Ground (Parenchyma) Tissues

Last updated: June 01, 2026EspañolРусский

Parenchyma (from Greek para — “beside” and enchein — “to pour in”), or ground tissue (ground tissue), is the most widespread, functionally diverse, and evolutionarily oldest type of tissues in vascular plants. In the Anglo-American tradition, the term “parenchyma” more often refers to a morphological cell type (thin-walled, living), whereas the concept ground tissue describes a topographical position — everything that lies between the dermal and vascular tissues (Evert, 2006; Mauseth, 2016). In the Russian- and German-language traditions, these concepts are often merged, referring to ground (parenchyma) tissues, which indeed occupy most of the volume of young roots, stems, leaves, fruits and seeds (Serebryakova et al., 2006).

Key differences from other tissues:

Cell wall type — only primary, thin (typically 0.1–0.5 µm), non‑lignified, retaining elasticity and ability to stretch. Unlike collenchyma, parenchyma has no uneven thickenings; unlike sclerenchyma, it never has a massive secondary lignified wall (Evert, 2006; Raven et al., 2013).
Protoplast state — parenchyma cells remain alive at maturity, until the whole organ dies. They contain a full protoplast with nucleus, vacuoles, plastids and mitochondria. This distinguishes them from most conducting (tracheids, vessels) and mechanical (fibers, sclereids) elements, which often lose their protoplast by the time they become functional (Evert, 2006).
Functional plasticity — parenchyma cells retain the ability to dedifferentiate: upon wounding or in tissue culture, they can resume division and give rise to any other cell type (totipotency). This property is absent in most other tissues (Mauseth, 2016; Stern & Jansky, 2020).
Main localization — parenchyma forms the pulp (mesophyll) of leaves, the pith and primary cortex of stems and roots, the endosperm and cotyledons of seeds, the flesh of fruits (Raven et al., 2013).

Evolutionary origin:

Parenchyma is considered the phylogenetic precursor of all other tissues of vascular plants. The first land plants (rhyniophytes, ~420 Mya) consisted exclusively of parenchyma‑like cells with thin walls (Evert, 2006). The transition to life on land required the appearance of dermal (epidermis), vascular (xylem, phloem) and mechanical (collenchyma, sclerenchyma) tissues. However, parenchyma remained the universal “building material” in which these specialized tissues are “embedded” (Morris et al., 2016; Raven et al., 2013).

Precursors of parenchyma cells — large thin‑walled parenchyma‑like cells in charophyte algae (Charophyta) that did not yet have true tissues but already performed storage and assimilation functions (Mauseth, 2016).

Analogues in other organisms:

Animals lack a direct histological analogue of parenchyma because their cells have no cell wall. However, in terms of functional role (filling space between organs, storing reserves, regeneration), plant parenchyma can be compared to animal connective tissue (loose and adipose). The similarity is strengthened by the fact that both tissue types participate in metabolism, energy storage and wound healing (Evert, 2006).

In flatworms (Plathelminthes) there is an evolutionarily convergent structure — parenchyma as a loose mass of cells between internal organs, performing supportive, storage and excretory functions. However, this structure is not homologous to plant parenchyma, merely demonstrating a similar solution to “filling the volume” with living, unspecialized cells (by analogy with Evert, 2006).

In mosses (Bryophyta) the mesophyll of “leaves” and “stems” is composed of parenchyma cells that are already partially differentiated but retain the ability to photosynthesize and store reserves (Mauseth, 2016).

Thus, parenchyma is an evolutionarily original, morphologically simple but functionally irreplaceable tissue that provides the metabolic activity, resource storage and regenerative capacity of the whole plant body.

1. Physiological significance and functions

Parenchyma tissues provide most of the vegetative processes in the plant (those not directly related to reproduction). Their functions are so diverse that no other tissue can fully replace them. Listed below are the main functions in order of their importance for understanding the plant’s overall operation and for applied agronomy.

1.1. Storage function

Cross sections of Brachypodium stem, iodine staining — Localization of starch in the storage parenchyma of a grass stem (<span lang="la" class="biological-name">Brachypodium distachyon</span>).
Cross sections at different stem levels (S-01 – S-04) stained with Lugol’s solution (iodine). Dark staining (arrows) indicates starch accumulation in the pith parenchyma, in the cortex, and around the vascular bundles. The largest amount of starch accumulates in the pith parenchyma – the main storage compartment of grass stems. Jensen & Wilkerson (2017), figure 3B–F. <a class="common-share-detail" rel='nofollow' href='https://creativecommons.org/licenses/by/4.0/' aria-label='Common Share CC BY 4.0' target='_blank'>CC BY 4.0</a>.

This is the key function for human economic activity. In parenchyma cells, storage nutrients are deposited: starch, proteins (as aleurone grains), oils (lipids), sugars, and water (Evert, 2006; Mauseth, 2016; Stern & Jansky, 2020).

Starch accumulates in amyloplasts (a special type of leucoplast) in the endosperm cells of cereals (wheat, rice, maize), potato tubers, root crops (beet, carrot), and legume seeds. It is this storage parenchyma that gives us flour, cereals, and potatoes (Morris et al., 2016; Serebryakova et al., 2006).
Proteins (prolamins, globulins) are stored in aleurone grains or protein bodies, often in the same cells as starch, but are especially abundant in the aleurone layer of cereal endosperm and in legume cotyledons (Evert, 2006).
Oils (lipids) are the most energy‑dense form of storage. They accumulate as spherosomes (oleosomes) in oilseed crop seeds (sunflower, rapeseed, flax, soybean) (Jensen & Wilkerson, 2017; Evert, 2006).
Water accumulates in the large vacuoles of parenchyma cells of succulents (cacti, aloe, agave), as well as in the water‑storage parenchyma of many epiphytes (Evert, 2006).

The volume and density of storage parenchyma directly affect yield: for example, the greater the proportion of parenchyma in the caryopsis endosperm, the higher the 1000‑grain weight and the flour yield (Jensen & Wilkerson, 2017).

1.2. Assimilation (photosynthetic) function

Parenchyma cells that contain chloroplasts are called chlorenchyma. They are the main site of photosynthesis in the plant (Evert, 2006).

In leaves, chlorenchyma forms the mesophyll, which is divided into palisade (columnar) and spongy layers. The palisade chlorenchyma with densely packed cells bearing many chloroplasts captures light; the spongy chlorenchyma with loosely arranged cells and extensive intercellular spaces ensures gas exchange (Raven et al., 2013; Stern & Jansky, 2020).
In young green stems (e.g., in grasses, horsetails, some cacti), chlorenchyma is also located under the epidermis, allowing the stem to perform photosynthesis (Mauseth, 2016).
Chloroplasts in chlorenchyma cells can move: under low light they are positioned at the cell periphery, under bright light they turn edge‑on or gather near the walls to avoid damage (Evert, 2006).

1.3. Transport (conducting) function

Parenchyma participates in short‑distance and long‑distance transport, complementing specialized conducting tissues.

Ray parenchyma (medullary rays) in wood and bast performs horizontal (radial) transport of water, mineral salts and organic assimilates between phloem and xylem, and also between the stem center and the cortex (Evert, 2006; Słupianek et al., 2021).
Vessel‑associated cells (VACs) in xylem, adjacent to vessels, participate in ion loading and unloading, and also play a role in restoring water‑conducting function after embolism (Słupianek et al., 2021; Morris et al., 2018).
Transfer cells — specialized parenchyma with plasma membrane ingrowths that increase the surface area for transport of solutes. They occur in places of intense exchange, e.g., in minor leaf veins, in the stalks of developing seeds (Evert, 2006).

1.4. Respiratory and gas‑exchange function

Parenchyma participates in gas exchange thanks to the presence of intercellular spaces (of schizogenous or lysigenous origin). This is especially pronounced in aerenchyma — a tissue with large air cavities, characteristic of aquatic and wetland plants (Evert, 2006; Stern & Jansky, 2020).

Aerenchyma supplies oxygen to roots under flooded conditions (rice paddies, mangroves). This is achieved by developing a system of interconnected air channels (Evert, 2006; Słupianek et al., 2021).
In the spongy chlorenchyma of leaves, intercellular spaces enable rapid diffusion of CO₂ to the chloroplasts and O₂ into the atmosphere (Raven et al., 2013).

1.5. Regenerative and wound‑healing functions

Parenchyma retains potential meristematic activity. When the plant is damaged, living parenchyma cells in the wound area dedifferentiate, start dividing and form callus — a wound tissue. From the callus, new vessels, roots or buds then develop (Mauseth, 2016; Evert, 2006).

This property underlies vegetative propagation by cuttings (e.g., in roses, grapevines), grafting, and clonal micropropagation in vitro (Stern & Jansky, 2020).
The totipotency of parenchyma cells (the ability to give rise to a whole plant) is used in biotechnology to produce virus‑free planting material (Evert, 2006).

1.6. Secretory and excretory function

In some parenchyma cells, called idioblasts, secondary metabolites are synthesized and accumulated — essential oils, resins, alkaloids, tannins, as well as calcium oxalate crystals (Evert, 2006; Stern & Jansky, 2020).

Such cells can be scattered among the ground tissue (e.g., essential oil cells in leaves of laurel, mint).
Tannins in the parenchyma of oak, willow, and bergenia bark protect the plant from herbivores and pathogens (Serebryakova et al., 2006).

Thus, parenchyma acts as the plant’s universal “workhorse”: it stores and synthesizes organic substances, ensures gas exchange and transport, responds to damage, and synthesizes protective compounds. All these functions together make parenchyma the central tissue on which the plant’s productivity and survival in changing environments are based.

2. Classification of parenchyma tissues

Parenchyma is classified according to several independent criteria: by cell shape, by function, and by topographical position in the plant. Such a multidimensional division reflects both the structural diversity and the functional specialization of this tissue.

2.1. Classification by cell shape

The shape of parenchyma cells varies greatly depending on mechanical loads, the nature of gas exchange, and the origin of the tissue (Evert, 2006).

Isodiametric (parenchymatous) shape — cells approximately equal in all dimensions (polyhedra, most often 14‑hedra). This is the classic form of parenchyma, occurring in the cortex, pith, and leaf mesophyll (except elongated cells). Such cells are usually tightly packed; intercellular spaces are either absent or small (Mauseth, 2016).
Prosenchymatous shape — cells elongated in length (length 5–10 times or more than width). Found in vascular bundles (ray parenchyma, xylem and phloem parenchyma), as well as in some types of chlorenchyma. Importantly, prosenchymatous parenchyma cells differ from fibers in that they retain living cytoplasm and thin cellulosic walls (Evert, 2006).
Stellate (astrosclereid‑like) shape — cells have numerous branching processes. This shape is often taken by cells of aerenchyma (e.g., in stems of rush Juncus effusus) which, with their processes, create a three‑dimensional network that reinforces the air spaces (Evert, 2006; Stern & Jansky, 2020).

2.2. Classification by function (main functional types)

This is the most commonly used classification in educational literature. It directly links the structure of parenchyma to its role in the plant’s life.

Functional type	Other name	Main characteristics	Localization	Physiological role
Assimilation	Chlorenchyma	Cells contain many chloroplasts; intercellular spaces well developed.	Leaf mesophyll, green young stems, some fruits (unripe tomatoes, peppers).	Photosynthesis, oxygen release, production of primary sugars.
Storage	—	Large cells with thin walls, vacuoles filled with starch (amyloplasts), oils (spherosomes) or proteins (aleurone grains).	Endosperm and cotyledons of seeds, tubers, root crops, stem pith, thickened roots (sweet potato, dahlia).	Deposition of nutrients during dormancy or for later use (germination, regrowth).
Water‑storage	Hydroparenchyma	Cells large, colorless, with very thin walls and huge central vacuoles containing mucilaginous substances that retain water.	Leaves and stems of succulents (cacti, aloe, agave, stonecrops), as well as some epiphytes (bromeliads, orchids).	Water storage for drought periods, maintenance of turgor.
Aerenchyma	Air‑storage parenchyma	Cells form a three‑dimensional network surrounding large air spaces (lacunae).	Submerged and floating organs of aquatic and wetland plants (water lily, pondweed, cattail, rice).	Oxygen supply to underground and submerged parts, provision of buoyancy, tissue ventilation.
Transfer	Transfer cells	Cells with plasma membrane ingrowths (labyrinth) that increase membrane area.	Minor leaf veins, nectaries, chalaza and endosperm tissues at the contact zone with the maternal plant.	Intensive short‑distance transport of solutes (phloem loading/unloading, apoplastic uptake).
Secretory (excretory)	Idioblasts	Individual parenchyma cells or groups that accumulate secondary metabolites (essential oils, resins, alkaloids, tannins) or form calcium oxalate crystals.	Scattered in various tissues of the cortex, pith, mesophyll, sometimes in bast parenchyma strands.	Chemical defense against herbivores and pathogens; binding of excess calcium and oxalic acid.

This classification is based on the works of Evert (2006), Mauseth (2016), as well as detailed reviews by Morris et al. (2016) and Słupianek et al. (2021).

2.3. Topographical classification (by position in the organ)

Cross sections of wood of different species with highlighted parenchyma types — Types of parenchyma in secondary xylem of gymnosperms and angiosperms.
Cross sections: (A) <span lang="la" class="biological-name">Pinus</span> sp., (B) <span lang="la" class="biological-name">Picea</span> sp., (C) <span lang="la" class="biological-name">Abies</span> sp. (conifers); (D) <span lang="la" class="biological-name">Aesculus</span> sp., (E) <span lang="la" class="biological-name">Populus</span> sp., (F) <span lang="la" class="biological-name">Fraxinus</span> sp., (G) <span lang="la" class="biological-name">Acer</span> sp., (H) <span lang="la" class="biological-name">Quercus</span> sp. (broadleaves). Colors highlight: green – ray parenchyma; red – paratracheal; blue – apotracheal; yellow – terminal. In conifers, ray parenchyma predominates; in broadleaves, abundant axial parenchyma. Słupianek et al. (2021), figure 2. <a class="common-share-detail" rel='nofollow' href='https://creativecommons.org/licenses/by/4.0/' aria-label='Common Share CC BY 4.0' target='_blank'>CC BY 4.0</a>.

In addition to functional types, parenchyma is often subdivided according to its location in the plant body. This approach is convenient for anatomical description of sections.

Primary cortex parenchyma (cortical parenchyma) — located between the epidermis and the central cylinder of the stem or root. Often contains chlorenchyma (in stems) or storage cells (in roots). In roots, the inner layer of the cortex differentiates into the endodermis (Evert, 2006).
Pith parenchyma (pith parenchyma) — occupies the central part of the stem. In grasses and many herbs, the pith may break down, forming a cavity (hollow stem). In plants with secondary growth, the pith parenchyma is gradually compressed and replaced by wood (Raven et al., 2013).
Ray parenchyma (ray parenchyma) — radial rays (medullary rays) passing through secondary xylem and phloem. Provides horizontal transport and storage of substances. In hardwood, rays can be uniseriate or multiseriate (wide rays of oak, beech). In conifers, ray parenchyma is often the only type of living cells in wood (Evert, 2006; Morris et al., 2016).
Xylem parenchyma and phloem parenchyma (xylem parenchyma, phloem parenchyma) — vertical strands of living cells accompanying vessels and sieve tubes. They participate in storage, regulation of the ionic composition of xylem sap, and in response to damage (formation of tyloses and callose) (Słupianek et al., 2021; Morris et al., 2018).

2.4. Additional remarks on classification

Transition between types. The same parenchyma cell may combine several functions. For example, palisade chlorenchyma cells at the end of the growing season may accumulate starch, turning into storage cells (Evert, 2006). Water‑storage parenchyma cells in succulents, when light is deficient, develop chloroplasts and become photosynthetic (Jensen & Wilkerson, 2017).
Relation to age. Young, growing organs are dominated by chlorenchyma and storage parenchyma. In old, woody parts of secondary phloem and xylem, parenchyma is preserved in the form of rays and vertical strands, but its volume fraction can vary greatly (in tropical trees up to 30–40%, in conifers only 5–10%) (Morris et al., 2016).
Ecological variability. In plants from arid habitats, water‑storage parenchyma is more developed; in shade plants, assimilatory parenchyma; in aquatic plants, aerenchyma (Evert, 2006).

Thus, the classification of parenchyma reflects its evolutionary plasticity: the same tissue can take different forms and perform different functions depending on its position in the plant and environmental conditions. This property makes parenchyma a key element of plant adaptation to diverse ecological niches.

3. Formation and development (Ontogeny)

Parenchyma tissues, like all permanent tissues of the plant, arise from meristems — generative tissues that retain the ability to divide. However, unlike highly specialized tissues (e.g., xylem or phloem), parenchyma retains features of meristematic activity throughout the life of the cell, allowing it to perform regenerative and storage functions even at maturity (Evert, 2006; Mauseth, 2016).

3.1. Source of origin: primary and secondary parenchyma tissues

Depending on the type of meristem from which the parenchyma originated, a distinction is made between primary and secondary parenchyma.

Type of parenchyma	Meristem source	Time of origin	Localization in the plant
Primary parenchyma	Apical meristems (shoot and root apex) → ground meristem and partly procambium.	Early stages of organ development, in the zone of primary tissue differentiation.	Cortex and pith of the primary stem, primary root cortex, leaf mesophyll, endosperm, cotyledons, fruit flesh.
Secondary parenchyma	Lateral meristems: - Vascular cambium (produces secondary xylem and phloem parenchyma, ray parenchyma) - Phellogen (cork cambium) (produces phelloderm – secondary cortex parenchyma)	Begins after primary growth is completed, often after several years.	Secondary phloem (phloem parenchyma, ray parenchyma), secondary xylem (xylem parenchyma, medullary rays), periderm (phelloderm).

Data in the table are summarized from Evert (2006), Raven et al. (2013), Serebryakova et al. (2006).

Important nuance: Secondary parenchyma produced by the cambium can be either axial (vertical strands of living cells in wood and bast) or radial (medullary rays). Radial parenchyma is especially important for horizontal transport and storage in perennial stems and roots (Morris et al., 2016; Słupianek et al., 2021).

3.2. Cytological changes during parenchyma cell differentiation

Differentiation of a parenchyma cell from a meristematic initial is a gradual process that includes the following key events (Evert, 2006; Mauseth, 2016):

Cessation of division (or sharp slowing of cytokinesis) — the cell exits the cell cycle but does not completely lose the ability to divide.
Increase in size (expansion growth). Vacuoles fuse into one large central vacuole, which can occupy up to 90% of the cell volume. The tonoplast (vacuolar membrane) is permeable to water, creating turgor pressure.
Thickening of the primary cell wall. It remains thin compared to sclerenchyma, but may additionally deposit layers of cellulose and hemicelluloses (e.g., in the storage parenchyma of date palm endosperm). Importantly, a secondary (lignified) wall is never formed in parenchyma, except in cases of sclerification (transition to sclereids during aging) (Evert, 2006).
Specialization of plastids. Depending on the function:
- Chlorenchyma — proplastids turn into chloroplasts.
- Storage parenchyma — leucoplasts turn into amyloplasts (starch storage) or elaioplasts (oil storage).
- Water‑storage parenchyma — plastids remain as low‑activity leucoplasts or degenerate.
Retention of the nucleus and all major organelles — unlike conducting cells (vessels, sieve tubes), the parenchyma nucleus does not disintegrate. The cell remains alive and metabolically active until its death (Evert, 2006).

3.3. Ability to dedifferentiate and totipotency

Parenchyma cells retain potential meristematic activity, i.e., the ability to resume division after having already fully differentiated. This process is called dedifferentiation.

Dedifferentiation occurs in response to wounding, infection, hormonal stimulation, or when placed in tissue culture. The cell loses specialized structures (chloroplasts may turn into proplastids, the vacuole fragments), its nucleus is activated, and it proceeds to mitosis (Evert, 2006; Mauseth, 2016).

The resulting mass of undifferentiated dividing cells is called callus. From the callus in vitro, a whole plant can be regenerated, demonstrating the totipotency of parenchyma cells (the ability to give rise to any cell type and to a whole organism). This property is widely used in plant biotechnology — clonal micropropagation, production of virus‑free material, transformation (Stern & Jansky, 2020).

Under natural conditions, dedifferentiation of parenchyma underlies:

Formation of wound cork (periderm) upon bark damage (Evert, 2006).
Formation of adventitious roots on stem cuttings (Mauseth, 2016).
Graft union — parenchyma cells of rootstock and scion dedifferentiate, form callus, from which new conducting tissues then differentiate (Stern & Jansky, 2020).

3.4. Some features of parenchyma development in different organs

In leaves: the mesophyll parenchyma arises from the ground meristem at early stages of primordium development. Differentiation into palisade and spongy parenchyma is determined by light and hormonal signals (in particular, auxin). Under shading, the palisade parenchyma is reduced, while the spongy parenchyma expands (Raven et al., 2013).

In grass stems (wheat, maize, bamboo): the pith and cortex parenchyma are formed from the ground meristem, but then the central parenchyma breaks down, forming a cavity (hollow stem). Parenchyma cells around the vascular bundles, on the contrary, may persist and even thicken their walls, participating in support function (Evert, 2006).

In roots (e.g., in Arabidopsis): cortical parenchyma cells arise from the ground meristem and live for a limited time, after which a programmed cell death (apoptosis) leads to cortical sloughing during secondary thickening. However, the parenchyma of the endodermis and pericycle is preserved, which then gives rise to lateral roots (Dubrovsky & Vissenberg, 2021).

3.5. Biological significance of ontogenetic plasticity of parenchyma

The ability of parenchyma to dedifferentiate and its totipotency are the plant’s evolutionary “backup plan”. Unlike animals, where the loss of specialized cells is often irreplaceable, a plant can regenerate lost organs and even a whole organism from a tissue fragment thanks to parenchyma. This is especially important for perennial plants that accumulate damage throughout their life (Evert, 2006).

Thus, the ontogeny of a parenchyma cell is not an irreversible path to narrow specialization, but a potentially reversible process that allows the plant to combine current physiological functions with the ability to regenerate and adapt to stresses.

4. Structural organization of the cell and tissue

Understanding the structure of parenchyma tissue at all levels — from organ to sub microscopic — is necessary to explain its diverse functions. In this section we will examine where exactly in the plant parenchyma is located, what cells it consists of, the characteristics of its cell wall and protoplast, and what specialized structures can be found in it.

4.1. Localization of parenchyma tissues in the plant

Multiseriate medullary rays in wood — ©Berkshire Community College Bioscience Image Library flickr.com CC0 1.0
Ray parenchyma in wood
Radial rays (medullary rays), composed of parenchyma cells, provide radial transport and storage of substances.

Parenchyma occupies all the spaces between other, more specialized tissues. Depending on the organ and its age, the following main zones of localization are distinguished (Evert, 2006; Raven et al., 2013):

In the primary stem: cortex (under the epidermis) and pith (in the centre, inside the ring of vascular bundles).
In the primary root: primary cortex (between the rhizodermis and the central cylinder). The inner layer of the cortex forms the endodermis (with Casparian strips), which is also parenchymatous but with special properties (Evert, 2006; Serebryakova et al., 2006).
In the leaf: mesophyll (between the upper and lower epidermis), subdivided into palisade and spongy parenchyma. In addition, the vascular bundles are surrounded by a parenchymatous bundle sheath (Evert, 2006).
In seeds and fruits: endosperm, cotyledons, fruit flesh — all consist mainly of storage parenchyma.
In secondary tissues (wood and bast): ray parenchyma (medullary rays) and axial parenchyma (vertical strands of living cells) (Morris et al., 2016; Słupianek et al., 2021).

4.2. Cellular composition and cytology of the parenchyma cell

The tissue is composed of uniform cells — parenchyma cells. This is a simple tissue type (as opposed to complex tissues such as xylem). Consider the structure of a typical parenchyma cell (Evert, 2006; Mauseth, 2016).

Shape and size:

In youth, cells are isodiametric, polyhedral (most often 14‑hedra — tetrakaidecahedra). During growth and differentiation, the shape may change: in leaf chlorenchyma the cells are rounded or elongated, in storage parenchyma they are rounded or oval, in ray parenchyma they are radially elongated.
Sizes range from 10–30 µm in meristematic derivatives to 100–200 µm in storage cells of tubers and fruits (e.g., watermelon cells are visible to the naked eye).

Protoplast:

Cell wall — only primary, thin (0.1–0.5 µm), non‑lignified. Its structure is devoted to a separate subsection (4.3).
Plasma membrane — typical lipoproteid membrane, regulating the entry and exit of substances.
Cytoplasm — a parietal layer, often with well‑developed endoplasmic reticulum and Golgi apparatus, especially in secretory and storage cells (Evert, 2006).
Nucleus — large, usually central in young cells; in mature large cells it may be pushed to the wall. The nucleus persists throughout the life of the cell (unlike sieve tubes). It is necessary for metabolic control and the ability to dedifferentiate (Mauseth, 2016).
Vacuole — occupies up to 90% of the volume of a mature cell. Surrounded by the tonoplast. Cell sap contains dissolved sugars, organic acids, pigments (anthocyanins), tannins, alkaloids. In water‑storage parenchyma, mucilaginous substances that retain water accumulate in the vacuole. In storage parenchyma, the vacuole serves as a depot for temporary sugar storage and may also contain aleurone grains (Evert, 2006; Stern & Jansky, 2020).
Plastids — one of the main features of a parenchyma cell. Depending on function, they may be:
- Chloroplasts (chlorenchyma) — contain chlorophyll and carotenoids.
- Leucoplasts (storage parenchyma) — colourless; specialized for synthesis of starch (amyloplasts), oils (elaioplasts) or protein (proteoplasts).
- Chromoplasts (fruit flesh cells, petals) — accumulate carotenoids, giving yellow, orange or red colour.
Mitochondria — numerous in metabolically active cells (chlorenchyma, transfer cells, secretory idioblasts). Provide energy for transport processes.

4.3. Cell wall of parenchyma: structure and chemical composition

The cell wall of parenchyma cells is only primary (Evert, 2006; Mauseth, 2016). It is characterized by the following features:

Thickness — usually 0.1–0.5 µm, rarely up to 2–3 µm (e.g., in collenchyma‑like parenchyma or in the storage endosperm of some palms).
Chemical composition:
- Cellulose microfibrils constitute about 15–30% of dry mass (in the primary wall). The microfibrils are randomly oriented (“reticulate” or “random” texture), allowing the cell to stretch in all directions during expansion growth.
- Hemicelluloses (xyloglucans, xylans) — bind cellulose fibrils together and to the matrix.
- Pectic substances (up to 30–50% of dry mass in the primary wall) give the wall hydrophilicity, plasticity and the ability to swell reversibly. The high pectin content makes the parenchyma wall permeable to water and solutes.
- Structural proteins (extensins, arabinogalactan proteins) — up to 10% of mass; involved in wall strengthening and protection against pathogens.
Absence of lignin. Normally, parenchyma cells do not lignify. Lignification can occur only in aging cells when they turn into sclereids (stone cells of pear) or in parenchyma bordering a wound (formation of wound periderm).
Plasmodesmata and primary pit fields:
- Plasmodesmata — cytoplasmic channels penetrating the walls of adjacent parenchyma cells (diameter about 30–50 nm). They provide symplastic transport of ions, sugars, amino acids and even signalling molecules (RNA, proteins) (Evert, 2006; Serebryakova et al., 2006).
- Often plasmodesmata are gathered in groups — primary pit fields — areas of the wall where secondary thickening is absent (if it were present) and where plasmodesmata are especially abundant. These are zones of active intercellular exchange.

4.4. Intercellular spaces: formation and types

Between parenchyma cells there are almost always intercellular spaces — spaces filled with gas (air) or, more rarely, liquid. They arise in two main ways (Evert, 2006; Raven et al., 2013):

Schizogenous intercellular spaces — formed by the separation of cells along the middle lamella (intercellular substance) at early stages of differentiation. Cells seem to “move apart” from each other, maintaining contact only in certain areas (in the region of primary pit fields). Such intercellular spaces are characteristic of the spongy parenchyma of the leaf and of aerenchyma. Their shape is usually triangular or rounded in cross section.
Lysigenous intercellular spaces — arise as a result of programmed destruction (lysis) of whole cells or parts thereof. For example, in the pith of grass stems, central parenchyma cells lyse, forming a cavity (hollow stem). Some secretory cavities (resin ducts) are also formed lysigenously (Evert, 2006).

Aerenchyma — a special type of parenchyma with very large, regularly arranged intercellular spaces (lacunae). It is found in aquatic and wetland plants (cattail, water lily, rice). Aerenchyma intercellular spaces can be schizogenous (cells push apart) or lysigenous (cells die). The system of intercellular spaces in rice and other flood‑tolerant plants ensures oxygen delivery to the roots by the principle of “inundation–oxygen transport” (Evert, 2006; Yeats & Rose, 2013).

4.5. Specific structures and specialized parenchyma cells

Individual cells or groups of cells with a special structure may occur in parenchyma tissues.

Transfer cells

These are specialized parenchyma cells in which the cell wall forms invaginations toward the protoplast. The invaginations have a labyrinthine appearance and are covered by the plasma membrane, dramatically increasing the membrane surface area (5–10 times). Such structures are called wall ingrowths (Evert, 2006; Mauseth, 2016).

Function: intensive short‑distance transport of solutes (e.g., phloem loading in minor leaf veins, absorption from the ovary into developing seeds).

Localization: in places with large fluxes of substances:

Phloem transfer cells in minor veins (in many dicots);
Xylem transfer cells in stem nodes and in root nodules of legumes;
Glandular transfer cells in nectaries and other secretory structures.

Idioblasts — specialized solitary cells

Idioblasts are individual parenchyma cells that differ sharply from neighbouring cells in shape, content or wall type (Evert, 2006; Serebryakova et al., 2006).

Crystal idioblasts (crystal‑bearing cells) — contain calcium oxalate crystals in the vacuole. The shape of the crystals varies:

Druses — spherical aggregates of small crystals (often in the cortex and pith, e.g., in linden, beet).
Raphides — bundles of needle‑like crystals (characteristic of many monocots, e.g., lilies, aroids; have a protective function — when the cell is damaged, raphides are “shot” and cause irritation).
Prismatic (single) crystals — found in the phloem parenchyma of some plants (Evert, 2006).
Crystal sand — many tiny crystals (in solanaceous plants).

Mucilage idioblasts — contain water‑soluble polysaccharides (mucilage) in their vacuoles. They are found in linden leaves, in many succulents. Mucilage plays a role in water retention and protection against damage.

Tannin (tanniferous) idioblasts — contain tannins (polyphenolic compounds) in their vacuoles. Give tissues a brown colour, have an astringent taste and protect the plant from herbivores and pathogens. Characteristic of the bark of oak, willow, bird cherry, many conifers.

Essential oil idioblasts — accumulate terpenoid essential oils (e.g., in leaves of mint, basil, rosemary). Such cells often have a suberized inner lining or are even completely isolated by a cork coat.

4.6. Variations in parenchyma structure in different organs (examples)

Leaf mesophyll (chlorenchyma) shows fine differentiation: the palisade layer consists of densely packed cylindrical cells with a large number of chloroplasts; the spongy layer consists of irregularly shaped cells with large intercellular spaces for gas exchange. Both layers, however, retain all the features of typical parenchyma (Evert, 2006).

Storage parenchyma of a potato tuber: cells large, almost isodiametric, with thin walls. Amyloplasts (leucoplasts that store starch) occupy almost the entire cell volume, the nucleus is pushed to the wall. Plasmodesmata are numerous (Evert, 2006; Stern & Jansky, 2020).

Aerenchyma of rush stem (Juncus): parenchyma cells are stellate, their processes connect, forming a three‑dimensional network, inside which large air channels remain. Such a construction, while being light, provides mechanical strength (Evert, 2006).

Ray parenchyma in oak wood: cells are prosenchymatous, elongated in the radial direction. Walls are thin, but primary pit fields are numerous. These cells remain alive for many years, storing starch and participating in remobilization of substances during spring awakening (Morris et al., 2016).

Thus, the structural organization of parenchyma is a combination of a simple structural plan (one primary wall, large vacuole, living organelles) with high diversity of specialized forms and inclusions. It is precisely this plasticity that allows parenchyma to perform such different functions in different parts of the plant and at different stages of its life.

5. Specialized physiological types (mechanisms of operation)

In the previous section (2) we examined the classification of parenchyma by function: assimilatory, storage, water‑storage, aerenchyma, etc. It is now important to understand how exactly these tissues are organized at the cellular level and by what mechanisms they perform their tasks. This knowledge is necessary for further discussion of factors affecting the state of parenchyma and for understanding the possibilities of agronomic management (sections 6 and 7).

5.1. Assimilatory parenchyma (chlorenchyma): light capture and gas exchange

Assimilatory parenchyma (chlorenchyma) is the tissue in which photosynthesis occurs. Its cellular mechanisms are aimed at maximum capture of light energy and efficient gas exchange.

Palisade (columnar) chlorenchyma

Orientation of chloroplasts. Palisade parenchyma cells are cylindrical in shape and closely appressed to each other. Chloroplasts in them are not evenly distributed but move depending on light intensity. Under low light, they are located at the cell periphery facing the light to capture as many photons as possible. Under bright light, chloroplasts move to the side walls or gather at the bottom of the cell, reducing the risk of photoinhibition and damage to photosystems. This movement is accomplished with the involvement of actin microfilaments and myosin (Evert, 2006; Mauseth, 2016).

Organization of thylakoids. In the chloroplasts of palisade cells, the system of grana (stacks of thylakoids) is well developed, creating a large membrane area for housing photosynthetic pigments and electron transport chain proteins. In shade‑tolerant plants, the grana are larger and the number of chloroplasts per cell is higher than in sun‑adapted plants (Evert, 2006).

Role of the vacuole. The large central vacuole pushes the cytoplasm with chloroplasts to the periphery, which promotes better illumination of the chloroplasts and facilitates the diffusion of CO₂ from the intercellular spaces to the fixation site (Raven et al., 2013).

Spongy chlorenchyma

Structure of intercellular spaces. Spongy parenchyma cells are rounded or irregular in shape, loosely attached to each other, forming an extensive system of schizogenous intercellular spaces. These intercellular spaces communicate with the atmosphere through stomata (mainly on the lower side of the leaf) and with the palisade parenchyma via plasmodesmata and the free space between cells (Evert, 2006).

Gas diffusion. Through the intercellular spaces of the spongy parenchyma, CO₂ rapidly diffuses from the stomata to the palisade parenchyma, and the O₂ released during photosynthesis diffuses in the opposite direction. This path of gas exchange is more efficient than diffusion through cell walls (Mauseth, 2016).

Adaptations to drought. In xerophytes (cacti, spurges), the spongy parenchyma may be reduced or replaced by water‑storage parenchyma, while the palisade chlorenchyma is arranged in several layers (multi‑layered palisade), which reduces water loss while maintaining photosynthesis (Evert, 2006).

Mechanisms regulating photosynthesis at the tissue level

Chloroplast movement (as described above) — a rapid (within minutes) response to changing light conditions.

Change in chlorophyll content. Under prolonged shading, the chlorenchyma cells increase their chlorophyll b content and the size of light‑harvesting antennae, which increases the efficiency of using diffuse light (Raven et al., 2013).

Photosynthetic memory (priming). Pre‑exposure to moderate stress (e.g., UV radiation) can induce accumulation of protective carotenoids and antioxidant defence enzymes, increasing the resistance of chlorenchyma to subsequent high light loads (Langensiepen et al., 2020).

5.2. Storage parenchyma: accumulation of starch, sugars and lipids

Electron micrograph of plastids in stem parenchyma — Specialization of plastids in the stem parenchyma of <span lang="la" class="biological-name">Brachypodium</span>.
Electron micrograph (TEM) of a cross section: in the cortical parenchyma (left), plastids contain well‑developed thylakoid membranes (chloroplasts). In the pith parenchyma (right), plastids lose thylakoids and accumulate large starch grains, turning into amyloplasts, which reflects the specialization of cells for the storage function. Jensen & Wilkerson (2017), figure 4D. <a class="common-share-detail" rel='nofollow' href='https://creativecommons.org/licenses/by/4.0/' aria-label='Common Share CC BY 4.0' target='_blank'>CC BY 4.0</a>.

Storage parenchyma serves as a depot for nutrients. The mechanisms of accumulation differ depending on the type of stored compound.

Starch accumulation (amyloplasts)

Vascular bundle in fern rhizome — commons.wikimedia.org CC BY-SA 4.0
Amyloplasts (starch grains) in parenchyma
Around the xylem cells (with red walls) one can see parenchyma in which dark‑stained amyloplasts (starch grains) serve as storage substance.

Synthesis and deposition. In amyloplasts (leucoplasts specialized for starch synthesis), from ADP‑glucose (a product of sucrose conversion) the enzymes starch synthase and starch branching enzyme first form amylose (linear chain), then amylopectin (branched). Starch grains are formed by layer‑by‑layer deposition around a hilum (formation centre). In potato tubers, an amyloplast contains a single simple grain; in oat endosperm, compound grains consisting of many simple ones (Evert, 2006; Stern & Jansky, 2020).

Osmotic regulation. Starch accumulation occurs from excess sugars, which at high concentrations would create dangerous osmotic pressure. By converting sugars into insoluble starch, the cell maintains a stable water balance. During mobilisation (e.g., during seed or tuber germination), starch is hydrolysed to sucrose, which is exported to the apoplast or transported via the phloem (Jensen & Wilkerson, 2017).

Energetics of the process. Starch synthesis requires ATP and UTP. In C₃ plants (e.g., wheat), the storage parenchyma of the endosperm accumulates starch from sucrose supplied by the maternal plant. In C₄ plants (maize, sugarcane), the stem storage parenchyma also actively participates in photosynthesis (chlorenchyma) and carbon reutilization (Jensen & Wilkerson, 2017).

Accumulation of soluble sugars (vacuole)

In some storage tissues (sugar beet roots, sugarcane stems, fruits), excess sucrose is accumulated not as starch but directly in dissolved form in the vacuoles. Sugar concentration can reach 15–20% or more. The tonoplast contains sucrose transporters (SUT proteins) that pump sucrose against the concentration gradient, expending energy (ATP) via a proton pump (Evert, 2006; Yeats & Rose, 2013).

Lipid accumulation (spherosomes/oleosomes)

In oilseed crop seeds (sunflower, rapeseed, flax), storage lipids are synthesized in the endoplasmic reticulum and accumulate as oleosomes (also called spherosomes). Each oleosome is bounded by a monolayer of phospholipids into which specific proteins — oleosins — are inserted, stabilizing the droplets and preventing their coalescence (Evert, 2006). During germination, lipases break down triacylglycerols, releasing fatty acids, which are then oxidized in glyoxysomes (specialized peroxisomes) to sugars via the glyoxylate cycle (Stern & Jansky, 2020).

5.3. Water‑storage parenchyma (hydroparenchyma): water retention and turgor

Vacuole features. Water‑storage parenchyma cells have very large vacuoles with a high content of mucilages (polysaccharides capable of retaining water). Mucilages increase the water‑holding capacity several times compared to pure water (Evert, 2006).

Walls and intercellular spaces. The walls of such cells are thin, but often have thickenings that prevent compression upon water loss. Intercellular spaces are absent or very small to reduce evaporation.

Water‑storage mechanism. During rainy periods, water‑storage parenchyma actively absorbs water through the roots (or leaves in epiphytes). Vacuoles swell, cells become turgid, the organ swells (cacti, aloe). During drought, water is slowly consumed, but the osmotic pressure in the cells is maintained by dissolved mucilages, allowing water to be released to neighbouring chlorenchyma cells (Mauseth, 2016; Serebryakova et al., 2006).

5.4. Aerenchyma (air‑storage parenchyma): ventilation and buoyancy

Formation of large cavities. Aerenchyma arises either schizogenously (cells separate without destruction) or lysigenously (programmed death of individual cells and dissolution of their contents). In rice, for example, aerenchyma is formed lysigenously under the action of ethylene, which accumulates in the roots during flooding (Evert, 2006; Yeats & Rose, 2013).

Connection of cavities. Air channels (lacunae) communicate with each other through perforations in the walls, forming a single ventilation system from leaves to root tips. Through these channels, oxygen entering through stomata in leaves or through lenticels in stems diffuses to the roots, where its concentration in flooded soil is negligible (Słupianek et al., 2021).

Role of diaphragms. In some aquatic plants (water lily, cattail), large lacunae are crossed by thin plates of living parenchyma cells — diaphragms — which prevent complete flooding of the cavities when damaged and provide mechanical strength (Evert, 2006).

Buoyancy. In free‑floating aquatic plants (duckweed, elodea), aerenchyma gives the tissues positive buoyancy, holding the leaves at the water surface.

5.5. Transfer parenchyma (transfer cells): short‑distance transport

Wood section with stained vessel‑associated cells — Vessel‑associated cells (VACs) in poplar wood (<span lang="la" class="biological-name">Populus</span> sp.).
Red arrows indicate contact cells that adjoin the vessel (V) through contact pits. Black arrows indicate isolation cells that have no direct contact with vessels. Contact cells play a key role in radial transport of water and ions, as well as in mechanisms restoring water‑conducting function after embolism. Słupianek et al. (2021), figure 3. <a class="common-share-detail" rel='nofollow' href='https://creativecommons.org/licenses/by/4.0/' aria-label='Common Share CC BY 4.0' target='_blank'>CC BY 4.0</a>.

Wall ingrowths. This is the main feature: the cell wall forms numerous outgrowths into the cell interior, which are covered by the plasma membrane. Such a surface increases the plasma membrane area by 5–10 times, allowing a large number of transport proteins (pumps, channels) to be placed (Evert, 2006).

Types of ingrowths. Two morphological types are distinguished: reticulate (papillate, branching) and flange (ridge‑like). Both types or their combination may occur in different species (Yeats & Rose, 2013).

Energy supply. Transfer cells contain many mitochondria located near the ingrowths. They provide ATP for the operation of proton pumps (H^\+-ATPase), which create a proton gradient used for coupled transport of sucrose, amino acids and ions (Evert, 2006).

Localization and examples. Transfer cells occur in places of intense exchange between the apoplast and symplast, for example, in minor leaf veins (phloem loading), in nectaries, in the endosperm at the contact zone with the maternal plant, in the salt glands of halophytes. They provide rapid “pumping” of substances from one tissue to another (Yeats & Rose, 2013).

5.6. Interrelation of specialized types using the leaf as an example

Chlorenchyma (palisade and spongy), transfer cells of veins and storage parenchyma (e.g., in fruit flesh) can coexist within the same organ and functionally complement each other. In the leaf, palisade chlorenchyma produces sugars, which are loaded into the phloem via plasmodesmata and transfer cells of the veins and transported to the storage parenchyma of the stem or root crop. In the fruit, conversely, the storage parenchyma accumulates sugars produced in the green parts of the plant.

Thus, the specialized physiological types of parenchyma are not isolated categories but an interconnected system, where each subgroup has its own cellular and tissue mechanisms ensuring the fulfilment of the plant’s overall survival and productivity strategy. Understanding these mechanisms allows targeted manipulation of agricultural crops (e.g., increasing starch content in tubers or sugar content in fruits) through regulation of growing conditions and breeding.

6. Factors affecting the state and quality of parenchyma

The state of parenchyma tissues — their cellular structure, metabolic intensity, composition of stored substances and resistance to stress — is not constant. It changes under the influence of both external (abiotic and biotic) factors and internal (hormonal, age‑related) ones. Understanding these dependencies is critical for managing the production process in agronomy.

6.1. Light

Light is the main regulator of the development and functioning of chlorenchyma (assimilatory parenchyma), and through it, of the entire parenchyma system.

Light intensity:

Under light deficiency (shading), the palisade (columnar) chlorenchyma is reduced: cells become shorter, rounded, and the number of chloroplasts in them decreases. In contrast, the spongy chlorenchyma expands and intercellular spaces increase — the leaf becomes thinner and acquires a “shade” structure (Evert, 2006; Raven et al., 2013). In grasses under shading, the proportion of chlorenchyma in the stem decreases, chlorophyll content falls, and photosynthetic productivity drops (Jensen & Wilkerson, 2017).
Under excessively high illumination (direct sunlight), chloroplasts in palisade cells move to the side walls (avoiding photodamage), and the thickness of the palisade layer in the leaf may increase (up to 3–4 rows of cells) through division and expansion. However, with prolonged overstress, photoinhibition occurs, chlorophyll breaks down, and reactive oxygen species accumulate, leading to ageing and death of chlorenchyma (Langensiepen et al., 2020).

Spectral composition:

Blue light stimulates chloroplast formation and chlorophyll synthesis, and also regulates chloroplast orientation in the cell (via phototropic receptors). Red light, absorbed by phytochrome, affects the expression of genes encoding proteins of the photosynthetic apparatus and enzymes of starch metabolism (Evert, 2006).

Photoperiod:

Day length affects the accumulation of storage substances in the parenchyma of tubers, root crops and bulbs. In short‑day plants (potato, Jerusalem artichoke), short days induce the outflow of assimilates from leaves into the storage parenchyma of underground organs. In long‑day plants (onion, radish) — the opposite. This effect is mediated by the phytochrome system and hormonal regulation (Mauseth, 2016).

6.2. Water regime (soil and air humidity)

Water is the most important factor for all parenchyma cells, since their turgor is provided by osmotic pressure in the vacuoles.

Water deficit (drought):

The size of chlorenchyma cells and the palisade layer decreases, vacuole volume declines, and the cell wall fraction increases. Cells may deform, and under severe drought — plasmolyse and die (Evert, 2006).
In xerophytes (cacti, agaves), in response to drought, the development of water‑storage parenchyma (hydroparenchyma) is enhanced — cells become larger and their vacuoles accumulate mucilages that retain moisture. Some succulents can accumulate up to 90% water on a fresh mass basis (Mauseth, 2016).
In the storage parenchyma of root crops (carrot, beet), drought reduces turgor, growth and sugar accumulation. However, in some varieties, moderate water deficit may increase sugar content due to osmotic concentration (Langensiepen et al., 2020).

Waterlogging (flooding, anoxia):

When roots are flooded, the primary cortex parenchyma may become hypoxic. In tolerant species (rice, cattail), ethylene synthesis is activated, which induces lysigenous aerenchyma formation — programmed death of cortical parenchyma cells and formation of large air spaces through which oxygen reaches the roots from the shoots (Evert, 2006; Yeats & Rose, 2013).
In intolerant species (wheat, maize), flooding causes anaerobic stress, leading to death of the cortical parenchyma and whole roots, sharply reducing yield (Jensen & Wilkerson, 2017).

6.3. Temperature

Temperature affects metabolic rate, the composition of lipids and proteins in parenchyma cells, as well as frost tolerance.

Low positive temperatures (chilling):

In temperate plants during cold acclimation, the parenchyma increases its content of soluble sugars and amino acids (osmolytes), which prevents ice crystal formation in vacuoles. The fatty acid composition of membranes changes — the proportion of unsaturated fatty acids, which maintain fluidity, increases (Evert, 2006).
Prolonged chilling of tropical crops (maize, tomato, pepper) causes damage to chloroplasts in chlorenchyma, accumulation of reactive oxygen species and cell death (Langensiepen et al., 2020).

Freezing (subzero temperatures):

During rapid freezing, extracellular ice crystals form in parenchyma cells, drawing water out of the protoplast, causing dehydration and protein coagulation. In winter‑hardy species (spruce, pine), parenchyma can undergo deep supercooling or tolerate extracellular ice formation due to high osmolyte concentrations (Evert, 2006).

High temperatures (heat):

At temperatures above 35–40 °C, most plants suffer damage to the photosynthetic apparatus proteins, reduced ribulose‑1,5‑bisphosphate carboxylase (RuBisCO) activity in chlorenchyma, and increased photorespiration. In the storage parenchyma of tubers (potato), high temperatures result in smaller tubers and reduced starch content (Jensen & Wilkerson, 2017).

6.4. Mineral nutrition

Nitrogen (N) — a key element for protein and chlorophyll synthesis. Under nitrogen deficiency, chlorenchyma pales (chlorosis), the number of chloroplasts decreases, and protein content in seed storage parenchyma falls. With excess nitrogen, vegetative mass (leaves, stems) grows excessively, but storage organs (tubers, root crops) may accumulate more nitrates and less starch/sugars (Mauseth, 2016).

Phosphorus (P) — is a component of ATP, membrane phospholipids and intermediate metabolites. Under phosphorus deficiency, the energy balance of photosynthesis is disrupted in chlorenchyma, and ATP synthesis for sucrose loading into the phloem is impaired in storage parenchyma. Phosphorus starvation leads to anthocyanin accumulation in parenchyma (purple coloration) and growth retardation (Evert, 2006).

Potassium (K) — participates in maintaining turgor and activating enzymes (including starch synthase). Under potassium deficiency, parenchyma cells lose turgor, the stomatal apparatus regulates transpiration poorly, and assimilate outflow from leaves to storage parenchyma is reduced (lower productivity) (Raven et al., 2013).

Calcium (Ca) — important for cell wall and membrane stability. Its deficiency leads to tissue softening (e.g., “blossom‑end rot” of tomatoes due to fruit parenchyma breakdown). Calcium also participates in stress signalling (Langensiepen et al., 2020).

Silicon (Si) — in grasses, silicon is deposited in the cell walls of parenchyma (and epidermis) as phytoliths (amorphous silica). This increases tissue stiffness, resistance to lodging, and to fungal damage. Silicon content in straw can reach 20% of dry mass (Evert, 2006; Stern & Jansky, 2020).

6.5. Phytohormones

Auxin — stimulates cell division in meristems (including in parenchyma that gives rise to callus) and cell expansion (by increasing cell wall plasticity). Auxin is necessary for differentiation of conducting elements, but also affects starch accumulation (high auxin concentrations may inhibit it) (Evert, 2006).

Cytokinins — delay chlorenchyma senescence in leaves, stimulate cell division and promote the conversion of leucoplasts into chloroplasts (greening). In potato storage parenchyma, exogenous cytokinins increase tuberization (Mauseth, 2016).

Gibberellins — enhance cell expansion (mainly through stem elongation). In grasses, gibberellins accelerate seed germination by activating hydrolysis of stored substances in the endosperm (the aleurone layer produces hydrolases). In grape berries, gibberellins increase fruit size by expanding parenchyma cells (Stern & Jansky, 2020).

Abscisic acid (ABA) — induces synthesis of enzymes that promote drought tolerance (accumulation of osmolytes in parenchyma), but inhibits cell division and growth. Under stress, ABA closes stomata, indirectly reducing photosynthesis intensity in chlorenchyma (Langensiepen et al., 2020).

Ethylene — under stress (flooding, wounding) stimulates aerenchyma formation and accelerates senescence of parenchyma cells. Ethylene is involved in fruit ripening: in fruit flesh (parenchyma), it activates hydrolytic enzymes that soften cell walls (Evert, 2006).

Brassinosteroids — regulate cell expansion and differentiation of xylem elements, but also affect starch accumulation in storage parenchyma (increasing tuber yield upon treatment) (Yeats & Rose, 2013).

6.6. Mechanical influences (wind, touch)

In plants growing in windy conditions (open spaces), thigmomorphogenesis occurs: stems thicken, the proportion of collenchyma and sclerenchyma increases, but parenchyma also responds — its cells become smaller, walls may thicken slightly (without lignification). Mechanical stimulation induces expression of genes involved in callose and cell wall protein synthesis, thereby increasing tissue strength (Evert, 2006).

6.7. Biotic factors (pathogens, insects)

Upon attack by phytopathogens, parenchyma activates defence mechanisms:

Wall thickening and deposition of callose around penetration sites.
Hypersensitive response (HR) — programmed death of several layers of parenchyma cells around the infection focus, isolating the pathogen.
Production of phytoalexins (antibiotic compounds, e.g., sclerotinin) in living parenchyma cells around the wound.
Accumulation of tannins and phenolic compounds in vacuoles, which suppresses the growth of fungi and bacteria (Evert, 2006; Yeats & Rose, 2013).

6.8. Organ age and natural senescence

With age, chloroplasts in chlorenchyma degrade (turn into chromoplasts or gerontoplasts), and chlorophyll content declines. This is natural leaf senescence, controlled by ethylene and ABA. Before leaf fall, valuable nutrients from the parenchyma (sugars, amino acids, phosphorus) are remobilized and transported to storage organs (Evert, 2006; Raven et al., 2013).

In perennial stems, ray and axial parenchyma may remain alive for decades. However, with age, waste products (crystals, tannins) accumulate in their cells, walls may lignify, and the parenchyma loses its ability to dedifferentiate (Morris et al., 2016).

6.9. Anthropogenic factors

Agronomic practices (irrigation, fertilization, pruning) purposefully alter the state of parenchyma: for example, irrigation increases the proportion of water‑storage parenchyma and cell turgor; nitrogen fertilisers enhance chlorenchyma development; growth retardants reduce stem parenchyma elongation (Stern & Jansky, 2020).

Air pollution (ozone, SO₂, heavy metals) causes oxidative stress, damage to chloroplasts, and death of parenchyma cells in leaves. Some plant species accumulate heavy metals in the vacuoles of parenchyma (phytoremediation) (Langensiepen et al., 2020).

7. Practical management in agroecosystems

Understanding the structure and physiology of parenchyma tissues is not only of academic interest. It is directly used in agronomy, breeding and biotechnology to increase yields, improve product quality and enhance plant resistance to stresses. The main directions of practical application of knowledge about parenchyma are given below.

7.1. Storage parenchyma as the target of cultivation for most crops

The economically valuable part of most agricultural crops are organs in which storage parenchyma predominates: seeds (endosperm, cotyledons), tubers, root crops, bulbs, fruits. The volume and properties of this parenchyma determine the yield and its quality.

Endosperm of cereal grains (wheat, rice, maize, barley)

The endosperm of cereals consists of two types of parenchyma cells: the aleurone layer (a single layer of cells with thick walls, rich in proteins and lipids, enzymatically active during germination) and starchy parenchyma (the central part, filled with amyloplasts containing starch grains). The ratio of the aleurone layer to the starchy parenchyma, as well as the size and shape of the starch grains, determine the baking properties of flour and flour yield (Evert, 2006; Stern & Jansky, 2020).

Thousand‑grain weight — a direct indicator of ear productivity. It depends on the number and size of starchy parenchyma cells. Genetic factors and growing conditions (especially water and nitrogen regimes during grain filling) affect the division and expansion of these cells. Increasing the number of amyloplasts and the size of starch grains leads to higher grain weight and flour yield (Jensen & Wilkerson, 2017).

Amylose/amylopectin ratio in the endosperm starch determines its technological properties (gelatinisation capacity, retrogradation resistance). Breeders and biotechnologists aim to change this ratio: for example, waxy maize contains almost 100% amylopectin and is used for producing pastes and modified starches (Evert, 2006).

Potato and sweet potato tubers

The storage parenchyma of potato tubers consists of large cells filled with amyloplasts (starch) and a small amount of protein in vacuoles. Starch content — a key quality indicator (for food, industrial and seed purposes). It depends on the variety, but also on growing conditions (photoperiod, temperature, potassium nutrition). Long days and high temperatures reduce starch content (Jensen & Wilkerson, 2017).

Regulation of tuberisation. The photoperiodic sensitivity of potato is associated with the protein SP6A (a florigen homologue), which is synthesised in leaves under short days and induces the transition of the stolon apical meristem to tuber formation. Understanding this mechanism makes it possible to manage planting dates and use varieties with different photoperiodic responses (Mauseth, 2016).

Tuber firmness (important for mechanised harvesting and storage) is determined not so much by the thickness of parenchyma cell walls as by turgor and the pectin content of the middle lamellae. Treatment of fields with retardants can strengthen covering tissues, but excessive compaction of the parenchyma leads to tuber cracking (Evert, 2006).

Root crops of sugar beet, carrot, radish

In root crops, storage parenchyma expands due to secondary thickening (cambium) or due to the formation of additional cambium rings (in beet). Sugar content (sucrose concentration) is determined not only by photosynthesis but also by the ability of storage parenchyma to load sucrose into vacuoles via specific transporters (SUT proteins) (Słupianek et al., 2021).

Agronomy. Moderate water deficit during root filling increases sugar content due to osmotic concentration. Excess nitrogen, on the contrary, reduces sugar content by stimulating vegetative growth at the expense of sucrose accumulation. Phosphorus‑potassium fertilisers improve the transport of assimilates into root crops (Langensiepen et al., 2020).

Oilseeds (sunflower, rapeseed, soybean)

In the cotyledons or endosperm of oilseeds, storage parenchyma accumulates oils (triacylglycerols) in oleosomes. Oil content (proportion of oil in dry mass) is a key breeding trait. It correlates positively with the proportion of parenchyma in the seed and the number of oleosomes per cell (Evert, 2006).

Managing oil content through agronomy: phosphorus supply (necessary for the synthesis of membrane phospholipids), moderate temperatures during seed filling (high temperatures reduce the synthesis of unsaturated fatty acids), protection from drought during flowering‑filling (Jensen & Wilkerson, 2017).

7.2. Managing parenchyma quality through agronomic practices and growth regulators

Irrigation: Regulated deficit irrigation (RDI) at certain developmental stages can increase sucrose accumulation in fruits (tomatoes, grapes) and root crops, stimulating osmotic adaptation of parenchyma cells. However, at early stages, drought reduces the number of storage parenchyma cells (Langensiepen et al., 2020).

Nitrogen fertilisation: Increased nitrogen doses increase the number and size of chlorenchyma cells in leaves, but may reduce the proportion of storage parenchyma in reproductive organs. For cereals, it is important to balance nitrogen nutrition so as not to cause excessive vegetative growth at the expense of grain filling (Mauseth, 2016).

Potassium fertilisers: Improve turgor of parenchyma cells, drought resistance, as well as sugar transport in phloem and their accumulation in storage parenchyma. Particularly important for potatoes, sugar beet and fruit crops (Evert, 2006).

Retardants (gibberellin inhibitors) — used on cereals to reduce lodging. They shorten stem internodes by reducing parenchyma cell expansion, but may reduce grain mass if applied during grain filling. For potatoes, retardants (chlormequat chloride) are sometimes used to form compact tubers (Stern & Jansky, 2020).

Rooting stimulants (auxins) — used in cuttings to induce adventitious roots from stem parenchyma cells (cambial and ray parenchyma are activated and give rise to root primordia) (Mauseth, 2016).

7.3. Breeding and biotechnology of parenchyma traits

Breeding for parenchyma volume: In cereals, forms with larger endosperm and loose starchy parenchyma (soft wheat varieties) are selected. In root crops — with a greater number of storage parenchyma rings (e.g., high‑sugar beet hybrids). In potatoes — with high dry matter (starch) content and resistance to parenchyma browning (Morris et al., 2016).

Marker‑assisted selection (QTL): Quantitative trait loci affecting the number of storage parenchyma cells and starch grain size have been identified. Use of markers accelerates the development of varieties with desired characteristics (Jensen & Wilkerson, 2017).

Genetic modification:

Changing starch composition: introduction of genes encoding different isoforms of starch synthase and starch branching enzyme makes it possible to obtain starch with high amylose content (for film production) or, conversely, amylopectin (waxy starch) (Evert, 2006).
Increasing oil content: overexpression of triacylglycerol biosynthesis genes in the seed parenchyma of rapeseed and soybean increases oil content (Stern & Jansky, 2020).
Improving drought tolerance: transformation with genes regulating the synthesis of osmolytes (proline, trehalose) and sucrose transporter activity can protect parenchyma cells from dehydration (Langensiepen et al., 2020).

7.4. Aerenchyma and flooding tolerance (rice cultivation)

Rice — the main crop cultivated under flooded conditions (paddies). The tolerance of rice to root anoxia is ensured by the strong development of lysigenous aerenchyma in the primary cortex. During flooding, ethylene synthesis induces programmed cell death of cortical parenchyma cells and the formation of air channels through which oxygen diffuses from leaves to roots (Evert, 2006; Yeats & Rose, 2013).

Rice breeding: Deep‑water and flood‑tolerant varieties are selected for their ability to form extensive aerenchyma and rapidly regenerate roots after water subsidence. Marker‑assisted selection for the Sub1 (submergence‑1) locus has made it possible to create varieties that maintain yield under two weeks of flooding (Jensen & Wilkerson, 2017).

Other crops: In wheat, barley, maize, aerenchyma forms poorly under flooding, leading to root death. Research is underway to transfer genes regulating aerenchyma formation from rice to other cereals (Langensiepen et al., 2020).

7.5. Regenerative capacity of parenchyma in agronomy and biotechnology

Cutting propagation (stem, leaf, root cuttings): The ability of cuttings to form adventitious roots is associated with dedifferentiation of parenchyma cells (most often from the cambium, ray parenchyma or pericycle) under the influence of auxin. Treatment of cuttings with rooting stimulators (heteroauxin, rootvin) increases rooting efficiency (Mauseth, 2016).

Grafting: When rootstock and scion unite, callus formed from living parenchyma cells (mainly from the cambial zone and ray parenchyma) differentiates into new conducting elements, restoring the unity of the vascular system (Stern & Jansky, 2020).

Clonal micropropagation (tissue culture): Pieces of parenchyma tissue (explants) are placed aseptically on a nutrient medium with hormones (auxin, cytokinin). Parenchyma cells dedifferentiate, form callus, and then, upon change of the hormonal balance, shoots and roots form from the callus — genetically identical regenerant plants are obtained. This method is used to propagate elite varieties of potato, strawberry, orchids and other crops (Evert, 2006).

Isolated protoplast culture: The cell wall can be enzymatically removed from parenchyma cells to obtain naked protoplasts, which, retaining totipotency, regenerate whole plants. This is used for somatic hybridisation (protoplast fusion of different species) (Mauseth, 2016).

7.6. Parenchyma as a source of phytochemicals and biostimulants

Valuable secondary metabolites are obtained from secretory (excretory) parenchyma cells (idioblasts): essential oils (mint, lavender, rosemary), resins (conifers), alkaloids (poppy, henbane, rauwolfia), cardiac glycosides (foxglove). Industrial cultivation of such crops requires managing the accumulation of these substances through harvest dates, stress treatments (mechanical damage, UV irradiation) (Evert, 2006; Yeats & Rose, 2013).

Biostimulants based on hydrolysates of algal parenchyma (kelp) or plant callus contain amino acids, polysaccharides and phytohormones. They improve root growth and stress tolerance in agricultural crops (Langensiepen et al., 2020).

7.7. Modelling of parenchyma tissue in decision support systems

Modern crop models (APSIM, DSSAT) include sub‑models of the development and functioning of storage parenchyma. For example, they simulate:

The effect of temperature and water deficit on the number and size of endosperm cells (prediction of grain weight).
The distribution of assimilates between leaf chlorenchyma and storage parenchyma of root crops/tubers (optimisation of harvest timing).
The formation of aerenchyma in roots under waterlogging (risk assessment of crop death) (Langensiepen et al., 2020; Jensen & Wilkerson, 2017).

References

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