Single‐vessel flow measurements indicate scalariform perforation plates confer higher flow resistance than previously estimated

During vessel evolution in angiosperms, scalariform perforation plates with many slit‐like openings transformed into simple plates with a single circular opening. The transition is hypothesized to have resulted from selection for decreased hydraulic resistance. Previously, additional resistivity of scalariform plates was estimated to be small – generally 10% or less above lumen resistivity – based on numerical and physical models. Here, using the single‐vessel technique, we directly measured the hydraulic resistance of individual xylem vessels. The resistivity of simple‐plated lumens was not significantly different from the Hagen–Poiseuille (HP) prediction (+6 ± 3.3% mean deviation). In the 13 scalariform‐plated species measured, plate resistivity averaged 99 ± 13.7% higher than HP lumen resistivity. Scalariform species also showed higher resistivity than simple species at the whole vessel (+340%) and sapwood (+580%) levels. The strongest predictor of scalariform plate resistance was vessel diameter (r² = 0.84), followed by plate angle (r² = 0.60). An equation based on laminar flow through periodic slits predicted single‐vessel measurements reasonably well (r² = 0.79) and indicated that Baileyan trends in scalariform plate evolution maintain an approximate balance between lumen and plate resistances. In summary, we found scalariform plates of diverse morphology essentially double lumen flow resistance, impeding xylem flow much more than previously estimated.     

Relationship between specific leaf area of aCryptomeria japonica foliage shoot segment and its diameter

Dependency of specific leaf area (SLA) on shoot diameter (x) was studied forCryptomeria japonica foliage shoot segments about 5 cm in length taken from 18 branches at different height levels on 3 trees in August 1987. The leaf area of a shoot segment (S) was defined as half the sum of the needle area (s) estimated by the allometric relationship betweens and needle length. The dry weight of woody tissue (W _w) and needles (W _n) of a segment and itsS were divided by the segment length (L) to give linear densities asW _w/L,W _n/L andS/L, respectively. The densities were related tox by power-form equations.S/L values tended to be constant around 1.2 [cm² cm⁻¹] within the discussed range ofx, whileW _w/L andW _n/L values clearly increased withx. The approximately reciprocal relationship between average needle area (s) and linear density of the number of needles supported the fact thatS/L values were roughly constant regardless ofx.SLA andSNA were defined asS/(W _w+W _n) andS/W _n, respectively. TheSLA-x relationship expected from the average value ofS/L and theW _w/L-x and theW _n/L-x relationships was well fitted to the observed decrease inSLA with increasingx. SNA also decreased asx increased. Variations inSLA andSNA among the shoot segments with similarx were not systematically related to their height levels. An empirical equation with a maximum value ofx (Xmax) was also proposed in order to formulate theSLA-x relationship.     

Statistical mechanical studies of polypeptides. 1. Theory of the helix-coil transition including right- and left-handed helical states

The Lifson-Roig and Zimm-Bragg theories of the helix–coil transition in polypeptides are generalized to include both right- and left-handed α-helical states. The partition functions for these more general theories are formulated in terms of the parameters u, v_R, V_L, W_R, and w_L for the generalized Lifson-Roig theory and σ_R, σ_L, s_R, and s_L for the generalized Zimm-Bragg theory. Matrix equations are derived for calculating such average molecular properties as the fraction of the amino acid residues hydrogen bonded into right- and left-handed α-helices, the average number of right- and left-handed helical sequences per molecule, the number-average length (in residues) of the right- and left-handed helical sequences, and the degree of solvent binding to the peptide NH and CO groups. These equations are shown to be conveniently adaptable to machine methods of calculation, thus avoiding the difficulty of solving an eigenvalue problem where the secular equation is of a high order. A discussion is given of the various energetic and entropic effects which determine the screw sense and stability of helices and of the extent to which it is valid to interpret experimental data by adjustment of the parameters of these statistical mechanical theories which include in their formulation only near-neighbor interactions between residues.     

Leaf morphology and leaf chemical composition in three Quercus (Fagaceae) species along a rainfall gradient in NE Spain

 Leaf features were examined in three Quercus species (Q. coccifera, Q. ilex and Q. faginea) along a steep rainfall gradient in NE Spain. The analyzed leaf traits were area, thickness, density, specific mass, leaf concentration of nitrogen, phosphorous, lignin, cellulose and hemicellulose, both on a dry weight basis (N_w, P_w, L_w, C_w, H_w) and on an area basis (N_a, P_a, L_a, C_a, H_a). These traits were regressed against annual precipitation and correlated with each other, revealing different response patterns in the three species. Q. faginea, a deciduous tree, did not show any significant correlation with rainfall. In Q. coccifera, an evergreen shrub, N_w, N_a, L_w, L_a and C_a increased with higher annual rainfall, while H_w decreased. In Q. ilex, an evergreen tree, leaf area, P_w and L_w increased with precipitation, whereas specific leaf mass, thickness and H_a showed the reverse response. Correlations between the leaf features revealed that specific mass variation in Q. faginea and Q. coccifera could be explained by changes in leaf density, while in Q. ilex specific leaf mass was correlated with thickness. Specific leaf mass in the three species appeared positively correlated with all the chemical components on a leaf area basis except with lignin in Q. ilex and with P in Q. ilex and Q. faginea. In these two tree species P_w showed a negative correlation with specific leaf mass. It is suggested that each species has a different mechanism to cope with water shortage which is to a great extent related to its structure as a whole, and to its habit. Received: 18 December 1995 / Accepted: 8 March 1996     

Changes in Structure and Hydraulic Conductivity for Root Junctions of Desert Succulents as Soil Water Status Varies

Variations in hydraulic conductivity (L_P) and the underlying anatomical and morphological changes were investigated for main root-lateral root junctions of Agave deserti and Ferocactus acanthodes under wet, dry, and rewetted soil conditions. During 21 d of drying, L_P and radial conductivity (L_R) increased threefold to fivefold at junctions of both species. The increase in L_R was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of the junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. During 7 d of rewetting, L_R decreased for junctions of A. deserti, as apoplastic water movement was not apparent, but L_R was unchanged for F. acanthodes. Axial conductance (K_h) decreased during drying for both species, largely because of embolism related to the degradation of unlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admit air bubbles at pressure differences of only 0.12-0.19 MPa. Rewetting restored K_h for both species, but not completely, due to blockage of xylem elements by tyloses. About 40% of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For the dicotyledon F. acanthodes, which can form new conduits in its secondary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral roots that persist during drought in the field compared with the case for the sympatric A. deserti.     

Genes determining leucine aminopeptidase and mildew resistance from the ornamental apple, ‘White Angel’

Mildew resistance in the ornamental apple White Angel was found to be determined by complementary genes. The gene R _w was found to be necessary for the expression of resistance controlled by the resistance gene Pl _w . The close linkage between the isoenzyme gene, Lap-2, for leucine aminopeptidase and P1 _w was confirmed. The efficiency of Lap-2 as a marker in screening for mildew resistance is limited, as it cannot account for susceptible plants with the r _w r _w P1 _w p1 _w genotype. It has, however, an important role to play in combining resistance genes from different sources. The genotypes of White Angel (R _w r _w , Pl _w pl _w , Lap-2an), Jester (R _w r _w , p1 _w p _w , Lap-2an) Katja (R _w r _w ,p1 _w p1 _w , Lap-2an) and Gloster 69 (r _w r _w , p1 _w p1 _w , Lap-2an) were determined. It also appeared that R _w might influence Lap-2 activity in young seedlings.     

Length to weight ratio of four fish species in estuary areas in the Northeast region of Brazil

The length to weight ratio (LWR) was used to estimate populations of four fish species sampled on the Amazon coast of the state of Maranhão, Brazil. The fishes were sampled in 2018 and 2019 by artisanal coastal fishing, using gillnets (0.20–0.60 mm). The parameters a and b of the equation W_T = a L_T^b were estimated. The obtained LWRs were W_T = 0.06LT^2.93, (r²) = .989 for Oligoplites saliens; W_T = 0.061L_T^2.57, (r²) = .982 for Peprilus crenulatus; W_T = 0.014L_T^2.80, (r²) = .985 for Bagre bagre; and W_T = 0.035L_T^3.21, (r²) = .981 for Nebris microps.     

Bestimmung der mittleren Energie WL zur Erzeugung eines Ionenpaares in Luft mit 60-MeV-bis 120-MeV-Bremsstrahlungen

Zusammenfassung Aus der kalorimetrisch bestimmten Energiedosis in Graphit und der unter gleichen Bedingungen gemessenen Hohlraum-Ionendosis wurde für Bremsstrahlungen mit Maximalenergien von 60, 90 und 120 MeV die mittlere EnergieW _L. zur Erzeugung eines Ionenpaares in Luft bestimmt. Der Wert vonW _L=34,0 eV für 60- und 90- MeV, sowie 33,9 eV für 120-MeV-Bremsstrahlung stimmt mit dem für Energien unter 2 MeV von der ICRU angegebenen Wert von w_L=33,73 eV überein.

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Determination of the average energy W_L, expended in air per ion pair formed for 60 MeV to 120 MeV Bremsstrahlung

Summary By calorimetric measurement of the absorbed dose in graphite and the measurement of ionization per unit mass of air under the same conditions the average energyW _L required to produce an ion pair in air was determined for X-rays with maximum energies of 60, 90 and 120 MeV. The value ofW _L=34.0 eV for 60 and 90 MeV-X-rays as well as 33.9 eV for 120 MeV-X-rays agrees with the well-known value ofW _l=33.73 eV as given by the ICRU for energies below 2 MeV.

     

The effects of feeding history and environment on condition,body composition and growth of bluegills Lepomis macrochirus

Initial relative mass (W_R, low v. high) and energetic trajectory in time (starved v. fed) were experimentally manipulated in bluegill Lepomis macrochirus. Fed fish starting at low W_R grew more and gained more W_R than fed fish starting at high W_R. Similarly, starved fish starting at high W_R lost more mass and W_R than did starved fish starting at low W_R. Temporal changes in other variables did not consistently match that of W_R, but, by the end of the experiment, proximate composition showed a high correlation to W_R. Regression slopes of W_R on proximate composition increased with time in the laboratory. Differences between wild and laboratory fish appeared to result from relaxation of environmental stress. When excess resources are available such that L. macrochirus grow, condition indices will increase, but individual response will depend on initial values and thus past environmental experience.     

Age,growth and length–weight relationships of Pinna bicolor Gmelin (Bivalvia: Pinnidae) in the seagrass beds of Sungai Pulai Estuary,Johor, Peninsular Malaysia

Age and growth of Pinna bicolor were examined in the seagrass beds of Merambong shoal (N 1°19′55.62″; E 103°35′57.75″) off the south‐western coast of Johor, Peninsular Malaysia between May 2006 and April 2007. Monthly growth increment data of P. bicolor were analyzed using FiSAT software (FAO‐ICLARM Stock Assessment Tools) to estimate the asymptotic length (L_∞) and growth coefficient (K). Average growth rate of P. bicolor was 1.42 (±0.01) cm per month; the estimated asymptotic length (L_∞) and growth coefficient (K) were 34.66 cm and 0.88 per year, respectively. In their natural habitat, P. bicolor attain shell heights of approximately 17, 25 and 30 cm at the end of their first, second and third years of growth. The length–weight relationship was estimated as Log W = ?5.397 + 3.111Log L, and in exponential form the equation was W = 0.000004L^3.111 (r² = 0.99, P < 0.01). Habitat temperature and salinity ranged between 27.47 and 29.66°C and 28.66–33.00 ppt with a mean of 29.10 (±0.66) m°C and 30.52 (±1.41) ppt, respectively.     

Cell specificity,anti-inflammatory activity,and plausible bactericidal mechanism of designed Trp-rich model antimicrobial peptides

To develop novel short Trp-rich antimicrobial peptides (AMPs) with potent cell specificity (targeting bacteria but not eukaryotic cells) and anti-inflammatory activity, a series of 11-meric Trp-rich model peptides with different ratios of Leu and Lys/Arg residues, XXWXXWXXWXX-NH₂ (X indicates Leu or Lys/Arg), was synthesized. K₆L₂W₃ displayed an approximately 40-fold increase in cell specificity, compared with the natural Trp-rich AMP indolicidin (IN). Lys-containing peptides (K₈W₃, K₇LW₃ and K₆L₂W₃) showed approximately 2- to 4-fold higher cell specificities than did their counterparts, the Arg-containing peptides (R₈W₃, R₇LW₃ and R₆L₂W₃), indicating that multiple Lys residues are more important than multiple Arg residues in the design of AMPs with good cell specificity. The excellent resistance of d-enantiomers (K₆L₂W₃-D and R₆L₂W₃-D) and Orn/Nle-containing peptides (O₆L₂W₃ and O₆L₂W₃) to trypsin digestion compared with the rapid breakdown of the l-enantiomers (K₆L₂W₃ and R₆L₂W₃), highlights the clinical potential of such peptides. K₆L₂W₃, R₆L₂W₃, K₆L₂W₃-D and R₆L₂W₃-D caused weak dye leakage from bacterial membrane-mimicking negatively charged EYPG/EYPE (7:3, v/v) liposomes. Confocal microscopy showed that these peptides penetrated the cell membrane of Escherichia coli and accumulated in the cytoplasm, as observed for buforin-2. Gel retardation studies revealed that the peptides bound more strongly to DNA than did IN. These results suggested that one possible peptide bactericidal mechanism may relate to the inhibition of intracellular functions via interference with DNA/RNA synthesis. Furthermore, some model peptides, containing K₆L₂W₃, K₅L₃W₃, R₆L₂W₃, O₆L₂W₃, O₆L₂W₃, and K₆L₂W₃-D inhibited LPS-induced inducible nitric oxide synthase (iNOS) mRNA expression, the release of nitric oxide (NO) following LPS stimulation in RAW264.7 cells and had powerful LPS binding activities at bactericidal concentrations. Collectively, our results indicated that these peptides have potential for future development as novel antimicrobial and anti-inflammatory agents.     

桂西南岩溶区八种适生植物光合性状的变异与关联

为探究岩溶植物的光合生理适应机制，采用Li-6400XT便携式光合作用测量系统，对广西平果市岩溶区8种适生植物的叶片净光合速率(P_n)、气孔导度(G_s)、胞间CO₂浓度(C_i)、蒸腾速率(T_r)、水分利用效率(WUE)和气孔限制值(L_s)等光合特征参数进行了测定分析。结果表明：(1)6个光合特征参数在种内和种间均存在不同程度的变异，并且种内变异均大于种间变异。(2)G_s和T_r的变化主要来源于种间变异(46.72%～49.76%),而P_n、C_i、WUE和L_s变化主要来源于种内变异(48.66%～64.50%)。在生活型水平上，P_n、G_s和T_r的种内变异表现为常绿植物小于落叶植物，而C_i、WUE和L_s则相反。(3)各参数的种间变异均表现为落叶植...     

Relations between vulnerability to xylem embolism and xylem conduit dimensions in young trees of Quercus corris

Measurements of xylem conduit length and width and the distribution of xylem conduit ends were made in inter-nodes (I), nodes (N) and twig junctions (J) of 1-, 2- and 3-year-old twigs of plants of Quercus cerris L. Parallel measurements were also made of the loss of hydraulic conductivity of twigs subjected to pressure differentials across conduit pit membranes, equalling the leaf water potential at the turgor loss point. The loss of theoretical hydraulic conductivity was calculated as the ratio of i esivr⁴ (where r is the conduit radius) of the non-conducting conduits to that of all the conduits in the outermost wood ring of I, N and J. Stem zones such as 1-year-old nodes and junctions were localized with narrower and shorter xylem conduits and with higher percentages of conduit ends than internodes. Such ‘constricted zonesrsquo; were less vulnerable to embolism than internodes. Latewood conduits were consistently narrower, shorter and less vulnerable to embolism than earlywood ones. A positive relation therefore existed between conduit diameter and length and vulnerability to embolism. The overall vulnerability to embolism of Q. cerris plants is discussed in terms of xylem conduit width and length and of the distribution of conduit ends.     

Structural analysis of purified human tracheobronchial mucins

Light scattering has been used to investigate the structure of human tracheobronchial mucin glycoproteins (HTBM) from the sputum of cystic fibrosis patients. The specimen was extracted using 6M guanidinium hydrochloride solution and fractionated by gel exclusion chromatography on Sephacryl S-1000. The fractionated HTBM was purified by density gradient ultracentrifugation. Purity of the resulting material was confirmed by SDS polyacrylamide gel electrophoresis and uv spectroscopy. Light scattering measurements on the fractionated mucins yield weight-average molecular weights M_w, and z-average radii of gyration R_{g, z}. The native cystic fibrosis HTBM consisted of a high molecular weight fraction with M_w = 9.3 × 10⁶ daltons and a lower molecular weight fraction contanining partly degraded mucins. After reduction and carboxymethylation of the high molecular weight native fraction, the resulting material was separated into three pools with M_w values of 5.1 × 10⁶, 1.6 × 10⁶, and 400,000. The derived molecular weights for the protein cores M_p,w, and the experimental radii of gyration are found to be consistent with the M_p,w – R_g relation established previously for submaxillary, cervical, and gastric mucins. These results imply that HTBM has the same extended-coil conformation reported for other mucins and has a molecular structure consisting of subunits, linked into linear chains via covalent (disulfide) bonds.     

The spatial pattern of air seeding thresholds in mature sugar maple trees

Air seeding threshold (P_a) of xylem vessels from current year growth rings were measured along the vertical axis of mature sugar maple trees (Acer saccharum Marsh.), with sampling points in primary leaf veins, petioles, 1-, 3-, and 7-year-old branches, large branches, the trunk and roots. The air seeding threshold was taken as the pressure required to force nitrogen gas through intervessel pit membranes. Although all measurements were made on wood produced in the same year, P_a varied between different regions of A. saccharum, with distal organs such as leaves and petioles having lower P_a than basal regions. Mean (SE) P_a ranged from 1.0 (± 0.1) MPa in primary leaf veins to 4.8 (± 0.1) MPa in the main trunk. Roots exhibited a P_a of 2.8 (± 0.2) MPa, lower than all other regions of the tree except leaf veins and petioles. Mean xylem vessel diameter increased basipetally, with the widest vessels occurring in the trunk and roots. Within the shoot, wider vessels had greater air seeding thresholds, contrasting with trends previously reported. However, further experimentation revealed that differences in P_a between regions of the stem were driven by the presence of primary xylem conduits, rather than differences in vessel diameter. In 1-year-old branches, P_a was significantly lower in primary xylem vessels than in adjacent secondary xylem vessels. This explained the lower values of P_a measured in petioles and leaf veins, which possessed a greater ratio of primary xylem to secondary xylem than other regions. The difference in P_a between primary and secondary xylem was attributed to the greater area of primary cell wall (pit membrane) exposed in primary xylem conduits with helical or annular thickening.     

A study on the relationship between leaf conductance,CO2 concentration and carboxylation rate in various species

Leaf conductance g_L is strongly influenced by environmental factors like CO₂, irradiance and air humidity. According to Ball et al. (1987), g_L is correlated with an index calculated as the product of net CO₂ exchange rate A and ambient water vapour concentration W_a, divided by ambient CO₂ concentration c_a. However, this empirical model does not apply to high values of g_L observed at c_a below CO₂ compensation concentration . Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P₁ and P₂, were determined by adding to A the quotient of and the sum of gas phase resistance r_g and intracellular resistance for CO₂ exchange r_i, P₁ = A+/(r_g + r_i), or the quotient of and r_i, P₂ = A + /r_i. If P₂ is chosen, c_a in the Ball index has to be replaced by the intercellular CO₂ concentration c_i. By using the modified indices P₁·W_a/c_a and P₂·W_a/c_i, we analysed data from the C₃ species Nicotiana tabacum and Nicotiana plumbaginifolia, the C₃–C₄ intermediate species Diplotaxis tenuifolia, and the C₄ species Zea mays. The data were collected at widely varying levels of irradiance and CO₂ concentration. For all species uniform relationships between g_L and the new indices were found for the whole range of CO₂ concentrations below and above . Correlations between g_L and P₁·W_a/c_a were closer than those between g_L and P₂·W_a/c_i because P₁/c_a implicitly contains g_L. Highly significant correlations were also obtained for the relationships between g_L and the ratios P₁/c_a and P₂/c_i.     

Allometric aspects of population dynamics: A symmetry approach

Summary The maximum intrinsic rate of increase (r _max ) shows a –0.25 scaling with adult body weight (W) in mammals (and others). Average adult life span (1/M) and age at maturity () show –0.25 scalings, independent of population size; these two lead to ther _max scaling, providedR _o is invariant with body size in rarified populations (=R _om ). Thus ther _max scaling follows from the existence of two population size symmetries (i.e. 1/M and ) and one body size symmetry (R _o ). The theory provides a formula to calculate the height (A ₃) of the scalingr _max =A ₃ ·W ^–0.25. The theory also helps to explain Fowler's Rules, which linkR _om to the relative position of the inflection point of the population growth curve.     

Photosynthetic pathway alters xylem structure and hydraulic function in herbaceous plants

Plants using the C₄ photosynthetic pathway have greater water use efficiency (WUE) than C₃ plants of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C₄ plants do not need to acquire and transport as much water as C₃ species. Because the structure of xylem tissue reflects hydraulic demand by the leaf canopy, a reduction in water transport requirements due to C₄ photosynthesis should affect the evolution of xylem characteristics in C₄ plants. In a comparison of stem hydraulic conductivity and vascular anatomy between eight C₃ and eight C₄ herbaceous species, C₄ plants had lower hydraulic conductivity per unit leaf area (K_L) than C₃ species of similar life form. When averages from all the species were pooled together, the mean K_L for the C₄ species was 1.60 × 10^?4 kg m^?1 s^?1 MPa^?1, which was only one‐third of the mean K_L of 4.65 × 10^?4 kg m^?1 s^?1 MPa^?1 determined for the C₃ species. The differences in K_L between C₃ and C₄ species corresponded to the two‐ to three‐fold differences in WUE observed between C₃ and C₄ plants. In the C₄ species from arid regions, the difference in K_L was associated with a lower hydraulic conductivity per xylem area, smaller and shorter vessels, and less vulnerable xylem to cavitation, indicating the C₄ species had evolved safer xylem than the C₃ species. In the plants from resource‐rich areas, such as the C₄ weed Amaranthus retroflexus, hydraulic conductivity per xylem area and xylem anatomy were similar to that of the C₃ species, but the C₄ plants had greater leaf area per xylem area. The results indicate the WUE advantage of C₄ photosynthesis allows for greater flexibility in hydraulic design and potential fitness. In resource‐rich environments in which competition is high, an existing hydraulic design can support greater leaf area, allowing for higher carbon gain, growth and competitive potential. In arid regions, C₄ plants evolved safer xylem, which can increase survival and performance during drought events.     

Photorespiration in Green Plants During Photosynthesis Estimated by Use of Isotopic CO(2)

Photosynthetic re-absorption of photorespired CO₂ causes underestimation in measured photorespiration and turnover rate of the substrate for photorespiration. Actual values of photorespiration exceed the measured by a factor greater than 1 + R′_w/r_p + [C_L]_x/(r_p·L_x). (R′_w and r_p are the partial resistances to CO₂ uptake between atmosphere, mesophyll evaporating surface, and photosynthetic sink, respectively; L_x is the measured flux of photorespired CO₂ and [C_L]_x is the ambient conc of photorespired CO₂). In 8 species, 1 + R′_w/r_p alone amounted to a correction ranging between 148% and 233%.     

Different vulnerabilities of Quercus ilex L. to freeze- and summer drought-induced xylem embolism: an ecological interpretation

Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by: (1) maintaining plants for 3h at ambient temperatures of 0, -1.5, -2.5, -5.0 and -11°C; and (2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials (Ψ₁) to that at the turgor loss point (Ψ_tlp) of 0.6, 0.9, 1.05, 1.20 and 1.33. The loss of hydraulic conductivity of one-year-old twigs reached 40% at -1.5°C and at Ψ₁/Ψ_tlP= 1.05. Recovery from these strains was almost complete 24 h after the release of thermal stress or after one irrigation, respectively. More severe stresses reduced recovery consistently. The percentages of xylem conduits embolized following application of the two stresses, were positively related to xylem conduit diameter. The capability of the xylem conduits to recover from stress was positively related to the conduit diameter in plants subjected to summer drought, but not in the plants subjected to winter freezing stress. The ecological significance of the different vulnerabilities to embolism of xylem conduits under naturally occurring climatic conditions is discussed.