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13.3: Eudicot Leaves - Biology

13.3: Eudicot Leaves - Biology


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Macroscopic Features

Eudicot leaves tend to have netted venation, with a larger central vein (the midrib or midvein) that branches off into a network of smaller veins. In the image below, you can see this branching pattern in a skeletal leaf.

Figure (PageIndex{1}): A skeletal holly leaf shows the network of vascular tissue. The lignified xylem and phloem fibers break down far more slowly than the parenchyma cells of the leaf. As the rest of the leaf tissues decompose, the lignified vascular tissue is left behind. This reveals the netted pattern of increasingly smaller side branches found in eudicot leaves. Photo by Maria Morrow, CC BY-NC.

Microscopic Features

Eudicot leaves can usually be distinguished by netted venation at the macroscopic level, but they also differ at the microscopic level. Note the difference in organization between the tissues in the leaf below and the leaves shown in the monocot section.

Figure (PageIndex{2}): A cross section through a eudicot leaf. The upper epidermis is a single layer of parenchyma cells. There are no stomata present in the upper epidermis of this leaf. Below the epidermis, cells (appearing pink due to staining of the nuclei and chloroplasts) are arranged in columns, forming the palisade mesophyll. Beneath the palisade mesophyll is the spongy mesophyll. The cells are approximately the same size as the palisade mesophyll, but there are large intercellular spaces between them. The lower epidermis is another single layer of parenchyma cells, but several stomata (flanked by guard cells) are visible in this epidermal layer. A large vascular bundle is in the center of the leaf. The xylem (stained pink) is on the top and the phloem is on the bottom. Photo by Maria Morrow, CC BY-NC.

Cuticle

You will often see a waxy cuticle coating the surface of most plant tissues. In leaves, the location and thickness of the cuticle can give you clues about the environment that the plant has adapted to.

Figure (PageIndex{3}): A cross section through the upper epidermis and palisade mesophyll. On top of the upper epidermis of this leaf, a transparent layer of cuticle is visible, sealing the top of the leaf. This waxy layer protects the leaf and forms a barrier to the movement of water. Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Vascular Bundles

Seeing vascular bundles of eudicots in cross sections can be confusing. The organization of tissues in the much larger midrib vascular bundle is often spread out into a semicircle, still with xylem on the top and phloem on the bottom, but they can be difficult to distinguish. In addition to this, the smaller veins are not oriented in the same direction, as they are in monocots.

In the image below, the vascular bundle just to the left of the midrib is coming more or less straight at us, so it is easy to distinguish the tissues. In contrast, the vascular bundle to the right of the midrib was moving diagonally and so was caught in an oblique section and looks more like a smear. Often with these oblique sections, you can distinguish the xylem cells by their strange secondary wall thickenings -- they look a bit like coiled springs.

Figure (PageIndex{4}): A cross section through the midvein of a eudicot leaf. The xylem tissue within the large vascular bundle is arranged in an arcing semi-circle, with phloem tissue in an arc traveling just below it. There are layers of collenchyma cells under the epidermis both above and below the midvein. What would be the function of these collenchyma cells? Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Figure (PageIndex{5}): Just to the right of the center of this image, you can see the coiled spring appearance of the xylem vessel elements that have been caught in an oblique section. Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Attributions

Content by Maria Morrow, CC BY-NC


Anatomy of a dicot leaf - Sunflower leaf

Leaves are very important vegetative organs because they are mainly concerned with photosynthesis and transpiration. Like stem and roots, leaves also have the three tissue systems - dermal, ground and vascular. The dermal tissue system consists of an upper epidermis and lower epidermis. Stomata occur in both the epidermis but more frequently in the lower epidermis. The ground tissue system that lies between the epidermal layers of leaf is known as mesophyll tissue. Often it is differentiated into palisade parenchyma on the adaxial (upper) side and spongy parenchyma on the abaxial (lower) side.

A leaf showing this differentiation in mesophyll is designated as dorsiventral. It is common in dicot leaves. If mesophyll is not differentiated like this in a leaf (i.e., made of only spongy or palisade parenchyma) as in monocots, it is called isobilateral. The mesophyll tissue, especially spongy parenchyma cells enclose a lot of air spaces. The presence of air spaces is a special feature of spongy cells. They facilitate the gaseous exchange between the internal photosynthetic tissue (mesophyll) and the external atmosphere through the stomata.

The vascular tissue system is composed of vascular bundles. They are collateral and closed. The vascular tissue forms the skeleton of the leaf and they are known as veins. The veins supply water and minerals to the photosynthetic tissue. Thus the morphological and anatomical features of the leaf help in its physiological functions.


It’s time to leaf: comparing monocot and dicot leaves

The leaves of flowering plants have an upper and lower surface, with the upper surface generally facing away from the ground and the lower surface facing toward it.

Leaf dermal tissue

Both monocot and dicot leaves have an outer, waxy layer called the cuticle that covers the dermal tissue of the upper and lower epidermis. The cuticle protects the leaf and helps it retain water. The epidermis, which is located beneath the cuticle, also protects the leaf. It plays a key role in gas exchange as well, because it contains pores called stomata. Stomata are also present in the plant’s stem and flowers, to some extent, but they are primarily a feature of the leaves.

The stomata allow carbon dioxide to enter the leaf and provide an avenue for water vapor and oxygen to exit the leaf. Each stoma is bordered by two specialized parenchymal cells, called guard cells. These cells open and close the stoma. When the turgor pressure in the guard cells is high, they bend outward, causing the stomatal pore to open. When the turgor pressure in the guard cells is low, due to a loss of water, the stomatal pore is closed.

Leaf ground tissue

A type of ground tissue called mesophyll fills the area between the leaf’s upper and lower epidermis. The cells in the mesophyll contain numerous chloroplasts, organelles that carry out photosynthesis, converting light, water, and carbon dioxide into sugar the plant can break down to generate energy. Oxygen is the main byproduct of photosynthesis—which is great for organisms like humans who need oxygen to breathe!

Leaf vascular tissue

In monocot and dicot leaves, vascular bundles are surrounded by one or more layers of parenchyma cells known as bundle sheaths. They protect the “veins” of the leaf. In monocot leaves, the cells of the bundle sheath carry out photosynthesis, but this isn’t always the case in dicot leaves.

Both types of vascular tissue have an important role to play in leaves. The xylem brings water and dissolved minerals up from the roots, and the cells in the mesophyll use the water when carrying out photosynthesis. Excess water is expelled through transpiration, the release of water vapor through the stomata. The phloem takes the dissolved sugars created by photosynthesis to the plant’s stem and roots to be used or stored.


MATERIALS AND METHODS

Plant material and growth conditions

A total of 54 species representing 13 of the 16 eudicot families known to contain C4 taxa were examined (Table 1). The species sampling included 21 C3 and 33 C4 eudicots. Of the C4 species, nine were already known to be NADP-ME, and 11 were known to be NAD-ME. Representative C3 (Phaseolus acutifolius) and C4 NADP-ME (Flaveria trinervia), NAD-ME (Amaranthus edulis), and PEP-CK (Melinis minutiflora) plants were included in the enzymic analyses to provide a clear reference for the comparisons. Taxa from three families known to possess C4 species (Scrophulariaceae, Molluginaceae, Gisekiaceae) were unavailable. Species names were confirmed by identification using appropriate regional floristic works.

With the exception of two species (Suaeda vera and Commicarpus africanus, for which leafy twigs and leaves were shipped from Germany and Jordan, respectively see also Table 1), plants were either germinated from seeds or propagated vegetatively from shoot cuttings and grown in a soil mixture of topsoil, sand, and organic potting soil (Promix, Sun Gro Horticulture Canada Ltd., Seba Beach, Alberta, Canada) (2 : 1 : 1 by volume) in 4-L plastic pots in the University of Toronto greenhouses. Maximum illumination on sunny days provided a photosynthetic photon flux density exceeding 1600 μmol·m −2 ·s −1 at plant height (natural light was supplemented with 400 W high pressure sodium lamps, PL light systems, Toronto, Ontario, Canada). Greenhouse temperatures ranged from 20°C to 30°C. The youngest, fully expanded leaves or mature stems from five replicate plants for each species were harvested around midday and used for enzyme assays and anatomical analysis.

Extraction and assay of decarboxylating enzymes

For the assays of NADP-ME, NAD-ME, and PEP-CK, crude leaf extracts were prepared using the extraction buffer of Ueno (1992). Leaf tissue (0.1 g fresh mass) was harvested from fully illuminated leaves, frozen in liquid N2, and rapidly ground to a fine powder using a pre-chilled mortar and pestle with the aid of acid-washed sand. One mL of ice-cold extraction medium was added to the powder, and grinding was continued at 4°C for about 30 s. The grinding solution contained 50 mM HEPES-KOH (pH 7.5), 10 mM MgCl2, 2.5 mM MnCl2, 5 mM diothiothreitol (DTT), 0.2 mM Na4-EDTA, 0.5% (w/v) BSA, and 2.5% (w/v) insoluble polyvinyl pyrrolidone (PVP). Tissues from Calligonum caput-medusae had very low extractability for each of these enzymes using the extraction buffer described. Instead, the grinding medium used contained 50 mM HEPES-KOH (pH 7.5), 2.5 mM MgCl2, 2.5 mM MnCl2, 5 mM DTT, 0.2 mM Na4-EDTA, 0.2% (w/v) Triton X-100, 0.7% (w/v) BSA, and 25 mg PVP (Ueno, 1998a). After removal of samples for chlorophyll estimation, crude extracts were immediately centrifuged in an Eppendorf microcentrifuge (Centrifuge 5415, Brinkmann Instruments, Westbury, New York, USA) at 10 000 × g for 45 s, and the supernatants were assayed at 30°C for enzymatic activity using a Diode array spectrophotometer set at 340 nm (model 8452A, Hewlett, Packard, Palo Alto, California, USA).

The activities of NADP-ME and NAD-ME were determined by monitoring the formation of NADPH and NADH, respectively. The reaction mixture for assay of NADP-ME contained 50 mM Tris-HCl (pH 8.2), 1 mM Na4-EDTA, 20 mM MgCl2, 0.5 mM NADP + , 5 mM Na-malate, and enzyme extract (Ku et al., 1991). The reaction was initiated by addition of malate. The assay medium for NAD-ME assay contained 25 mM HEPES-KOH (pH 7.2), 5 mM DTT, 0.2 mM Na4-EDTA, 2.5 mM NAD + , 5 mM Na-malate, 8 mM (NH4)2SO4, 75 μM coenzyme A or 25 μM acetyl coenzyme A, 8 mM MnCl2, 25 μM NADH, and enzyme extract (modified from Hatch and Kagawa, 1974 Hatch et al., 1982). The reaction was started by addition of MnCl2.

PEP-CK was assayed in the carboxylase direction (Reiskind and Bowes, 1991 Chen et al., 2002 Walker et al., 2002) following NADH depletion in a reaction mixture containing 100 mM HEPES-KOH (pH 7.0), 4% (v/v) 2-mercaptoethanol, 100 mM KCl, 90 mM NaHCO3, 5 mM PEP, 1 mM ADP, 10 μM MnCl2, 4 mM MgCl2, 0.14 mM NADH, 6 units malate dehydrogenase (MDH) and enzyme extract. All reaction rates were recorded within a range where the increase in A340 was linear. Chlorophyll content in the extracts was determined spectrophotometrically in 96% (v/v) ethanol according to Wintermans and De Mots (1965).

Leaf anatomy

Small pieces of leaf tissue (approx. 1–2 mm 2 , five leaves per species) containing only high-order veins were excised from the midportion of laminate leaves. For stems and cylindrical leaves, tissue pieces, 1–2 mm 3 , were sampled. Cut tissues were fixed in FAA (70% ethanol : glacial acetic acid : formalin [18 : 1 : 1, v/v]) overnight at room temperature. Following a standard dehydration in a graded ethanol–acetone series, samples were infiltrated through acetone–Spurr's epoxy resin mixtures and cured in pure Spurr's resin (Spurr, 1969). Transverse sections, 2 μm thick, were cut with a glass knife on a Porter Blum MT-2 ultramicrotome (Ivan Sorvall Inc., Norwalk, Connecticut, USA), dried onto poly-l-lysine (100 μg·ml −1 , MW 560 000)-coated slides, and stained with 0.5% (w/v) toluidine blue O in 0.1% (w/v) Na2CO3. Sections were observed and viewed with a Reichert-Jung Polyvar microscope (Reichert-Jung, Vienna, Austria), and images were obtained using a Nikon DXM-1200 digital camera and ACT-1 software (Nikon, Tokyo, Japan). Images of 14 species not illustrated are available on request.

Captured images containing two to four veins for laminate leaves and four to seven veins for semicylindrical and cylindrical leaves and stems were used for quantitative analyses (Dengler et al., 1994). Images were digitized using PCI Image-Pro Plus software (Media Cybernetics, Silver Spring, Maryland, USA). Measured variables included cross-sectional areas (as a proxy for volume) of (1) PCA (or C3 mesophyll [M], including all parenchymatous ground tissues, intercellular space, and substomatal cavities), (2) PCR (or C3 bundle sheath [BS]), (3) intercellular space (described next), and (4) epidermis (sum of upper and lower epidermal layers). Ratios of PCA to PCR (or C3 M to BS) were calculated, and areas were also expressed as percentages of total cross-sectional leaf area.

The area of intercellular space (ICS) was estimated stereometrically using a grid superimposed on a cross section and counting the proportion of dots falling within the spaces and cells (Parkhurst, 1982). Leaf thickness at the thickest part of each vein sector was determined for laminate leaves but not for cylindrical leaves and stems. PCR tissue perimeter and length of PCR (or C3 BS) outer tangential walls exposed to ICS were also measured using Image Pro Plus software. Ratios of PCR (or C3 BS) surface area and PCR perimeter exposed to ICS to PCR area were also determined. Means of quantitative data for each species are available in supplemental data accompanying online version of this article (Appendix S1).

Vein pattern

Vein density was measured as total vein length per unit leaf surface area (Roth-Nebelsick et al., 2001). The middle third of each replicate leaf was washed thoroughly with 70% (v/v) ethanol, bleached with 5% (w/v) NaOH, cleared in saturated chloral hydrate, and mounted in the same solution. Slides were examined under bright field and differential image contrast optics on a Reichert-Jung Polyvar microscope. Two representative images containing only higher order veins were taken for each cleared leaf. A total of 10 images per species (two images per leaf) were digitized as described for leaf sections.

Data analysis

Data were analyzed using nested analysis of variance on raw and transformed data sets using Proc GLM in the SAS program (SAS Institute, Cary, North Carolina, USA). The model used was: variable = meanoverall + photosynthetic type + species (type) + error. To meet the assumptions of ANOVA for normality and homoscedasticity of variance, we employed square-root or ln transformations for non-ratio variables, and an arcsine (square root) transformation for ratio and percentage-based variables, when needed. P values with a probability level ≤ 0.05 are reported and regarded as indicators of significant differences among means of the tested groups. ANOVA was used to compare biochemical types (Table 4, Appendix S1, N = 14–32 species per type), but not to compare anatomical types within each biochemical type because n ranged from 1 (constrained by occurrence of types) to 16 species (Table 5).

A multivariate canonical discriminant analysis (CDA) using Statistica software (StatSoft, 2001, Tulsa, Oklahoma, USA) was used to differentiate between photosynthetic types on the transformed data. Generally, CDA is used to determine which variables discriminate between two or more naturally occurring groups however, for this study we selected nine variables that were known to contribute robustly to discrimination amongst photosynthetic types in grasses (Dengler et al., 1994). The included variables were leaf thickness, PCA (C3 M) and PCR (C3 BS) tissue areas, PCA to PCR area ratio, proportion of ICS, proportion of PCR (C3 BS) perimeter exposed to ICS, PCR area to volume ratio, proportion of epidermis tissue, and vein density. Multivariate CDA rotates a set of nv variables to maximize differences among ng groups chosen a priori. Specifically, it derives ng − 1 or nv (whichever is smaller) orthogonal canonical roots, which are linear combinations of the chosen variables. The first canonical root represents the combination of variables that describes the greatest amount of discrimination among groups. The second root defines the next largest amount of discrimination and is independent of the first root, and so on. Canonical coefficients define the linear projection of each variable onto each root. Raw canonical coefficients were standardized to a mean of zero and variance of one to simplify comparison between variables.

Both the univariate and multivariate analyses raise the question as to whether discrimination among the three photosynthetic types in eudicots is confounded by evolutionary history. To resolve this, we used a Mantel test (Sokal and Rohlf, 1995 after Mantel, 1967) to evaluate the independence of two matrices. A correlation coefficient is calculated for the pair of matrices, and a randomization test is used to estimate the significance of the correlation. The first matrix in our analysis was a data matrix of pairwise distances among species, calculated from the first canonical root of the canonical discriminant analysis. This matrix was compared to a matrix of (1) family membership or (2) photosynthetic type group membership (coded as 1 for the same family or group and 0 when different).


Difference between Dicot and Monocot Leaf

Sl. No.Dicot Leaf
Dorsiventral Leaf
Monocot Leaf
Isobilateral Leaf
1Dicot leaves are dorsiventralMonocot leaves are isobilateral
2Upper surface of the leaf is dark green and the lower surface is light greenBoth the surfaces of the leaf are equally green
3Epidermal cells are not silicified (silica deposition absent)Epidermal cells are silicified (heavy deposition of silica)
4Bulliform (motor) cells absent in the epidermisBulliform cells are present
5Leaves usually hypostomatic (stomata present on the lower surface of the leaf)Leaves usually amphistomatic (stomata present on both the surface of leaf)
6Stomata are arranged randomly on the epidermisStomata are arranged in parallel rows in the epidermis
Stomata
7Stomatal guard cells are kidney-shapedStomatal guard cells are dumb-bell shaped
Guard Cell
8Mesophyll is differentiated into palisade and spongy tissuesMesophyll undifferentiated (composed of loosely packed isodiametric cells with intercellular spaces)
Mesophyll
9Leaf veins are reticulateLeaf veins are parallel
10Protoxylem elements are indistinguishableProtoxylem elements are distinguishable as protoxylem lacuna
11Bundle sheath with single layer of cellsBundle sheath with single or multiple layers
12Bundle sheath cells usually lack chloroplastBundle sheath cells usually possess chloroplasts
13Bundle sheath extension is parenchymatousBundle sheath extension is sclerenchymatous
14Lower portion of the mid-rib is collenchymatousLower portion of mid-rib is sclerenchymatous

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Watch the video: Structure Of The Leaf. Plant. Biology. The FuseSchool (February 2023).