Design and mechanical properties of insect cuticle

Since nearly all adult insects fly, the cuticle has to provide a very efficient and lightweight skeleton. Information is available about the mechanical properties of cuticle-Young's modulus of resilin is about 1 MPa, of soft cuticles about 1 kPa to 50 MPa, of sclerotised cuticles 1-20 GPa; Vicker's Hardness of sclerotised cuticle ranges between 25 and 80 kgf mm(-2); density is 1-1.3 kg m(-3)-and one of its components, chitin nanofibres, the Young's modulus of which is more than 150 GPa. Experiments based on fracture mechanics have not been performed although the layered structure probably provides some toughening. The structural performance of wings and legs has been measured, but our understanding of the importance of buckling is lacking: it can stiffen the structure (by elastic postbuckling in wings, for example) or be a failure mode. We know nothing of fatigue properties (yet, for instance, the insect wing must undergo millions of cycles, flexing or buckling on each cycle). The remarkable mechanical performance and efficiency of cuticle can be analysed and compared with those of other materials using material property charts and material indices. Presented in this paper are four: Young's modulus-density (stiffness per unit weight), specific Young's modulus-specific strength (elastic hinges, elastic energy storage per unit weight), toughness-Young's modulus (fracture resistance under various loading conditions), and hardness (wear resistance). In conjunction with a structural analysis of cuticle these charts help to understand the relevance of microstructure (fibre orientation effects in tendons, joints and sense organs, for example) and shape (including surface structure) of this fibrous composite for a given function. With modern techniques for analysis of structure and material, and emphasis on nanocomposites and self-assembly, insect cuticle should be the archetype for composites at all levels of scale.     

Penetration of chemicals into the Malus leaf cuticle

The adaxial leaf cuticle of Malus pumila was examined by electron microscopy to determine possible avenues for transcuticular movement of foliarly applied chemicals. Cutin-embedded polysaccharide microfibrils originated at the outer epidermal cell wall and occasionally extended to the cuticle surface. Lamellae, ca. 4 nm wide, usually were oriented parallel to the cuticle surface. When oriented perpendicular to the surface, they extended nearly to the subjacent wall layer from the surface. Aqueous solutions of uranyl acetate, silver nitrate and phenyl mercuric acetate applied to the cuticle surface of leaf segments floated on solutions of phosphate salts or thiocarbohydrazide (TCH) reacted within the cuticle to form insoluble electron-opaque deposits indicative of their avenues of transcuticular movement. Uranyl phosphate deposits were observed only in the polysaccharide microfibrils of chloroform: methanolextracted leaves. Silver-TCH deposits were observed in the microfibrils of both extracted and nonextracted leaf cuticles. Phenyl mercuric acetate-TCH deposits were randomly dispersed throughout the extracted cuticle and not associated with the polysaccharide microfibrils.Abbreviations TCH thiocarbohydrazide - PMA phenyl mercuric acetate     

The citrus leaf cuticle in relation to measurement of leaf water potential using thermocouple psychrometers*

Abstract. Cuticular resistance to water vapour diffusion is an important aspect of thermocouple psychrometry and may introduce significant error in the measurement of leaf water potential (Ψ). The effect of the citrus (Citrus mitis Blanco) leaf cuticle on water vapour movement was studied using the times required for vapour pressure equilibration during thermocouple psychrometric measurement of Ψ. Cuticular abrasion with various carborundum powders was used to reduce the diffusive resistance of both the adaxial and abaxial leaf surfaces, and the extent of the disruption to the leaf was investigated with light and electron microscopy. Cuticular abrasion resulted in reduced equilibration times due to decreased cuticular resistance and greater water vapour movement between the leaf and the psychrometer chamber. Equilibration times were reduced from over 5 h in the unabraded control leaves to 1 h with cuticle abrasion. This was associated with the decrease in diffusive resistance with cuticular abrasion from over 55 s cm^?1 to less than 8 s cm^?1 for both the adaxial and abaxial leaf surfaces. Scanning electron micrographs of the abraded leaf tissue revealed considerable disruption of the stomatal ledge and of the guard cells, surface smoothing and displacement of waxes into the stomatal aperture, and damage to veins. Observations with the transmission electron microscope revealed frequent disruption of epidermal cell walls, and damage to both the cytoplasmic and vacuolar membranes.     

The mechanical properties of the abdominal cuticle of Rhodnius larvae.

1. The mechanical properties of loops of cuticle cut from the abdomens of 5th instar Rhodnius have been investigated. The cuticle shows pronounced viscoelastic behaviour. 2. Stress-relaxation tests show a continuously falling modulus over a wide range of times after the imposition of a strain. 3. Plasticized samples of cuticle show stress-relaxation curves which are shifted along the time axis towards earlier times by up to times 10-3. The modulus at any particular time after the imposition of strain is about 10 times lower than that of the unplasticized cuticle. 4. It is concluded that the mechanical properties of this cuticle are determined, at least for maintained stresses, largely by the matrix material. Chitin microfibrils may act as a reinforcing filler for short-term, rapid stresses. The cuticular macromolecules are probably not extensively cross-linked by primary bonds, though secondary interactions between them are probably important in the viscoelastic properties of the cuticle. 5. Plasticization probably involves a change in either the number or the strength of secondary interactions between the cuticle macromolecules, or both.     

Xylem and Phloem transport and the functional economy of carbon and nitrogen of a legume leaf

Exchanges of CO₂ and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to one-third area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. ¹⁴C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9 to 12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two% of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation, and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 milligrams C per milligram N; phase 2, 24.7 milligrams C per milligram N; phase 3, 91.9 milligrams C per milligram N; and phase 4, 47.7 milligrams C per milligram N.     

Consideration of the importance of hydrophobic interactions in stabilizing insect cuticle

Protein fractions of insect cuticles with different mechanical properties have related values of polarity and hydrophobicity. Hydrophobicity is important for the self-assembly of cuticle which is produced prior to the moult and in plasticization of cuticle. The cuticles of soft-bodied fly larvae are quite distinct from those of exopterygotes (e.g. locusts) and this can be related to the chemistry and mode of tanning. The properties of cuticular proteins are compared: the proteins of the pliant cuticles most closely resemble globulins, and the proteins in stiff cuticles are more like fibrous and hydrophobic structural proteins. Changes in the environment of the proteins may alter their shape and hence the mechanical properties of the cuticle.     

Diurnal export and carbon economy in an expanding source leaf of cucumber at contrasting source and sink temperature

Effects of contrasting temperatures of an expanding leaf (source) and of remaining plant parts (sink) on diurnal export and distribution of carbon were studied in seedlings of Cucumis sativus L., cv. Farbio. The time course of the rate of export was calculated by measuring simultaneously the exchange of ¹⁴CO₂ and the amount of ¹⁴C in the source leaf by means of a Geiger-Müller detector using a steady-state labelling technique. In all treatments average export rate during the night (16 h) was maximally 50% of the average rate during the 8-h day. Temperature affected the diurnal course of export via the source leaf and the sink in different ways. At a source leaf temperature of 25 or 30°C export stopped 12 h after start of the night, whereas at 20°C export continued throughout the night. However, the total amount of carbon exported during a 24 h cycle, expressed as a proportion of the amount of carbon assimilated, was the same at source leaf temperatures of 20 or 30°C. Thus source leaf temperature did not affect the distribution of assimilates between source and sink, in contrast to sink temperature. After 24 h at a sink temperature of 30°C, 20% more ¹⁴C was exported to plant parts below the source leaf than with a sink temperature of 20°C, at the expense of carbon remaining in the source. During the day less starch and more structural dry matter was formed at a source leaf temperature of 30°C than at 20°C. After a complete day/night cycle, however, there was no difference between the treatments. Starch was the primary carbon source during the night, and the decline in the rate of export coincided with the depletion of starch. Thus the decline in the rate of export at a source leaf temperature of 25 or 30°C at 12 h after the start of the night was due to the depletion of starch at that time. Similarly, at 20°C export could continue until the end of the night as the starch degradation supplied assimilates during the whole night.     

Taxonomy of the genus Ephedra. Significance of stem and leaf epidermis and cuticle

Epidermal and cuticular characters of 30 taxa of Ephedra (including 24 species and six additional varieties) are described and their taxonomic value is discussed.     

The importance of mechanical loading in bone biology and medicine

This paper discusses the premise that the skeleton is primarily a mechanical organ, and reviews the reasons that mechanical factors play a major role in bone biology. It begins by considering three basic observations: (1) Galileo's observation that bone proportions become more robust as the species' overall size increases; (2) da Vinci's observation that larger structures are inherently weaker than smaller structures subjected to the same stress; and (3) the general observation that each unit of bone mass provides structural support for about 15 units of soft tissue organ mass. Together, these observations lead to the concept that it can be advantageous to minimize bone mass, consistent with constraints on other factors. This premise is discussed here in relation to the phenomenon of bone remodeling, which is seen to serve two purposes: the adjustment of bone mass and geometry to maintain peak bone strains at their maximum tolerable values, and the continual removal of fatigue damage produced at those strain levels. Finally, it is observed that bone remodeling apparently originated approximately 250 million years ago when the first vertebrates of substantial size became weight-bearing on land, suggesting that mechanical forces associated with weight-bearing were instrumental in the evolution of bone remodeling.     

A simulation study on the importance of size-related changes in leaf morphology and physiology for carbon gain in an epiphytic bromeliad

This study addresses the question of how size-related changes in leaf morphology and physiology influence light absorption and carbon gain of the epiphytic bromeliad Vriesea sanguinolenta. A geometrically based computer model, Y-plant, was used for the three-dimensional reconstruction of entire plants and for calculation of whole plant light interception and carbon gain. Plants of different sizes were reconstructed, and morphological and physiological attributes of young and old leaves, and small and large plants were combined to examine the individual effects of each factor on light absorption and carbon gain of the plant. The influence of phyllotaxis on light absorption was also explored. Departure of measured divergence angles between successive leaves from the ideal 137.5 degrees slightly decreased light absorption. The only morphological parameter that consistently changed with plant size was leaf shape: larger plants produced more slender foliage, which substantially reduced self-shading. Nevertheless, self-shading increased with plant size. While the maximum rate of net CO(2) uptake of leaves increased linearly with plant size by a factor of two from the smallest to the largest individual, the potential plant carbon gain (based on total foliage area) showed a curvilinear relationship, but with similar numerical variation. We conclude that leaf physiology has a greater impact on plant carbon gain than leaf and plant morphology in this epiphytic bromeliad.     

The mechanical transduction of physiological strength electric fields

In this article it is proposed that electric fields of physiological strength (approximately 100 V/m) are transduced by the mechanical torque they exert on glycoproteins. The resulting mechanical signal is then transmitted to the cytoskeleton and propagated throughout the cell interior. This mechanical coupling is analyzed for transmembrane glycoproteins, such as integrins and the glycocalyx, and for glycoproteins in the extracellular matrix of cartilage. The applied torque is opposed by viscous fluid drag and restoring forces exerted by adjacent molecules in the membrane or cartilage. The resulting system represents a damped, driven harmonic oscillator. The amplitude of oscillation is constant at low frequencies, but falls off rapidly in the range 1-1000 Hz. The transition frequency depends on parameters such as the viscosity of the surrounding fluid and the restoring force exerted by the surrounding structure. The amplitude increases as the fourth power of the length of the transmembrane glycoproteins and as the square of the applied field. This process may operate in concert with other transduction mechanisms, such as the opening of voltage-gated channels and electrodiffusion/osmosis for DC fields.     

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores. 2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values. 5. Even at low density (approximately 2 m^–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.     

Extended leaf senescence promotes carbon gain and nutrient resorption: importance of maintaining winter photosynthesis in subtropical forests

The relative advantages of being deciduous or evergreen in subtropical forests and the relationship between leaf phenology and nutrient resorption efficiency are not well understood. The most successful deciduous species (Lyonia ovalifolia) in an evergreen-dominated subtropical montane cloud forest in southwest (SW) China maintains red senescing leaves throughout much of the winter. The aim of this study was to investigate whether red senescing leaves of this species were able to assimilate carbon in winter, to infer the importance of maintaining a positive winter carbon balance in subtropical forests, and to test whether an extended leaf life span is associated with enhanced nutrient resorption and yearly carbon gain. The red senescing leaves of L. ovalifolia assimilated considerable carbon during part of the winter, resulting in a higher yearly carbon gain than co-occurring deciduous species. Its leaf N and P resorption efficiency was higher than for co-occurring non-anthocyanic deciduous species that dropped leaves in autumn, supporting the hypothesis that anthocyanin accumulation and/or extended leaf senescence help in nutrient resorption. Substantial winter carbon gain and efficient nutrient resorption may partially explain the success of L. ovalifolia versus that of the other deciduous species in this subtropical forest. The importance of maintaining a positive carbon balance for ecological success in this forest also provides indirect evidence for the dominance of evergreen species in the subtropical forests of SW China.     

The value of leaf cuticle characteristics in the identification and classification of Iberian Mediterranean members of the genus Pinus

This study reports the value of leaf cuticle characteristics in the identification and classification of Iberian Mediterranean species of the genus Pinus (P. nigra subsp. salzmannii, P. pinaster, P. pinea and P. halepensis), with the aim of using these characters to identify isolated cuticles and stomata in palynology slides. Preparations were made of the cuticles of pine needles belonging to one natural Iberian population of each of the above species. A number of epidermal morphological characteristics were then recorded with the aim of distinguishing these species from one another. The structure of the stomatal complex (the shape and arrangement of the subsidiary cells) was different in each species. The aperture of the epistomatal chamber was significantly smaller in P. pinea than in the other species examined, and the variables recorded for the thickening of the guard cells provided relationships that clearly distinguished all four taxa. The width and length of the stomata and the upper woody lamellae, the central distance between the external limits of the medial lamellae borders and the length of the stem were the most useful variables in this respect. The present results contribute to the ongoing discussion regarding the taxonomic classification of the members of Pinus, and provide valuable clues for the identification of Iberian Mediterranean pine species from small pine needle fragments or isolated stomata. After validation of the present results for multiple populations, these results could also be used to help identify fossil leaf macroremains and the scattered/isolated stomata commonly observed in palaeopalynological samples. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 161 , 436–448.     

Ellipticity of He-Ne-laser linearly polarized radiation reflected by leaves of plants and leaf cuticle characteristics

The elliptical polarization of He-Ne laser radiation reflected by plant leaves (Begonia Fista and Hedera Helix) was investigated. It was supposed that the elliptic polarization component of reflected radiation is caused by the acts of total reflection of incident radiation on cuticle inhomogeneities. By using the model of the phase-shifting ability of a scattering layer, the cuticle refractive index and the mean angle of inclination of cuticle roughness microfacets to leaf surface was evaluated.     

Elliptical polarization of laser light reflected from plant leaves and characteristics of the leaf cuticle

Polarization was examined in laser light reflected from samples of plant leaves. Elliptical polarization is supposed to result from multiple total reflections of the incident light on the inhomogeneities of the leaf cuticle. A model of the phase-shifting ability of the scattering layer allows assessment of the refractive index and the mean facet slope of cuticular roughness.