Compartmentation in Vicia faba Leaves: I. Kinetics of C in the Tissues following Pulse Labeling

Leaflets of Vicia faba were pulse-labeled with ¹⁴CO₂ to follow the subsequent movement of photosynthate between leaf tissues. Samples were taken during a ¹²CO₂ chase, quick frozen, freeze-substituted, and embedded in methacrylate. Paradermal sections provided tissue samples consisting only of upper epidermis, palisade parenchyma, spongy parenchyma and veins, spongy parenchyma, or lower epidermis. Most CO₂ fixation occurred in the palisade parenchyma, but its ¹⁴C content declined rapidly after labeling. Concomitant with the decline of activity in the palisade parenchyma, there was an increase in activity in the spongy parenchyma and upper epidermis and a slow increase in the lower epidermis. Activity in the palisade parenchyma and spongy parenchyma eventually reached similar levels and remained constant. Tissue samples containing veins were consistently the most radioactive, and activity in those samples showed a decline. Very little change occurred in the insoluble fraction from any tissue. The results support previous assumptions regarding the pathway of assimilate transport to the veins, and demonstrate the rapidity of such transport. Sucrose is apparently the principal mobile compound.     

Sucrose Compartmentation in the Palisade Parenchyma of Vicia faba L

Intracellular sucrose compartmentation in the palisade parenchyma of Vicia faba L. leaflets was investigated by comparing the specific radioactivity of photosynthetically labeled [¹⁴C]sucrose in samples enriched in vacuole to that in samples enriched in cytoplasm. Brief centrifugation of leaflet punches was used to sediment most of the palisade parenchyma cytoplasm in the adaxial ends of the cells. The punches were quick-frozen, freeze-substituted, and embedded in methacrylate. Samples enriched in cytoplasm or in vacuoles were obtained from paradermal sections. After pulse-labeling, the sucrose specific radioactivity in vacuole-enriched samples was fairly constant. Sucrose specific radioactivity in cytoplasm-enriched samples was about 2.5 times that in vacuole-enriched samples initially and declined thereafter. Earlier interpretation of intracellular sucrose compartmentation (Plant Physiol 1975 55: 704-711) had predicted larger specific activity differences (up to 20 times) between the cytoplasm and vacuole. The difference between the actual and predicted behavior is ascribed to the observed extent of cross-contamination in samples and, more importantly, to the confinement of sucrose to extrachloroplastic regions of the cytoplasm.     

Structural Adaptation of the Leaf Mesophyll to Shading

Structural characteristics of the mesophyll were studied in five boreal grass species experiencing a wide range of light and water supply conditions. Quantitative indices of the palisade and spongy mesophyll tissues (cell and chloroplast sizes, the number of chloroplasts per cell, the total cell and chloroplast surface area per unit leaf surface area) were determined in leaves of each of the species. The cell surface area and the cell volume in spongy mesophyll were determined with a novel method based on stereological analysis of cell projections. An important role of spongy parenchyma in the photosynthetic apparatus was demonstrated. In leaves of the species studied, the spongy parenchyma constituted about 50% of the total volume and 40% of the total surface area of mesophyll cells. The proportion of the palisade to spongy mesophyll tissues varied with plant species and growth conditions. In a xerophyte Genista tinctoria, the total cell volume, cell abundance, and the total surface area of cells and chloroplasts were 30–40% larger in the palisade than in the spongy mesophyll. In contrast, in a shade-loving species Veronica chamaedris, the spongy mesophyll was 1.5–2 times more developed than the palisade mesophyll. In mesophyte species grown under high light conditions, the cell abundance and the total cell surface area were 10–20% greater in the palisade mesophyll than in the spongy parenchyma. In shaded habitats, these indices were similar in the palisade and spongy mesophyll or were 10–20% lower in the palisade mesophyll. In mesophytes, CO₂ conductance of the spongy mesophyll accounted for about 50% of the total mesophyll conductance, as calculated from the structural characteristics, with the mesophyll CO₂ conductance increasing with leaf shading.     

Leaf morphology of 40 evergreen and deciduous broadleaved subtropical tree species and relationships to functional ecophysiological traits

We explored potential of morphological and anatomical leaf traits for predicting ecophysiological key functions in subtropical trees. We asked whether the ecophysiological parameters stomatal conductance and xylem cavitation vulnerability could be predicted from microscopy leaf traits. We investigated 21 deciduous and 19 evergreen subtropical tree species, using individuals of the same age and from the same environment in the Biodiversity‐Ecosystem Functioning experiment at Jiangxi (BEF‐China). Information‐theoretic linear model selection was used to identify the best combination of morphological and anatomical predictors for ecophysiological functions. Leaf anatomy and morphology strongly depended on leaf habit. Evergreen species tended to have thicker leaves, thicker spongy and palisade mesophyll, more palisade mesophyll layers and a thicker subepidermis. Over 50% of all evergreen species had leaves with multi‐layered palisade parenchyma, while only one deciduous species (Koelreuteria bipinnata) had this. Interactions with leaf habit were also included in best multi‐predictor models for stomatal conductance (g_s) and xylem cavitation vulnerability. In addition, maximum g_s was positively related to log ratio of palisade to spongy mesophyll thickness. Vapour pressure deficit (vpd) for maximum g_s increased with the log ratio of palisade to spongy mesophyll thickness in species having leaves with papillae. In contrast, maximum specific hydraulic conductivity and xylem pressure at which 50% loss of maximum specific xylem hydraulic conductivity occurred (Ψ₅₀) were best predicted by leaf habit and density of spongy parenchyma. Evergreen species had lower Ψ₅₀ values and lower maximum xylem hydraulic conductivities. As hydraulic leaf and wood characteristics were reflected in structural leaf traits, there is high potential for identifying further linkages between morphological and anatomical leaf traits and ecophysiological responses.     

Effect of Phosphorus Nutrition on the Cellular Distribution of Acid Phosphatase in the Leaves of Lycopersicon esculentum L.

A histochemical study using light microscopy has been made ofthe distribution of acid phosphatase (EC 3.1.3.2 [EC] ) activity intransverse sections of fully expanded leaves of Lycopersiconesculentum grown in phosphate-deficient or sufficient media.Leaf tissues were prepared by two methods and were embeddedin paraffin wax. The location of acid phosphatase activity inleaf sections was determined by trapping orthophosphate releasedfrom p-nitrophenyl phosphate with lead acetate and subsequentlyconverting the lead phosphate to optically dense lead sulphide.In leaf sections from control tissue lead sulphide depositswere larpely confined to the spongy mesophyll cells. Whereasthe staining of the palisade cells was limited and of a granularnature, the staining of the spongy mesophyll cells was heavierand coincident with the outline of the individual cells. Moreover,the minor veins were more heavily stained than the surroundingmesophyll cells. Sections of phosphorus-deficient tissues wereheavily stained in both the palisade and spongy mesophyll layersand heavy deposits of lead sulphide were present in the regionsof the minor veins. It is suggested that the enhanced acid phosphataseactivity of the mesophyll cells in fully expanded leaves couldbe involved in the remobilization of phosphate within phosphorus-deficientplants, or be part of a phosphate transporting system, concentratingthe intracellular phosphate from the limiting supply in thesolution bathing the mesophyll cells. Lycopersicon esculentum L., tomato, acid phosphatase, phosphorus nutrition     

The plant growth regulator activity of the fungicide,epoxiconazole, on Galium aparine L. (cleavers)

The plant growth regulator activity of epoxiconazole, a new triazole fungicide, was investigated by time-course, dose-response and histology experiments with Galium aparine L. (cleavers). Seven days after treatment with 125g ai ha^–1 epoxiconazole (field rate), plant height was reduced by 43%. After seventeen days, leaflet area was reduced by 27% but leaflet fresh weight was not significantly influenced. This was partly because leaflet thickness had increased by 20% following epoxiconazole application. Chlorophyll concentrations were also increased on a unit area basis. Examination of leaflet anatomy showed that epoxiconazole elongated palisade, spongy mesophyll and upper epidermal cells. For example, 125g ai ha^–1 caused a 35% increase in the length of spongy mesophyll cells. Epoxiconazole also prevented cell separation as there were significantly more palisade and spongy mesophyll cells per unit area than in leaflets sprayed with water. Stem development was reduced and 125g ai ha^–1 inhibited the elongation of pith cells in stem tissue by 53%. However, the simultaneous application of gibberellin A₃ (GA₃) with epoxiconazole resulted in stem elongation similar to that of control plants. These observations are consistent with the expected effects following the inhibition of cytochrome P-450 dependent enzyme activity.Abbreviations GA₃ gibberellin A₃ - g ai ha^–1 grams of active ingredient per hectare - L ha^–1 litres per hectare - PPFD photosynthetic photon flux density - RH relative humidity - SE standard error     

Anatomical responses of leaves of Black Locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> L.) to urban pollutant gases and climatic factors

This study investigates responses in the leaf anatomy of Black Locust (Robinia pseudoacacia L.) to the atmospheric pollutants, SO₂, NO₂ and O₃ and climate in Tehran. The anatomical variables studied include thickness of the leaf lamina and of its main constituent tissues and the length and density of stomata. We present evidence that, in response to urban air pollution, the spongy mesophyll layer is thinner, the upper cuticle of the leaf thicker and stomatal density and the ratio of palisade parenchyma to spongy parenchyma are increased. Similar responses were also detected in relation to a climatic gradient. Stomatal density and thickness of the leaf lamina and of its mesophyll layer were all higher under warmer drier conditions. This overlap in anatomical response to two very different suites of environmental variables may reflect a functional overlap between mechanisms designed to restrict water loss in dry climates and those that minimize the uptake of toxic gases in polluted habitats.     

STRUCTURE OF THE LEAF OF PYROSSIA LONGIFOLIA—A FERN EXHIBITING CRASSULACEAN ACID METABOLISM

The leaf of Pyrossia longifolia (Burm.) Morton, an epiphytic fern known to exhibit CAM, was examined by light and electron microscopy. The relatively thick leaf contains a single-layered epidermis, “water-storage” tissue, and a reticulate vascular system embedded in mesophyll tissue not differentiated into palisade and spongy layers. Mesophyll is composed of large, slightly elongate cells each with a thin, parietal layer of cytoplasm and a large central vacuole. The chloroplast-microbody ratio in mesophyll cells indicates that Pyrossia may be a high photorespirer and thus similar in that sense to C₃ plants. Mesophyll is separated from the vascular tissue by a tightly-arranged layer of endodermal cells with Casparian strips. The inner layer of mesophyll cells and the endodermal cells lack suberin lamellae. The collateral veins contain sieve elements, tracheary elements, pericycle and vascular parenchyma cells, the latter conspicuously larger than the sieve elements. The vascular parenchyma is the only cell type in the leaf which contains plastids with a peripheral reticulum. The parenchymatic elements of the leaf are connected by plasmodesmata, all of which lack neck constrictions and sphincters, or sphincter-like structures. The connections between sieve elements and adjacent parenchymatic elements are pore-plasmodesmata characterized by prominent wall thickenings on the parenchymatic-element side of the wall. The distribution and relative frequencies of plasmodesmata between the various cell types of the leaf indicate photoassimilates may move either symplastically or by a combination of symplast and apoplast from the mesophyll to the site of phloem loading in the veins.     

Untangling metabolic and spatial interactions of stress tolerance in plants. 1. Patterns of carbon metabolism within leaves

The localization of the key photoreductive and oxidative processes and some stress-protective reactions within leaves of mesophytic C₃ plants were investigated. The role of light in determining the profile of Rubisco, glutamate oxaloacetate transaminase, catalase, fumarase, and cytochrome-c-oxidase across spinach leaves was examined by exposing leaves to illumination on either the adaxial or abaxial leaf surfaces. Oxygen evolution in fresh paradermal leaf sections and CO₂ gas exchange in whole leaves under adaxial or abaxial illumination was also examined. The results showed that the palisade mesophyll is responsible for the midday depression of photosynthesis in spinach leaves. The photosynthetic apparatus was more sensitive to the light environment than the respiratory apparatus. Additionally, examination of the paradermal leaf sections by optical microscopy allowed us to describe two new types of parenchyma in spinach—pirum mesophyll and pillow spongy mesophyll. A hypothesis that oxaloacetate may protect the upper leaf tissue from the destructive influence of active oxygen is presented. The application of mathematical modeling shows that the pattern of enzymatic distribution across leaves abides by the principle of maximal ecological utility. Light regulation of carbon metabolism across leaves is discussed.     

Changes in chloroplast morphology of different parenchyma cells in leaves of Haberlea rhodopensis Friv. during desiccation and following rehydration

The size, shape, and number of chloroplasts in the palisade and spongy parenchyma layers of Haberlea rhodopensis leaves changed significantly during desiccation and following rehydration. The chloroplasts became smaller and more rounded during desiccation, and aggregated in the middle of the cell. The size and number of chloroplasts in the palisade parenchyma cells were higher than in spongy parenchyma. The good correlation observed between the size or number of chloroplasts and the cross-sectional area of mesophyll cells, the cross-sectional width of the leaf and its water content suggested that the palisade cells were more responsive to water availability than the spongy cells. Changes in chloroplast number during desiccation and rehydration process are characteristic features for desiccation-tolerant plants (especially in homoiochlorophyllous strategy).     

Ultrastructural effects in potato leaves due to antisense-inhibition of the sucrose transporter indicate an apoplasmic mode of phloem loading

To study the export of sugars from leaves and their long-distance transport, sucrose-proton/co-transporter activity of potato was inhibited by antisense repression of StSUT1 under control of either a ubiquitously active (CaMV 35S ) or a companion-cell-specific (rolC) promotor in transgenic plants. Transformants exhibiting reduced levels of the sucrose-transporter mRNA and showing a dramatic reduction in root and tuber growth, were chosen to investigate the ultrastructure of their source leaves. The transformants had a regular leaf anatomy with a single-layered palisade parenchyma, and bicollateral minor veins within the spongy parenchyma. Regardless of the promoter used, source leaves from transformants showed an altered leaf phenotype and a permanent accumulation of assimilates as indicated by the number and size of starch grains, and by the occurrence of lipid-storing oleosomes. Starch accumulated throughout the leaf: in epidermis, mesophyll and, to a smaller degree, in phloem parenchyma cells of minor veins. Oleosomes were observed equally in mesophyll and phloem parenchyma cells. Companion cells were not involved in lipid accmulation and their chloroplasts developed only small starch grains. The similarity of ultrastructural symptoms under both promotors corresponds to, rather than contradicts, the hypothesis that assimilates can move symplasmically from mesophyll, via the bundle sheath, up to the phloem. The microscopical symptoms of a constitutively high sugar level in the transformant leaves were compared with those in wild-type plants after cold-girdling of the petiole. Inhibition of sugar export, both by a reduction of sucrose carriers in the sieve element/companion cell complex (se/cc complex), or further downstream by cold-girdling, equally evokes the accumulation of assimilates in all leaf tissues up to the se/cc complex border. However, microscopy revealed that antisense inhibition of loading produces a persistently high sugar level throughout the leaf, while cold-girdling leads only to local patches containing high levels of sugar. Received: 4 March 1998 / Accepted: 7 April 1998     

Kinetics of C-14 Translocation in Soybean: II. Kinetics in the Leaf

The kinetics of ¹⁴C-assimilates in the soybean leaf were studied in pulse labeling and steady state labeling experiments. ¹⁴C-Sucrose apparently served as the ultimate source, at least, of translocated ¹⁴C-sucrose. However, since the specific activity of leaf sucrose reached a maximum within 5 minutes after pulse labeling, whereas that of exported sucrose did not reach a maximum until at least 20 minutes, it appeared that there were two sucrose compartments in the leaf. A possible physical basis for the two compartments may be the mesophyll (a photosynthetic compartment) and a specialized “paraveinal mesophyll” (a nonphotosynthetic compartment), through which photosynthate must pass on its way to the veins.     

Translocation of C Sucrose in Sugar Beet during Darkness

The time-course of arrival of ¹⁴C translocate in a sink leaf was studied in sugar beet (Beta vulgaris L. cultivar Klein Wanzleben) for up to 480 minutes of darkness. Following darkening of the source leaf, translocation rapidly declined, reaching a rate approximately 25% of the light period rate by 150 minutes. Comparison of data from plants that were girdled 1 cm below the crown with data from ungirdled plants indicates that after about 150 minutes darkness the beet root becomes a source of translocate to the sink leaf. After about 90 minutes darkness, starch-like reserve polysaccharide from the source leaf begins to contribute ¹⁴C to ethanol soluble pools in that leaf. Because of a 15% isotope mass effect, sucrose, at isotopic saturation, reaches a specific activity which is about 85% of the level of the supplied CO₂. The source leaf sucrose specific activity remains at the isotopic saturation level for about 150 minutes of darkness, after which time input from polysaccharide reserves causes the specific activity to drop to about 55% of that of the supplied CO₂. Sucrose specific activity determinations, polysaccharide dissolution measurements, and pulse labeling experiments indicate that following partial depletion of the sucrose pool, source leaf polysaccharide contributes to dark translocation. Respired CO₂ from the source leaf appears to be derived from a pool which, unlike sucrose, remains at a uniform specific activity.     

Probable mechanical transmission of a virus-like agent from rose rosette disease-infected multiflora rose to Nicotiana species

As part of efforts to identify the causal agent of the rose rosette disease (RRD) of multiflora rose (Rosa multiflora Thunb.), root tip extracts from both symptomatic and nonsymptomatic roses were used to mechanically inoculate leaves of Nicotiana glutinosa. Pale green spots were observed along the margins of the major leaf veins only on leaves inoculated with extracts prepared from symptomatic rose plants. Light microscopy revealed abnormal development of the palisade and spongy mesophyll cells in the symptomatic tissue, although no virus‐like particles (VLPs) were observed by electron microscopy. However, VLPs were observed in cells from tissue adjacent to the leaf veins and bordered by the pale green spots. Inoculation of N. benthamiana with extracts from symptomatic N. glutinosa initially did not result in visible symptoms on N. benthamiana inoculated leaves. However, approximately 4 wk post inoculation, splitting of leaf tissue across and along major leaf veins in expanding leaves occurred. In later stages of leaf expansion some leaves split in regions not associated with veins. Light microscopy of thick sections revealed separation between palisade cells and groups of small dead cells in the mesophyll tissue of expanding systemically infected leaf blades. Electron microscopy revealed crystalline arrays in the cytoplasm of mesophyll cells. No abnormal cellular changes were observed in plants inoculated with extracts prepared from nonsymptomatic rose plants.     

STUDIES ON THE LEAF OF POPULUS DELTOIDES (SALICACEAE): ULTRASTRUCTURE,PLASMODESMATAL FREQUENCY,AND SOLUTE CONCENTRATIONS

The minor veins and contiguous tissues of mature leaves of Populus deltoides Bartr. ex Marsh. were examined with the electron microscope to determine the ultrastructural characteristics of the component cells and to determine the structure, distribution, and frequency of plasmodesmata between the various cell types. In addition, plasmolytic studies were carried out to determine the solute concentrations of the various cell types of the minor veins and contiguous tissues. The cells comprising the mesophyll and bundle sheath contain all the components typical of photosynthetic cells. Paraveinal mesophyll cells and bundle-sheath cells have fewer microbodies and smaller chloroplasts than do palisade parenchyma cells. Vascular parenchyma and companion cells tend to intergrade with one another structurally but can be distinguished from one another by their characteristic plastids. The mature, enucleate sieve-tube member is lined by a parietal layer of cytoplasm consisting of plasmalemma, endoplasmic reticulum, mitochondria, plastids, and P-protein. Plasmodesmata occur along all possible routes from the palisade parenchyma cells to the sieve tubes of the minor veins, and their frequency increases with increasing proximity to the sieve-tube members. Plasmolytic studies revealed that the paraveinal mesophyll cells had a higher C₅₀ (estimated mannitol concentration plasmolyzing, on the average, 50% of a given cell type) than any other cell type of the leaf. Concentration gradients existed along the palisade cell/bundle-sheath cell/companion cell (or vascular parenchyma cell) route as well as along the paraveinal mesophyll cell/bundle-sheath cell/companion cell (or vascular parenchyma cell) route. Considering the frequency of plasmodesmata along these routes, it is conceivable that photosynthate diffuses from palisade cells to the companion cells along concentration gradients. Within the minor veins, the C₅₀ was higher for sieve-tube members than for either companion cells or vascular parenchyma cells, indicating that loading of the sieve tubes is an active, energy-dependent process.     

Histological Compartmentation of Phosphate in Vicia faba L. Leaflet : Possible Significance to Stomatal Functioning

Guard cells, epidermal cells, palisade parenchyma cells, and spongy parenchyma cells of Vicia faba L. leaflet were analyzed for inorganic phosphate and phosphorus. On a molar basis, cells in the epidermal layer contained about 15-fold more inorganic phosphate than mesophyll cells did. Although a metabolic role for this asymmetric distribution cannot be defined unequivocally, we note that high epidermal inorganic phosphate would buffer against pH changes in the epidermis during stomatal movements.     

Different Patterns of Vein Loading of Exogenous [C]Sucrose in Leaves of Pisum sativum and Coleus blumei

Vein loading of exogenous [¹⁴C]sucrose was studied using short uptake and wash periods to distinguish between direct loading into veins and loading via mesophyll tissue. Mature leaf tissue of Pisum sativum L. cv Little Marvel, or Coleus blumei Benth. cv Candidum, was abraded and leaf discs were floated on [¹⁴C]sucrose solution for 1 or 2 minutes. Discs were then washed for 1 to 30 min either at room temperature or in the cold and were frozen, lyophilized, and autoradiographed. In P. sativum, veins were clearly labeled after 1 minute uptake and 1 minute wash periods. Autoradiographic images did not change appreciably with longer times of uptake or wash. Vein loading was inhibited by p-chloromercuribenzenesulfonic acid. These results indicate that uptake of exogenous sucrose occurs directly into the veins in this species. When C. blumei leaf discs were floated on [¹⁴C]sucrose for 2 minutes and washed in the cold, the mesophyll was labeled but little, if any, minor vein loading occurred. When discs were labeled for 2 minutes and washed at room temperature, label was transferred from the mesophyll to the veins within minutes. These results indicate that there may be different patterns of phloem loading of photosynthetically derived sucrose in these two species.     

Leaf anatomy of highbush blueberry grownin vitro and during acclimatization toex vitro conditions

Leaves of micropropagated highbush blueberry (Vaccinium corymbosum) cv. ‘Bluetta’ have been observed during the acclimatization phase. In vitro-developed leaf cells were circular and small, the spongy parenchyma was discontinuous and disorganized and formed by 1–2 layers of cells with large intercellular spaces and the palisade to spongy mesophyll thickness ratio was 1:1.5. After rooting ex vitro, the first leaves formed under natural conditions showed substantial changes in the anatomical characteristics. After 6 months, the plants produced leaves similar to those in field-grown plants. The palisade cells were rectangular, the spongy parenchyma was formed by 3–4 layers of cells and the intercellulars were around the stomata. Leaves from field-grown plants lost 24 % of water during 150 min after excision while leaves from in vitro shoots lost about 50 % of water in the same time. Leaves from in vitro shoots showed a higher number of smaller stomata (361 per mm²), with the guard cells forming a circular ring; the stomata frequency in field-grown leaves was 241 per mm² and the guard-cells were elliptical.     

Leaf anatomy during leaf development of photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis>-grown tobacco plants as affected by growth irradiance

Tobacco (Nicotiana tabacum L.) plants were cultured in vitro photoautotrophically at three levels of irradiance (PAR 400–700 nm): low (LI, 60 μmol m⁻² s⁻¹), middle (MI, 180 μmol m⁻² s⁻¹) and high (HI, 270 μmol m⁻² s⁻¹). Anatomy of the fourth leaf from bottom was followed during leaf development. In HI and MI plants, leaf area expansion started earlier as compared to LI plants, and both HI and MI plants developed some adaptations of sun species: leaves were thicker with higher proportion of palisade parenchyma to spongy parenchyma tissue. Furthermore, in HI and MI plants palisade and spongy parenchyma cells were larger and relative abundance of chloroplasts in parenchyma cells measured as chloroplasts cross-sectional area in the cell was lower than in LI plants. During leaf growth, chloroplasts crosssectional area in both palisade and spongy parenchyma cells in all treatments considerably decreased and finally it occupied only about 5 to 8 % of the cell cross-sectional area. Thus, leaf anatomy of photoautotrophically in vitro cultured plants showed a similar response to growth irradiance as in vivo grown plants, however, the formation of chloroplasts and therefore of photosynthetic apparatus was strongly impaired.     

Leaf biomechanics, morphology, and anatomy of the deciduous mesophyte Prunus serrulata (Rosaceae) and the evergreen sclerophyllous shrub Heteromeles arbutifolia (Rosaceae)

Leaf tensile properties were compared between the mesic deciduous tree Prunus serrulata (var. "Kwanzan") and the xeric and sclerophyllous chaparral evergreen shrub Heteromeles arbutifolia (M. Roem). All values for biomechanical parameters for H. arbutifolia were significantly greater than those of P. serrulata. The fracture planes also differed between the two species with P. serrulata fracturing along the secondary veins, while H. arbutifolia most often fractured across the leaf irrespective of the vein or mesophyll position, thus yielding qualitative differences in the stress-strain curves of the two species. Anatomically, P. serrulata exhibits features typical for a deciduous mesophytic leaf such as a thin cuticle, a single layer of palisade mesophyll, isodiametric spongy mesophyll, and extensive reticulation of the laminar veins. Heteromeles arbutifolia leaves, however, are typically two- to three-fold thicker with a 35% higher dry mass/fresh mass ratio. The vascular tissue is restricted to the interface of the palisade and spongy mesophyll near the center of the leaf. Both epidermal layers have a thick cuticle. The palisade mesophyll is tightly packed and two to three layers thick. The spongy mesophyll cells are ameboid in shape and tightly interlinked both to other spongy cells as well as to the overlying palisade layer. We conclude that the qualitative and quantitative biomechanical differences between the leaves of these two species are likely due to a complex interaction of internal architectural arrangement and the physical/chemical differences in the properties of their respective cell walls. These studies illustrate the importance that morphological and anatomical correlates play with mechanical behavior in plant material and ultimately reflect adaptations present in the leaves of chaparral shrubs that are conducive to surviving in arid environments.