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Mechanical Tissues

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Mechanical tissues (from Greek mechanē — tool, machine), or armature tissues, are a complex of specialized plant tissues whose main function is to provide mechanical strength and stability of the plant body against static (gravity) and dynamic (wind, precipitation, mechanical impacts) loads (Yakovlev et al., 2008). Figuratively speaking, mechanical tissues act as an internal skeleton (“armature”) that allows the plant to maintain an upright position, resist bending, compression, tension, and maintain the shape of organs (Mauseth, 2016). Unlike protective tissues, which protect against the external environment, and conducting tissues, which ensure the transport of substances, mechanical tissues are specialized exclusively for a supportive function, although they often combine this with storage or protective roles.

The key distinguishing features of mechanical tissues are:

  1. High strength of cell walls, achieved through their thickening (often due to the deposition of additional layers of cellulose) and, in many cases, lignification (impregnation with lignin) (Evert, 2006).

  2. Special arrangement within the organ, obeying biomechanical laws: in stems and leaves, mechanical elements are generally placed at the periphery (for maximum bending resistance), while in roots, which experience predominantly tensile loads, they are concentrated in the center (Masrahi et al., 2023).

  3. Different capacity for stretching: some mechanical tissues (collenchyma) retain plasticity and can stretch together with the growing organ; others (sclerenchyma) possess elasticity and perform their supporting function after growth ceases (Mauseth, 2016).

Evolutionary origin. The emergence of mechanical tissues is one of the key aromorphoses that allowed plants to move from water to land and reach large sizes. In early land plants (rhyniophytes), the supporting function was performed by central strands of tracheids, combining water conduction and support (Strasburger, 1971). Gradually, differentiation occurred: specialization of some cells for water conduction (xylem) and others exclusively for support. Collenchyma likely arose as an evolutionary compromise – a strong but plastic tissue enabling organ elongation (Evert, 2006). Sclerenchyma (fibers and sclereids) represents a later acquisition associated with the transition to secondary thickening and the appearance of woody forms (Beck, 2010). Complete separation of functions occurred in angiosperms, where libriform fibers became purely supporting elements, and vessels became conducting elements.

Analogues in other organisms. In animals, mechanical support is provided by the endoskeleton (bones, cartilage) or exoskeleton (chitinous cover of arthropods). In plants, due to the absence of a true skeleton, mechanical tissues perform a similar role, but their operating principle is different: they work not as a rigid framework, but as a network of reinforced elements embedded in an elastic mass of turgid parenchyma cells (the “reinforced concrete” principle) (Serebryakova et al., 2006). Bacteria and fungi lack discrete mechanical tissues; the strength of their body is provided by strong cell walls (in fungi – chitin, in bacteria – peptidoglycan). In algae, especially large brown algae (Laminaria, Macrocystis), the supporting function is performed by cortical layers of cells with thickened walls, which can be considered an evolutionary precursor to true mechanical tissues of land plants (Strasburger, 1971).

1. Functions and significance in agrocenosis

Mechanical tissues perform several fundamental functions in the plant, which directly affect the productivity and resistance of agricultural crops.

1.1. Counteracting lodging

Lodging is the deviation of stems from the vertical position under the influence of gravity of their own mass (especially during the grain-filling phase in cereals) or under the influence of wind and precipitation. Lodging sharply reduces yield by 20-50% or more, complicates mechanized harvesting, and impairs product quality. The main role in lodging resistance is played by stem mechanical tissues: the sclerenchyma ring under the epidermis and the collenchyma strands along the ridges (Evert, 2006). The breaking strength of the stem directly correlates with the content of sclerenchyma fibers and lignin in cell walls. In cereals, for example, the strong development of sclerenchyma in the culm is a key breeding trait for lodging resistance (Yakovlev et al., 2008).

1.2. Shaping canopy architecture and maintaining the photosynthetic surface

Mechanical tissues create a framework that allows leaves and branches to occupy an optimal position in space for capturing sunlight. Without sufficient mechanical strength of petioles and branches, leaves would droop and shade each other, reducing photosynthetic efficiency. In fruit crops, the strength of branches, provided by layers of sclerenchyma and xylem, determines the tree’s ability to hold fruit weight without breaking (Mauseth, 2016). Additionally, the vertical position of cereal stems improves air circulation within the crop and reduces the risk of fungal diseases.

1.3. Protection of conducting tissues and transport function

Sclerenchyma fibers often surround vascular bundles, forming so-called sclerenchyma sheaths or bundles sheaths (Serebryakova et al., 2006). These sheaths play a dual role: first, they protect the delicate phloem cells and xylem vessels from compression and damage; second, they themselves may participate in water conduction (in gymnosperms – tracheids) or in storing nutrients (living fibers). In many plants, sclerenchyma strands (“bast fibers”) form around large vascular bundles, simultaneously reinforcing the petiole or stem (Strasburger, 1971).

1.4. Significance for the feed value of plant mass

For the agronomist-livestock specialist, mechanical tissues are not only a plant support but also a factor limiting the digestibility of feed. Sclerenchyma fibers, especially lignified ones, are rich in lignin and cellulose, which are poorly digested by enzymes in the digestive tract of ruminants. Increased lignin content and sclerenchyma sheaths in the leaves and stems of cereal and legume grasses reduce the nutritional value of hay, silage, and green fodder (Su et al., 2023). Conversely, the presence of living, non-lignified collenchyma elements does not reduce digestibility. Therefore, breeding for reduced lignin content and tender leaf blades (e.g., brown midrib mutants of maize and sorghum) aims to improve feed quality.

1.5. Ecological plasticity and adaptation to stresses

In agrocenoses, plants constantly experience mechanical stresses: wind load, soil cultivation, machinery movement, trampling. Mechanical tissues can adaptively change their structure in response to these influences – a phenomenon known as thigmonasty (stigmotropism) (Beck, 2010). With regular mechanical impact (e.g., “training” by wind), the proportion of collenchyma and sclerenchyma in stems increases, cell walls thicken, and the plant becomes more stocky and strong. However, excess nitrogen fertilizers, while stimulating growth, often lead to a decrease in the relative content of mechanical tissues and, consequently, to lodging (Yakovlev et al., 2008). Thus, managing the development of mechanical tissues is an important technique in modern crop production.

2. Classification of mechanical tissues

Based on origin, cell wall structure, and physical-mechanical properties, mechanical tissues are divided into collenchyma (from Greek kolla — glue) and sclerenchyma (from Greek skleros — hard). This division is based on three key characteristics: the presence of lignin, the nature of wall thickening, and the state of the protoplast at maturity (Evert, 2006; Mauseth, 2016).

2.1. Collenchyma – living plastic supporting tissue

Cross-section of Eleutherococcus stem
commons.wikimedia.orgCC BY-SA 4.0

Collenchyma under the periderm in the stem of _Eleutherococcus_

The image shows the periderm (cork, phellogen, phelloderm) and underlying layers: collenchyma with thickened walls, parenchyma with calcium oxalate crystals, as well as tissues of the vascular bundle.

Collenchyma is a simple mechanical tissue composed of living, usually elongated (prosenchymatous) cells with unevenly thickened primary cell walls (Serebryakova et al., 2006). These walls are never lignified and retain the ability to undergo plastic deformation, so collenchyma functions in growing organs – young stems, leaf petioles, flower stalks. Collenchyma cells usually remain alive, containing cytoplasm, a nucleus, and often chloroplasts (Strasburger, 1971). Due to their high content of pectins and water, collenchyma walls swell readily and maintain elasticity only in a turgid state (Yakovlev et al., 2008). When plants wilt, turgor drops, and the support from collenchyma is temporarily lost.

Depending on the pattern of thickening and the presence of intercellular spaces, several types of collenchyma are distinguished (Evert, 2006; Masrahi et al., 2023):

  • Angular collenchyma – thickenings are concentrated in the corners of cells (at the junctions of 3–5 cells). Intercellular spaces are absent or very small. This is the most common type; found in the stems of many dicotyledons (potato, tomato, sunflower) and in leaf petioles (beet, celery). In cross-section, the thickened corners create a characteristic “star-shaped” pattern.

  • Lamellar collenchyma – thickenings develop on the tangential (parallel to the surface) walls, while the radial walls remain thin. Cells are arranged in regular concentric layers. Well expressed in the stem bark of elderberry (Sambucus nigra) and rhubarb (Rheum).

  • Lacunar collenchyma – characterized by the presence of large intercellular spaces; thickenings are concentrated precisely on the walls facing the intercellular spaces. This type provides a combination of strength and aeration; found in some Asteraceae and Malvaceae, for example, in lettuce (Lactuca).

  • Annular collenchyma (distinguished by some authors) – cells have an almost regular rounded shape in cross-section, and thickenings are distributed relatively evenly around the entire circumference (Metcalfe, 1979, in Evert, 2006). Strictly speaking, this is a transitional form to sclerenchyma, and its independence is not recognized by all anatomists.

Collenchyma is almost never found in roots (except for aerial roots of epiphytes). In monocotyledons, especially cereals, collenchyma is absent or poorly developed; its place soon after growth begins is taken by sclerenchyma (Evert, 2006).

2.2. Sclerenchyma – dead elastic supporting tissue

Sclerenchyma is a mechanical tissue whose cells at maturity have uniformly thickened, typically lignified secondary walls. The protoplast in most cases dies after the wall formation is complete, and the cell becomes empty (Evert, 2006; Beck, 2010). Due to lignin, sclerenchyma walls possess high rigidity and elasticity: they resist compression and bending but return to their original shape when the load is removed (unlike plastic collenchyma). Sclerenchyma is a tissue that functions in organs that have finished growing, so its elements never stretch.

Based on cell shape, sclerenchyma is divided into fibers and sclereids (Serebryakova et al., 2006; Mauseth, 2016).

Fibers

Fibers are long (often tens and hundreds of times longer than their width) prosenchymatous cells with pointed ends. They can be either living or dead, with simple or bordered pits, usually arranged in bundles, strands, or continuous cylinders.

Based on origin and location, they are distinguished:

  • Phloem fibers (bast fibers) – develop from procambium or cambium cells at the periphery of the phloem. In flax (Linum usitatissimum), hemp (Cannabis sativa), nettle (Urtica dioica), these fibers do not lignify (or weakly lignify), retain a living protoplast, but most importantly – they are very elastic and strong in tension. They are used for textile production (linen fabric, hemp) (Evert, 2006).

  • Xylem fibers (libriform fibers) – form from the cambium inward, towards the xylem. They are almost always lignified, with thick walls and a very narrow lumen. In most angiosperms, xylem fibers are the main reinforcing element, giving wood its hardness and rigidity (Beck, 2010). Among them, libriform fibers (with simple pits) and fibrous tracheids (with bordered pits) are distinguished, which are a transitional form between tracheids and true fibers.

  • Perivascular (pericyclic) fibers – located outside the phloem, sometimes forming a continuous cylinder (in many Fabaceae and Malvaceae) (Strasburger, 1971).

  • Cortical fibers – arise in the cortex (primary cortex of the stem), not associated with vascular bundles.

A special group consists of septate fibers – cells in which, after the formation of the secondary wall, the protoplast divides by transverse septa into several compartments. Such fibers remain alive and perform, along with a supporting function, also a storage function (accumulating starch). They are characteristic of many legumes (e.g., robinia) and bamboos (Evert, 2006; Su et al., 2023).

Sclereids

Sclereids are short, often isodiametric or irregularly shaped cells with very thick lignified walls and branched pits (radiform pits). In most cases, sclereids are dead. Based on shape, they are distinguished (Yakovlev et al., 2008; Serebryakova et al., 2006):

  • Brachysclereids (stone cells) – almost rounded or slightly elongated; found in large numbers in pear flesh (giving it its characteristic “crunch”), in nut shells, in fruit stones (cherry, peach).

  • Macrosclereids – columnar, elongated, forming a palisade layer in the seed coat of legumes (e.g., in beans).

  • Osteosclereids – “bone-like” cells with expanded ends, often forming a mechanical layer in the seed coat (pea).

  • Astrosclereids – star-shaped, with processes penetrating intercellular spaces. Typical of water lily leaves (Nymphaea) and tea (Camellia).

  • Trichosclereids – thin-walled, thread-like, branched, resembling hairs; found in aerial roots of some Araceae (Monstera).

Sclereids can arise either directly from meristematic cells (primary sclereids) or as a result of secondary lignification of ordinary parenchyma cells (secondary sclereids). Their role is to create a rigid, “armor” coating protecting seeds and fruits, as well as to impart rigidity to xerophyte leaves (Evert, 2006).

The following table summarizes the key differences between collenchyma and the two main types of sclerenchyma.

Characteristic Collenchyma Fibers (sclerenchyma) Sclereids (sclerenchyma)
Life state Living cells Usually dead (exceptions – septate) Dead
Cell wall type Primary, non-lignified Secondary, lignified (except some bast) Secondary, heavily lignified
Nature of thickening Uneven Uniform, often multilayered Uniform, very thick
Cell shape Prosenchymatous, slightly elongated Long, spindle-shaped (high L/D) Short, isodiametric or star-shaped
Typical localization Under epidermis of growing stems, petioles In cortex (bast), in xylem (wood fibers), in leaves In fruits, seeds, bark, leaves
Mechanical properties Plastic, stretchable Elastic, work in tension and bending Brittle, work in compression

2.3. Evolutionary dynamics of classification

From an evolutionary perspective of land plants, collenchyma can be viewed as a “primitive” mechanical tissue that arose earlier and retains characteristics of parenchyma (living protoplast, primary wall). Sclerenchyma, especially libriform fibers and sclereids, is a later adaptation providing maximum strength with minimal maintenance costs (Serebryakova et al., 2006). In some modern plants, transitional forms occur, e.g., collenchyma-like fibers with thickened but not yet lignified walls, illustrating the continuity of the evolutionary transition (Evert, 2006).

3. Ontogenesis and formation

The process of formation of mechanical tissues is a strictly regulated sequence of events, beginning with the division of precursor cells in meristems and ending with the formation of specialized supporting elements with specific physical-mechanical properties. The ontogenesis of collenchyma and sclerenchyma has both common features and fundamental differences, due to their different roles in the plant’s life (Evert, 2006; Serebryakova et al., 2006).

3.1. Origin from meristems

All mechanical tissues are derivatives of meristems – formative tissues that retain the ability to divide.

  • Collenchyma develops from the ground meristem – the same as the cortex and pith parenchyma. This occurs at early stages of organ development, in the zone immediately behind the apical meristematic cone. Precursor cells of collenchyma differ from neighboring parenchyma cells by somewhat denser cytoplasm and begin to unevenly thicken their primary walls even before organ elongation is complete (Evert, 2006; Yakovlev et al., 2008).

  • Sclerenchyma can have both primary and secondary origin:

  • Primary fibers (e.g., flax bast fibers) arise from procambium – the primary meristem that gives rise to vascular bundles. Their differentiation occurs directly from spindle-shaped procambial initial cells. In many plants, primary sclereids (stone cells) can also develop from the ground meristem, as well as from the protoderm (e.g., macrosclereids of legume seed coats) (Beck, 2010).

  • Secondary fibers (xylem and phloem fibers, as well as fibers in the periderm) form from cambium (vascular cambium) – a lateral meristem that ensures secondary thickening. The cambium deposits derivative cells both inward (towards the xylem) and outward (towards the phloem). Some of these derivatives, under the influence of hormonal signals, follow the path of differentiation into fibers (Chano et al., 2015).

  • Secondary sclereids often arise through secondary lignification of mature parenchyma cells (e.g., in the bark and pith of aging stems) – a process called sclereidification (Strasburger, 1971).

3.2. Cell differentiation and growth

After a precursor cell receives a signal to differentiate, its development proceeds along two main trajectories, which determine the future type of mechanical tissue.

Collenchyma differentiation

Collenchyma cells begin to thicken their primary walls during organ elongation (Evert, 2006). This process is closely coordinated with surrounding cells – so-called coordinated (symplastic) growth. Thickening occurs unevenly: additional layers of cellulose and pectin substances are deposited in those areas of the wall that experience the greatest mechanical stress. Importantly, collenchyma cells remain living and capable of division (although they divide rarely). Their walls contain a lot of water and pectins, providing them with plasticity – they can stretch following the elongation of the entire organ without breaking (Mauseth, 2016). The plasticity of collenchyma is a key property that allows young shoots and petioles to maintain strength while not hindering growth.

Hormonal regulation of collenchyma differentiation is less studied than that of sclerenchyma, but it is known that gibberellins (GA) stimulate cell elongation, while auxins (IAA) can influence the unevenness of wall thickening (Aloni, 1979, cited in Evert, 2006). Mechanical stimuli (wind, touch) noticeably accelerate and enhance collenchyma development – a phenomenon of thigmonasty (Evert, 2006).

Sclerenchyma fiber differentiation

Fiber development is a more complex and multi-stage process, including intrusive growth and programmed cell death (PCD) (Beck, 2010).

Stage 1: Initiation and polarization. A derivative cell of the cambium or procambium destined to become a fiber initially does not differ from neighboring parenchyma cells. Under the influence of an auxin gradient (high concentration near leaves, low near roots), genes responsible for the synthesis of cell wall proteins and secondary metabolism are activated (Evert, 2006; Beck, 2010). Cytokinins coming from the roots increase the sensitivity of cells to auxin and stimulate the differentiation of vascular and fiber elements (Su et al., 2023).

Stage 2: Intrusive growth. Fibers achieve their enormous length (in flax – up to 40–60 mm, in ramie – up to 350–550 mm) not only through coordinated growth with neighboring cells but also through apical intrusive growth (Evert, 2006; Su et al., 2023). The tips of the growing fiber intrude between cells, pushing and dissolving (with the help of enzymes) the middle lamellae. The nucleus and dense cytoplasm are located at the growing tip, while the vacuole is in the older part of the cell. Intrusive growth continues even after the organ has stopped elongating, so fibers can be significantly longer than the internodes in which they are located. In some species, fibers increase their length thousands of times compared to the initial meristematic cell (Mauseth, 2016).

Stage 3: Formation of the secondary cell wall. After growth (both coordinated and intrusive) is complete, the cell begins to synthesize the secondary wall. Microtubules align under the plasmalemma, determining the orientation of cellulose microfibrils. In fibers, microfibrils are oriented predominantly longitudinally (or at a small angle to the axis), which provides maximum tensile strength (Beck, 2010; Su et al., 2023). The secondary wall usually consists of three layers (S1, S2, S3), with the middle layer (S2) being the thickest. Then lignin (a polymer of phenylpropanoids) begins to be deposited in the wall, imparting rigidity, water resistance, and resistance to microbial degradation.

Stage 4: Programmed cell death. After the formation of the secondary wall is complete, the protoplast of the fiber generally dies (Evert, 2006; Beck, 2010). This is not accidental death, but a genetically programmed process (PCD), analogous to apoptosis in animals. The tonoplast (vacuolar membrane) ruptures, hydrolase enzymes exit the vacuole and degrade the cytoplasm and organelles. The nucleus also undergoes fragmentation and degradation. Ultimately, only a hollow, thick-walled tube remains of the cell, which performs a supporting function after death. In some fibers (e.g., in some bast fibers not used for weaving, or in septate fibers), the protoplast is retained, and the nucleus may divide, forming septa. Such living fibers, along with support, perform a storage function (Evert, 2006).

3.3. Formation of sclereids

Sclereids can develop in two ways (Serebryakova et al., 2006; Evert, 2006):

  1. Direct differentiation from meristematic or young parenchyma cells. In this case, the precursor cell stops dividing relatively early, increases in size (but not much), and then deposits a very thick, heavily lignified secondary wall with numerous branched pits. The protoplast dies. This is how, for example, macrosclereids of legume seed coats arise.

  2. Secondary sclereidification of mature parenchyma cells. In aging tissues (e.g., in the pith of the stem or in fruit flesh), ordinary parenchyma cells may suddenly begin to deposit a thick lignified secondary wall and turn into stone cells (brachysclereids). This process is often induced by stress or aging. A classic example is the formation of sclereids in pear flesh at the end of the growing season (Evert, 2006).

3.4. Hormonal regulation of sclerenchyma differentiation

Numerous experiments (mainly on tissue cultures and intact plants) have revealed the key role of phytohormones:

  • Auxin (IAA) is necessary for the initiation of fiber and vessel differentiation. It acts polarly (from leaves to roots). An increased concentration of auxin stimulates the differentiation of fibers with thicker walls (Evert, 2006; Beck, 2010).

  • Gibberellins (GA) promote fiber elongation (influencing intrusive growth) and, in high concentrations, lead to the formation of longer but thin-walled fibers (Evert, 2006). The combined action of auxin and gibberellin is necessary for normal fiber development.

  • Cytokinins (especially zeatin) enhance the action of auxin and stimulate the division of cambial cells, increasing the number of derivatives that can differentiate into fibers (Su et al., 2023).

  • Ethylene and jasmonates are often activated by mechanical damage and can induce the formation of traumatic (wound) fibers and resin ducts, as well as enhance lignification (Chano et al., 2015).

3.5. Genetic control

In recent years, regulatory genes for sclerenchyma formation have been identified. Key roles are played by transcription factors of the NAC family (e.g., NST1, NST3, VND6, VND7) and MYB (e.g., MYB46, MYB83). They activate cascades of genes responsible for the synthesis of cellulose, hemicelluloses, lignin, and for programmed cell death (Su et al., 2023; Beck, 2010). Mutations in these genes lead to impaired secondary wall formation in fibers and vessels. For example, in nst1 nst3 mutants of Arabidopsis, sclerenchyma fibers in the stem are completely absent (Mitsuda et al., 2007, cited in Su et al., 2023).

Thus, the ontogenesis of mechanical tissues is a finely regulated process in which the interaction of genetic programs and environmental signals (mechanical stimuli, hormones) ensures the formation of an effective support system adapted to the growing conditions of the particular plant.

4. Topographic anatomy: arrangement of mechanical tissues in the plant body

Stem cross-section diagram
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Public Domain

Diagram of a stem cross-section

The diagram labels the main tissues of the stem, including collenchyma and sclerenchyma, demonstrating their topography in the organ. 1 – pith, 2 – protoxylem, 3 – xylem II, 4 – phloem I, 5 – sclerenchyma fibers, 6 – cortex, 7 – epidermis.

The distribution of mechanical tissues in the plant is not random – it is strictly governed by biomechanical principles and reflects adaptation to the specific types of loads experienced by different organs. In stems and leaves, where the main danger is bending force, mechanical elements are brought to the periphery (according to the principle of a hollow beam or I-beam construction). In roots, which mainly work in tension (anchoring the plant in the soil), the mechanical armature, conversely, is concentrated in the center (Masrahi et al., 2023; Yakovlev et al., 2008). These patterns are observed both in the primary and secondary plant body.

4.1. Arrangement in the leaf

The leaf is a flat organ experiencing bending loads (under its own weight and wind pressure) and tension (in the wind). Therefore, the mechanical “armature” of the leaf is placed in areas of greatest stress:

  • Along the veins – sclerenchyma fibers (usually in strands) accompany the vascular bundles, especially the large ones. They form sclerenchyma sheaths that not only protect the vessels and sieve tubes but also give rigidity to the veins (Evert, 2006; Serebryakova et al., 2006). In many grasses and palms, a powerful sclerenchyma ring develops around each vascular bundle, and the bundles themselves are additionally connected by sclerenchyma strands to the epidermis, creating a ribbed structure (Fig. 2E in Masrahi et al., 2023).

  • Along the leaf blade margin – a continuous strand of sclerenchyma or collenchyma often runs here, preventing edge tearing. This is especially pronounced in grasses and rigid xerophytic leaves (Evert, 2006).

  • Under the epidermis – in many dicotyledons with fleshy or shade leaves, collenchyma is located under the upper and/or lower epidermis, imparting elasticity to the leaf without hindering its growth (Mauseth, 2016). In monocot leaves (e.g., grasses), a layer of sclerenchyma (hypodermis) is often located under the epidermis, creating a rigid external framework.

  • In the mesophyll (in some xerophytes and aquatic plants), astrosclereids or other sclereids may occur, which, besides support, sometimes participate in light conduction (e.g., in olive) or ventilation (Evert, 2006).

Specifics of grass leaf structure (using wheat, maize as examples) – one of the most perfect mechanical constructions in the plant world. The grass leaf has parallel venation; each vascular bundle is surrounded by a powerful sclerenchyma sheath, which on both sides (upper and lower) connects to subepidermal sclerenchyma strands (Masrahi et al., 2023; Evert, 2006). This creates a system of longitudinal ribs providing high rigidity with low mass. In some grasses, sclereids are also present in the leaf blade (e.g., in the seed coat and leaves).

4.2. Arrangement in the stem

Cross-section of three-year-old linden stem
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Sclerenchyma within the phloem of a three-year-old linden stem

The image illustrates the location of thick-walled sclerenchyma cells between sieve tubes and companion cells in the phloem.

The stem is an axial organ bearing leaves, flowers, and fruits. The main mechanical task is resistance to bending and compression. According to the laws of physics, maximum stress during bending occurs at the periphery, so mechanical tissues in the stem develop closer to the surface.

Primary structure (herbaceous stems)

  • Collenchyma is located directly under the epidermis, often as a continuous cylinder (in many dicotyledons – mint, sunflower) or as separate strands along the ridges (in quadrangular stems, e.g., in deadnettle, nettle). This arrangement allows collenchyma to effectively resist bending while remaining plastic and not hindering stem elongation (Serebryakova et al., 2006; Evert, 2006).

  • Sclerenchyma in primary stems occurs as:

  • Sheaths of vascular bundles – sclerenchyma fibers (most often primary bast fibers) are located on the outer side of the bundles (outside the phloem), and sometimes on the inner side (toward the pith). This creates a rigid support for each bundle individually (Beck, 2010).

  • Continuous sclerenchyma ring – in many dicotyledons (e.g., sunflower), fibers of adjacent bundles fuse, forming a continuous (or nearly continuous) cylinder between the cortex and pith (Evert, 2006).

  • Peripheral strands – in grasses (culm), external strength is provided by a powerful sclerenchyma ring located immediately under the epidermis and connected by strands to the sheaths of the vascular bundles. The inside of the grass stem is usually hollow (or filled with parenchyma), which reduces mass (Masrahi et al., 2023; Evert, 2006).

Examples in dicotyledons (according to Masrahi et al., 2023):

  • Type 1 (rounded stem, Heliotropium) – collenchyma arranged in three layers (angular, lamellar, lacunar) under the epidermis, and sclerenchyma in the form of incomplete “semi-gear” structures surrounds the vascular bundles.

  • Type 2 (rounded with projections, Trianthema) – angular and lacunar collenchyma, sclerenchyma in the form of vertical dividing layers inside the vascular bundles.

  • Type 3 (undulating, Suaeda) – two layers of collenchyma (lamellar and angular), and sclerenchyma forms a triple ring around the phloem and xylem.

  • Type 4 (ribbed, Aerva) – collenchyma thicker in the ribs than in the intercostal regions; sclerenchyma – strands parallel to the ribs.

  • Type 5 (quadrangular, Indigofera) – each face has layers of collenchyma (lacunar, angular, sclerenchyma, angular), providing maximum bending strength.

Secondary structure (woody stems and trunks)

In perennial plants with secondary thickening, the main mechanical function is performed by wood (secondary xylem), especially its thick-walled elements:

  • Libriform fibers (xylem fibers) – the main reinforcing component of angiosperm wood. They impart hardness and rigidity to the trunk, working in compression and bending (Beck, 2010).

  • Thick-walled tracheids – in gymnosperms (conifers), the supporting function, along with water conduction, is performed by tracheids, especially in latewood (latewood has thicker walls and narrower lumens) (Evert, 2006).

  • Medullary rays (ray parenchyma) – although not themselves a mechanical tissue, their numerous living cells impart additional elasticity to the wood and prevent cracking (Beck, 2010).

The peripheral bark (phloem) may also contain bast fibers, which are arranged in groups or bands (especially in linden, hemp, flax). In old trunks, the outer layers of the phloem die and turn into rhytidome, which also possesses some mechanical strength, but its main role is protective.

4.3. Arrangement in the root

The root experiences different mechanical loads: mainly tension (when the stem pulls the root upward) and compression (when the root grows in dense soil). Additionally, the root is surrounded by soil, which supports it from all sides, so the risk of bending is minimal. Based on these conditions:

  • Mechanical tissues in the root are located centrally – in the axial cylinder (stele). There, thick-walled tracheids and xylem vessels are found, and sometimes specialized sclerenchyma fibers (Evert, 2006; Mauseth, 2016).

  • Collenchyma is practically absent in roots. In very rare cases, it may develop in the cortex of aerial roots or in roots growing in the light (Van Fleet, 1950, cited in Evert, 2006).

  • In the primary root, mechanical strength is mainly provided by the central radial strand of xylem (protoxylem and metaxylem). Sclerenchyma may be absent or weakly expressed (Serebryakova et al., 2006).

  • During secondary thickening of the root (in perennial dicotyledons and gymnosperms), wood (secondary xylem) with thick-walled fibers forms, which performs the main supporting function. The roots of such trees become very strong and can have a large mass of wood.

Specifics of aerial roots (in epiphytes, e.g., orchids, Monstera). These roots are not immersed in soil and bear the mass of the epiphytic plant. They experience not only tension but also bending (under their own weight). Therefore, collenchyma or a sclerenchyma hypodermis (velamen) sometimes develops in their cortex, while in the center, powerful sclerenchyma strands are found alongside conducting elements (Evert, 2006).

4.4. Diagnostic significance of mechanical tissue topography

For the botanist-anatomist, the location, shape, and type of mechanical tissues are crucial diagnostic features for identifying the species, genus, and even family of plants (Masrahi et al., 2023; Evert, 2006). For example:

  • By the presence and nature of collenchyma (angular, lamellar, lacunar), closely related species of dicotyledons can be distinguished.

  • By the structure of the sclerenchyma sheaths of vascular bundles in the grass stem, genera and species are identified.

  • In wood, by the type of fiber arrangement (ring-porous or diffuse-porous wood) and the structure of the rays, the taxonomic identity of the tree is judged.

Thus, the topographic anatomy of mechanical tissues is not just a description of their placement, but a key to understanding the plant’s biomechanics and its adaptation to the environment.

5. Biomechanics and the operating principle of mechanical tissues

Plant mechanical tissues are not just passive “props,” but highly organized biocomposite materials whose properties (strength, elasticity, plasticity) are determined by a combination of cell wall architecture, chemical composition, and the physiological state of the cells (Evert, 2006; Beck, 2010). Understanding their biomechanics is important not only for botany but also for agronomy (breeding for lodging resistance) and materials science (creating biomimetic composites).

5.1. Biomechanical principles of placement: beam and lever theory

The plant, especially the stem and leaf, can be considered as a cantilever beam fixed at one end (root collar). When a force acts (wind, gravity of its own mass), the following occur in such a beam:

  • Tensile stresses on the convex side.

  • Compressive stresses on the concave side.

  • A neutral line in the center, where stresses are minimal.

Therefore, maximum strength with minimum mass is achieved if the reinforcing (mechanical) elements are placed at the periphery and the light parenchyma is in the center. This is the hollow beam principle (or thin-walled tube). This is exactly how mechanical tissues are placed in plant stems and leaves – collenchyma and sclerenchyma are located immediately under the epidermis or near it (Masrahi et al., 2023; Serebryakova et al., 2006). In the root, which is surrounded by soil and works mainly in tension, the mechanical elements, conversely, are concentrated in the center (central cylinder) (Evert, 2006).

5.2. Operating principle of collenchyma: hydraulic reinforcement and plasticity

Collenchyma is a unique mechanical tissue combining the properties of a solid body and a hydraulic system. Its operation is inextricably linked to turgor – the internal hydrostatic pressure in the cells (Mauseth, 2016; Evert, 2006).

  1. Wall plasticity. The unevenly thickened primary walls of collenchyma contain many pectins and hemicelluloses, giving them the ability for plastic deformation – they can stretch and not return to their original state after the load is removed. This allows collenchyma to follow organ growth (Evert, 2006). If the stretching of a collenchyma cell exceeds the limit of plasticity, the wall does not rupture (as in sclerenchyma), but remains in a stretched state.

  2. Role of turgor. Living collenchyma cells maintain high osmotic pressure in the vacuole (due to dissolved sugars and salts). Water enters the cell, creating turgor pressure on the wall. This turgor component gives collenchyma longitudinal stiffness (Mauseth, 2016). When water is lost (wilting), turgor drops, collenchyma becomes soft, and the organ droops. Thus, collenchyma is an “active” mechanical tissue whose operation depends on the plant’s water regime.

  3. Analogy with a tire and air pressure. Mauseth (2016) offers a figurative analogy: collenchyma works like an automobile tire. The tire itself (the collenchyma walls) has some stiffness, but it acquires full load-bearing capacity only after being inflated with air (turgor). Without air pressure, the tire (and the stem with collenchyma) collapses.

  4. Mechanical indicators. According to classical studies (Ambronn, 1881, cited in Evert, 2006), collenchyma strands withstand a tensile load of 10-12 kg/mm2 (comparable to some fibers), but their elongation at break is 2-2.5%. Moreover, after removing the load, collenchyma does not return to its original length (plastic deformation), whereas sclerenchyma fibers do return (elasticity). This is a fundamental difference.

5.3. Operating principle of sclerenchyma: elastic composite

Sclerenchyma (fibers and sclereids) is a dead (in most cases) tissue whose operation is based solely on the properties of the secondary lignified cell wall. It does not depend on turgor or water regime.

  1. “Reinforced concrete” composite structure. The sclerenchyma cell wall is a multilayered composite (Evert, 2006; Beck, 2010):

    • Reinforcement – cellulose microfibrils with high tensile strength. They are oriented predominantly longitudinally (in fibers) or at an angle, determining the anisotropy of mechanical properties.

    • Matrix – lignin, an amorphous polymer that binds the microfibrils, imparts compressive stiffness, and makes the wall hydrophobic (water-resistant). Lignin is the “concrete” filling the space between the “reinforcement” (microfibrils).

    • The result is a material whose strength is comparable to steel (tensile strength up to 15-20 kg/mm2 in some fibers), but which also possesses elasticity (Evert, 2006).

  2. Elasticity. Unlike plastic collenchyma, sclerenchyma fibers behave like an elastic body. When stretched, they elongate (within the limit of elastic deformation), and after the load is removed, they return to their original length. This property is critically important for woody plants: trunks and branches can bend under gusts of wind without breaking and then straighten (Beck, 2010). If wood were plastic, trees would remain permanently bent after every storm.

  3. Function of fibers and sclereids. Fibers, especially libriform, are specialized for resistance to tension and bending. Their great length and interweaving into bundles create “cables” that penetrate the plant. Sclereids (stone cells) are brittle elements designed to resist compression and squeezing. They form hard shells (nuts, stones, seed coats) protecting the contents (Evert, 2006; Serebryakova et al., 2006).

  4. Microfibrillar angle and anisotropy. The orientation of cellulose microfibrils in the wall (microfibrillar angle) determines the direction of maximum strength. In flax and hemp fibers, which experience strong tension, the microfibrils are oriented almost parallel to the cell axis. In gymnosperm tracheids, which work in both tension and compression, the angle is larger, giving them additional bending stiffness (Beck, 2010). The complex layered structure of the secondary wall (layers S1, S2, S3 with cross-oriented microfibrils) increases fracture toughness (Su et al., 2023).

5.4. Cooperative function of tissues: the reinforced parenchyma principle

In the living plant, mechanical tissues do not work in isolation. There is synergism between them and the parenchyma (Yakovlev et al., 2008; Serebryakova et al., 2006).

  • Parenchyma (especially the pith and cortex), consisting of living turgid cells with thin walls, acts as an elastic padding that redistributes the load between mechanical elements and prevents their local overstress.

  • When the stem bends, the parenchyma on the convex side stretches, and on the concave side compresses, transmitting forces to the collenchyma or fibers. Without this “packaging,” the fibers could shift or bend individually.

  • Collenchyma + turgid parenchyma = a system analogous to an inflated tire. Sclerenchyma + parenchyma = a reinforced concrete structure, where fibers are the reinforcement and parenchyma is the concrete, but unlike real concrete, the parenchyma cells are living and can store substances.

5.5. Adaptive changes in biomechanical properties

Mechanical tissues are capable of plastic changes in response to external conditions (Evert, 2006; Beck, 2010):

  • Mechanical stimulation (thigmonasty, wind training). Plants regularly subjected to wind load form thicker stems with an increased proportion of collenchyma and sclerenchyma, as well as thickened cell walls. This phenomenon underlies the “hardening” of seedlings.

  • Effect of nitrogen. Excess nitrogen fertilizers stimulate the growth of soft parenchyma tissues at the expense of mechanical ones, leading to lodging. Optimization of nitrogen nutrition and the use of retardants (e.g., chlormequat chloride) make it possible to manage the tissue ratio in favor of sclerenchyma (Yakovlev et al., 2008).

  • Reaction wood (traumatic). When a trunk or branch bends, tension wood with special gelatinous fibers (G-fibers) forms on the upper side in angiosperms, which contract and straighten the trunk. In gymnosperms, compression wood with increased lignin content and an abnormal microfibril angle forms on the lower side (Beck, 2010; Su et al., 2023). This is a striking example of active biomechanical adaptation.

5.6. Extreme biomechanical constructions in nature

  • Grass culm – a classic example of a hollow tube with a ribbed wall. The outer sclerenchyma ring is connected by radial strands to the sheaths of the vascular bundles, giving the culm exceptional rigidity with low weight (Masrahi et al., 2023).

  • Ramie fibers (Boehmeria nivea) reach lengths of up to 55 cm and strength comparable to steel. Their secret lies in the special multilayered wall structure and high content of crystalline cellulose (Evert, 2006).

  • Pear stone cells – an example of how rigid “islands” (sclereids) form from soft parenchyma by secondary sclereidification, giving the flesh its characteristic crunchiness and protecting the seeds from crushing (Serebryakova et al., 2006).

Conclusion on biomechanics

Thus, the biomechanics of mechanical tissues is not just the sum of the properties of individual cells, but the result of complex interactions at several levels: molecular (microfibril orientation, lignification), cellular (turgor, shape, wall thickening), tissue (arrangement, ratio with parenchyma), and organ (stem, leaf shape). Understanding these principles allows the agronomist to consciously manage plant resistance to lodging and mechanical damage.

6. Ecological plasticity and management by agro-factors

The development of mechanical tissues is not a rigidly determined trait – it is largely plastic and depends on a complex of environmental factors (light, temperature, humidity, wind) and mineral nutrition conditions. Understanding these dependencies allows the agronomist to purposefully manage the formation of the plant’s support system, increasing their resistance to lodging, drought, and mechanical damage (Yakovlev et al., 2008; Serebryakova et al., 2006).

6.1. Effect of nitrogen nutrition: the growth-strength trade-off

Nitrogen is a crucial element stimulating cell division and overall vegetative growth. However, excess nitrogen (especially in readily available nitrate form) causes an imbalance: plants produce large, succulent, but loose tissues with thin parenchyma walls and a reduced proportion of sclerenchyma and collenchyma (Evert, 2006; Yakovlev et al., 2008). Secondary wall cells (fibers) do not have time to become sufficiently lignified. As a result:

  • Stems become more brittle but at the same time – less stiff, losing elasticity.

  • The risk of lodging in cereals, legumes, and industrial crops increases sharply, especially during grain filling.

  • The strength of roots and their anchoring ability decrease.

Agronomic solutions:

  1. Balanced nitrogen nutrition – split application of nitrogen according to the developmental stage. In the second half of the growing season (before flowering and grain filling), nitrogen doses are reduced, while emphasis is placed on phosphorus-potassium fertilizers, which promote lignification and wall thickening (Serebryakova et al., 2006).

  2. Use of retardants (e.g., chlormequat chloride, tetcyclacis, modified triazoles). These substances inhibit gibberellin synthesis, which slows cell elongation, shortens internodes, and stimulates wall thickening, including through enhanced deposition of lignin and cellulose in sclerenchyma (Yakovlev et al., 2008; Evert, 2006). The result is a more stocky, strong stem resistant to lodging, while yield does not decrease, and grain or fruit quality may even improve.

6.2. Wind load (thigmonasty, “training”)

Regular mechanical impact (wind, touch, rain) induces a specific morphogenetic reaction in plants – thigmonasty (or stigmotropism) (Evert, 2006; Beck, 2010). Its mechanism involves increased synthesis of ethylene and altered auxin distribution. Anatomical changes include:

  • Thickening of cell walls (especially of collenchyma and sclerenchyma) due to increased content of cellulose and lignin.

  • Increase in the total amount of mechanical tissues (percentage of fibers and collenchyma in the stem cross-section).

  • Change in stem shape – it becomes more stocky, often ribbed, which increases the moment of inertia of the cross-section and resistance to bending.

  • In some species, increased branching occurs (due to suppression of apical dominance), which further distributes the mechanical load.

Practical significance:

  • Hardening of seedlings (vegetable, flower, forest crops) by periodic blowing with wind or mechanical shaking promotes the formation of strong, lodging-resistant stems.

  • In protective afforestation and park management, wind load naturally “trains” trees, making their trunks stronger and more elastic.

  • In agrocenoses, strong wind can act as a stressor, so windbreaks, shelterbelts, and the selection of resistant varieties are used to reduce mechanical damage.

6.3. Silicon fertilizers

Silicon (Si) is not an essential macronutrient for all plants, but for many species (especially cereals, horsetails, some dicotyledons) it plays an important structural and protective role. Silicon, in the form of monosilicic acid (H4SiO4), is absorbed by roots and transported to shoots, where it polymerizes into amorphous silica (SiO2·nH2O) in cell walls, including those of sclerenchyma fibers, the epidermis, and vascular bundle sheaths (Evert, 2006; Su et al., 2023).

Effects of silicon nutrition:

  1. Strengthening of mechanical tissues. Deposition of silica in the walls of sclerenchyma and collenchyma significantly increases their hardness and resistance to bending and compression (Evert, 2006). In cereals, this is especially important for the culm and leaf blade.

  2. Increased lodging resistance (confirmed for rice, wheat, oats). Stems become stiffer and more elastic.

  3. Protection against pests and diseases. Silica deposits create a physical barrier against fungal penetration and against feeding by phytophagous insects (e.g., aphids and leafrollers).

  4. Reduced transpiration and improved water regime. Silicon strengthens the epidermis and promotes more efficient stomatal function, which is especially important under dry conditions.

Agronomic practices: The application of silicon-containing fertilizers (diatomite, rice husks, silicates) in rice cultivation, as well as for cereals, sugarcane, and some vegetable crops, is becoming an increasingly common practice to increase stem strength and overall stress tolerance (Yakovlev et al., 2008).

6.4. Other factors

  • Light. Lack of light (shading) leads to etiolation: stems elongate, but mechanical tissues develop poorly, with collenchyma and sclerenchyma represented by few layers. Increased light (sunny locations), conversely, stimulates wall thickening and lignin accumulation (Evert, 2006). This is associated with the activation of photoreceptors (phytochrome, cryptochrome), which regulate the expression of lignin and cellulose synthesis genes.

  • Temperature. Low positive temperatures (cold stress) often slow growth and can reduce the degree of lignification, making tissues more vulnerable. High temperatures (within the optimal range) usually accelerate growth, but under extreme heat, lignification can be disrupted (Chano et al., 2015).

  • Water regime. Water deficit (drought) causes accumulation of abscisic acid (ABA), which stimulates lignin synthesis and wall thickening, increasing the proportion of sclerenchyma (this is part of the adaptation to drought). However, with severe dehydration, turgor is lost, and collenchyma ceases to function (Mauseth, 2016). Excessive moisture (waterlogging) can cause aerenchymatous loosening of tissues and reduced mechanical strength.

  • Injury (wound stress). When the bark or cambium is damaged (e.g., by girdling or burns), in response to ethylene and jasmonate, traumatic sclerenchyma and traumatic resin ducts (in conifers) form. This increases local strength and protects against pathogens (Chano et al., 2015).

6.5. Relationship with the production process and breeding

Managing the development of mechanical tissues is always a compromise between strength and productivity. Too strong development of sclerenchyma requires expenditure of assimilates for the synthesis of lignin and cellulose, reducing the proportion of storage tissues. However, an optimal ratio allows obtaining lodging-resistant varieties without loss of yield.

Breeding approaches:

  • Selection for increased content of lignin and cellulose in the stem (but not in the leaves of forage crops, so as not to reduce digestibility).

  • Creation of varieties with optimal architecture (short, thick stems, powerful vascular bundle sheaths).

  • Use of donor genes controlling lignin synthesis (e.g., genes of the CAD, CCoAOMT family) to increase strength.

  • In forage crops – breeding for reduced lignin and sclerenchyma content in leaves (brown midrib, bmr mutants) to improve digestibility (Su et al., 2023).

Thus, the ecological plasticity of mechanical tissues opens up wide opportunities for agronomic practice: from optimizing mineral nutrition and using retardants to employing silicon fertilizers and selecting resistant genotypes.

7. Practical management in agrosystems

Knowledge of the patterns of mechanical tissue development allows the agronomist and breeder to purposefully influence plant resistance to lodging, fiber quality, and the feed value of plant mass. Below are key techniques for managing the support system in modern crop production.

7.1. Management of lodging resistance in cereal crops

Lodging is one of the main causes of yield loss in cereals (wheat, barley, oats, rice). Biomechanically, the resistance of the culm is determined by three factors: (1) strength of the sclerenchyma ring, (2) degree of cell wall lignification, (3) ratio of stem height to its diameter (Evert, 2006; Yakovlev et al., 2008).

Practical practices:

  1. Use of retardants (antigibberellins). The most studied are chlormequat chloride (CCC, chlormequat chloride), modified triazoles (tebuconazole, paclobutrazol), and trinexapac-ethyl. Mechanism of action:

    • Inhibition of gibberellin synthesis (especially GA1) at early stages (blocking ent-kaurene synthase or ent-kaurene oxidase).

    • Shortening of internodes – the stem becomes more stocky.

    • Thickening of cell walls in sclerenchyma and collenchyma, increased lignification (Yakovlev et al., 2008).

    • Increased proportion of sclerenchyma fibers in the stem cross-section (Evert, 2006).

    • Increase in culm diameter and, consequently, the moment of inertia of the cross-section.

    • Result: reduction of lodging by 30-60% without a drop in grain yield, and often with an increase due to better grain filling.

  2. Optimization of nitrogen nutrition. As mentioned in section 6, excess nitrogen reduces the proportion of mechanical tissues. Therefore, in intensive technologies, nitrogen is applied in split doses, shifting the main share to early stages (tillering – stem elongation), and by the flag leaf and heading stage, doses are reduced while simultaneously increasing phosphorus-potassium top dressing (Yakovlev et al., 2008). Phosphorus promotes vigorous root development and ATP synthesis, necessary for the formation of secondary walls; potassium activates enzymes of cellulose and lignin synthesis.

  3. Use of silicon fertilizers (diatomite, rice husks, silicates). For cereals, silicon is a quasi-essential element. Deposition of silica (SiO2) in the cell walls of sclerenchyma and the epidermis sharply increases culm stiffness (Su et al., 2023; Evert, 2006). In rice paddies, silicon application is a standard practice to prevent lodging and increase yield by 10-20%.

  4. Breeding for “dwarfism” and “thick stem”. The Green Revolution of the 1960s was based on the introduction of dwarfing genes (e.g., Rht-B1b, Rht-D1b in wheat; sd1 in rice). These genes reduce sensitivity to gibberellins, leading to shortened internodes, but often also reduce stem diameter. Modern breeding aims to combine short stems with thickened culm and increased lignin content. QTLs controlling the thickness of the sclerenchyma ring and the degree of lignification are being sought (Yakovlev et al., 2008).

  5. Application of traumatic stress (mechanical impact) during the seedling period. For vegetable and flower crops (tomatoes, peppers, asters), periodic shaking or light mechanical irritation of the stems (e.g., by hand brushing, fan blowing) is beneficial. This stimulates the production of ethylene and jasmonates, leading to stem thickening and increased proportions of collenchyma and sclerenchyma (thigmonasty) (Evert, 2006; Beck, 2010). Seedlings become stocky, less elongated, and better withstand transplanting and lodging in the open field.

7.2. Management of feed value (proportion of sclerenchyma in the leaf)

For ruminants, the main limitation on the palatability and digestibility of grasses (cereals and legumes) is the content of lignin and hard-to-digest sclerenchyma fibers (Su et al., 2023; Evert, 2006). Lignin blocks the access of cellulolytic enzymes of the rumen microflora to cellulose. Additionally, mechanical tissues (especially sclereids and thick-walled fibers in leaves) physically complicate chewing.

Ways to improve feed quality:

  1. Breeding for brown midrib (bmr). In maize, sorghum, and Sudan grass, mutants with reduced lignin content in leaves and stems (by 15-40%) are known. The cause is mutations in lignin biosynthesis genes (e.g., bmr2 – defect in COMT – caffeic acid O-methyltransferase; bmr3 – defect in CAD – cinnamyl alcohol dehydrogenase). Such hybrids have higher dry matter digestibility (+10-20%) and better palatability, but are often inferior in yield and lodging resistance (Su et al., 2023; Yakovlev et al., 2008). Breeders are working to create balanced lines.

  2. Optimization of harvest time. In early stages (stem elongation – heading), the proportion of sclerenchyma and lignin in leaves is minimal, and digestibility is maximal. Delaying harvest (full heading – milk ripeness) causes plants to direct assimilates to form supporting tissues in the stem and leaf sheaths, and feed quality sharply declines (Evert, 2006).

  3. Use of retardants on forage grasses. Treatment with retardants (e.g., modified triazoles) not only reduces lodging but can also affect the lignin-cellulose ratio. However, this practice requires caution, as excessive stimulation of lignification will reduce digestibility (Yakovlev et al., 2008).

  4. Selection for reduced number of sclereids in the leaf. In legume crops (alfalfa, clover), varieties with thinner leaf blades and fewer stone cells in the mesophyll are desirable. Modern phenotyping methods (spectroscopy, NIRS) allow rapid assessment of lignin content and the proportion of indigestible fibers (Su et al., 2023).

7.3. Management of fiber quality in industrial crops

Flax, hemp, kenaf, ramie, jute – sources of natural cellulosic fibers (bast). Fiber quality is determined by its length, tensile strength, elasticity, and yield of clean fiber after retting (dew retting) (Evert, 2006).

Agronomic practices for improving fiber:

  1. Plant density. Dense stands of fiber flax stimulate more active intrusive growth of primary bast fibers (cells elongate, competing for light). As a result, fibers become longer, thinner, and finer. However, too dense sowing reduces stem diameter and the total fiber mass per unit area (Evert, 2006).

  2. Nitrogen and phosphorus nutrition. High doses of nitrogen reduce fiber strength (cell walls become thinner). The optimal N:P:K ratio for flax is about 1:2:2 (in contrast to cereals). Phosphorus promotes the formation of dense, well-lignified walls, which increases strength (Yakovlev et al., 2008). But lignification of bast fibers must be moderate, otherwise the fiber becomes coarse and poorly separates from the wood (shive).

  3. Growth regulators. Retardants are not used on flax (since high stem length is needed), but gibberellins (GA3) are used to stimulate stem elongation, which indirectly increases fiber length (Evert, 2006). However, excessive application of GA3 leads to lodging, so doses are carefully selected.

  4. Harvest and retting timing. Flax is harvested at the early yellow maturity stage, when the fiber is fully formed but the wood is not yet too lignified. Retting (dew retting) in the field is an enzymatic process breaking down the pectins that glue the fibers into bundles. The quality of retting depends on the activity of microorganisms and weather conditions; modern technologies use chemical retting (e.g., treatment with enzyme solutions) to accelerate the process and improve fiber uniformity (Evert, 2006).

7.4. Shaping the yield framework of fruit and berry crops

In fruit trees and shrubs, the mechanical tissues of the skeletal branches (perennial wood) determine the tree’s ability to bear fruit weight without breaking. Management of this framework is carried out through pruning and canopy shaping.

  • Proper pruning ensures an optimal ratio of the central leader to the skeletal branches, as well as an even distribution of mechanical stresses (Beck, 2010).

  • Wind load “trains” the wood, stimulating the deposition of latewood with thick-walled fibers. Therefore, a too dense, closed canopy is less ventilated, and branches become less strong (Evert, 2006).

  • For the grapevine, where the perennial wood of the arms must be both flexible and strong, balanced crop load and irrigation are important. Excess nitrogen and water lead to loose, weak wood (Evert, 2006).

7.5. Biotechnological approaches and genetic engineering

Modern science offers tools for direct gene editing to improve mechanical properties:

  • Editing promoters of NST1, NST3, and MYB46 genes to enhance the expression of these transcription factors and, consequently, secondary wall synthesis (Su et al., 2023; Ma et al., 2022). This allows increasing the content of cellulose and lignin in fibers without altering plant architecture.

  • Knockout or suppression of lignin pathway genes (e.g., 4CL, CCoAOMT, CAD) to create “soft” varieties with low lignin content suitable for forage production (Su et al., 2023). However, such varieties often become more susceptible to lodging and diseases, so a balanced approach is required.

  • Introduction of silicon transporter biosynthesis genes (e.g., Lsi1, Lsi2 in rice) into other crops to increase their ability to accumulate silicon in cell walls, thereby strengthening mechanical tissues (Evert, 2006; Su et al., 2023).

References

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  2. Bidlack, J.E. and Jansky, S.H. (2021) Stern’s Introductory Plant Biology, 15th edn, McGraw-Hill Education, New York, Chapter 4 ‘Tissues’, pp. 51–60.

  3. Chano, V., Lopez, R., Pita, P., Collada, C. and Soto, A. (2015) ‘Proliferation of axial parenchymatic xylem cells is a key step in wound closure of girdled stems in Pinus canariensis’, BMC Plant Biology, 15, article 64. DOI: 10.1186/s12870-015-0447-z PubMed

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  5. Evert, R.F. (2006) ‘Esau’s Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body – Their Structure, Function, and Development’, 3rd edn, John Wiley & Sons, Hoboken, Chapter 7 ‘Parenchyma and Collenchyma’, pp. 1–19; Chapter 8 ‘Sclerenchyma’, pp. 1–22.

  6. Evert, R.F. and Eichhorn, S.E. (2013) ‘Raven Biology of Plants’, 8th edn, W.H. Freeman and Company, New York, Chapter 23 ‘Cells and Tissues of the Plant Body’, pp. 544–569.

  7. Ma, Y., MacMillan, C.P., de Vries, L., Mansfield, S.D., Hao, P., Ratcliffe, J., Bacic, A. and Johnson, K.L. (2022) ‘FLA11 and FLA12 glycoproteins fine-tune stem secondary wall properties in response to mechanical stresses’, New Phytologist, 233(4), pp. 1750–1767. DOI: 10.1111/nph.17898 PubMed

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  10. Serebryakova, T.I., Voronin, N.S., Elenevsky, A.G., Batygina, T.B., Shorina, N.I. and Savinykh, N.P. (2006) ‘Botany with fundamentals of phytocenology: Anatomy and morphology of plants’, IKC «Akademkniga», Moscow, Chapter 2 ‘Tissues’, pp. 100–137.

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  12. Su, Q., Chen, L., Dai, C., Fei, B., Chen, X., Luo, X., Fang, C., Ma, X., Zhang, X. and Liu, H. (2023) ‘Structure and mechanisms of foam-like bamboo parenchyma tissue’, Industrial Crops and Products, 203, 117046. DOI: 10.1016/j.jmrt.2023.10.028

  13. Yakovlev, G.P., Chelombitko, V.A. and Dorofeev, V.I. (2008) ‘Botany’, textbook, 2nd edn, Publishing house of the St. Petersburg State Chemical-Pharmaceutical Academy, St. Petersburg, Chapter 2 ‘Tissues’, pp. 87–107.

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