Constitutive expression of apple endo-POLYGALACTURONASE1 in fruit induces early maturation, alters skin structure and accelerates softening.

During fruit ripening, polygalacturonases (PGs) are key contributors to the softening process in many species. Apple is a crisp fruit that normally exhibits only minor changes to cell walls and limited fruit softening. Here, we explore the effects of PG overexpression during fruit development using transgenic apple lines overexpressing the ripening-related endo-POLYGALACTURONASE1 gene. MdPG1-overexpressing (PGox) fruit displayed early maturation/ripening with black seeds, conversion of starch to sugars and ethylene production occurring by 80 days after pollination (DAP). PGox fruit exhibited a striking, white-skinned phenotype that was evident from 60 DAP and most likely resulted from increased air spaces and separation of cells in the hypodermis due to degradation of the middle lamellae. Irregularities in the integrity of the epidermis and cuticle were also observed. By 120 DAP, PGox fruit cracked and showed lenticel-associated russeting. Increased cuticular permeability was associated with microcracks in the cuticle around lenticels and was correlated with reduced cortical firmness at all time points and extensive post-harvest water loss from the fruit, resulting in premature shrivelling. Transcriptomic analysis suggested that early maturation was associated with upregulation of genes involved in stress responses, and overexpression of MdPG1 also altered the expression of genes involved in cell wall metabolism (e.g. ß-galactosidase, MD15G1221000) and ethylene biosynthesis (e.g. ACC synthase, MD14G1111500). The results show that upregulation of PG not only has dramatic effects on the structure of the fruit outer cell layers, indirectly affecting water status and turgor, but also has unexpected consequences for fruit development.

Time course of changes in the transcriptome during russet induction in apple fruit.

BACKGROUND: Russeting is a major problem in many fruit crops. Russeting is caused by environmental factors such as wounding or moisture exposure of the fruit surface. Despite extensive research, the molecular sequence that triggers russet initiation remains unclear. Here, we present high-resolution transcriptomic data by controlled russet induction at very early stages of fruit development. During Phase I, a patch of the fruit surface is exposed to surface moisture. For Phase II, moisture exposure is terminated, and the formerly exposed surface remains dry. We targeted differentially expressed transcripts as soon as 24 h after russet induction. RESULTS: During moisture exposure (Phase I) of 'Pinova' apple, transcripts associated with the cell cycle, cell wall, and cuticle synthesis (SHN3) decrease, while those related to abiotic stress increase. NAC35 and MYB17 were the earliest induced genes during Phase I. They are therefore linked to the initial processes of cuticle microcracking. After moisture removal (Phase II), the expression of genes related to meristematic activity increased (WOX4 within 24 h, MYB84 within 48 h). Genes related to lignin synthesis (MYB52) and suberin synthesis (MYB93, WRKY56) were upregulated within 3 d after moisture removal. WOX4 and AP2B3 are the earliest differentially expressed genes induced in Phase II. They are therefore linked to early events in periderm formation. The expression profiles were consistent between two different seasons and mirrored differences in russet susceptibility in a comparison of cultivars. Furthermore, expression profiles during Phase II of moisture induction were largely identical to those following wounding. CONCLUSIONS: The combination of a unique controlled russet induction technique with high-resolution transcriptomic data allowed for the very first time to analyse the formation of cuticular microcracks and periderm in apple fruit immediately after the onset of triggering factors. This data provides valuable insights into the spatial-temporal dynamics of russeting, including the synthesis of cuticles, dedifferentiation of cells, and impregnation of cell walls with suberin and lignin.

Neck shrivel in European plum is caused by cuticular microcracks, resulting from rapid lateral expansion of the neck late in development.

MAIN CONCLUSION: Susceptibility to neck shrivel in European plum is due to cuticular microcracking resulting from high surface area growth rates in the neck region, late in development. Susceptibility to the commercially important fruit disorder 'neck shrivel' differs among European plum cultivars. Radial cuticular microcracking occurs in the neck regions of susceptible cultivars, but not in non-susceptible ones, so would seem to be causal. However, the reason for the microcracking is unknown. The objective was to identify potential relationships between fruit growth pattern and microcracking incidence in the neck (proximal) and stylar (distal) ends of selected shrivel-susceptible and non-susceptible cultivars. Growth analysis revealed two allometric categories: The first category, the 'narrow-neck' cultivars, showed hypoallometric growth in the neck region (i.e., slower growth than in the region of maximum diameter) during early development (stages I + II). Later (during stage III) the neck region was 'filled out' by hyperallometric growth (i.e., faster than in the region of maximum diameter). The second category, the 'broad-neck' cultivars, had more symmetrical, allometric growth (all regions grew equally fast) throughout development. The narrow-neck cultivars exhibited extensive radial cuticular microcracking in the neck region, but little microcracking in the stylar region. In contrast, the broad-neck cultivars exhibited little microcracking overall, with no difference between the neck and stylar regions. Across all cultivars, a positive relationship was obtained for the level of microcracking in the neck region and the difference in allometric growth ratios between stage III and stages I + II. There were no similar relationships for the stylar region. The results demonstrate that accelerated stage III neck growth in the narrow-neck plum cultivars is associated with more microcracking and thus with more shrivel.

Apple fruit periderms (russeting) induced by wounding or by moisture have the same histologies, chemistries and gene expressions.

Russeting is a cosmetic defect of some fruit skins. Russeting (botanically: induction of periderm formation) can result from various environmental factors including wounding and surface moisture. The objective was to compare periderms resulting from wounding with those from exposure to moisture in developing apple fruit. Wounding or moisture exposure both resulted in cuticular microcracking. Cross-sections revealed suberized hypodermal cell walls by 4 d, and the start of periderm formation by 8 d after wounding or moisture treatment. The expression of selected target genes was similar in wound and moisture induced periderms. Transcription factors involved in the regulation of suberin (MYB93) and lignin (MYB42) synthesis, genes involved in the synthesis (CYP86B1) and the transport (ABCG20) of suberin monomers and two uncharacterized transcription factors (NAC038 and NAC058) were all upregulated in induced periderm samples. Genes involved in cutin (GPAT6, SHN3) and wax synthesis (KCS10, WSD1, CER6) and transport of cutin monomers and wax components (ABCG11) were all downregulated. Levels of typical suberin monomers (ω-hydroxy-C20, -C22 and -C24 acids) and total suberin were high in the periderms, but low in the cuticle. Periderms were induced only when wounding occurred during early fruit development (32 and 66 days after full bloom (DAFB)) but not later (93 DAFB). Wound and moisture induced periderms are very similar morphologically, histologically, compositionally and molecularly.

Self-Assembly of Nanocrystals into Ring-like Superstructures: When Shape, Size, and Material Do Not Matter.

This manuscript describes a universal method for the spontaneous self-assembly of nanostructures ranging from 2-4 nm spherical particles to ∼440 nm long anisotropic nanorods into ring-like superstructures. The nanostructures composed of Au, Pt, and Pd as surface materials were synthesized in an aqueous cetyltrimethyl ammonium bromide (CTAB) solution. The ligand exchange technique with 4-mercaptophenol was applied to replace CTAB from the surface of nanostructures with a functional thiol. The esterification reaction was carried out to covalently attach carboxy-terminated long-chain polystyrene (PS) molecules to the surface of nanostructures. The high grafting density of PS chains around nanocrystals made them highly soluble in a wide range of organic solvents. When a drop of nanostructure solution in a volatile nonpolar solvent was dried on a solid surface, the nanostructures spontaneously arranged themselves in the form of ring-like assemblies. The condensation of microscopic water droplets from the atmosphere on the surface of an evaporating solvent creates templates for the self-assembly of nanostructures into rings. We demonstrate that this self-assembly method is highly universal and can be extended to various nanostructures regardless of their shapes, sizes, and surface materials.

Direct Evidence for a Radial Gradient in Age of the Apple Fruit Cuticle.

The pattern of cuticle deposition plays an important role in managing strain buildup in fruit cuticles. Cuticular strain is the primary trigger for numerous fruit-surface disorders in many fruit crop species. Recent evidence indicates a strain gradient may exist within the apple fruit cuticle. The outer layers of the cuticle are more strained and thus more susceptible to microcracking than the inner layers. A radial gradient in cuticle age is the most likely explanation. Our study aimed to establish whether (or not) deposition of new cutin in a developing apple fruit occurs on the inner surface of the cuticle, i.e., immediately abutting the outward-facing epidermal cell wall. Developing apples were fed with 13C oleic acid through the skin. Following a 14-d period for incorporation, the fruit was harvested and the cuticular membranes (CMs) isolated enzymatically. The CMs were then ablated to varying extents from the inner or the outer surfaces, using a cold atmospheric pressure plasma (CAPP). Afterwards, the ablated CMs were dewaxed and the 13C contents were determined by mass spectrometry. The incorporation of 13C in the cutin fraction was higher than in the wax fraction. The 13C content was highest in non-ablated, dewaxed CM (DCM) and decreased as ablation depth from the inner surface increased. There was no change in 13C content when ablation was carried out from the outer surface. As fruit development proceeded, more 13C label was found towards the middle of the DCM. These results offered direct evidence for deposition of cutin being on the inner surface of the cuticle, resulting in a radial gradient in cuticular age-the most recent deposition (youngest) being on the inner cuticle surface (abutting the epidermal cell wall) and the earliest deposition (oldest) being on the outer surface (abutting the atmosphere).

Low cuticle deposition rate in 'Apple' mango increases elastic strain, weakens the cuticle and increases russet.

Russeting compromises appearance and downgrades the market value of many fruitcrops, including of the mango cv. 'Apple'. The objective was to identify the mechanistic basis of 'Apple' mango's high susceptibility to russeting. We focused on fruit growth, cuticle deposition, stress/strain relaxation analysis and the mechanical properties of the cuticle. The non-susceptible mango cv. 'Tommy Atkins' served for comparison. Compared with 'Tommy Atkins', fruit of 'Apple' had a lower mass, a smaller surface area and a lower growth rate. There were little differences between the epidermal and hypodermal cells of 'Apple' and 'Tommy Atkins' including cell size, cell orientation and cell number. Lenticel density decreased during development, being lower in 'Apple' than in 'Tommy Atkins'. The mean lenticel area increased during development but was consistently greater in 'Apple' than in 'Tommy Atkins'. The deposition rate of the cuticular membrane was initially rapid but later slowed till it matched the area expansion rate, thereafter mass per unit area was effectively constant. The cuticle of 'Apple' is thinner than that of 'Tommy Atkins'. Cumulative strain increased sigmoidally with fruit growth. Strains released stepwise on excision and isolation (εexc+iso), and on wax extraction (εextr) were higher in 'Apple' than in 'Tommy Atkins'. Membrane stiffness increased during development being consistently lower in 'Apple' than in 'Tommy Atkins'. Membrane fracture force (Fmax) was low and constant in developing 'Apple' but increased in 'Tommy Atkin'. Membrane strain at fracture (εmax) decreased linearly during development but was lower in 'Apple' than in 'Tommy Atkins'. Frequency of membrane failure associated with lenticels increased during development and was consistently higher in 'Apple' than in 'Tommy Atkins'. The lower rate of cuticular deposition, the higher strain releases on excision, isolation and wax extraction and the weaker cuticle account for the high russet susceptibility of 'Apple' mango.

Synthesis of Asymmetric One-Dimensional Pd on Au Bimetallic Nanostructures.

Nanostructures composed of a gold nanorod (AuNR) core and a Pd/Pt shell are of great interest due to their potential application as plasmon resonance-enhanced catalysts. However, the synthesis of well-defined one-dimensional bimetallic nanostructures with precise control over shell thickness and length remains a challenge. In this study, we report a detailed and systematic study on the chemical synthesis of a uniform Pd shell on single crystalline and pentahedrally twinned (PHT) AuNRs of various lengths. AuNRs were used as a template, and the slow and controlled reduction of Pd(II) ions on preformed AuNRs was carried out for the formation of rectangular-shaped Au@Pd bimetallic nanorods. The Pd shell thickness around the AuNRs was controlled by the supply of Pd(II) ions in the growth solution. We were able to grow a ∼20 nm uniform Pd shell around the AuNR, keeping the rod-like morphology intact without local nucleation to form irregular shapes and randomly overgrown nanostructures. The formation of bimetallic nanorods was also extended beyond typical single crystalline nanorods to PHT high aspect ratio gold nanorods and nanowires, using them as templates. To our surprise, unusually curved asymmetric nanorods were formed when the Pd deposition was carried out on AuNRs longer than ∼800 nm which could be possibly due to a Pd and Au lattice mismatch at the interface and higher flexibility of the nanorods when they exceeded certain lengths.

Cutin Synthesis in Developing, Field-Grown Apple Fruit Examined by External Feeding of Labelled Precursors.

An intact skin is essential in high-quality apples. Ongoing deposition of cuticular material during fruit development may decrease microcracking. Our objective was to establish a system for quantifying cutin and wax deposition in developing apple fruit. Oleic acid (13C and 14C labelled) and palmitic acid (14C labelled) were fed to developing apples and the amounts incorporated in the cutin and wax fractions were quantified. The incorporation of 14C oleic acid (C18) was significantly higher than that of 14C palmitic acid (C16) and the incorporation in the cutin fraction exceeded that in the wax fraction. The amount of precursor incorporated in the cutin increased asymptotically with time, but the amount in the wax fraction remained about constant. Increasing the concentration of the precursor applied generally increased incorporation. Incorporation in the cutin fraction was high during early development (43 days after full bloom) and decreased towards maturity. Incorporation was higher from a dilute donor solution (infinite dose feeding) than from a donor solution subjected to drying (finite dose feeding) or from perfusion of the precursor by injection. Feeding the skin of a developing apple with oleic acid resulted in significant incorporation in the cutin fraction under both laboratory and field conditions.

Gold Nanowires from Nanorods.

Gold nanowires (AuNWs) possess strong potential application in micro- and nanoelectronics as well as in plasmonic waveguides because of their low electrical resistance. However, the synthesis of pure solvent-dispersible AuNWs with full control over their length still remains a challenge. All the previously reported methods produce AuNWs with other impurities such as smaller nanorods, platelets, and spherical particles and are limited to a certain length (typically below 10 µm). This article describes a one-step synthesis of extremely long AuNWs (up to 25 µm) with great control over their dimensions by using pentahedrally twinned gold nanorods (AuNRs) as seed particles. To induce the AuNW growth, the reduction of Au(I) to Au(0) was carried out on the surface of AuNRs at a very low pH by introducing HCl into the growth solution. The slow conversion of Au(I) to Au(0) due to the increase in reduction potential at lower pH promoted the preferential deposition of metallic gold on the more reactive tips of AuNRs compared to their sides, resulting in the formation of AuNWs. In analogy to the "living" polymerization reaction, the length of the AuNWs was proportional to the amount of Au(I) added to the growth solution; thus, the desired length of AuNWs was achieved by controlling the supply of Au(I) ions in the reaction mixture. The AuNWs longer than 6 µm were found to be responsive to microwave radiation. When an aqueous solution of AuNWs was exposed to microwaves, the formation of sharp kinks was observed in several locations of AuNWs without their disintegration into smaller pieces.

Russeting in Apple Is Initiated After Exposure to Moisture Ends-I. Histological Evidence.

Russeting (periderm formation) is a critical fruit-surface disorder in apple (Malus × domestica Borkh.). The first symptom of insipient russeting is cuticular microcracking. Humid and rainy weather increases russeting. The aim was to determine the ontogeny of moisture-induced russeting in 'Pinova' apple. We recorded the effects of duration of exposure to water and the stage of fruit development at exposure on microcracking, periderm formation and cuticle deposition. Early on (21 or 31 days after full bloom; DAFB) short periods (2 to 12 d) of moisture exposure induced cuticular microcracking-but not later on (66 or 93 DAFB). A periderm was not formed during moisture exposure but 4 d after exposure ended. A periderm was formed in the hypodermis beneath a microcrack. Russeting frequency and severity were low for up to 4 d of moisture exposure but increased after 6 d. Cuticle thickness was not affected by moisture for up to 8 d but decreased for longer exposures. Cuticular ridge thickness decreased around a microcrack. In general, moisture did not affect cuticular strain release. We conclude that a hypodermal periderm forms after termination of moisture exposure and after microcrack formation. Reduced cuticle deposition may cause moisture-induced microcracking and, thus, russeting.

Solution synthesis of anisotropic gold microcrystals.

Wet chemistry methods have proven to be successful for the synthesis of nanocrystals. However, the size and shape control is often lost when the dimensions of the particles exceed several hundred nanometers. Therefore, a synthetic transformation of nano- to microcrystals in solution remains to be a challenge. Here we report a very efficient chemical synthesis of anisotropic microcrystals of gold and the set of experimental conditions that enables precise control over their size and aspect ratio.

Russet Susceptibility in Apple is Associated with Skin Cells that Are Larger, More Variable in Size, and of Reduced Fracture Strain.

Russeting is an economically important surface disorder in apple (Malus × domestica Borkh.). Indirect evidence suggests an irregular skin structure may be the cause of the phenomenon. The objective of this study was to characterize epidermal and hypodermal cell morphology and the mechanical properties of the skins of apple cultivars of differing russet susceptibility. Dimensions of epidermal and hypodermal cells were determined using microscopy. Stiffness (S), maximum force (Fmax), and maximum strain (ɛmax) at failure were quantified using uniaxial tensile tests of skin strips. Particularly during early fruit development, epidermal cells (EC) and hypodermal cells (HC) in russet non-susceptible cultivars occurred in greater numbers per unit area than in russet-susceptible ones. The EC and HC were lower in height, shorter in length, and of reduced tangential surface area. There were little differences in S or Fmax between non-susceptible and susceptible cultivars. However, the ɛmaxwere higher for the skins of non-susceptible cultivars, than for those of susceptible ones. This difference was larger for the young than for the later growth stages. It is concluded that russet-susceptible cultivars generally have larger cells and a wider distribution of cell sizes for both EC and HC. These result in decreased ɛmax for the skin during early fruit development when russet susceptibility is high. This increases the chances of skin failures which is known to trigger russeting.

Russeting in Apple is Initiated after Exposure to Moisture Ends: Molecular and Biochemical Evidence.

Exposure of the fruit surface to moisture during early development is causal in russeting of apple (Malus × domestica Borkh.). Moisture exposure results in formation of microcracks and decreased cuticle thickness. Periderm differentiation begins in the hypodermis, but only after discontinuation of moisture exposure. Expressions of selected genes involved in cutin, wax and suberin synthesis were quantified, as were the wax, cutin and suberin compositions. Experiments were conducted in two phases. In Phase I (31 days after full bloom) the fruit surface was exposed to moisture for 6 or 12 d. Phase II was after moisture exposure had been discontinued. Unexposed areas on the same fruit served as unexposed controls. During Phase I, cutin and wax synthesis genes were down-regulated only in the moisture-exposed patches. During Phase II, suberin synthesis genes were up-regulated only in the moisture-exposed patches. The expressions of cutin and wax genes in the moisture-exposed patches increased slightly during Phase II, but the levels of expression were much lower than in the control patches. Amounts and compositions of cutin, wax and suberin were consistent with the gene expressions. Thus, moisture-induced russet is a two-step process: moisture exposure reduces cutin and wax synthesis, moisture removal triggers suberin synthesis.

Chemical Transformation of Nanorods to Nanowires: Reversible Growth and Dissolution of Anisotropic Gold Nanostructures.

This manuscript describes a reversible wet chemical process for the tip-selective one-dimensional (1D) growth and dissolution of gold nanorods (AuNRs) and gold nanowires (AuNWs). Tip-selective dissolution was achieved by oxidation of AuNRs with a Au(III)/CTAB complex, whereas the growth of AuNRs was carried out by the reduction of Au(I) ions on the AuNR surface with a mild reducing agent, ascorbic acid (AA). Both the dissolution and growth processes are highly tip selective and proceed exclusively in one dimension. A decrease in the aspect ratio (AR) of AuNRs during the dissolution resulted in a blue shift in the longitudinal plasmon band (LPB) position, and red shifts in the LPB position were achieved by increasing the AR by 1D growth of AuNRs. Both growth and dissolution processes are fully controllable and can be stopped and resumed at any given time when the desired AR and/or LPB position is achieved. In addition, the tip-selective 1D growth of AuNRs can be continued with the additional supply of Au(I)/CTAB/AA solution to produce extremely long AuNWs.

Gram-Scale Synthesis of Isolated Monodisperse Gold Nanorods.

Although gold nanorods (AuNRs) have strong potential applications in nanotechnology, plasmonics, and sensing, the scale-up synthesis of isolated AuNRs in gram quantities remains a challenge. Nearly all previously reported methods produce aqueous solutions of cetyltrimethylammonium bromide (CTAB)-coated AuNRs in milligram quantities with yields of approximately 20-30 % in terms of AuI to Au0 conversion. In addition, it is difficult to remove the CTAB bilayer from the surface of AuNRs and yet make them soluble and functionalized for further processing and chemical modification. This report describes the synthesis of monodisperse functionalized AuNRs (standard deviation, σ≈5 %) in gram quantities. Our approach involved increasing the concentration of HAuCl4 ⋅3 H2 O in the growth solution to produce larger quantities of starting AuNRs and further reducing the remaining AuI ions onto the surface of AuNRs. The slow and controlled addition of ascorbic acid as a reducing agent continued the conversion of AuI into Au0 (through a disproportionation reaction) onto the surface of the nanorods, which maintained their uniform morphology without creating any unwanted impurities of various shapes. In addition, this approach significantly narrowed the size distribution owing to continuous growth of the partially grown AuNRs during the initial stage of the synthesis. To isolate a 1 g quantity of the AuNRs and to make them functionalized for further chemical reactions, a ligand-exchange approach was utilized, in which the CTAB surfactant was replaced with 4-mercaptophenol. The thiol group from 4-mercaptophenol formed a covalent bond with the surface of the AuNRs, leaving free functional OH groups available for further chemical coupling reactions. For the ligand-exchange process, a concentrated solution of 4-mercaptophenol in tetrahydrofuran solution was introduced into the AuNRs solution. Pure AuNRs functionalized with 4-mercaptophenol were isolated by dispersion and rinsing with an excess amount of THF, followed by centrifugation.

Russeting partially restores apple skin permeability to water vapour.

MAIN CONCLUSION: The higher water loss of russeted fruit results from the higher permeance of the periderm of the russeted skin as compared to that of the intact cuticle and epidermis. Apple fruit surfaces are often in-parallel composites, comprising areas of intact cuticle (atop a healthy epidermis) adjacent to areas covered by periderm (so-called russet). The occurrence of non-russeting and russeting genotypes makes this species an ideal model to study the barrier properties of its composite skin. The objective was to quantify the water vapour permeances of non-russeted ([Formula: see text]) and russeted fruit skins ([Formula: see text]). Rates of water loss from whole fruit ([Formula: see text]) and excised epidermal skin segments (ES) or peridermal skin segments (PS) were quantified gravimetrically. The [Formula: see text] was larger in russeting than in non-russeting genotypes because [Formula: see text] exceeded [Formula: see text] by about twofold. Also, the [Formula: see text] of russeting genotypes was larger than that of non-russeting genotypes. Generally, [Formula: see text] was more variable than [Formula: see text]. These differences were consistent across seasons and genotypes. The lower [Formula: see text] as compared to [Formula: see text] resulted primarily from the higher wax content of the cuticle of the [Formula: see text]. For non-russeted genotypes, the value of [Formula: see text] was significantly related to the permeance determined on the same intact fruit ([Formula: see text]). Close relationships were also found between the [Formula: see text] calculated from [Formula: see text] determined on the same fruit and the measured [Formula: see text]. For russeting genotypes, the [Formula: see text] or [Formula: see text] were not correlated with [Formula: see text]. The [Formula: see text] calculated from [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] (all determined on an individual-fruit basis) was significantly correlated with the measured [Formula: see text]. Our results demonstrate that the periderm permeance exceeds the cuticle permeance and that permeances of non-russeted surfaces of russeting genotypes exceed those of non-russeting genotypes.

Patterns of microcracking in apple fruit skin reflect those of the cuticular ridges and of the epidermal cell walls.

MAIN CONCLUSION: Microcracks in the cuticle of developing apples are aligned with ridges on the inner cuticle surface and are indicative of stress-strain concentrations above the anticlinal cell walls. Microcracks occur in cuticles of most fruits. Growth strains are considered causal. In apples (Malus × domestica), microcracks usually form a mesh pattern similar to that formed by cuticular ridges. Ridge patterns are similar to those of the epidermal cells' anticlinal walls. Our aim was to identify the mechanistic bases for these pattern similarities. By quantifying ridge depth, ridge width, and the areas enclosed by ridges, we reveal the presence of major and minor ridges. Major ridges enclose two-to-four epidermal cells, minor ridges only one cell. There are similar and overlying patterns of microcracking on the cuticle's outer surface and of ridges on its inner surface-microcracks generally follow the outlines of the major ridges. In biaxial tensile tests at 20 kPa, strains were low and microcracks shallow, but at > 40 kPa, strains were higher and microcracks deeper. Microcracks traversing the cuticle are usually aligned with the anticlinal walls of the underlying epidermal cells. In general, increased skin strain is associated with increased skin transpiration. Transpiration increases are reversible for low strains but irreversible for high strains. The alignment of cuticular microcracks with the major ridges, and these with the anticlinal cell walls, indicates associated stress/strain concentrations.

Mechanical properties of cuticles and their primary determinants.

Cuticles envelope primary surfaces of the above-ground portion of plants. They function as barriers to water movement and to gas exchange, and in pathogen defense. To serve as a barrier on growing organs, cuticles must remain intact but at the same time must accommodate ongoing growth. Minimizing cuticle failure has stimulated significant research on the cuticle's mechanical properties. The objective here is to review the literature on the mechanical properties of isolated fruit and leaf cuticles. Cuticles are viscoelastic polymers. Viscoelasticity results mainly from the cutin matrix. Impregnation by waxes, flavonoids, and cutan increases stiffness and strength but decreases extensibility. On the inner side, the cutin matrix is impregnated by cell wall polysaccharides, which are responsible for its elastic behavior. Across species, the maximum forces sustainable by hydrated cuticles in uniaxial tensile tests averaged 0.82 N (range 0.15-1.63 N), the maximum stresses averaged 13.2 MPa (range 2.0-29.0 MPa), the maximum strains averaged 8.8% (range 1.6-28.0%), and the moduli of elasticity averaged 224 MPa (range 60-730 MPa). Among the environmental factors, high temperature and hydration both decreased stiffness. Therefore, the mechanical properties of cuticles in vivo depend largely on the relative proportions of their constituents. These proportions change during development and are also affected by environmental factors such as temperature.

An atomistic view of the interfacial structures of AuRh and AuPd nanorods.

In this work we address the challenge of furthering our understanding of the driving forces responsible for the metal-metal interactions in industrially relevant bimetallic nanocatalysts, by taking a comparative approach to the atomic scale characterization of two core-shell nanorod systems (AuPd and AuRh). Using aberration-corrected scanning transmission electron microscopy, we show the existence of a randomly mixed alloy layer some 4-5 atomic layers thick between completely bulk immiscible Au and Rh, which facilitates fully epitaxial overgrowth for the first few atomic layers. In marked contrast in AuPd nanorods, we find atomically sharp segregation resulting in a quasi-epitaxial, strained interface between bulk miscible metals. By comparing the two systems, including molecular dynamics simulations, we are able to gain insights into the factors that may have influenced their structure and chemical ordering, which cannot be explained by the key structural and energetic parameters of either system in isolation, thus demonstrating the advantage of taking a comparative approach to the characterization of complex binary systems. This work highlights the importance of achieving a fundamental understanding of reaction kinetics in realizing the atomically controlled synthesis of bimetallic nanocatalysts.