Spongy Mesophyll Research Articles

Do anthocyanins protect leaves of New Zealand native species from UV-B?

Anthocyanin pigments must reside in the uppermost tissues of a leaf if they are to be effective as UV -B filters. However, in our survey of leaves and phylloclades from 25 native New Zealand plants, only four species held anthocyanins in the upper epidermis and/or hypodermis. For 18 species, anthocyanins were located in vacuoles of the palisade and/or spongy mesophyll, the same tissues that are potentially susceptible to UV-B-induced photoinhibition. Leaf pigmentation patterns varied among species and were correlated to the histological distributions ofanthocyanins. Most species held cyanidin-derived pigments. UV-B filtration cannot be regarded as a unified theory for anthocyanin function in leaves.

Ontogenetic differences in mesophyll structure and chlorophyll distribution in Eucalyptus globulus ssp. globulus (Myrtaceae)

Mesophyll structure has been associated with the photosynthetic performance of leaves via the regulation of internal light and CO(2) profiles. Differences in mesophyll structure and chlorophyll distribution within three ontogenetically different leaf types of Eucalyptus globulus ssp. globulus were investigated. Juvenile leaves are blue-grey in color, dorsiventral (adaxial palisade layer only), hypostomatous, and approximately horizontal in orientation. In contrast, adult leaves are dark green in color, isobilateral (adaxial and abaxial palisade), amphistomatous, and nearly vertical in orientation. The transitional leaf type has structural features that appear intermediate between the juvenile and adult leaves. The ratio of mesophyll cell surface area per unit leaf surface area (A(mes)/A) of juvenile leaves was maximum at the base of a single, adaxial palisade layer and declined through the spongy mesophyll. Chlorophyll a + b content showed a coincident pattern, while the chlorophyll a:b ratio declined linearly from the adaxial to abaxial epidermis. In comparison, the mesophyll of adult leaves had a bimodal distribution of A(mes)/A, with maxima occurring beneath both the adaxial and abaxial surfaces within the first layer of multiple palisade layers. The distribution of chlorophyll a + b content had a similar pattern, although the maximum ratio of chlorophyll a:b occurred immediately beneath the adaxial and abaxial epidermis. The matching distributions of A(mes)/A and chlorophyll provide further evidence that mesophyll structure may act to influence photosynthetic performance. These changes in internal leaf structure at different life stages of E. globulus may be an adaptation for increased xeromorphy under increasing light exposure experienced from the seedling to adult tree, similar to the characteristics reported for different species according to sunlight exposure and water availability within their native habitats.

Effects of elevated ozone on CO2 uptake and leaf structure in sugar maple under two light environments

The interactive effects of ozone and light on leaf structure, carbon dioxide uptake and short‐term carbon allocation of sugar maple (Acer saccharum Marsh.) seedlings were examined using gas exchange measurements and 14C‐macroautoradiographic techniques. Two‐year‐old sugar maple seedlings were fumigated from budbreak for 5 months with ambient or 3 × ambient ozone in open‐top chambers, receiving either 35% (high light) or 15% (low light) of full sunlight. Ozone accelerated leaf senescence, and reduced net photosynthesis, 14CO2 uptake and stomatal conductance, with the effects being most pronounced under low light. The proportion of intercellular space increased in leaves of seedlings grown under elevated ozone and low light, possibly enhancing the susceptibility of mesophyll cells to ozone by increasing the cumulative dose per mesophyll cell. Indeed, damage to spongy mesophyll cells in the elevated ozone × low light treatment was especially frequent. 14C macroautoradioraphy revealed heterogeneous uptake of 14CO2 in well defined areole regions, suggesting patchy stomatal behaviour in all treatments. However, in seedlings grown under elevated ozone and low light, the highest 14CO2 uptake occurred along larger veins, while interveinal regions exhibited little or no uptake. Although visible symptoms of ozone injury were not apparent in these seedlings, the cellular damage, reduced photosynthetic rates and reduced whole‐leaf chlorophyll levels corroborate the visual scaling of whole‐plant senescence, suggesting that the ozone × low light treatment accelerated senescence or senescence‐like injury in sugar maple.

Leaf Anatomy of Wollemi Pine (Wollemia nobilis, Araucariaceae)

Leaves of adult morphology from Wollemi pine(Wollemia nobilis W.G.Jones, K.D.Hill & J.M.Allen)possess a thick cuticle, sunken stomata, abundant hypodermal fibres, distinctpalisade and spongy mesophyll with most palisade development on the adaxialside, compartmented cells, resin canals, sclereids, and vascular bundles withtransfusion tissue and a fibre cap abaxial to the phloem. Stomata are presenton both leaf surfaces, although in greater density on the abaxial surface, andusually have an oblique orientation and four or five subsidiary cells. At thelight microscope level, Araucaria can be distinguishedfrom Agathis as it possesses unusual compartmented cellsin the mesophyll, while Agathis does not. In addition,most Agathis species are hypostomatic, while mostAraucaria species have stomata on both the abaxial andadaxial surfaces. Thus W. nobilis has a leaf anatomywhich has a greater similarity to Araucaria than toAgathis.

Thylakoid membrane architecture

Confocal scanning laser microscopic observations were made on live chloroplasts in intact cells and on mechanically isolated, intact chloroplasts. Chlorophyll fluorescence was imaged to observe thylakoid membrane architecture. C3 plant species studied included Spinacia oleracea L., Spathiphyllum sp. Schott, cv. ‘Mauna Loa’, and Pisum sativum L. C4 plants were also investigated: Saccharum officinarum L., Sorghum bicolor L. Moench, Zea mays L. and Panicum miliaceum L. Some Spinacia chloroplasts were treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to enhance or sodium dithionite (SD) to reduce the photosystem II fluorescence signal. Confocal microscopy images of C3 chloroplasts differed from electron microscopy pictures because they showed discrete spots of bright fluorescence with black regions between them. There was no evidence of fluorescence from stroma thylakoids. The thylakoid membrane system at times appeared to be string-like, with brightly fluorescing grana lined up like beads. C4 bundle sheath chloroplasts were imaged from three different types of C4 plants. Saccharum and Sorghum bundle sheath chloroplasts showed homogeneous fluorescence and were much dimmer than mesophyll chloroplasts. Zea had rudimentary grana, and dim, homogeneous intergrana fluorescence was visualised. Panicum contained thylakoids similar in appearance and stringlike arrangement to mesophyll chloroplasts. Isolated Pisum chloroplasts, treated with a drop of 5 mM MgCl2 showed a thylakoid membrane system which appeared to be unravelling. Spongy mesophyll chloroplasts of Spinacia treated with 5 mM sodium dithionite showed a granal thylakoid system with distinct regions of no fluorescence. A time-series experiment provided evidence of dynamic membrane rearrangements over a period of half an hour.

First report of oil cavities in Scrophulariaceae and reinvestigation of air spaces in leaves of Leucophyllum frutescens

Conspicuous air spaces in Leucophyllum (Scrophulariaceae; Leucophylleae) leaves have been suggested to be developmentally transformed secretory cavities. We reinvestigated air space development in Leucophyllum frutescens, using freehand sections of mature fresh leaves and paraffin sections of several leaf stages. Each of the numerous air spaces per leaf forms because greater separation occurs within a local group of spongy mesophyll cells than in the developing spongy mesophyll elsewhere. We found no anatomical evidence of transitory epithelial cells or lysis of cells in developing air spaces, thus the hypothesis that air spaces are transformed secretory cavities is not supported. However, an important finding was that all leaves had one pair of conspicuous true secretory cavities flanking the midvein at the apex, each lined by an epithelium and filled with oil. We also found conspicuous apical cavities in freehand sections of herbarium specimens of this and three other Leucophyllum species. Cavities were not seen in L. revolutum or in the related Eremogeton grandiflorus. This is the first report and description of a true internal secretory cavity in Scrophulariaceae. In the related family Myoporaceae, we found epithelium-lined cavities scattered abundantly in leaves of cleared samples of three genera.

Changes in leaf structure in relation to crown position and tree size of Betula papyrifera within fire-origin stands of interior cedar-hemlock

Dimensions of anatomical and morphological attributes of leaves can influence physiological response to changes in environment over time. Linking structural attributes of leaves to crown position and tree size within naturally developing cohorts of trees can provide a clearer understanding of changes in crown morphology for a species. This study examined leaf anatomy and morphology of Betula papyrifera Marsh. growing in two stages of stand development of interior cedar-hemlock forest, northern British Columbia. Anatomical and morphological measurements of leaves located at six different positions within the crown were made on trees selected from stands that originated 15 years (small tree size) and 145 years (large tree size) after catastrophic fire. Leaf area and mass were measured in the field. Microscopic measures were made in the laboratory of thicknesses of leaves, cuticle, upper and lower epidermis, palisade, and spongy mesophyll. Stomatal density and stomatal aperture lengths were also determined. Leaf anatomy and morphology varied significantly with position in the crown and among size-classes of trees. Changes in leaf anatomy observed among positions within the crown reflect the changing availability of light and moisture experienced during crown development. For both size-classes of tree the largest anatomical dimensions of leaves were at the outer and uppermost parts of the crown, whilst the smallest were at the lower and innermost parts. Foliage of large trees (145 years) exhibit leaf attributes characteristic of the sun-shade dichotomy reported in the literature, but this was not shown for foliage of young saplings (15 years). For the small trees (15 years) the largest leaves were located at the top of the crown while the smallest were located at the bottom. This has been reported for many tropical pioneers but is in contrast to the usual sun-shade dichotomy of temperate pioneers. The large trees (145 years) followed the typical pattern with the smallest leaves at the top of the crown. Information from this study contributes to our understanding of foliar development of tree crowns by demonstrating how leaf structure changes with crown position and tree size under stand competition.Key words: Betula papyrifera, British Columbia, crown development, cuticle thickness, leaf area, palisade mesophyll, paper birch, stand dynamics.

Light-guiding function of foliar sclereids in the evergreen sclerophyll Phillyrea latifolia: a quantitative approach

Abstract Key words: Phillyrea latifolia L. (incl. P. media L.), foliarsclereids, light guiding, image analysis.The anatomy and orientation of the foliar sclereids ofthe evergreen sclerophyll Phillyrea latifolia suggest alight-guiding function. Light microscope observations Introductionof enzymatically isolated sclereids showed that they The Mediterranean climate region is characterized by anpossessed very thick cell walls, lobes and branchesevergreen sclerophyllous vegetation. The mostly smallwhich occurred mainly at the end of the idioblastsand thick leaves of these plants (low surface to volumereaching the abaxial epidermis. Leaf cross-sections ratio), possess very similar basic anatomical organization:showed that sclereids occurred diffusely within the densely packed mesophyll with a thick palisade layer,mesophyll and were oriented vertically with respect to protective tissue (epidermis, cuticle) with considerablethe lamina. In paradermal sections, the cut cell walls thickness, the occurrence of hairs and, as a rule, well-of the sclereids appeared as bright light spots among developed sclerenchymatic tissue (Kummerow, 1973;the dark-green background of the mesophyll cells. Turner, 1994a). Although these anatomical characteristicsThe heterogeneity of the radiation field transmitted require high construction cost (Ehleringer and Mooney,through the same paradermal section was quantified 1983), they seem to be necessary for the survival ofby image analysis and two- or three-dimensional rep- the plant, especially during the dry summer monthsresentations. The amount of light transmitted through (Kummerow, 1973; Turner, 1994a). It is generallythe sclereids was found to be up to 30-fold higher accepted that the occurrence of abutant sclerenchymaticcompared to that transmitted through the neigh- leaf tissues oVers an eVective protection against collapsebouring mesophyll cells. The light guiding capacity of after severe dehydration (Turner, 1994a). The scleren-the sclereids at the spongy mesophyll level was estim- chymatic cells are located in distinct regions of the laminaated to be 40–80%. In leaves illuminated from the (such as the vascular bundle sheaths and the leaf margins),adaxial surface, light passing through the ends of the or diVusely within the mesophyll, as sclereid idioblastssclereids seemed to be reflected from the internal (Fahn and Cutler, 1992; Turner, 1994a).surface of the abaxial epidermis. In sunny conditions Sclereids may be distributed widely in the plant body,when leaf thickness tends to increase, the number of supporting and protecting the plant tissues throughsclereids per unit leaf area was increased significantly mechanical strengthening (Esau, 1965; Fahn, 1990; Raocompared to the shaded ones. It is proposed that the and Das, 1979). In some special cases, their role is notanatomy and orientation of the foliar osteosclereids of restricted just to a mechanical one, but they may contrib-P. latifolia, are suitable for a light-guiding function. ute to water transport to the mesophyll and epidermalThus foliar sclereids, besides other roles, may con- cells (Heide-Jorgensen, 1990; Schanderl, 1973). Recently,tribute both qualitatively and quantitatively, to the it was suggested that foliar sclereids may have an opticalenhancement of the light microenvironment within the role—that of guiding light within the mesophyllmesophyll of these sclerophyllous leaves. (Karabourniotis et al., 1994). Evidence for this suggestion

The Ultrastructure of Glandular Trichomes ofPhillyrea latifoliaL. (Oleaceae) Leaves

Abstract The anatomy and ultrastructure of glandular trichomes at different developmental stages were investigated in Phillyrea latifolia L. leaves by transmission electron microscopy and histochemical techniques. The trichome consisted of a multicellular secretory head, a unicellular stalk and a collecting cell surrounded by epidermal cells and spongy mesophyll cells. There were numerous plasmodesmata across the cell walls of trichome cells, and especially between the stalk cell and the collecting cell. The collecting cell and stalk cell contained few chloroplasts. Mitochondria, elements of the endoplasmic reticulum and small vacuoles were abundant in the secretory cells. Crystals were present in the secretory cells and the collecting cell, especially at the mature and senescent stages of trichome development. As the cuticle, which covered the secretory cells, did not show pores or perforations, it is proposed that secretion occurred by accumulation of products in subcuticular spaces followed by diffusion through the cuticle. Callose accumulation was observed between the stalk cell and the collecting cell of senescent trichomes, especially in salt-treated plants. Trichome ontogeny was accelerated in salt-treated plants.

Leaf Form and Photosynthesis

M orphological and anatomical features of plant leaves are commonly associated with metabolic type (e.g., Kranz anatomy of C4 species), amount of sun exposure (e.g., sun and shade leaves), or water stress (e.g., xeromorphism). However, although the primary function of the leaf is to absorb and process sunlight and carbon dioxide for photosynthesis, few structural features of leaves have been related mechanistically to these tasks. For example, it has been known for over a century that the internal anatomy of leaves is characterized by different cell layers (e.g., the palisade and spongy mesophyll) and that stomatal pores can be located on one or both sides of a leaf. Yet, only recently has any functional relationship between leaf form and photosynthetic performance been suggested. A variety of ecological studies have correlated numerous leaf structural parameters with photosynthetic performance (e.g., Abrams and Kubiske 1990, 1994, Hinckley et al. 1989,

Effects of Simulated Acid Rain on Anatomy of Primary Leaves of Phaseolus Vulgaris

Ten-days-old bean plants (Phaseolus vulgaris L., cv. Cheren Starozagorski) were treated with simulated acid rain (pH 2.4, 2.2, 2.0 and 1.8). Anatomical changes in the primary leaves were studied 3, 48 and 168 h after a single treatment. This treatment induced: 1) change in the shape of palisade cells, contraction of their contact surfaces and expansion of spongy cells (pH 1.8, 3 h after treatment); 2) reduction of symplast connections among palisade cells and of apoplast in the spongy mesophyll (pH 1.8, 48 h after treatment); 3) destruction of adaxial epidermis and portions of palisade mesophyll, plasmolysis of spongy cells (pH 1.8, 168 h after treatment); 4) full destruction of mesophyll (pH 2.4, 2.2, 2.0 and 1.8, 168 h after treatment). The structure of abaxial epidermis was more stable than that of the adaxial one. With respect to anatomical parameters the studied species could be considered as comparatively resistant to acid rain.

Changes in morphological and anatomical structure of cabbage (Brassica oleracea L.) outer leaves and in ultrastructure of their chloroplasts caused by an in vitro excess of nickel

Morphological, anatomical and ultrastructural changes, the quantity and quality of stomata, and the chlorophyll (Chl) content in primary outer leaves of cabbage plants cv. Slawa from Enkhouizen were examined. The plants were grown in agar with basic MS medium containing added nickel (as NiSO4×7 H2O) in concentrations of 0, 5, 10, and 20 g m-3 (Ni0, Ni5, Ni10, Ni20). Reduction of leaf blade area, of succulence and of leaf density, and growth of specific leaf area were noticed in plants treated with all concentrations of Ni. In Ni-treated plants the total number of stomata and open stomata decreased, and the number of defective stomata in both adaxial and abaxial side of leaves was higher. In all Ni-treated samples the volume of spongy and palisade mesophyll cells was smaller in comparison to control, and it was decreasing when the Ni concentration was increasing; at the same time, the number of mesophyll cells on the same area of cross sections of leaves was increasing. In comparison to control, the intercellular spaces of mesophyll tissue decreased in Ni10 and Ni20 plants and increased in Ni5 plants. In Ni5 plants the number of chloroplasts in mesophyll cells was higher than in the Ni0 control. Reduction of grana size and increase of number of non-appressed lamellae, which often had central arrangement, were observed. In the Ni10 and Ni20 plants, the number and size of chloroplasts decreased, and their internal membranes (especially grana) were reduced and swollen. In Ni5 plants the concentration of Chl in leaves was slightly higher than in the control; in Ni10 and Ni20 plants it was lower than in the control.

Factors affecting shoot organogenesis in leaf disc culture of African violet

Shoot organogenesis in African Violet was affected by factors including leaf age, wounding and leaf disc orientation. A gradient of regenerative potential was found among individual leaves on a plant. This regenerative potential was not directly proportional to the juvenility of leaf tissues. Wound treatment increased shoot organogenesis only when the wound was in direct contact with the medium, possibly by increasing nutrient and hormone uptake. Leaf disc orientation did not affect shoot number per disc but did determine the position of shoot organogenesis. Reduced gas exchange significantly reduced shoot production. This study also showed that adventitious roots originated from spongy mesophyll cells.

Inclination of sun and shade leaves influences chloroplast light harvesting and utilization

Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy and leaf inclination. Leaves of Thermopsis montana had well‐developed palisade and spongy mesophyll whereas the photosynthetic tissue of Smilacina stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low‐light levels, leaves of S. stellata and T. montana were nearly horizontal, whereas under high‐light levels, S. stellata leaves and T. montana leaves were inclined 600 and 300, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial leaf surface of T. montana plants grown under high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxial surface of low‐light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem II (PSII) down regulation, measured from each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was found in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2–3 times higher than those in T. montana, while the portion of the PSII population which was found to be QB nonreducing was 4–6 times lower in high light S. stellata leaves than in T. montana. All values of Stern‐Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher than the irradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxial and abaxial leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf inclination.

Responses of Leaf Anatomy to Light Environment in the Tree Fern Cyathea caracasana (Cyatheaceae) and Its Application to Some Ancient Seed Ferns

The ecological interpretation of extinct plants requires that function-based ecological analogy be made between extant and extinct organisms. Particularly for non-angiosperms, such analogy has been reluctantly assumed. Using a nonangiosperm model, this study identifies a specific suite of characters that can be used to interpret light environments for ancient plants. The tree fern Cyathea caracasana is used as a living model because it offers structural and architectural analogy with ancient tree ferns and some Paleozoic free-standing arborescent pteridosperms. A variety of leaf anatomical and morphological measurements were made on 34 individuals growing in secondary forest and abandoned pasture at Reserva Natural La Planada, Nariho, Colombia. Leaf characters were correlated with sitelight conditions quantified from hemispherical photographs. Spearman correlation coefficients show that blade size, petiole length, and palisade/ spongy mesophyll ratio are well correlated with light environment. Significant relationships were also observed in frond surface area, thickness of ultimate segment, epidermis thickness, and hypodermis thickness. Although some features may be influenced by nutrient and water status, a well-developed palisade layer and thick hypodermis are specific indicators of light environment that vary plastically in Cyathea caracasana. These data can be used to refine ecological interpretations of extinct foliage; Alethopterisand Neuropteristype medullosan pteridosperms of the Middle Pennsylvanian (Westphalian) of Euramerica provide an illustrative example. INTRODUCTION

The Intercellular Distribution of Glycine Decarboxylase in Leaves of Cassava in Relation to the Photosynthetic Mode and Leaf Anatomy.

There have been conflicting reports on the photosynthetic mode of cassava (Manihot esculenta Crantz). It has been suggested that cassava is a C3-C4 intermediate plant and, alternatively, that it is a C3 plant. Glycine decarboxylase (GDC), a photorespiratory enzyme, is known to exhibit different patterns of intercellular distribution between C3-C4 intermediate and C3 leaves. In order to elucidate the photosynthetic mode of cassava, we investigated the localization of GDC in the leaves of three cultivars by immunogold electron microscopy and the anatomical structures of the leaves. The leaves of cassava are essentially hypostomatous, and the photosynthetic tissues consist of palisade mesophyll cells (PMCs), spongy mesophyll cells (SMCs) and bundle sheath cells (BSCs). Although the BSCs include centrifugally located chloroplasts, they show no increase in mitochondrial frequcncy. Labeling specific for GDC was found on all mitochondria of all three types of cell. However, the density of labeling in the PMCs was always higher than in the SMCs and BSCs. In leaves that developed under a water deficit, the difference in labeling density became even clearer. These data suggest that at least the cultivars of cassava examined here are not C3-C4 intermediates and should be regarded as C3 plants. However, the intercellular differences in the level of accumulation of GDC seem to merit further investigation with respect to an internal gradient of photorespiratory capacity in the leaves.

Relationships between anatomical characteristics and ozone sensitivity of leaves of several herbaceous dicotyledonous plant species at Great Smoky Mountains National Park

Many plant species in the Great Smoky Mountains National Park have experienced cell death, visible leaf injury, and reductions in overall growth attributable to ambient ozone exposure levels. The overall objective of this research was to determine if ozone sensitivity of leaves of herbaceous dicotyledonous species was related to differences in leaf anatomical characteristics such as stomatal density, cross-sectional cell areas, percentage of intercellular spaces, number of layers of palisade parenchyma and thicknesses of epidermal and mesophyll tissues. Results demonstrate that three ozone sensitive species, Sassafras albidum, Rudbeckia laciniata, and Rubus canadensis had higher stomatal densities and more intercellular spaces among palisade cells compared with less sensitive species ( Magnolia tripetela, Aster divaricatus, and Liquidamber styraciflua). There were no statistically significant relationships between ozone sensitivity and cross-sectional area of spongy mesophyll cells, number of palisade parenchyma cell layers or thicknesses of epidermal or mesophyll tissues. These data support the hypothesis that ozone sensitivity is associated with leaf characteristics that relate to the ability of ozone to diffuse into leaves (high stomatal densities) and the ability of ozone to diffuse among the target cells (high percentage of intercellular spaces among palisade parenchyma cells).

Three-dimensional radiation transfer modeling in a dicotyledon leaf

The propagation of light in a typical dicotyledon leaf is investigated with a new Monte Carlo ray-tracing model. The three-dimensional internal cellular structure of the various leaf tissues, including the epidermis, the palisade parenchyma, and the spongy mesophyll, is explicitly described. Cells of different tissues are assigned appropriate morphologies and contain realistic amounts of water and chlorophyll. Each cell constituent is characterized by an index of refraction and an absorption coefficient. The objective of this study is to investigate how the internal three-dimensional structure of the tissues and the optical properties of cell constituents control the reflectance and transmittance of the leaf. Model results compare favorably with laboratory observations. The influence of the roughness of the epidermis on the reflection and absorption of light is investigated, and simulation results confirm that convex cells in the epidermis focus light on the palisade parenchyma and increase the absorption of radiation.

High‐light effects on CO2 fixation gradients across leaves

ABSTRACTChlorophyll fluorescence and internal patterns of 14CO2 fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m−2 s−1 for 2h, FV/FM decreased by about 15%, but 14CO2 fixation was unaffected, whereas shade leaves exhibited a 21% decrease in Fv/FM and a 25% decrease in 14CO2 fixation. Irradiation of sun and shade leaves with 4000μmol m−1 for 4 h decreased FV/FM by 30% in sun leaves and 40% in shade leaves, while total 14CO2 fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO2 fixation across leaves were determined by measuring 14CO2 fixed in paradermal leaf sections after a 10s pulse of 14CO2. Gradients of 14CO2 fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m−2 s−1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m−2 s−1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.

A Study to Evaluate Links between Leaf Anatomy and Environmental Stress in Acer saccharum Cultivars

An anatomical study was conducted to determine and compare the internal leaf surface to volume (S/V) ratio of the spongy mesophyll in selected cultivars of Acer saccharum. A low S/V ratio is one of several selectively advantageous characteristics associated with xerophytic plants. It has been proposed that a correlation exists between certain injuries produced by environmental stress and plant leaf anatomy. The taxa included in the study were Acer saccharum cvs. Green Mountain and Legacy, A. saccaharum Caddo and Wichita Mountain (seedlings of Caddo County and Wichita Mountains, Oklahoma relictual populations respectively), and A. saccharum ssp. nigrum (syn. Acer nigrum). Leaf samples were taken from five different representative trees for each of the five cultivars. The results showed that Caddo, which is highly stress tolerant, had a significantly lower internal S/V ratio than the other four cultivars. `Legacy' which is intermediate in stress tolerance had the next lowest S/V ratio. As expected the highly injury-susceptible selections A. saccharum `Green Mountain' and A. saccharum nigrum had high S/V ratios. However, the Wichita Mountain variety which exhibits a stress tolerance similar to Caddo, also had a high S/V ratio. These results suggest that other factors may be involved in determining environmental stress tolerance.