration to be 20.8 ± 0.3 M⁻¹ at 25 °C and ionic strength 0.5 M.

These kinetic results lead to an interpretation of the data, and a reinterpretation of other data for aquo-Ga(III) complex formation kinetics from the literature which support the assignment of a dissociative interchange mechanism for these reactions rather than the associative activation mode sometimes proposed.     

Dopamine receptors in human foetal brains: Characterization, regulation and ontogeny of [H]spiperone binding sites in striatum

Eighteen corpora striata from normal human foetal brains ranging in gestational age from 16 to 40 weeks and five from post natal brains ranging from 23 days to 42 years were analysed for the ontogeny of dopamine receptors using [³H]spiperone as the ligand and 10 mM dopamine hydrochloride was used in blanks. Spiperone binding sites were characterized in a 40-week-old foetal brain to be dopamine receptors by the following criteria: (1) It was localized in a crude mitochondrial pellet that included synaptosomes; (2) binding was saturable at 0.8 nM concentration; (3) dopaminergic antagonists spiperone, haloperidol, pimozide, trifluperazine and chlorpromazine competed for the binding with IC₅₀ values in the range of 0.3–14 nM while agonists—apomorphine and dopamine gave IC₅₀ values of 2.5 and 10 μM, respectively suggesting a D₂ type receptor.

Epinephrine and norepinephrine inhibited the binding much less efficiently while mianserin at 10 μM and serotonin at 1 mM concentration did not inhibit the binding. Bimolecular association and dissociation rate constants for the reversible binding were 5.7 × 10⁸ M⁻¹ min⁻¹ and 5.0 × 10⁻² min⁻¹, respectively. Equilibrium dissociation constant was 87 pM and the K_D obtained by saturation binding was 73 pM.

During the foetal age 16 to 40 weeks, the receptor concentration remained in the range of 38–60 fmol/mg protein or 570–1080 fmol/g striatum but it increased two-fold postnatally reaching a maximum at 5 years Significantly, at lower foetal ages (16–24 weeks) the [³H]spiperone binding sites exhibited a heterogeneity with a high (K_D, 13–85 pM) and a low (K_D, 1.2–4.6 nM) affinity component, the former accounting for 13–24% of the total binding sites. This heterogeneity persisted even when sulpiride was used as a displacer. The number of high affinity sites increased from 16 weeks to 24 weeks and after 28 weeks of gestation, all the binding sites showed only a single high affinity.

GTP decreased the agonist affinity as observed by dopamine competition of [³H]spiperone binding in 20-week-old foetal striata and at all subsequent ages. GTP increased IC₅₀ values of dopamine 2 to 4.5 fold and Hill coefficients were also increased becoming closer to one suggesting that the dopamine receptor was susceptible to regulation from foetal life onwards.     

Receptor binding profiles of NPY analogues and fragments in different tissues and cell lines

To investigate receptor selectivity and possible species selectivity of a number of NPY analogues and fragments, receptor binding studies were performed using cell lines and membranes of several species. NPY displays 4–25-fold higher affinity for the Y₂ receptor than for the Y₁ receptor. The affinity of [Leu³¹,Pro³⁴]NPY is 7–60-fold higher for the Y₁ receptor when compared with the Y₂ subtype. Species selectivity within the Y₂ receptors is demonstrated by PYY(3–36), NPY(2–36), NPY(22–36), and NPY(26–36). It is shown that NPY(22–36) is species selective for the human Y₂ subtype (K_i of 0.3 nM) compared with the rabbit and rat Y₂ receptor (K_i of 2 and 10 nM, respectively). PYY(3–36) displays highest affinity for the human and rabbit Y₂ subtype (K_i of 0.03 and 0.17 nM). The screening of NPY analogues and fragments revealed that highest affinity for the human Y₂ receptor is shown by NPY(2–36) and PYY(3–36). In addition, PYY(3–36) and NPY(2–36) are not only subtype selective, but also species selective.     

Temperature-dependent regulation of d-cis-[H]diltiazem binding to Ca channels by 1,4-dihydropyridine channel agonists and antagonists

The binding of the Ca²⁺-channel blocker d-cis-[³H]diltiazem to guinea pig skeletal muscle microsomes is temperature-dependent. At 2°C the K_D is 39 nM and B_max is 11 pmol/mg protein. The binding is fully reversible (K₋₁ = 0.02 min⁻¹). The binding sites discriminate between the diastereoisomers 1- and d-cis-diltiazem, recognize verapamil, gallopamil and tiapamil, and are sensitive to La³⁺-inhibition. At 30°C the K_D is 37 nM and the B_max is 2.9 pmol/mg protein. D-cis-diltiazem-labelling is regulated by the 1,4-dihydropyridine Ca²⁺-channel blockers and a novel Ca²⁺-channel activator in a temperature-dependent manner. At 30°C an enhancement of d-cis-diltiazem binding by the channel blockers is observed. This is attributed to a B_max increase. EC₅₀-values for enhancement and the maximal enhancement differ for the individual 1,4-dihydropyridines. At 2°C 1,4-dihydropyridines inhibit d-cis-[³H]diltiazem binding. This is attributed to a B_max decrease. We have directly labelled one of the drug receptor sites within the Ca²⁺-channel which can allosterically interact with the 1,4-dihydropyridine binding sites.     

Positive cooperativity induces multimodal site and thermodynamic affinity distributions in multivalent proteins

The characterization of the stoichiometric and site-affinity distributions for the reaction of hemoglobin with O(2) and CO is presented as an example of a multivalent receptor system which exhibits positive site-site interactions. The distributions of stoichiometric constants, T(i)(K(i))'s, are obtained assuming that the distribution of site constants, N(k), is known. The importance of these distributions is that they can be directly related to quantities measured experimentally and that they represent affinity distributions for each ligation step. In hemoglobin, positive site-site interactions generate both stoichiometric and site-affinity distributions with complex and previously unrecognized multimodal patterns that are very different from the theoretical distributions obtained in the absence of interactions. These distributions are related to the generation of heterogeneity during the ligand binding process. Experimental binding data show that these complex distributions can be related to the physiological functions of uptake, transport, and release of gaseous ligands by hemoglobin.     

Characterization of a new leiurotoxin I-like scorpion toxin: PO5 from Androctonus mauretanicus mauretanicus

Three novel peptide inhibitors of the SK_Ca channels were purified to homogeneity from the venom of the scorpion Androctonus mauretanicus mauretanicus using one step of RP-HPLC and competition assays with [¹²⁵I]apamin to rat brain synaptosomes. PO_i, PO₂ and PO₅ have K_0.5 of 100,100 and 0.02 nM, respectively, for the apamin binding site. The sequence of PO₅ was established and compared to that of other scorpion toxins active on K⁺ channels: it contains 31 residues and has a free carboxyl end. It shares sequence similarity with apamin and leiurotoxin I.     

AT4 receptor structure-binding relationship: N-terminal-modified angiotensin IV analogues

The effect of structural changes in the N-terminal amino acid of AIV, with respect to AT₄ receptor binding, was examined by competition with [¹²⁵I]AIV in bovine adrenal membranes. Analogues with modifications of the first residue -amino group possessed lower affinities than the primary amine-containing parent compound. Peptides with a residue 1 -carbon in the conformation exhibited poor affinity for the AT₄ receptor. Modifications of the residue 1 R-group demonstrate that a straight chain aliphatic moiety containing four carbons is optimal for receptor-ligand binding, as evidenced by the extremely high affinity of [Nle¹]AIV (K_i = 3.59±0.51 pM). Replacement of the 1–2 peptide bond of AIV with the methylene bond isostere Ψ (CH₂-NH), increased the K_i approximately fivefold, indicating that the peptide bond may be replaced wihle maintaining relatively high-affinity receptor binding.     

Leaf hydraulic traits and their trade-offs for nine Chinese temperate tree species with different wood properties

Aims Trees with different wood properties display variations in xylem anatomy and leaf vein structure, which may influence tree water transport efficiency and water-use strategy, and consequently constrain tree survival, growth and distribution. However, the effects of wood properties on leaf hydraulic conductance and vulnerability and their potential trade-offs at leaf level are not well understood. Our aims were to examine variations in leaf hydraulic traits of trees with different wood properties and explore potential trade-offs between leaf hydraulic efficiency and safety.
Methods Nine tree species with different wood properties were selected for measuring the leaf hydraulic traits, including three diffuse-porous species (Populus davidiana, Tilia amurensis, Betula platyphylla), three ring-porous species (Quercus mongolica, Fraxinus mandshurica, Juglans mandshurica), and three non-porous species (Picea koraiensis, Pinus sylvestris var. mongolica, Pinus koraiensis). Four dominant and healthy trees per species were randomly selected. The hydraulic traits measured included leaf hydraulic conductance on leaf area (K_area) and dry mass (K_mass) basis, leaf hydraulic vulnerability (P₅₀), and leaf water potential at turgor loss point (TLP), while the leaf structural traits were leaf dry mass content (LDMC), leaf density (LD) and leaf mass per unit area (LMA).
Important findings The K_area, K_mass, and P₅₀ differed significantly among the tree species with different woody properties (p < 0.05). Both K_area and K_mass were the lowest for the non-porous trees, and did not differ significantly between the diffuse-porous and ring-porous trees. The ring-porous trees had the highest P₅₀ values, while the diffuse-porous and non-porous trees showed no significant differences in P₅₀. Both K_area and K_mass were negatively correlated with P₅₀ (p < 0.05) for all the trees, and the relationships for the diffuse-porous, ring-porous, and non-porous trees were fitted into linear, power, exponential functions, respectively. This indicates that significant trade-offs exist between leaf hydraulic efficiency and safety. The K_mass was correlated (p < 0.01) with TLP in a negative linear function for the diffuse- and ring-porous trees and in a negative exponential function for the non-porous trees. The P₅₀ increased with increasing TLP. These results suggest that apoplastic and symplastic drought resistance are strictly coordinated in order to protect living cells from approaching their critical water status under water stresses. The K_mass was negatively correlated (p < 0.01) with LDMC, LD, or LMA, while the P₅₀ was positively correlated with LDMC and LD; this suggests that variations in K_mass and P₅₀ are driven by similar changes in structural traits regardless of wood traits. We conclude that the tree tolerance to hydraulic dysfunction increases with increasing carbon investment in the leaf hydraulic system.     

九种不同材性的温带树种叶水力性状及其权衡关系

不同材性树种的解剖、叶脉分布等结构性状差异会影响树木的水分运输效率和水分利用策略, 进而限制树木的生存、生长和分布。然而, 材性对叶导水率、水力脆弱性及其潜在的权衡关系的影响尚不清楚。该研究选择东北温带森林中不同材性的9种树种(散孔材: 山杨(Populus davidiana)、紫椴(Tilia amurensis)、白桦(Betula platyphylla); 环孔材: 蒙古栎(Quercus mongolica)、水曲柳(Fraxinus mandshurica)、胡桃楸(Juglans mandshurica); 无孔材: 红皮云杉(Picea koraiensis)、樟子松(Pinus sylvestris var. mongolica)、红松(Pinus koraiensis), 测量其基于叶面积和叶质量的叶导水率(K_area和K_mass)、水力脆弱性(P₅₀)、膨压丧失点水势(TLP)及叶结构性状, 以比较不同材性树种叶水力性状的差异, 并探索叶水力效率与安全的权衡关系。结果表明: 3种材性树种的K_area、K_mass和P₅₀均差异显著(p < 0.05)。无孔材树种的K_area和K_mass最低, 而散孔材和环孔材树种差异不显著; 环孔材树种P₅₀最高, 而散孔材和无孔材树种差异不显著。K_area和K_mass均与P₅₀显著负相关(p < 0.05), 但散孔材、环孔材和无孔材树种的相关关系分别呈线性、幂函数和指数函数关系。这表明叶水力效率与安全之间存在一定的权衡关系, 但该关系受树木材性的影响。K_mass与TLP显著负相关(p < 0.01), 其中散孔材和环孔材树种呈线性负相关, 无孔材树种呈负指数函数关系; P₅₀随TLP的增加而增加, 这表明树木在面临水分胁迫时, 其质外体和共质体抗旱阻力共同协调保护叶片活细胞, 防止其水分状况到达临界阈值。K_mass与叶干物质含量、叶密度、比叶重均显著负相关, 而P₅₀与之显著正相关(p < 0.01, P₅₀与比叶重的关系除外), 表明树木叶水力特性的变化受相同叶结构特性驱动, 树木增加对水力失调的容忍需要在叶水力系统构建上增加碳投资。