Characterising sclerophylly: some mechanical properties of leaves from heath and forest

Mitigating drought stress in wheat plants (Triticum Aestivum L.) through grain priming in aqueous extract of spirulina platensis

BACKGROUND

The study focuses on the global challenge of drought stress, which significantly impedes wheat production, a cornerstone of global food security. Drought stress disrupts cellular and physiological processes in wheat, leading to substantial yield losses, especially in arid and semi-arid regions. The research investigates the use of Spirulina platensis aqueous extract (SPAE) as a biostimulant to enhance the drought resistance of two Egyptian wheat cultivars, Sakha 95 (drought-tolerant) and Shandawel 1 (drought-sensitive). Each cultivar's grains were divided into four treatments: Cont, DS, SPAE-Cont, and SPAE + DS. Cont and DS grains were presoaked in distilled water for 18 h while SPAE-Cont and SPAE + DS were presoaked in 10% SPAE, and then all treatments were cultivated for 96 days in a semi-field experiment. During the heading stage (45 days: 66 days), two drought treatments, DS and SPAE + DS, were not irrigated. In contrast, the Cont and SPAE-Cont treatments were irrigated during the entire experiment period. At the end of the heading stage, agronomy, pigment fractions, gas exchange, and carbohydrate content parameters of the flag leaf were assessed. Also, at the harvest stage, yield attributes and biochemical aspects of yielded grains (total carbohydrates and proteins) were evaluated.

RESULTS

The study demonstrated that SPAE treatments significantly enhanced the growth vigor, photosynthetic rate, and yield components of both wheat cultivars under standard and drought conditions. Specifically, SPAE treatments increased photosynthetic rate by up to 53.4%, number of spikes by 76.5%, and economic yield by 190% for the control and 153% for the drought-stressed cultivars pre-soaked in SPAE. Leaf agronomy, pigment fractions, gas exchange parameters, and carbohydrate content were positively influenced by SPAE treatments, suggesting their effectiveness in mitigating drought adverse effects, and improving wheat crop performance.

CONCLUSION

The application of S. platensis aqueous extract appears to ameliorate the adverse effects of drought stress on wheat, enhancing the growth vigor, metabolism, and productivity of the cultivars studied. This indicates the potential of SPAE as an eco-friendly biostimulant for improving crop resilience, nutrition, and yield under various environmental challenges, thus contributing to global food security.

Disentangling leaf structural and material properties in relationship to their anatomical and chemical compositional traits in oaks (Quercus L.)

BACKGROUND AND AIMS

The existence of sclerophyllous plants has been considered an adaptive strategy against different environmental stresses. Given that it literally means 'hard-leaved', it is essential to quantify the leaf mechanical properties to understand sclerophylly. However, the relative importance of each leaf trait for mechanical properties is not yet well established.

METHODS

Genus Quercus is an excellent system to shed light on this because it minimizes phylogenetic variation while having a wide variation in sclerophylly. We measured leaf anatomical traits and cell wall composition, analysing their relationship with leaf mass per area and leaf mechanical properties in a set of 25 oak species.

KEY RESULTS

The upper epidermis outer wall makes a strong and direct contribution to the leaf mechanical strength. Moreover, cellulose plays a crucial role in increasing leaf strength and toughness. The principal component analysis plot based on leaf trait values clearly separates Quercus species into two groups corresponding to evergreen and deciduous species.

CONCLUSIONS

Sclerophyllous Quercus species are tougher and stronger owing to their thicker epidermis outer wall and/or higher cellulose concentration. Furthermore, section Ilex species share common traits, although they occupy different climates. In addition, evergreen species living in mediterranean-type climates share common leaf traits irrespective of their different phylogenetic origin.

AusTraits, a curated plant trait database for the Australian flora

We introduce the AusTraits database - a compilation of values of plant traits for taxa in the Australian flora (hereafter AusTraits). AusTraits synthesises data on 448 traits across 28,640 taxa from field campaigns, published literature, taxonomic monographs, and individual taxon descriptions. Traits vary in scope from physiological measures of performance (e.g. photosynthetic gas exchange, water-use efficiency) to morphological attributes (e.g. leaf area, seed mass, plant height) which link to aspects of ecological variation. AusTraits contains curated and harmonised individual- and species-level measurements coupled to, where available, contextual information on site properties and experimental conditions. This article provides information on version 3.0.2 of AusTraits which contains data for 997,808 trait-by-taxon combinations. We envision AusTraits as an ongoing collaborative initiative for easily archiving and sharing trait data, which also provides a template for other national or regional initiatives globally to fill persistent gaps in trait knowledge.

Vein Distribution on the Deformation Behavior and Fracture Mechanisms of Typical Plant Leaves by Quasi In Situ Tensile Test under a Digital Microscope

Angiosperm leaf venation is based on two major patterns, typically dicotyledonous branching and monocotyledonous parallel veins. The influence of these patterns on deformation and fracture properties is poorly understood. In this paper, three species of dicotyledons with netted venation and three species of monocots with parallel venation were selected, and the effect of vein distribution of leaves on their mechanical properties and deformation behavior was investigated. Whole images of leaves were captured using a digital camera, and their vein traits were measured using the image processing software Digimizer. A self-developed mechanical testing apparatus with high precision and low load was used to measure the tensile properties of leaves. The deformation behavior of the leaf was captured using a digital microscope during the tensile test. Results showed that the vein architecture of monocots and dicots is different, which had a remarkable effect on their mechanical properties, deformation behavior, and crack propagation behavior. The greater the diameter and the more the number of veins parallel to the tensile direction, the higher the tensile force, tensile strength, and elastic modulus of the leaves. The netted venation leaves evinced the elastic-plastic fracture type, and the hierarchy venation provided resistance to fracture propagation of cracks in the leaves by lengthening the crack path. The leaves with parallel venation behaved in a predominantly brittle manner or elastic fracture type, and the parallel venation inhibited the initiation of cracks in the leaves by increasing the load at complete fracture of the leaves. The investigation provides reference for a stiffened plate/shell structure and bionic anticrack design.

Effect of drought on yield of ten wheat cultivars linked with their flag leaf water status, fatty acid profile and shoot vigor at heading

Flag leaf and shoot growth at heading stage as well as ultimate yield capacity of ten wheat cultivars were assessed in a pot experiment under normal and drought conditions. Drought was imposed by withholding 25% of field capacity from the 45- day old plants for 21 days followed by normal irrigation until maturity. Leaf succulence degree and stomatal opening area as well as shoot biomass, density and distribution decreased in all cultivars in response to drought but to different degrees. On contrary, leaf sclerophylly degree and water saturation deficit increased in all cultivars as a result of drought. At the same time, drought caused marked alterations in leaf transpiration rate, hair features, abscisic acid content, osmotic adjustment and fatty acid profile of the concerned cultivars; with ultimate variable capacity for yield. The drought- induced changes in the estimated traits were graphically represented in a single map then they were correlated with each other. The considered cultivars could be eventually clustered based on their drought response; with Sids cultivars being the most drought tolerant whereas Shandaweel 1 and Giza 168 being the most sensitive.

Time-resolved nano-Newton force spectroscopy in air and vacuum using a load cell of ultra micro-balance

We demonstrate a simple and versatile nanomechanical force measuring setup with 1 nN precision in air and vacuum using a load cell of an ultra-microbalance. We validate stability, precision, and linearity of the load cell with simple tests. The setup is customized to measure stress-strain response of biomaterials (silk, leaf, and flower) and capillary force in fluids. We isolated an optical pull force induced by a Watt-level laser reflected from a mirror/solid surface in air, in addition to optical push force. Furthermore, we add an interferometric probe to directly measure nanoscale deflection of cantilever of the load cell in real-time, thus bypassing its conventional electromagnetic readout, to improve speed and precision of the instrument. We demonstrate nanomechanical force measurement in high vacuum with the same precision and employ radiation pressure to calibrate the load cell for various precision measurements.

The relationship between physiological and mechanical properties of Acer platanoides L. and Tilia cordata Mill. leaves and their seasonal senescence

The seasonal senescence of leaves in the phenological cycle coincides with the change of their strength properties which determine resistance to environmental conditions and the efficiency of the photosynthesis process. That affects the development, growth and condition of the plant. Therefore, the aim of this paper was to observe and compare the results of strength tests performed on the leaves of two species of trees popular in Poland - lime and maple. As well as chlorophyll fluorescence and photosynthetic pigments content in the context of the changes occurring during the entire leaf life cycle. Obtained results showed that the strength properties of the tested leaves reached the minimum values in spring and the maximum in the summer similarly to the leaf greenness index. Whereas the fluorescence increased which the seasonal senescence in opposition to the photosynthesis efficiency of the leaves. Collected data revealed that strength parameters and photosynthetic pigment content were significantly higher for maple leaves than for lime leaves. Studies showed differences between physiological and mechanical properties of the leaves of two trees species, even if they grew under the same environmental conditions. It is concluded from the results that phenotype and physical parameters of leaves are related to seasonal senescence.

Clusia criuva Cambess. (Clusiaceae): anatomical characterization, chemical prospecting and antioxidant activity

This study aims the anatomical description and chemical characterization of aerial parts of Clusia criuva Cambess., Clusiaceae in addition to the evaluation of the antioxidant activity of crude extracts, correlated to the flavonoid content. The morphological characterization was performed using traditional techniques of plant anatomy. For phytochemical studies, crude extracts were obtained by static maceration and analyzed by thin layer chromatography. The antioxidant activity and the flavonoids content were determined by colorimetric methods involving, respectively, 2,2-diphenyl-1-picrylhydrazyl free radical and aluminum chloride. C. criuva has uniseriate epidermis, paracytic stomata, hypostomatic leaves, cuticular flanges and cordiform vascular cylinder with accessory bundles. Chemical prospecting confirmed the abundant presence of terpenes and phenols in the extracts of leaves and of fruits. The methanolic extract of seeds showed the lowest EC50 value, but the methanolic extract of pericarps exhibited the highest maximum antioxidant activity. The results suggested a high percentage of flavonoids in the hexanic extract of pericarps, however, this could represent, in fact, the presence of benzophenones. Secretory ducts and the shape of the midrib are diagnostic for C. criuva. The antioxidant activity is not directly related to the flavonoids. The results indicate the importance of future studies with C. criuva chemical constituents.

Plants and Light Manipulation: The Integrated Mineral System in Okra Leaves

Calcium oxalate and silica minerals are common components of a variety of plant leaves. These minerals are found at different locations within the leaf, and there is little conclusive evidence about the functions they perform. Here tools are used from the fields of biology, optics, and imaging to investigate the distributions of calcium oxalate, silica minerals, and chloroplasts in okra leaves, in relation to their functions. A correlative approach is developed to simultaneously visualize calcium oxalates, silica minerals, chloroplasts, and leaf soft tissue in 3D without affecting the minerals or the organic components. This method shows that in okra leaves silica and calcium oxalates, together with chloroplasts, form a complex system with a highly regulated relative distribution. This distribution points to a significant role of oxalate and silica minerals to synergistically optimize the light regime in the leaf. The authors also show directly that the light scattered by the calcium oxalate crystals is utilized for photosynthesis, and that the ultraviolet component of light passing through silica bodies, is absorbed. This study thus demonstrates that calcium oxalates increase the illumination level into the underlying tissue by scattering the incoming light, and silica reduces the amount of UV radiation entering the tissue.

Leaf mechanical resistance in plant trait databases: comparing the results of two common measurement methods

BACKGROUND AND AIMS

The influence of leaf mechanical properties on local ecosystem processes, such as trophic transfer, decomposition and nutrient cycling, has resulted in a growing interest in including leaf mechanical resistance in large-scale databases of plant functional traits. 'Specific work to shear' and 'force to tear' are two properties commonly used to describe mechanical resistance (toughness or strength) of leaves. Two methodologies have been widely used to measure them across large datasets. This study aimed to assess correlations and standardization between the two methods, as measured by two widely used apparatuses, in order to inter-convert existing data in those global datasets.

METHODS

Specific work to shear (W(SS)) and force to tear (FT) were measured in leaves of 72 species from south-eastern Australia. The measurements were made including and excluding midribs. Relationships between the variables were tested by Spearman correlations and ordinary least square regressions.

KEY RESULTS

A positive and significant correlation was found between the methods, but coefficients varied according to the inclusion or exclusion of the midrib in the measurements. Equations for prediction varied according to leaf venation pattern. A positive and significant (r = 0·90, P < 0·0001) correlation was also found between W(SS) values for fresh and rehydrated leaves, which is considered to be of practical relevance.

CONCLUSIONS

In the context of broad-scale ecological hypotheses and used within the constraints recommended here, leaf mechanical resistance data obtained with both methodologies could be pooled together into a single coarser variable, using the equations provided in this paper. However, more detailed datasets of FT cannot be safely filled in with estimations based on W(SS), or vice versa. In addition, W(SS) values of green leaves can be predicted with good accuracy from W(SS) of rehydrated leaves of the same species.

Salt stress mitigation by seed priming with UV-C in lettuce plants: growth, antioxidant activity and phenolic compounds

Seeds of Lactuca sativa L. 'Romaine' were subjected to priming treatments with UV-C radiation at 0.85 or 3.42 kJ m(-2). Seedlings obtained from both primed (Pr) and non-primed (NPr) seeds were grown in an hydroponic culture system supplemented with 0 (control) or 100 mM NaCl. After 21 days of NaCl treatment, root and leaf biomass, root lengths, leaf numbers, and leaf surface area were measured. Ions (Na(+) and K(+)) accumulation was determined in roots and leaves. Total phenolic compound and flavonoid concentrations, as well as antioxidant and antiradical activities were measured in L. sativa leaves. Salt stress resulted in a lower increase in fresh weight of roots and leaves, which was more pronounced in roots than in leaves, due to reduced root elongation, leaf number and leaf expansion, as well as leaf thickness. The lower increase in fresh weight was accompanied by a restriction in tissue hydration and K(+) ion uptake, as well as an increase in Na(+) ion concentrations in all organs. These effects were mitigated in plants from the UV-C primed seeds. The mitigating effect of UV-C was more pronounced at 0.85 than at 3.42 kJ m(-2). Salt stress also resulted in an increase in total phenolic compounds and flavonoid concentrations and in the total antioxidant capacity in leaves. The highest diphenylpicrylhydrazyl radical (DPPH) scavenging activity was found in the leaves of plants from both Pr seeds. Our results suggest that plants grown from seed primed by exposure to moderate UV-C radiation exhibited a higher tolerance to salinity stress.

Disturbance regimes, gap-demanding trees and seed mass related to tree height in warm temperate rain forests worldwide

For tropical lowland rain forests, Denslow (1987) hypothesized that in areas with large-scale disturbances tree species with a high demand for light make up a larger proportion of the flora; results of tests have been inconsistent. There has been no test for warm temperate rain forests (WTRFs), but they offer a promising testing ground because they differ widely in the extent of disturbance. WTRF is dominated by microphylls sensu Raunkiaer and has a simpler structure and range of physiognomy than tropical or subtropical rain forests. It occurs in six parts of the world: eastern Asia, New Zealand, Chile, South Africa, SE Australia and the Azores. On the Azores it has been mostly destroyed, so we studied instead the subtropical montane rain forest (STMRF) on the Canary Islands which also represents a relict of the kind of WTRF that once stretched across southern Eurasia. We sought to find whether in these six regions the proportion of tree species needing canopy gaps for establishment reflects the frequency and/or extent of canopy disturbance by wind, landslide, volcanic eruptions (lava flow and ash fall), flood or fire. We used standard floras and ecological accounts to draw up lists of core tree species commonly reaching 5 m height. We excluded species which are very rare, very localized in distribution, or confined to special habitats, e.g. coastal forests or rocky sites. We used published accounts and our own experience to classify species into three groups: (1) needing canopy gaps for establishment; (2) needing either light shade throughout or a canopy gap relatively soon (a few months or years) after establishment; and (3) variously more shade-tolerant. Group 1 species were divided according the kind of canopy opening needed: tree-fall gap, landslide, lava flow, flood or fire. Only some of the significant differences in proportion of Group 1 species were consistent with differences in the extent of disturbance; even in some of those cases other factors seem likely to have had a major determining influence during evolution. We also sought to determine whether the species that are at least 'short-term persistent' in the soil seed bank (lasting 2-4 years) are all species needing canopy gaps for establishment. The answer was negative; large numbers of seeds of some shade-tolerants accumulate in the soil, and these species are able to benefit from soil disturbance in deep shade. We found a significant and strong positive relationship in Japan between mean seed mass and mature tree height, a weak positive relationship in New Zealand and no relationship in any of the other four regions. When comparing the seed mass values of Group 1 and Group 3 species we obtained different answers depending on whether or not we confined ourselves to taxonomically controlled contrasts. In only two of the four regions with an appreciable number of species in Group 1 is the mean seed mass of such species significantly lower than that of Group 3 species when taxonomic relatedness is ignored.

In tropical lowland rain forests monocots have tougher leaves than dicots, and include a new kind of tough leaf

BACKGROUND AND AIMS

There has been little previous work on the toughness of the laminae of monocots in tropical lowland rain forest (TLRF) despite the potential importance of greater toughness in inhibiting herbivory by invertebrates. Of 15 monocot families with >100 species in TLRF, eight have notably high densities of fibres in the lamina so that high values for toughness are expected.

METHODS

In north-eastern Australia punch strength was determined with a penetrometer for both immature leaves (approx. 30 % final area on average) and fully expanded, fully toughened leaves. In Singapore and Panama, fracture toughness was determined with an automated scissors apparatus using fully toughened leaves only.

KEY RESULTS

In Australia punch strength was, on average, 7x greater in shade-tolerant monocots than in neighbouring dicots at the immature stage, and 3x greater at the mature stage. In Singapore, shade-tolerant monocots had, on average, 1.3x higher values for fracture toughness than neighbouring dicots. In Panama, both shade-tolerant and gap-demanding monocots were tested; they did not differ in fracture toughness. The monocots had markedly higher values than the dicots whether shade-tolerant or gap-demanding species were considered.

CONCLUSIONS

It is predicted that monocots will be found to experience lower rates of herbivory by invertebrates than dicots. The tough monocot leaves include both stiff leaves containing relatively little water at saturation (e.g. palms), and leaves which lack stiffness, are rich in water at saturation and roll readily during dry weather or even in bright sun around midday (e.g. gingers, heliconias and marants). Monocot leaves also show that it is possible for leaves to be notably tough throughout the expansion phase of development, something never recorded for dicots. The need to broaden the botanist's mental picture of a 'tough leaf' is emphasized.

Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption

Leaf venation is a showcase of plant diversity, ranging from the grid-like network in grasses, to a wide variety of dendritic systems in other angiosperms. A principal function of the venation is to deliver water; however, a hydraulic significance has never been demonstrated for contrasting major venation architectures, including the most basic dichotomy, "pinnate" and "palmate" systems. We hypothesized that vascular redundancy confers tolerance of vein breakage such as would occur during mechanical or insect damage. We subjected leaves of woody angiosperms of contrasting venation architecture to severing treatments in vivo, and, after wounds healed, made detailed measurements of physiological performance relative to control leaves. When the midrib was severed near the leaf base, the pinnately veined leaves declined strongly in leaf hydraulic conductance, stomatal conductance, and photosynthetic rate, whereas palmately veined leaves were minimally affected. Across all of the species examined, a higher density of primary veins predicted tolerance of midrib damage. This benefit for palmate venation is consistent with its repeated evolution and its biogeographic and habitat distribution. All leaves tested showed complete tolerance of damage to second- and higher-order veins, demonstrating that the parallel flow paths provided by the redundant, reticulate minor vein network protect the leaf from the impact of hydraulic disruption. These findings point to a hydraulic explanation for the diversification of low-order vein architecture and the commonness of reticulate, hierarchical leaf venation. These structures suggest roles for both economic constraints and risk tolerance in shaping leaf morphology during 130 million years of flowering plant evolution.

Novel way of measuring the fracture toughness of leaves and other thin films using a single inclined razor blade

A new test for measuring leaf fracture toughness by cutting with a single inclined razor blade is described here, this having been developed to overcome some of the inadequacies of conventional double-bladed cutting tests, such as scissoring and shearing. The accuracy and precision of this test were determined by measuring the fracture toughness of various leaf types and homogeneous films, and comparing the results with those obtained by scissoring. The new test was found to display a low friction of cutting with great precision in measurements. Fracture toughness measurements of the specimens were considerably lower for the new test than those obtained by scissoring, owing to greater blade sharpness and reduced damage to the specimens during cutting. Despite this, the rankings of fracture toughness measurements for the specimens are similar for both the new test and scissoring, thus demonstrating the test's consistency with scissoring. The new test was found to be successful in measuring the fracture toughness of leaf blades and other thin, film-like materials. It was also able to overcome some of the difficulties of conventional double-bladed cutting tests, especially the estimation of energy expenditure that is extraneous to the work of cutting.

The growth-defense trade-off and habitat specialization by plants in Amazonian forests

Tropical forests include a diversity of habitats, which has led to specialization in plants. Near Iquitos, in the Peruvian Amazon, nutrient-rich clay forests surround nutrient-poor white-sand forests, each harboring a unique composition of habitat specialist trees. We tested the hypothesis that the combination of impoverished soils and herbivory creates strong natural selection for plant defenses in white-sand forest, while rapid growth is favored in clay forests. Recently, we reported evidence from a reciprocal-transplant experiment that manipulated the presence of herbivores and involved 20 species from six genera, including phylogenetically independent pairs of closely related white-sand and clay specialists. When protected from herbivores, clay specialists exhibited faster growth rates than white-sand specialists in both habitats. But, when unprotected, white-sand specialists outperformed clay specialists in white-sand habitat, and clay specialists outperformed white-sand specialists in clay habitat. Here we test further the hypothesis that the growth defense trade-off contributes to habitat specialization by comparing patterns of growth, herbivory, and defensive traits in these same six genera of white-sand and clay specialists. While the probability of herbivore attack did not differ between the two habitats, an artificial defoliation experiment showed that the impact of herbivory on plant mortality was significantly greater in white-sand forests. We quantified the amount of terpenes, phenolics, leaf toughness, and available foliar protein for the plants in the experiment. Different genera invested in different defensive strategies, and we found strong evidence for phylogenetic constraint in defense type. Overall, however, we found significantly higher total defense investment for white-sand specialists, relative to their clay specialist congeners. Furthermore, herbivore resistance consistently exhibited a significant trade-off against growth rate in each of the six phylogenetically independent species-pairs. These results confirm theoretical predictions that a trade-off exists between growth rate and defense investment, causing white-sand and clay specialists to evolve divergent strategies. We propose that the growth-defense trade-off is universal and provides an important mechanism by which herbivores govern plant distribution patterns across resource gradients.

Chemical and mechanical changes during leaf expansion of four woody species of dry Restinga woodland

Young leaves are preferential targets for herbivores, and plants have developed different strategies to protect them. This study aimed to evaluate different leaf attributes of presumed relevance in protection against herbivory in four woody species (Erythroxylum argentinum, Lithrea brasiliensis, Myrciaria cuspidata, and Myrsine umbellata), growing in a dry restinga woodland in southern Brazil. Evaluation of leaf parameters was made through single-point sampling of leaves (leaf mass per area and leaf contents of nitrogen, carbon, and pigments) at three developmental stages and through time-course sampling of expanding leaves (area and strength). Leaves of M. umbellata showed the highest leaf mass per area (LMA), the largest area, and the longest expansion period. On the other extreme, Myrc. cuspidata had the smallest LMA and leaf size, and the shortest expansion period. Similarly to L. brasiliensis, it displayed red young leaves. None of the species showed delayed-greening, which might be related to the high-irradiance growth conditions. Nitrogen contents reduced with leaf maturity and reached the highest values in the young leaves of E. argentinum and Myrc. cuspidata and the lowest in M. umbellata. Each species seems to present a different set of protective attributes during leaf expansion. Myrciaria cuspidata appears to rely mostly on chemical defences to protect its soft leaves, and anthocyanins might play this role at leaf youth, while M. umbellata seems to invest more on mechanical defences, even at early stages of leaf growth, as well as on a low allocation of nitrogen to the leaves. The other species display intermediate characteristics.

Biomechanical properties of insects in relation to insectivory: cuticle thickness as an indicator of insect 'hardness' and 'intractability'

The concept of ‘hardness’ has long been used to describe the biomechanical properties of the diet of many animals. However, due to the lack of a consistent definition, and the multitude of uses to which the term has been put, the use of the term ‘intractability’ has been advocated here to represent the extent to which the structural strength, stiffness and toughness are increased in a foodstuff. The thickness of the cuticle of an insect was a good measure of the intractability of cuticle. The tremendous advantage of the use of cuticle thickness as a measure of the biomechanical properties of invertebrates means that the dietary properties of a living insectivore can be directly quantified according to the thickness of the cuticle in its faeces. The quantitative measurement of intractability obtained through this technique can be used in correlations with adaptations of the masticatory apparatus, including tooth and skull morphology, as well as more general considerations of ecology. This is a major advance on previous measures of the biomechanical properties of insectivore diets, and may represent the best technique of any dietary group in assessing the properties of its diet.

Adaptation, niche conservatism, and convergence: comparative studies of leaf evolution in the California chaparral

Small leaves and low specific leaf area (SLA) have long been viewed as adaptations to Mediterranean-type climates in many species of evergreen woody plants. However, paleobotanical and floristic evidence suggests that in many cases these traits originated prior to the advent of the summer-drought climate regime. In this study, molecular phylogenies and ancestral state reconstructions were used to test the hypothesis of adaptive leaf evolution in 12 lineages of evergreen shrubs in the California chaparral. Across all lineages there was a small but significant shift toward lower SLA, but there were no trends in leaf size evolution. For individual lineages, adaptive changes were detected in only three cases for SLA and in one case for leaf size. Three of these cases of evolutionary change were observed in taxa derived from cool temperate ancestors (e.g., Heteromeles). In contrast, most lineages originating from subtropical ancestors exhibited relative stasis in leaf trait evolution (e.g., Ceanothus). The absence of change suggests that ancestors of chaparral taxa had already acquired appropriate traits that contributed to their success under Mediterranean-type climates. These results illustrate how biogeographic history may influence patterns of trait evolution and adaptation and highlight the contribution of ecological sorting processes to the assembly and functional ecology of regional biotas.

Characterizing sclerophylly: the mechanical properties of a diverse range of leaf types

•  Although sclerophylly is defined by textural properties, its adaptive significance has been debated without a strong base of mechanical data. We measured a wide range of mechanical properties across a diverse range of species and leaf forms, including highly scleromorphic leaves, and compared these with sclerophylly indices to determine the mechanical properties of sclerophylls. •  Fracture and flexure tests were used to determine leaf strength, toughness (work to fracture) and flexural stiffness ('structural' properties), and specific strength, specific toughness and Young's modulus of elasticity ('material' properties, i.e. normalized per unit leaf thickness). •  Leaves varied considerably in all properties tested, and in the way they combined various 'structural' and 'material' properties. However, on average, highly scleromorphic leaves were stronger, tougher and stiffer than soft leaves. 'Structural' properties correlated more strongly with sclerophylly than 'material' properties, and the ratio of stiffness to strength and toughness increased in sclerophyllous species. •  Of the structural properties, strength, toughness and flexural stiffness each made substantial independent contributions to the variation in sclerophylly indices, but the best individual explanators were flexural stiffness and strength, with the best predictive model being a combination of these two properties. This model should now be tested on leaves from contrasting environments.

Leaves at low versus high rainfall: coordination of structure, lifespan and physiology

•  Across species, leaf lifespan (LL) tends to be correlated with leaf mass per area (LMA). Previously we found that Australian perennial species from low-rainfall sites had c . 40% shorter LL at a given LMA than high-rainfall species. •  Here we relate indices of leaf strength (work to shear, W _shear , and tissue toughness) to LL and LMA across the same suite of species. W _shear is the work required to cut a leaf with a blade; W _shear divided by leaf thickness gives tissue toughness. •  Low- and high-rainfall species did not differ in their LL at a given W _shear , but dry-site species had lower W _shear at a given LMA, leading to the observed LL - LMA shift with rainfall. These patterns were driven by 50% lower tissue toughness in dry-site species. •  The lower toughness was linked with high leaf N concentration, which is known to enhance water conservation during photosynthesis in low-rainfall species. Our results suggest that a significant cost of this strategy is reduced LL for a given investment in leaf tissue (LMA).