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Palisade cell
Palisade cell
Palisade cell
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Diagram of the internal structure of a leaf

Palisade cells, also called palisade mesophyll cells, are plant cells located inside the mesophyll of most green leaves. They are vertically elongated and are stacked side by side, in contrast to the irregular and loosely arranged spongy mesophyll cells beneath them. Palisade cells are responsible for carrying out the majority of the photosynthesis in a leaf.[1]

Palisade cells occur in dicotyledonous plants, and also in the net-veined monocots: the Araceae and Dioscoreaceae.[citation needed]

Structure

[edit]

Palisade cells are located beneath the upper epidermis and cuticle but above the spongy mesophyll cells.

Palisade cells contain a high concentration of chloroplasts, particularly in the upper portion of the cell, making them the primary site of photosynthesis in the leaves of plants that contain them. Their vacuole also aids in this function: it is large and central, pushing the chloroplasts to the edge of the cell, maximising the absorption of light.[2]

References

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Palisade cell

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Palisade cells are elongated, columnar-shaped cells found in the upper layer of the mesophyll tissue in the leaves of many vascular plants, particularly dicotyledons, where they form one to three tightly packed layers just below the upper . These cells, typically measuring about 50 micrometers in height and 10 micrometers in width, are densely arranged to maximize light absorption and contain three to five times more chloroplasts than spongy mesophyll cells, enabling them to serve as the primary site for by efficiently converting light energy into . Their vertical orientation and close packing optimize the capture of penetrating the surface, while small air spaces between cells allow for minimal gas . In , cells contrast with the underlying spongy mesophyll cells, which are irregularly shaped, loosely packed, and feature larger intercellular air spaces to facilitate with the atmosphere through stomata. This layered organization in the mesophyll—palisade above and spongy below—enhances overall leaf efficiency by balancing light harvesting in the palisade layer with CO₂ uptake and O₂ release in the spongy layer. The abundance of chloroplasts in palisade cells, visible as small green organelles, underscores their specialized role in production, making them vital for plant growth and survival. Recent studies highlight how the of palisade cells influences not only light absorption but also broader physiological processes like water use efficiency, though their core function remains centered on in most species. In some plants, variations in palisade cell layers adapt to environmental conditions, such as shade or high light intensity, demonstrating evolutionary flexibility in leaf structure.

Location and Occurrence

Position in Leaf Anatomy

Palisade cells constitute the primary layer of the mesophyll tissue in leaves, positioned directly beneath the upper in typical dicotyledonous (dorsiventral) leaves. This strategic placement allows them to receive maximal penetration while being protected by the , which often features a waxy to minimize loss. In such leaves, the mesophyll forms the uppermost portion of the internal , transitioning below to the spongy mesophyll and eventually the lower . These cells are organized into one to three tightly packed layers, arranged vertically in a palisade-like (stake-like) formation that optimizes absorption across the surface. The columnar or cylindrical of the cells, with their long axes oriented perpendicular to the surface, contributes to this configuration, enhancing the of capture in the upper regions. This arrangement is particularly prominent in sun-exposed leaves, where multiple layers may develop to increase photosynthetic capacity. Palisade cells maintain connectivity to the leaf's vascular tissues through the extensive system, which branches throughout the mesophyll to deliver and nutrients while removing photosynthetic products. This integration via cell-to-cell contacts and plasmodesmata ensures sustained support for the high metabolic demands of these cells. Typically, palisade cells measure 40–100 μm in length and 10–20 μm in width, allowing for dense packing without compromising intercellular spaces for .

Distribution Across Plant Species

Palisade cells are predominantly found in dicotyledonous plants, where they form a distinct layer in the mesophyll of dorsiventral leaves, as exemplified by the Arabidopsis thaliana. In these species, the palisade consists of elongated, tightly packed cells optimized for light absorption beneath the upper . While less common, palisade cells also occur in certain monocotyledons, particularly in C4 grasses exhibiting Kranz anatomy, where the outer mesophyll cells are elongated and function similarly to palisade tissue, surrounding bundle sheath cells. This distribution reflects the evolutionary divergence between dicots and monocots, with dicots more consistently displaying differentiated palisade and spongy mesophyll layers. In gymnosperms, such as , the mesophyll often consists of homogeneous palisade-like cells with minimal differentiation. In aquatic plants, particularly submerged hydrophytes, palisade cells are typically absent or greatly reduced, as the mesophyll lacks differentiation into palisade and spongy tissues to accommodate diffuse penetration in water. Submerged leaves often feature homogeneous mesophyll with spherical cells and extensive for , adapting to low-intensity, scattered rather than direct capture. This reduction enhances and oxygen in aquatic environments but limits compared to terrestrial forms. The evolutionary origin of palisade cells is tied to the terrestrial of vascular during the period, approximately 400 million years ago, when early tracheophytes developed complex leaves to optimize capture in shaded understories of primitive forests. These columnar cells likely emerged as an innovation in megaphyllous leaves, allowing efficient penetration of canopy-filtered and contributing to the radiation of land . Across habitats, palisade layer thickness varies significantly: heliophytes in sun-exposed environments develop thicker layers with multiple tiers of elongated cells to maximize absorption, whereas sciophytes in shaded conditions exhibit thinner, single-layered palisade to balance harvesting with energy conservation. This plasticity underscores the role of environmental regimes in shaping palisade distribution and morphology.

Structure and Ultrastructure

Cellular Morphology

Palisade cells are characterized by their elongated, cylindrical or prismatic shape, which facilitates efficient vertical orientation within the tissue. These cells typically measure 50 to 100 micrometers in and 10 to 20 micrometers in width, resulting in a length-to-width ranging from 5:1 to 10:1 that enables maximal stacking and light penetration through the leaf layers. This columnar form is oriented perpendicular to the leaf surface, promoting a compact arrangement that minimizes shading among adjacent cells. The cells are tightly packed with minimal intercellular spaces, typically occupying 15-20% of the tissue depending on hydration status, which optimizes the transmission of to the photosynthetic apparatus. This dense packing reduces air-filled gaps while maintaining sufficient contact between cells for structural integrity. The primary cell walls of palisade cells are thin and primarily composed of , along with hemicelluloses and pectins, providing flexibility to accommodate expansion during growth. Palisade cells display apical-basal polarity that varies with conditions, with the nucleus positioned at the basal end near the cell's lower extremity in darkness but relocating to anticlinal walls under blue light. In contrast, the is more concentrated toward the apical (upper) end, supporting the distribution of cellular components along the cell's long axis. This polarized organization contributes to the cell's overall efficiency in resource allocation within the constrained space of the .

Organelles and Components

Palisade cells exhibit a high density of , containing 50 to 200 per mature cell depending on species and conditions, with these organelles predominantly concentrated in the peripheral near the upper cell walls to optimize light absorption. Each contains stacks of thylakoids organized into grana, structures that house the and facilitate the of by providing sites for electron transport and ATP/NADPH production. A prominent feature of palisade cells is the large central vacuole, which can occupy up to 90% of the cell's volume, primarily functioning to store water, ions, and metabolites while maintaining turgor pressure essential for cell rigidity and leaf expansion. This substantial vacuole displaces much of the cytoplasm to the cell periphery, enhancing the packing efficiency of chloroplasts. The endoplasmic reticulum (ER) and Golgi apparatus are extensive in palisade cells, supporting the synthesis, folding, and vesicular transport of nuclear-encoded proteins and lipids required for chloroplast biogenesis and maintenance of photosynthetic machinery. These organelles process enzymes such as those involved in carbon fixation, ensuring their proper delivery to target membranes. Mitochondria in palisade cells are distributed along the anticlinal and inner periclinal walls, particularly under strong conditions, to conduct and generate ATP while minimizing shading of chloroplasts and interference with capture. This strategic positioning reflects adaptations to balance energy demands of and respiration within the constrained cytoplasmic space.

Function and Adaptations

Role in

Palisade cells function as the principal site for the of , where s house I and II. These absorb light energy to drive electron transport, ultimately producing ATP and NADPH, which serve as energy and reducing power for subsequent metabolic processes. This localization maximizes the efficiency of light capture in the upper layers, as the cells' high chloroplast density supports robust photochemical activity. In addition to generating ATP and NADPH, palisade cells host the light-independent reactions of the within the stroma. They contain elevated concentrations of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (), which catalyzes the of ribulose-1,5-bisphosphate with CO₂ to form 3-phosphoglycerate, initiating the fixation of carbon into sugars. This high content, combined with the ample supply of ATP and NADPH from the light reactions, enables palisade cells to efficiently convert atmospheric CO₂ into carbohydrates, supporting plant growth and energy storage. Palisade cells account for the majority of a 's photosynthetic output, particularly in sun leaves, due to their predominant abundance and optimized metabolic machinery. Although CO₂ enters the through stomata primarily on the lower , its occurs mainly via the air spaces in the spongy mesophyll before reaching the palisade layer for assimilation. This coordinated ensures a steady supply of CO₂ to sustain the high photosynthetic rates in palisade cells.

Structural Adaptations for Light Capture

Palisade cells exhibit a columnar or cylindrical morphology that significantly enhances capture by increasing the surface area available for compared to more spherical cell types found in shade-adapted leaves. This elongated shape positions numerous chloroplasts along the length of the cell, aligning them perpendicular to incoming and facilitating deeper penetration of into the interior. In sun-grown plants, such as , these cells form multiple tiers, allowing to reach lower layers efficiently without excessive attenuation. The anticlinal walls, which run parallel to the direction of incident , further minimize inter-cell shading by reducing lateral light blockage, ensuring uniform illumination across the palisade layer. The compact arrangement of cells, characterized by minimal intercellular air spaces, plays a crucial role in optimizing transmission by limiting and reflection within the tissue. Unlike the spongy mesophyll, where extensive air spaces promote diffuse , the layer efficiently channels direct toward chloroplasts, enhancing absorption efficiency and reducing energy loss. This structural feature is particularly adaptive in high- environments, where it supports rapid photosynthetic responses by directing photons straight to photosynthetic machinery. Chloroplast motility within palisade cells provides a dynamic for modulating exposure based on environmental conditions. Actin-based filaments, mediated by proteins such as CHUP1 and phototropins (phot1 and phot2), enable s to reposition rapidly—typically at speeds of 0.3–1.5 µm/min—in response to blue light signals. Under low intensities, s accumulate along the periclinal walls (facing the ) to maximize capture and , while in high , they migrate to the anticlinal walls to shade themselves and prevent photodamage. This movement is essential for balancing harvesting with photoprotection, particularly in the elongated where space constraints influence positioning. Recent studies have identified lobed variants of cells in certain , such as those in the genus , which deviate from the typical columnar form to enhance light utilization in shaded or variable environments. These lobed structures reduce cell packing density, promoting internal light scattering that distributes photons more evenly to chloroplasts and boosts per-cell photosynthetic productivity. Post-2020 research demonstrates that such adaptations can yield higher overall leaf efficiency in conditions compared to strictly columnar cells, highlighting evolutionary flexibility in palisade architecture.

Comparisons and Variations

With Spongy Mesophyll Cells

Palisade mesophyll cells form a compact layer of elongated, cylindrical cells aligned to the surface, densely packed with chloroplasts to maximize absorption and . In contrast, spongy mesophyll cells are irregularly shaped, often branched or lobed, and loosely arranged with extensive interconnected intercellular air spaces that occupy a substantial portion of the tissue volume, promoting gas and exchange. These structural differences underpin their complementary roles in leaf function: the palisade mesophyll accounts for the majority of photosynthetic CO2 fixation due to its high chloroplast density and minimal shading from air spaces, while the spongy mesophyll contributes to photosynthesis but features a higher proportion of mitochondria, supporting elevated respiration rates and facilitating CO2 supply to the palisade layer. Both cell types originate from the ground meristem in the leaf primordium, where positional cues and gradients during development drive their differentiation—the upper cells elongate into under higher light exposure, while lower cells form the porous spongy layer to optimize internal gas flow.

Variations in Different Plants

In C4 plants like , Kranz features mesophyll cells surrounding bundle sheath cells arranged in a ring around vascular bundles, with the bundle sheath handling CO2 concentration for the via in centrifugally arranged chloroplasts, while mesophyll cells perform initial CO2 fixation via PEP carboxylase. This differs from the uniform columnar cells in C3 plants that perform both reactions. This arrangement boosts by reducing in high-light, arid conditions. Palisade cell organization adapts to light availability, with sun-exposed plants developing multiple layers (typically 2–3) of straight, cylindrical cells to enhance light penetration and CO2 diffusion deep into the leaf. In contrast, shade-adapted plants feature fewer layers (1–2) of lobed or funnel-shaped cells, which promote diffuse light capture through increased surface area and chloroplast mobility. These structural differences optimize photosynthesis under varying irradiance levels. Monocots and dicots exhibit phylogenetic variations in palisade placement due to leaf ; dicot leaves are dorsiventral with palisade mesophyll restricted to the adaxial side for upper exposure. Some monocots with amphistomatic leaves bearing stomata on both surfaces and isobilateral have palisade-like mesophyll on adaxial and abaxial sides, enabling balanced absorption from multiple angles in upright or horizontal orientations. Recent 2024 studies on reveal genetic variations in drought-responsive F-box genes that enhance photosynthetic capacity under climate stresses like . These variations promote denser cell packing and maintained CO2 uptake, improving resilience in warming environments.

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