A Conewise Linear Elasticity mixture model for the analysis of tension-compression nonlinearity in articular cartilage

A biphasic mixture model is developed that can account for the observed tension-compression nonlinearity of cartilage by employing the continuum-based Conewise Linear Elasticity (CLE) model of Curnier et al. (J. Elasticity, 37, 1-38, 1995) to describe the solid phase of the mixture. In this first investigation, the orthotropic octantwise linear elasticity model was reduced to the more specialized case of cubic symmetry, to reduce the number of elastic constants from twelve to four. Confined and unconfined compression stress-relaxation, and torsional shear testing were performed on each of nine bovine humeral head articular cartilage cylindrical plugs from 6 month old calves. Using the CLE model with cubic symmetry, the aggregate modulus in compression and axial permeability were obtained from confined compression (H-A = 0.64 +/- 0.22 MPa, k2 = 3.62 +/- 0.97 x 10(-16) m4/N.s, r2 = 0.95 +/- 0.03), the tensile modulus, compressive Poisson ratio, and radial permeability were obtained from unconfined compression (E+Y = 12.75 +/- 1.56 MPa, v- = 0.03 +/- 0.01, kr = 6.06 +/- 2.10 x 10(-16) m4/N.s, r2 = 0.99 +/- 0.00), and the shear modulus was obtained from torsional shear (mu = 0.17 +/- 0.06 MPa). The model was also employed to predict the interstitial fluid pressure successfully at the center of the cartilage plug in unconfined compression (r2 = 0.98 +/- 0.01). The results of this study demonstrate that the integration of the CLE model with the biphasic mixture theory can provide a model of cartilage that can successfully curve-fit three distinct testing configurations while producing material parameters consistent with previous reports in the literature.     

A COMPARATIVE ANALYSIS OF CELL MORPHOLOGY IN MATURING TISSUES OF ROOTS FROM SHOCKED AND UNSHOCKED BULBS OF ALLIUM CEPA L.

A shock pressure pulse of 60 pounds per square inch (psi; 4.22 kg/cm²) effectively inhibited root growth of onion bulbs. Morphological changes observed after shock included a reduction in cell number in transverse section, a decrease in cell length, a decrease in cell volume, and an increase in cell cross-sectional area. Mitotic activity was consistently increased one day after shock, and this may have accounted for the increased cell number per millimeter of root tip segment 8 days after shock. The development of the tissues appeared normal after shock exposure; however, the tissue response to pressures seems to depend on whether they are exposed to prolonged confining pressures or a brief pressure pulse. Responses which were unique to shock treatment include a decrease in cell number in transverse section, reduced cell volume, and increased radial enlargement of the cell. These responses have not been observed under prolonged pressure treatment.     

Mechanobiology of soft skeletal tissue differentiation—a computational approach of a fiber-reinforced poroelastic model based on homogeneous and isotropic simplifications

The material properties of multipotent mesenchymal tissue change dramatically during the differentiation process associated with skeletal regeneration. Using a mechanobiological tissue differentiation concept, and homogeneous and isotropic simplifications of a fiber-reinforced poroelastic model of soft skeletal tissues, we have developed a mathematical approach for describing time-dependent material property changes during the formation of cartilage, fibrocartilage, and fibrous tissue under various loading histories. In this approach, intermittently imposed fluid pressure and tensile strain regulate proteoglycan synthesis and collagen fibrillogenesis, assembly, cross-linking, and alignment to cause changes in tissue permeability (k), compressive aggregate modulus (H_A), and tensile elastic modulus (E). In our isotropic model, k represents the permeability in the least permeable direction (perpendicular to the fibers) and E represents the tensile elastic modulus in the stiffest direction (parallel to the fibers). Cyclic fluid pressure causes an increase in proteoglycan synthesis, resulting in a decrease in k and increase in H_A caused by the hydrophilic nature and large size of the aggregating proteoglycans. It further causes a slight increase in E owing to the stiffness added by newly synthesized type II collagen. Tensile strain increases the density, size, alignment, and cross-linking of collagen fibers thereby increasing E while also decreasing k as a result of an increased flow path length. The Poisson's ratio of the solid matrix, _s, is assumed to remain constant (near zero) for all soft tissues. Implementing a computer algorithm based on these concepts, we simulate progressive changes in material properties for differentiating tissues. Beginning with initial values of E=0.05 MPa, H_A=0 MPa, and k=1×10^–13 m⁴/Ns for multipotent mesenchymal tissue, we predict final values of E=11 MPa, H_A=1 MPa, and k=4.8×10^–15 m⁴/Ns for articular cartilage, E=339 MPa, H_A=1 MPa, and k=9.5×10^–16 m⁴/Ns for fibrocartilage, and E=1,000 MPa, H_A=0 MPa, and k=7.5×10^–16 m⁴/Ns for fibrous tissue. These final values are consistent with the values reported by other investigators and the time-dependent acquisition of these values is consistent with current knowledge of the differentiation process.     

Microhabitat,water relations,and photosynthesis of a desert fern,Notholaena parryi

Summary Interrelationships between morphology, microhabitat, water relations, and photosynthesis of a xeric fern, Notholaena parryi D.C. Eat. (Pteridaceae), were examined in the western Colorado desert. In its typical microhabitat rock outcroppings protected N. parryi from direct sunlight and moderated the diurnal variations in air temperature. For example, frond temperature at noon in late winter was 15.3° C, which was 7.3° C cooler than an energy budget simulation predicting frond temperature at an exposed site. The lowest soil water potential leading to daytime stomatal opening was about-1.5 MPa (-15 bars). Rainfall runoff that was channeled to the periphery of the rocks caused ^soil near the fern roots to rise above-1.5 MPa even after light rainfalls, and it remained above-1.5 MPa longer after rainfall than in non-rocky sites.The water potential gradient along the stipe necessary to support the observed rates of transpiration was about-10 MPa m^-1; such a large gradient reflected the small conducting area in the xylem. The water vapor conductance decreased as the frond temperature was raised, an effect that became proportionally greater as the soil dried out. The daytime water-use efficiency (mass CO₂ fixed/mass water transpired) was 0.0058 for a spring day. Individual fronds reached 90% of light saturation for photosynthesis at only 100 Einsteins m^-2 s^-1, a photosynthetically active radiation similar to that from the diffuse sunlight incident on the generally north-facing microhabitat of the fern. Below 50 E m^-2 s^-1 the quantum requirement was 13 Einsteins absorbed/mole CO₂ fixed. The ratio of chlorophyll to P700 was 552, indicating a fairly large photosynthetic unit that is characteristic of plants adapted to shaded habitats. The temperature optimum for net photosynthesis shifted from 13° C in midwinter (mean daily air temperature of 11° C) to 19° C in early fall (air temperature of 23° C).     

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira,Guajira, Colombia

Summary Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m^-2 day^-1 during cloudy days and between 532 and 944 g m^-2 day^-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.     

Effect of fiber orientation and strain rate on the nonlinear uniaxial tensile material properties of tendon

Tendons are exposed to complex loading scenarios that can only be quantified by mathematical models, requiring a full knowledge of tendon mechanical properties. This study measured the anisotropic, nonlinear, elastic material properties of tendon. Previous studies have primarily used constant strain-rate tensile tests to determine elastic modulus in the fiber direction. Data for Poisson's ratio aligned with the fiber direction and all material properties transverse to the fiber direction are sparse. Additionally, it is not known whether quasi-static constant strain-rate tests represent equilibrium elastic tissue behavior. Incremental stress-relaxation and constant strain-rate tensile tests were performed on sheep flexor tendon samples aligned with the tendon fiber direction or transverse to the fiber direction to determine the anisotropic properties of toe-region modulus (E0), linear-region modulus (E), and Poisson's ratio (v). Among the modulus values calculated, only fiber-aligned linear-region modulus (E1) was found to be strain-rate dependent. The E1 calculated from the constant strain-rate tests were significantly greater than the value calculated from incremental stress-relaxation testing. Fiber-aligned toe-region modulus (E(1)0 = 10.5 +/- 4.7 MPa) and linear-region modulus (E1 = 34.0 +/- 15.5 MPa) were consistently 2 orders of magnitude greater than transverse moduli (E(2)0 = 0.055 +/- 0.044 MPa, E2 = 0.157 +/- 0.154 MPa). Poisson's ratio values were not found to be rate-dependent in either the fiber-aligned (v12 = 2.98 +/- 2.59, n = 24) or transverse (v21 = 0.488 +/- 0.653, n = 22) directions, and average Poisson's ratio values in the fiber-aligned direction were six times greater than in the transverse direction. The lack of strain-rate dependence of transverse properties demonstrates that slow constant strain-rate tests represent elastic properties in the transverse direction. However, the strain-rate dependence demonstrated by the fiber-aligned linear-region modulus suggests that incremental stress-relaxation tests are necessary to determine the equilibrium elastic properties of tendon, and may be more appropriate for determining the properties to be used in elastic mathematical models.     

Climate-driven changes in biomass allocation in pines

Future increases in air temperature resulting from human activities may increase the water vapour pressure deficit (VPD) of the atmosphere. Understanding the responses of trees to spatial variation in VPD can strengthen our ability to predict how trees will respond to temporal changes in this important variable. Using published values, we tested the theoretical prediction that conifers decrease their investment in photosynthetic tissue (leaves) relative to water‐conducting tissue in the stem (sapwood) as VPD increases. The ratio of leaf/sapwood area (A_L/A_S) decreased significantly with increasing VPD in Pinus species but not in Abies, Pseudotsuga, Tsuga and Picea, and the average A_L/A_S was significantly lower for pines than other conifers (pines: 0.17 m² cm^?2; nonpines: 0.44 m² cm^?2). Thus, pines adjusted to increasing aridity by altering above‐ground morphology while nonpine conifers did not. The average water potential causing a 50% loss of hydraulic conductivity was ?3.28 MPa for pines and ?4.52 MPa for nonpine conifers, suggesting that pines are more vulnerable to xylem embolism than other conifers. For Pinus ponderosa the decrease in A_L/A_S with high VPD increases the capacity to provide water to foliage without escalating the risk of xylem embolism. Low A_L/A_S and plasticity in this variable may enhance drought tolerance in pines. However, lower A_L/A_S with increasing VPD and an associated shift in biomass allocation from foliage to stems suggests that pines may expend more photosynthate constructing and supporting structural mass and carry less leaf area as the climate warms.     

Day/night variations in turgor pressure in individual cells of Mesembryanthemum crystallinum L.

Summary Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C₃-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAMAbbreviations CAM crassulacean acid metabolism - V volume of the cells (mm³) - P turgor pressure (MPa) - volumetric elastic modulus (MPa) - ⁱ osmotic pressure of the cell sap (MPa) - T _1/2 half-time of water exchange (s) - Lp hydraulic conductivity of the cell membrane (m·s^-1·MPa^-1) - A surface area of cells (mm²) - P pressure changes (MPa) - V volume changes (mm³) - nocturanal nighttime - diurnal daytime     

Seasonal patterns of gas exchange,water relations and growth of it Roupala montana,an evergreen savanna species

Roupala montana is an evergreen species widespread in the seasonal savannas of the central plains of Brazil. I examined the degree of coupling of photosynthetic gas-exchange characteristics, water relations and growth responses of R. montana with regard to seasonal changes in soil water availability. Despite a rainless period of over three months soil water potential at 60 cm depth reached values of only about -1.0 MPa, while pre-dawn leaf water potential (^l) reached about -0.4 MPa by the end of the three-month drought. Thus, R. montana had access to deep soil water in the dry period, but pre-dawn ^l did not reach the high wet season values of -0.2 MPa. Most of the shoot growth was concluded in the onset of the rainy season. Although some individual branches might have shown some extension thereafter, most of them remained inactive during the rest of the rainy season and the subsequent dry season. New leaf production was also restricted to the first part of the wet period. R. montana remained evergreen in the dry season, but there was a 27% decrease in the number of leaves and herbivory removed about 16% of the leaf area still present in the plant. CO₂-exchange rates of these leaves reached only ca. 55% of the maximum rainy season values of 14 µmol m^-2 s^-1. Thus, the estimated potential daily carbon gain was about 34% of the maximum by the end of the dry period. These values will be even lower, if we considered the decrease in photosynthetic rates that occurred around midday. These reductions in photosynthetic rates as a result of partial stomatal closure were measured both in the wet and dry season and they were related to increases in the evaporative demand of the atmosphere. In conclusion, the combined effect of herbivory, leaf loss and reductions in photosynthetic rates limited plant productivity in the dry season.     

Material characterization of human medial collateral ligament.

The objectives of this study were to determine the longitudinal and transverse material properties of the human medial collateral ligament (MCL) and to evaluate the ability of three existing constitutive models to describe the material behavior of MCL. Uniaxial test specimens were punched from ten human cadaveric MCLs and tensile tested along and transverse to the collagen fiber direction. Using load and optical strain analysis information, the tangent modulus, tensile strength and ultimate strain were determined. The material coefficients for each constitutive model were determined using nonlinear regression. All specimens failed within the substance of the tissue. Specimens tested along the collagen fiber direction exhibited the typical nonlinear behavior reported for ligaments. This behavior was absent from the stress-strain curves of the transverse specimens. The average tensile strength, ultimate strain, and tangent modulus for the longitudinal specimens was 38.6 +/- 4.8 MPa, 17.1 +/- 1.5 percent, and 332.2 +/- 58.3 MPa, respectively. The average tensile strength, ultimate strain, and tangent modulus for the transverse specimens was 1.7 +/- 0.5 MPa, 11.7 +/- 0.9 percent, and 11.0 +/- 3.6 MPa, respectively. All three constitutive models described the longitudinal behavior of the ligament equally well. However, the ability of the models to describe the transverse behavior of the ligament varied.     

Novel regenerated cellulose films prepared by coagulating with water: Structure and properties

Regenerated films were successfully prepared from cellulose/NaOH/urea solution by coagulating with water at temperature from 25 to 45 °C. The results of solid ¹³C NMR, wide angle X-ray diffraction, scanning electron microscopy (SEM) and tensile testing revealed that the cellulose films possessed homogeneous structure and cellulose II crystalline, similar to that prepared previously by coagulating with 5 wt% H₂SO₄. By changing the coagulation temperature from 25 to 45 °C, tensile strength of the films was in the range of 85-139 MPa. Interestingly, the RC35 film coagulated at 35 °C exhibited the highest tensile strength (σ_b = 139 MPa). The inclusion complex associated with cellulose, NaOH and urea hydrates in the cellulose solution were broken by adding water (non-solvent), leading to the self-association of cellulose to regenerate through rearrangement of the hydrogen bonds. This work provided low-cost and “green” pathway to prepare cellulose films, which is important in industry.     

Seasonal changes of osmotic pressure, symplasmic water content and tissue elasticity in the blades of dune grasses growing in situ along the coast of Oregon

Abstract Midday water potentials of blades of the dune grasses Ammophila arenaria (L.) Link and Elymus mollis Trin. ex Spreng. growing in situ declined over the summer growing period, indicating a trend of increasing water stress. An analysis of the water relations characteristics of these blades using pressure-volume techniques demonstrated that both species increased bulk osmotic pressure at full hydration () and, therefore, bulk turgor as an acclimation response. In A. arenaria, however, the increase of osmotic pressure (+ 0.35 MPa) was entirely the result of decreasing symplasmic water content. The increase of osmotic pressure (+ 0.54 MPa) observed in E. mollis blades was due to solute accumulation (72% of Δ) and to a lesser degree, decreased symplasmic water content (28% of Δ). Osmotic adjustment in E. mollis blades was accompanied by a significant decrease in tissue elasticity (_max went from 12 to 19 MPa). The elastic properties of A. arenaria blades remained constant over the same period and had a maximum modulus (10 MPa) that was always less than that of E. mollis, As estimated from Höfler plots, these seasonal adjustments of osmotic pressure and differences in tissue elasticity enabled plants in situ to maintain turgor pressure in the range of 0.5–0.6 MPa at the lowest water potentials of mid-August. Laboratorygrown plants exhibited the species-specific differences in osmotic pressure, turgor pressure, and tissue elasticity observed in field plants. Although certain alterations of leaf structure were expected to coincide with the observed changes and species-specific differences in symplasmic water content and tissue elasticity, these could not be detected by measurements of specific leaf weight or the ratio of dry matter to saturated water content.     

PH-DEPENDENCE OF CHLORTETRACYCLINE(CTC)-INDUCED PLUG FORMATION IN NITELLA FLEXILIS (CHARACEAE)1

The formation of chlortetracycline(CTC)-induced wall appositions (callose plugs) in Nitella flexilis (L.)Ag. was pH-dependent in the range between 4.3-8.3. Plug number and plug diameter increased with the pH of the CTC solution. At pH 4.3 plug formation was light-dependent and occurred below the alkaline regions of the cell surface which form during photo synthetic assimilation of HCO₃^?. Inhibition of photosynthesis by 3–(3′,4′-dichlorophenyl)-1, 1-dimethylurea prevented plug formation in the light. Dark-treated cells could be induced to form plugs by raising the pH of the CTC solution. The formation of large but incomplete plugs in the presence of cytochalasin B is explained by the formation of numerous weak alkaline sites. I suggest that CTC enhances locally the Ca²⁺content at the cytoplasm near the plasmamembrane. The ionophoric character of CTC is probably more pronounced at high pH mainly because of a weaker binding with cations and a closer contact with the membrane.     

Photosynthetic Characteristics of Portulaca grandiflora, a Succulent C(4) Dicot : CELLULAR COMPARTMENTATION OF ENZYMES AND ACID METABOLISM

The succulent, cylindrical leaves of the C₄ dicot Portulaca grandiflora possess three distinct green cell types: bundle sheath cells (BSC) in radial arrangement around the vascular bundles; mesophyll cells (MC) in an outer layer adjacent to the BSC; and water storage cells (WSC) in the leaf center. Unlike typical Kranz leaf anatomy, the MC do not surround the bundle sheath tissue but occur only in the area between the bundle sheath and the epidermis. Intercellular localization of photosynthetic enzymes was characterized using protoplasts isolated enzymatically from all three green cell types.     

The effects of elevated atmospheric [CO<Subscript>2</Subscript>] on Norway spruce needle parameters

Studies of selected morphological needle parameters were carried out on young (17–19 year old) Norway spruce trees cultivated inside glass domes at ambient (A, 370 μmol (CO₂) mol⁻¹) and elevated (E, 700 μmol (CO₂) mol⁻¹) atmospheric CO₂ concentrations [CO₂] beginning in 1997. Annual analyses performed from 2002 to 2004 revealed higher values for needle length (especially for current needles, up to 18%) and projected needle area (up to 13%) accompanied by lower values for specific needle area (up to 15% lower, as quantified by needle mass to projected area ratio) in the E treatment compared to the A treatment. Statistically significant differences for most of the investigated morphological parameters were found in young needles in the well irradiated sun-adapted crown parts, particularly under water-limiting soil conditions in 2003. This was likely a result of different water relations in E compared to A trees as investigated under temperate water stress (Kuper et al. in Biol Plantarum 50:603–609, 2006). Furthermore, E trees had much higher absorbing root area, which modified and enhanced root:shoot as well as root:conductive stem area proportions. These hydraulic properties and early seasonal stimulation of photosynthesis forced advanced needle development in E trees, particularly under limited soil water conditions. The number of needles per unit shoot length was found to be unaffected by elevated [CO₂].     

Biomechanical properties and holdfast morphology of coenocytic algae (Halimedales, Chlorophyta) in Bocas del Toro, Panama

For attached marine organisms, specific biomechanical properties may result in detachment or in tissue loss, when sufficient tensile force is applied. Algae experience such forces through water movement, which may thus act to limit size, abundance, and species composition, of populations of algae.Coenocytic construction is uncommon in the algae, but it occurs relatively more frequently in green algae found in shallow subtidal sediments associated with coral reefs, e.g., at our study site of Isla Colon, Bocas del Toro, Panama. We studied the biomechanical properties of some tropical coenocytic algae (Udotea flabellum (Ellis et Solander) Lamouroux, Penicillus capitatus Lamarck, P. pyriformis A. and E.S. Gepp, and Halimeda gracilis Harvey) anchored in sediments. We compare our results with published data on other coenocytic algae, as well as with multicellular algae. Our results show that properties of sand-dwelling coenocytes, such as mean force to dislodge (4.9-12.7 N), mean force to break (6.6-22.1 N), and mean strength (1.0-7.0 MN m^− 2), are all within the range reported for temperate, multicellular, algae. In contrast, the coenocytes differed markedly from the temperate non-coenocytes in the consequences of applied tensile force: coenocytes were removed whole, while most temperate algae attached to rocks break within the thallus. Some multicellular algae can regrow from tissue left on the substratum; three of the four coenocytic species we examined had rhizoids connecting closely adjacent (0.1-0.15 m) individuals, and these rhizoids may serve to regrow a new individual. While our experiments indicated that sufficient tensile force results in dislodgment, calculations using the experimentally determined variables led us to conclude that water velocities sufficient to dislodge individuals are unlikely to occur. Since dislodgment is usually fatal for algae, the role of the holdfast is a critical one. All of the species we investigated had similar holdfast morphology, a mass of rhizoids which entrained sand, the entire unit forming a hemispherical to cylindrical mass. Despite the consistency in holdfast form, and the initial prediction that this was an optimal form for anchoring these algae, our data suggest that this is not the case.     

Wall appositions induced by ionophore A 23187, CaCl2, LaCl3, and nifedipine in characean cells

Summary The formation of wall appositions (plugs) by ionophore A 23187, CaCl₂, LaCl₃, and nifedipine was studied in mature internodal cells of characeaen algae. CaCl₂ at concentrations above 10^–2M induces thick fibrillar plugs without callose inNitella flexilis. InChara corallina andNitella flexilis ionophore A 23187 (1.25×10^–5 to 5×10^–5M) and LaCl₃ (7.5×10^–5 to 2.5×10^–4M) cause flat appositions which contain callose and have a more granular structure. Plug formation by ionophore A 23187, CaCl₂, and LaCl₃ is pH-dependent and occurs beneath the alkaline regions of the cell. Nifedipine (10^–4 to 10^–5M) induces plugs inNitella flexilis after previous injury. These callose-containing wall appositions consist of a heterogeneous granular core which is covered by a fibrillar layer. The results of this work are compared with previous studies on wound wall formation and chlortetracycline (CTC)-induced plug formation which reveal that abundant coated vesicles occur only when a thick fibrillar wall layer is formed. Neither LaCl₃ nor nifedipine inhibit the formation of CaCl₂- or CTC-plugs. The unusual effects of these substances, which normally act as Ca²⁺ antagonists and therefore should prevent and not induce plug formation, are discussed. It is suggested that La³⁺ mimicks the effects of calcium and that nifedipine binding to the Ca²⁺ channels is altered in the alkaline regions of characean internodes and allows an influx of Ca²⁺.Abbreviations AFW artificial fresh water - CTC chlortetracycline - DCMU dichlorphenyldimethylurea - DMSO dimethylsulfoxide - EGTA ethyleneglycoltetraacetic acid - MES 2-(N-morpholino) ethanesulfonic acid - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - TAPS N-tris[hydroxymethyl]methyl-3-aminopropanesulfonic acid     

FLEXURAL RIGIDITY OF CHIVE AND ITS RESPONSE TO WATER POTENTIAL

The biomechanical relationship between the ability of a plant organ to resist bending and the extent to which tissues are hydrated is illustrated for the cylindrical leaves of chive (Allium schoenoprasnum var. schoenoprasnum L.). The flexural rigidity (EI), which measures the ability to resist bending, is maximum when leaves are fully turgid and decreases monotonically as a function of water potential (r² = 0.99). Dehydration results in a reduction in the elastic modulus (E) of leaves. Reductions in E are correlated with geometric distortion in the transverse geometry of leaves which influences their second moment of inertia (I). The traditional theory of elastic stability (developed on the basis of the mechanical behavior of nonbiological systems) is shown to be inadequate to distinguish the behavior of E as plant organs geometrically distort during dehydration. This inadequacy results from the violation of a principal assumption made by the theory (= uniform cross-sectional geometry). A derivation is presented that accommodates the localized geometric distortions in cylindrical plant organs and permits a valid estimate of reductions in E as tissues dehydrate. Based on this derivation, the Young's modulus of chive leaves just before mechanical failure due to buckling is shown to be less than 50% of that calculated for fully turgid leaves.