Conducting Tissues

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Conducting tissues are complexes of specialized cells that enable long‑distance transport (from tens of centimeters to tens of meters) of water, dissolved minerals, and organic substances throughout the body of a vascular plant (Evert, 2006). Together with dermal and mechanical tissues, they form the basis of the homoiohydric (water‑regulated) organization that allowed plants to colonize terrestrial habitats (Strasburger, 1971).

The key difference between conducting tissues and all other tissues is the presence of two types of conducting elements:

Tracheary elements (vessels and tracheids) of the xylem – dead hollow tubes with lignified walls that carry the upward flow of water and mineral salts (Evert, 2006; Beck, 2010).
Sieve elements (sieve cells in gymnosperms and sieve tubes in angiosperms) of the phloem – living but enucleated cells that carry the downward flow of organic assimilates (Raven et al., 2013; Evert, 2006).

Evolutionarily, conducting tissues appeared in rhyniophytes – the first vascular plants of the Devonian period. The primary water‑conducting element was the tracheid – a long, tapered cell with bordered pits that combined transport and support functions. Later, in angiosperms, vessels (vessel elements joined into long tubes with complete dissolution of transverse walls – perforations) evolved from tracheids, which dramatically increased hydraulic efficiency but reduced safety against air embolism (Beck, 2010; Evert, 2006). The evolution of the phloem proceeded from primitive sieve cells to more specialized sieve tubes with companion cells, providing more efficient mass transport of sucrose (Evert, 2006).

In the animal kingdom, the closed circulatory system can be considered an analogue of the conducting system, but plants lack a central pump (heart); movement of phloem sap is driven by osmotically generated pressure (Münch’s hypothesis), and xylem sap by transpirational tension (cohesion‑tension theory) (Graham et al., 2014; Steudle, 2000). The following sections will be devoted to the detailed structure, function, ontogeny, and regulation of xylem and phloem.

1. Functional significance

Conducting tissues integrate the entire plant body as a unified whole, connecting distant organs – roots, stems, leaves, and reproductive structures. Without an efficient long‑distance transport system, the existence of large land plants would be impossible. The functions of xylem and phloem complement each other, forming a single transport network (Evert, 2006; Raven et al., 2013).

Xylem performs three main functions:

Upward transport of water and minerals. The main function of xylem is to conduct water with dissolved ions from the roots to the leaves and other photosynthetic organs. This flow is called the transpiration stream; its driving force is the evaporation of water from the leaf surface (transpiration) and the negative hydrostatic pressure (tension) thereby created (Steudle, 2000; Beck, 2010). The speed of water movement through vessels can reach tens of meters per hour (Graham et al., 2014).
Transport of mineral elements and signaling molecules. Not only water and essential nutrients (N, P, K, Ca, etc.) move through the xylem, but also some organic compounds (e.g., hormones – abscisic acid, cytokinins) and even peptides involved in long‑distance signaling under stress (Lacombe & Achard, 2016; Ruiz-Medrano et al., 2001).
Mechanical (support) function. Because the walls of tracheary elements (tracheids and vessels) are impregnated with lignin, they have high compressive and bending strength. In gymnosperms, whose xylem lacks specialized fibers, the mechanical function falls almost entirely on tracheids (Evert, 2006). In angiosperms, this role is often performed by libriform fibers – xylem derivatives with very thick walls and narrow lumens.

Phloem specializes in the transport of organic substances, primarily the products of photosynthesis:

Downward flow of assimilates. The main transport form of carbohydrates in the phloem of angiosperms is sucrose; some plants additionally transport raffinose and stachyose (about 90% of all organic carbon transported in the plant is accounted for by sucrose) (Lalonde et al., 2003; Turgeon, 2010). Phloem sap moves from sources – mature leaves where intensive photosynthesis occurs, or storage organs during reserve mobilization – to sinks – growth points, developing fruits, seeds, roots, tubers (Graham et al., 2014).
Long‑distance signaling and transport of regulatory molecules. Phloem sap contains not only carbohydrates but also proteins (including transcription factors), RNA (mRNA, microRNA), hormones (auxins, gibberellins, jasmonates, strigolactones) and peptides (e.g., systemin, CLE peptides) (Ruiz-Medrano et al., 2001; Lucas et al., 2013; Agustí & Blázquez, 2020). These macromolecules enable rapid coordination of development and defense responses at the whole‑plant level.
Redistribution of nutrients. The phloem participates in the recycling of mineral elements (especially K, Mg, P), which can be leached from senescing leaves and transported to young tissues or to storage organs (Marschner, 2012).

Thus, xylem and phloem are functionally coupled: the xylem supplies water and minerals to the leaves where organic substances are synthesized, and the phloem carries these substances away to non‑photosynthetic parts of the plant. Disruption of any of the conducting tissues inevitably affects the productive process (yield), which explains the close attention paid to their anatomy and physiology in agronomy (see Section 7).

2. Classification and types of diversity

Conducting tissues are divided into two main types: xylem (wood) and phloem (bast). Each of them, in turn, includes several types of conducting elements that differ in structure, origin, and function. The classification is based on comparative anatomy and takes into account the evolutionary specialization of cells (Evert, 2006; Beck, 2010).

2.1. Xylem

Xylem is a complex tissue that includes conducting (tracheary), mechanical (fibers), and parenchyma (storage) elements. The classification of tracheary elements is presented in the table.

Table. Types of tracheary elements of xylem (summarized from Evert, 2006; Beck, 2010; Mauseth, 2017).

Character	Tracheids	Vessels (vessel elements)
Cell shape	Long, with tapered (closed) ends	Short and wide, with transverse or oblique ends
Perforations	Absent; water moves through pits	Present – large openings on end walls (perforation plates)
Wall thickening pattern	Annular, spiral, scalariform, reticulate, pitted (bordered pits)	Same as in tracheids, plus simple and multiple perforation plates
Protoplast at maturity	Absent (dead cells)	Absent (dead cells)
Main evolutionary group	All vascular plants (including ferns, gymnosperms, angiosperms)	Gnetophytes, angiosperms (occasionally in some ferns and horsetails)
Function	Water conduction + support	Efficient water conduction

In addition, xylem is further distinguished:

Annular and spiral vessels – characteristic of protoxylem (primary xylem formed in growing organs). Their walls can stretch as the stem or root elongates.
Scalariform, reticulate, and pitted (with bordered pits) vessels – found in metaxylem and in secondary xylem (wood). They do not stretch, providing maximum strength and hydraulic conductivity (Evert, 2006; Beck, 2010).

In most angiosperms, both types of tracheary elements are present in the xylem; however, in some primitive families (Winteraceae, Trochodendraceae) vessels are absent, and the water‑conducting function falls entirely on tracheids (Feild et al., 2002; Carlquist, 2001).

2.2. Phloem

Cross section of a three‑year‑old linden stem — ©Berkshire Community College Bioscience Image Library flickr.com
Public Domain
Phloem and phloem rays in a three‑year‑old linden stem
The image shows the structure of secondary phloem: sieve tubes and companion cells. Phloem parenchyma rays (phloem rays) for lateral transport are visible.

Phloem is also a complex tissue, consisting of conducting (sieve), parenchyma (companion cells in angiosperms, albuminous cells in gymnosperms), and mechanical (bast fibers) elements. The main types of sieve elements are presented in the table.

Table. Types of sieve elements of phloem (summarized from Evert, 2006; Raven et al., 2013; Mauseth, 2017).

Character	Sieve cells	Sieve tubes
Cell shape	Long, spindle‑shaped, with overlapping ends	Sieve elements joined into long tubes; end walls transformed into sieve plates
Sieve areas	Located on all walls (including lateral); pores relatively narrow	Large pores gathered on sieve plates (end walls); lateral areas small
Companion cells	Absent; support function performed by albuminous (Strasburger) cells	Companion cells present (ontogenetically linked to the sieve‑tube element)
Protoplast at maturity	Living cell, enucleated, with retained plastids and ER	Living cell, lacking nucleus and vacuole, with P‑protein (in most angiosperms)
Distribution	Gymnosperms, ferns, some primitive angiosperms	Angiosperms (main type of phloem)

Additional types of diversity:

By location within phloem: protophloem (primary, formed in growing parts, often short‑lived) and metaphloem (primary but maturing after growth ceases, long‑functioning) (Evert, 2006).
By presence of callose: in intact, functioning sieve elements, callose (β‑1,3‑glucan) is absent or present in trace amounts; upon injury or at the end of the season, it is deposited in the pores, blocking flow (Evert, 2006).
By types of P‑protein: in different angiosperm taxa, P‑protein (phloem protein) can be dispersed or crystalline (non‑dispersed), which has systematic significance (Evert, 2006; Behnke, 1991).

2.3. Anomalies and variations

Some plants exhibit anomalies in the structure of conducting tissues:

Bicollateral bundles (e.g., in Cucurbitaceae, Solanaceae) – phloem is located on both the outer and inner sides of the xylem (Evert, 2006).
Included (interxylary) phloem – islands of phloem within the secondary xylem (characteristic of some lianas and plants of the family Chenopodiaceae) (Carlquist, 2001).
Vessel‑less xylem – in some parasitic and aquatic plants (e.g., Ceratophyllum) vessels may be completely reduced (Evert, 2006).

This classification allows one to identify not only the tissue type but also the taxonomic affinity of the plant based on anatomical features, as well as to draw conclusions about its growing conditions (Morris et al., 2016; Baas, 1982).

3. Ontogeny (Formation and development)

The development of conducting tissues is a complex process that includes successive stages: meristem initiation, cell proliferation, cell growth, differentiation, and, in the case of tracheary elements, programmed cell death. The ontogeny of xylem and phloem is closely linked to the activity of two types of meristems – primary (procambium) and secondary (cambium) (Evert, 2006; Fischer et al., 2019).

3.1. Origin of conducting tissues: procambium and cambium

Primary conducting tissues (primary xylem and primary phloem) are laid down from the procambium – a primary meristem that arises from the apical meristems of the shoot and root. The procambium forms at early stages of embryogenesis (globular stage) and then differentiates acropetally (from the base toward the tip) (Busse & Evert, 1999; Agustí & Blázquez, 2020). In the stem, procambial strands are arranged in a ring; in leaves they form a continuous network of future veins (Scarpella et al., 2006).

The cambium is a secondary lateral meristem that arises from the remnants of procambium between the primary xylem and phloem (fascicular cambium) and from parenchyma cells of the medullary rays (interfascicular cambium). By fusing, they form a continuous cambial ring that provides secondary growth (thickening) of the stem and root (Fischer et al., 2019). Cambial division is bifacial: cells deposited inward (centripetally) differentiate into secondary xylem (wood), and cells deposited outward (centrifugally) differentiate into secondary phloem (bast) (Evert, 2006; Fischer et al., 2019). This is shown in Fig. 1 of Agustí & Blázquez (2020).

3.2. Differentiation of tracheary elements of xylem

Differentiation of xylem cells from procambium or cambium involves several stages (Fukuda, 1997; Evert, 2006):

Cell elongation and expansion. The precursor cell increases in size: in tracheids and fibers, growth in length (including intrusive growth) predominates; in vessel elements, growth in width (lateral expansion) prevails. Hormones play an important role: auxin stimulates elongation, while gibberellins promote division and expansion (Fischer et al., 2019).
Deposition of the secondary cell wall. The cell stops growing and begins synthesizing cellulose, hemicelluloses, and lignin. Cortical microtubules align along the future thickenings (annular, spiral, scalariform, reticulate), directing the movement of cellulose‑synthesizing complexes (Evert, 2006; Beck, 2010).
Formation of pits and perforations. In areas not covered by the secondary wall, portions of the primary wall – pits – are preserved. In vessel elements, hydrolytic removal of the primary wall and middle lamella occurs on the end walls, forming openings – perforations (simple, scalariform, reticulate) (Evert, 2006; Heo et al., 2016).
Programmed cell death (PCD). Tracheary elements complete differentiation by death of the protoplast. Sequence of events: disruption of the tonoplast, release of hydrolytic enzymes from the vacuole, degradation of the nucleus and all organelles. Only lignified cell walls remain, forming hollow water‑conducting tubes (Fukuda, 1997; Heo et al., 2016). This type of PCD is called autophagic.

3.3. Differentiation of sieve elements of phloem

Differentiation of sieve elements (sieve cells in gymnosperms and sieve‑tube members in angiosperms) differs significantly from that in xylem:

Unequal division of the mother cell. In angiosperms, the mother cell (formed from procambium or cambium) divides longitudinally into two unequal products: the smaller becomes the companion cell, the larger becomes the sieve‑tube member (Evert, 2006; Mauseth, 2017). The companion cell retains a dense cytoplasm, a nucleus, and numerous mitochondria; it will supply energy to the enucleated sieve tube.
Selective autolysis. Unlike PCD in xylem, sieve elements undergo partial destruction of the protoplast. The following disappear: nucleus, vacuole (tonoplast breaks down), ribosomes, Golgi apparatus, cytoskeleton. The following are retained: plasma membrane, smooth endoplasmic reticulum (agranular ER), mitochondria, plastids, and – in most angiosperms – P‑protein (phloem protein) (Evert, 2006; Heo et al., 2016). P‑protein is synthesized in companion cells and enters the sieve‑tube members via pore‑plasmodesma connections.
Formation of sieve plates and sieve areas. Plasmodesmata on the end (and sometimes lateral) walls widen to form open pores, which become lined with callose (β‑1,3‑glucan). Regions with groups of pores are called sieve areas; at the ends of sieve‑tube members they are transformed into large sieve plates – simple (single area) or compound (several areas) (Evert, 2006; Raven et al., 2013).
Retained viability. Thanks to a functioning plasma membrane and preserved mitochondria, the sieve tube remains alive and capable of maintaining the osmotic pressure gradient necessary for mass transport of sucrose (see Section 5) (Steudle, 2000; Knoblauch & van Bel, 1998).

3.4. Role of hormones in the ontogeny of conducting tissues

Auxin (IAA) – a key regulator of differentiation and polarity. It is synthesized in young leaves and apical meristems, transported basipetally (toward the roots), and creates a concentration gradient. High auxin concentrations maintain cambial proliferation, low concentrations trigger differentiation (Aloni, 2001; Fischer et al., 2019). Auxin‑dependent transcription factors (e.g., MONOPTEROS) regulate the formation of primary vessels (Agustí & Blázquez, 2020).
Cytokinins – come from the roots, act synergistically with auxin, stimulate cambial cell division and phloem differentiation (Fischer et al., 2019; Matsumoto-Kitano et al., 2008).
Gibberellins – influence xylem fiber elongation and secondary wall formation; their overproduction in poplars leads to accelerated formation of xylem fibers (Eriksson et al., 2000; Fischer et al., 2019).
Brassinosteroids and peptides (TDIF/CLE41) – interact with the PXY/TDR receptor and regulate cambial proliferation, suppressing premature xylem differentiation (Hirakawa et al., 2008; Etchells & Turner, 2010; Fischer et al., 2019).

Thus, the ontogeny of conducting tissues is a highly coordinated process in which hormonal signals, cytoskeletal rearrangements, and strictly programmed cell death (or preservation) ensure the formation of an efficient plant transport system.

4. Structural organization

Conducting tissues have a complex three‑dimensional architecture integrated into the overall anatomy of the plant organs. Their structural features are determined by functional requirements – transport efficiency, mechanical strength, and the ability to regenerate.

4.1. Localization in plant organs

Cross section of a buttercup root — ©Berkshire Community College Bioscience Image Library commons.wikimedia.org CC0 1.0
Radial vascular bundle in a buttercup root
The radial structure is clearly visible: xylem rays alternate with phloem regions. The endodermis with Casparian strips is stained red.

In different organs, xylem and phloem are arranged differently, which is related to the direction of transport flows and operating conditions (Evert, 2006; Esau, 1977):

In the root: xylem and phloem form a radial vascular bundle (Fig. 121A in Strasburger, 1971). Xylem occupies the center (usually as a star‑shaped figure with projections – actinostele), while phloem is located in the gaps between the xylem arms. This arrangement provides maximum contact area with the cortex and uniform distribution of water by the absorbing zones of the root (Evert, 2006; Mauseth, 2017).
In the stem (in dicots and gymnosperms): xylem and phloem form collateral vascular bundles arranged in a ring. Xylem faces the center of the stem (inner side), phloem faces the periphery (outer side) (Fig. 150 in Strasburger, 1971; Evert, 2006). Between them in dicots lies the cambium (open collateral bundles), enabling secondary growth. In monocots, the cambium is absent and the bundles are closed collateral, scattered throughout the stem mass (Mauseth, 2017).
In the leaf: vascular bundles form veins, in which xylem is located on the adaxial (upper) side and phloem on the abaxial (lower) side (Sack et al., 2013; Evert, 2006). This order optimizes water supply to the photosynthetic mesophyll cells and the export of assimilates.
In the secondary bark and wood: during secondary growth, secondary xylem (wood) is deposited inward from the cambium and accumulates as annual rings, while secondary phloem (bast) is deposited outward. Because new layers are constantly deposited, the outer phloem layers become compressed and die, turning into bark (Evert, 2006; Beck, 2010).

4.2. Cellular composition and cytology

Xylem and phloem are complex tissues, i.e., they consist of several cell types (Evert, 2006; Esau, 1977).

Xylem

Cross section of a pine stem — ©Berkshire Community College Bioscience Image Library flickr.com
Public Domain
Cambium and xylem structure in a pine stem
The image of a pine stem shows the ring of vascular cambium. The xylem has an endarch arrangement: protoxylem toward the center of the stem, metaxylem toward the periphery.

Tracheids (in gymnosperms and some angiosperms). Spindle‑shaped cells 0.5 to 11 mm long (up to 5 mm in pine). On radial walls they contain bordered pits with a torus (thickening in the center of the pit membrane) and margo (porous regions). The torus acts as a valve: when a pressure difference occurs, it shifts, closing the pit and preventing the spread of air embolisms (Evert, 2006; Beck, 2010).
Vessel elements (in angiosperms). Short (0.2–1 mm), wide (up to 0.5 mm in lianas). End walls have perforation plates: simple (one large opening, Fig. 10.3A in Evert, 2006) or compound (scalariform, reticulate, foraminate). Lateral walls may have spiral, annular, scalariform, reticulate or pitted thickenings, as well as bordered pits without a torus (pit membrane homogeneous) (Evert, 2006; Beck, 2010).
Xylem fibers (libriform fibers, fiber‑tracheids). Long (up to several mm), with very thick lignified walls and a narrow lumen. Provide mechanical strength. In many species they remain alive and participate in starch storage (Evert, 2006; Mauseth, 2017).
Xylem parenchyma (wood parenchyma, xylem rays). Living cells with simple pits; store starch, fats, proteins, and also synthesize and secrete protective substances (phenols, resins) (Evert, 2006; Morris et al., 2016). Rays (uniseriate or multiseriate) perform radial transport and communication with the phloem.

Phloem

Cross section of a one‑year‑old linden stem — ©Berkshire Community College Bioscience Image Library commons.wikimedia.org
Public Domain
Conducting tissues in a linden stem (first year)
Differentiation is clearly visible: outer ring of primary phloem, vascular cambium, and larger inner ring of primary xylem.

Sieve cells (in gymnosperms). Long, spindle‑shaped, with tapered ends. Sieve areas (groups of narrow pores) are scattered over all walls. The pores are traversed by strands of smooth ER that act as a “filter.” The nucleus is absent, but mitochondria, plastids, and ER are retained (Evert, 2006; Raven et al., 2013).
Sieve‑tube members (in angiosperms). Short, with transverse or slightly oblique end walls transformed into sieve plates (simple or compound). The pores are larger than those of sieve cells and do not contain ER in the lumen (only callose when damaged). Along the walls are located mitochondria, plastids (S‑type with starch or P‑type with protein inclusions), and networks of smooth ER (Behnke, 1991; Evert, 2006).
Companion cells (in angiosperms). Derivatives of the same mother cell as the sieve‑tube member. They have a large nucleus, dense cytoplasm, numerous mitochondria and ribosomes, often with folded walls (transfer cells). They are connected to the sieve‑tube member by pore‑plasmodesma channels (up to 1000 pores per cell) and ensure the loading/unloading of assimilates, synthesis of P‑protein, and maintenance of energy status (Evert, 2006; Oparka & Turgeon, 1999).
Albuminous cells (in gymnosperms). Analogous to companion cells, but do not share an ontogenetic origin with the sieve cell. They are located at the ends of rays. They contain a nucleus and are metabolically active (Evert, 2006).
Phloem fibers (bast fibers). In many dicots and gymnosperms. Provide mechanical support to the bast; can be living (with storage substances) or dead (Evert, 2006; Mauseth, 2017).
Phloem parenchyma. Storage tissue, often containing calcium oxalate crystals and phenolic compounds (Morris et al., 2016; Evert, 2006).

4.3. Extracellular matrix and intercellular spaces

Cross section of juniper wood — commons.wikimedia.org CC BY-SA 4.0
Cross section of juniper xylem
The image shows annual rings, tracheid cells, and ray parenchyma.

In conducting tissues, intercellular spaces are practically absent because cells are tightly appressed to each other, which is necessary for the continuity of the transport pathway (Evert, 2006). Exceptions include:

resin ducts in conifers (in xylem and phloem) – schizogenous intercellular spaces lined with epithelial cells that synthesize resin (Evert, 2006; Chano et al., 2015);
intercellular spaces in the cambial zone may temporarily arise during rapid cell division (Fischer et al., 2019).

The bulk of the extracellular matrix in conducting tissues is concentrated in the cell walls and in specialized pits. The cell walls of tracheary elements have three main layers: middle lamella (pectin), primary wall (cellulose, hemicelluloses, pectins), and secondary wall (cellulose, hemicelluloses, lignin) (Evert, 2006; Beck, 2010). Lignification of the secondary wall makes it water‑impermeable, so water can enter and exit only through pits (areas not covered by the secondary wall). Pits of adjacent tracheary elements form pit pairs separated by a pit membrane (modified primary wall + middle lamella) (Evert, 2006).

4.4. Specific structures

For xylem:

Bordered pits (bordered pits). Characteristic of tracheids and vessels. They have a dome‑shaped overgrowth of the secondary wall (“border”) above the pit membrane. In conifers, the pit membrane contains a torus (thickening in the center) and margo (bundles of cellulose microfibrils). Upon aspiration, the torus shifts and closes the pit aperture, preventing the spread of gas bubbles (Evert, 2006; Beck, 2010). In angiosperms, the pit membrane is homogeneous and lacks a torus, but may have vestured pits (with projections – vestures) that reduce the risk of air entry (Evert, 2006; Jansen et al., 2004).
Tyloses (tyloses). In many woody angiosperms (oak, walnut, black locust), when sapwood transitions to heartwood, parenchyma cells adjacent to vessels form outgrowths (tyloses) that enter the vessel lumen through pits and fill it, blocking water conduction (Evert, 2006; Beck, 2010). Tyloses also participate in defense against pathogens by limiting the spread of fungi and bacteria through vessels (Chano et al., 2015).
Protective layer (protective layer). In contact parenchyma cells adjacent to vessels, after completion of secondary thickening, a thin, non‑lignified layer (pectin‑cellulose) is deposited through which metabolite exchange occurs (Evert, 2006).

For phloem:

Sieve plates (sieve plates). Several types exist: simple (one large sieve area on the end wall; characteristic of the most evolutionarily advanced angiosperms, e.g., apple, maple) and compound (several areas arranged in a scalariform or reticulate pattern; more common in primitive angiosperms and in the metaphloem of monocots) (Evert, 2006; Behnke, 1991). The pores of functioning sieve plates are free of callose (except in damaged or dormant cells).
Callose (callose). β‑1,3‑glucan, a physiologically active polysaccharide. Normally, small amounts are deposited at the pore margins. Upon injury, during winter dormancy, or during sieve element senescence, massive callose deposition occurs, completely blocking the pores (wound callose, definitive callose) (Evert, 2006). Callose also plays a role in sieve plate development, forming callose pads around growing pores (Esau & Thorsch, 1985).
P‑protein (phloem protein). A protein system unique to the phloem of angiosperms. In young sieve‑tube members it forms discrete bodies (P‑bodies) which, upon cell maturation, disperse into fibrils or tubules lining the walls. When a sieve tube is damaged, P‑protein rapidly aggregates, plugging the pores together with callose (slime plug) and preventing loss of phloem sap (Evert, 2006; Knoblauch & van Bel, 1998). In legumes, P‑protein forms crystalline forisomes capable of reversible condensation in response to changes in Ca²⁺ concentration (Evert, 2006).
Pore‑plasmodesma units (PPU). The connection between a sieve tube and a companion cell occurs through specialized plasmodesmata: on the sieve‑tube side it is a wide pore (diameter up to 100 nm), on the companion cell side it is branched plasmodesmata. Proteins (including P‑protein), RNA, and ATP pass through PPUs (Evert, 2006; Oparka & Turgeon, 1999).
Plastids of sieve elements (P‑type and S‑type). All sieve elements have plastids that have lost thylakoids but contain inclusions: starch (S‑type) or protein crystals (P‑type). The type of plastid has taxonomic significance (Behnke, 1991; Evert, 2006).

Thus, the structural organization of conducting tissues represents a complex ensemble of specialized cell types, intercellular contacts, and specific molecular structures that together ensure reliable and controlled long‑distance transport of substances in the plant.

5. Physiological processes (Flow theories)

The movement of water and dissolved substances through xylem and phloem obeys different physical laws and biological mechanisms. Understanding these processes is necessary to explain how plants raise water to heights of tens of meters and distribute organic substances among organs.

5.1. Upward flow in xylem: the transpiration‑tension theory

The movement of water and mineral salts from roots to leaves (upward flow) is explained by the cohesion‑tension theory, proposed by Dixon and Joly (1894) and later developed by Tyree (1997) and other authors (Steudle, 2000; Beck, 2010).

Main postulates of the theory:

Transpiration creates tension. In leaves, water evaporates through stomata into the atmosphere. As a result, a water deficit arises in mesophyll cells, and water begins to move from the endings of xylem vessels. Because vessel walls are rigid and do not collapse, a negative hydrostatic pressure (tension) develops in the xylem water – from −1 to −10 MPa (10–100 atm), and in trees during drought it can reach −15 MPa or more (Steudle, 2000; Tyree & Zimmermann, 2002).
Cohesion (stickiness) of water molecules. Water molecules are held together by hydrogen bonds (cohesion force). Thanks to this, the water column does not break under tension and transmits the force from leaves to roots – like a cable pulling water upward (Steudle, 2000; Graham et al., 2014). Experimentally, water in narrow capillaries has been shown to withstand tension up to −30 MPa without breaking (Zimmermann et al., 1995).
Adhesion (wetting) to vessel walls. Water molecules are also attracted to the hydrophilic walls of xylem vessels (via hydrogen bonds with cellulose and hemicelluloses). This prevents the water column from sliding downward under gravity (Evert, 2006).
Continuity of the water column. Xylem is a continuous network of dead hollow tubes (vessels and tracheids) connected by pits and perforations. Pit membranes with small pores (5–20 nm) retain air but allow water to pass, preserving hydraulic integrity (Evert, 2006; Hacke & Sperry, 2001).

Driving force: the water potential gradient from the soil (high, near zero) to the leaves (low, strongly negative). Water moves passively, without ATP expenditure, due to the potential difference created by transpiration (Steudle, 2000).

Criticism and modern additions: The cohesion‑tension theory has repeatedly been questioned (Zimmermann et al., 1995; Canny, 1998), because in some cases sufficient tension could not be measured. However, most physiologists accept it as correct, with the caveat that the negative pressure in the xylem may be partially compensated by positive pressure generated by the root pump (especially in herbaceous plants and during spring sap flow) (Steudle, 2000; Beck, 2010). In maple, birch, and grapevine, spring positive root pressure (up to 0.1–0.3 MPa) is known to raise water before leaf emergence (Evert, 2006).

5.2. Cavitation and embolism: disruption of water flow

When tension becomes too high (drought, freezing), the water column may break – this is cavitation (formation of a cavity filled with water vapor). After the gas expands and fills the lumen, it is called embolism (air bubble) (Tyree & Zimmermann, 2002; Evert, 2006).

Factors causing cavitation: exceeding the critical tension (>−1.5…−10 MPa depending on the species), freezing (gas bubbles are released upon water crystallization), mechanical damage, air entry through pits upon drying.
Consequences: an embolized vessel or tracheid is taken out of service, hydraulic conductivity decreases.
Mechanisms limiting damage: in conifers, bordered pits with torus, upon aspiration, block gas access to adjacent tracheids (Evert, 2006; Beck, 2010). In angiosperms, pits are homogeneous, but thin pit membranes do not allow bubbles to pass due to surface tension (increased resistance to air‑seeding). In addition, vestured pits and tyloses can block the spread of embolism (Jansen et al., 2004; Choat et al., 2004). Some plants are able to refill embolized vessels by actively releasing sugars and creating an osmotic gradient that draws water through parenchyma (Salleo et al., 2004; Brodersen et al., 2010).

5.3. Downward flow in phloem: the pressure‑flow hypothesis (Münch)

The transport of organic substances (primarily sucrose) from source leaves to sink organs is explained by the pressure‑flow hypothesis, proposed by Ernst Münch in 1930 and repeatedly confirmed experimentally (Münch, 1930; Eschrich et al., 1972; Knoblauch & van Bel, 1998; Knoblauch et al., 2016).

Main postulates:

Phloem loading in the source. In leaves (source), sucrose is actively loaded into sieve tubes (via companion cells, using sucrose transporters and H^\+-ATPase). The concentration of sucrose in phloem sap reaches 0.5–1 M (sometimes up to 1.5 M) (Lalonde et al., 2003; Turgeon, 2010). As a result, a low water potential is created inside the sieve tube.
Water entry from xylem. Following the osmotic gradient, water from adjacent xylem vessels enters the sieve tubes through aquaporins (and partly through pits). Water entry increases the hydrostatic pressure inside the phloem (up to 1–3 MPa) (Steudle, 2000; Evert, 2006).
Phloem unloading in the sink. In roots, growing tips, fruits (sink), sucrose is actively taken up from sieve tubes (either through plasmodesmata – symplastic unloading, or via the apoplast using transporters). Sucrose concentration decreases, water exits the sieve tube back into the xylem or into surrounding cells following the osmotic gradient.
Generation of a pressure gradient. In the source, pressure is high (due to sucrose loading and water inflow); in the sink, pressure is low (due to unloading). A hydrostatic pressure gradient arises, and phloem sap (sucrose + water + other substances) moves through sieve tubes from source to sink (bulk flow, mass flow) (Knoblauch & van Bel, 1998; Knoblauch et al., 2016).

Important consequences and refinements:

Flow velocity in angiosperms is 0.5–1.5 m/h, in some trees up to 1–2 m/h, which corresponds to viscous flow through long tubes (Evert, 2006; Jensen et al., 2016).
Phloem loading is of two types: apoplastic (crossing membranes, ATP‑expending, characteristic of many herbs and trees) and symplastic (through plasmodesmata, often in plants with oligosaccharide transporters, e.g., cucurbits) (Turgeon, 2010; Evert, 2006).
Unloading in different sinks can also be symplastic (e.g., in root tips, young leaves) or apoplastic (in ripening fruits, seeds) (Evert, 2006; Oparka & Turgeon, 1999).
Pressure recovery: water that exits the phloem returns to the xylem and rises upward, creating a water cycle (hydraulic cycle) (Steudle, 2000; Graham et al., 2014).

5.4. Transport of signaling molecules and RNA

Phloem sap contains not only sugars and ions but also proteins (more than 200 species), mRNA, microRNA, peptides, and hormones (Ruiz-Medrano et al., 2001; Lucas et al., 2013). The transport of these macromolecules occurs by the same mass‑flow mechanisms, but with additional control through pore‑plasmodesma units. It has been demonstrated that proteins and RNA synthesized in companion cells enter sieve tubes and are then unloaded at distant growth points, affecting transcription (e.g., transport of the transcription factor KNOTTED1, CmNACP1 mRNA in pumpkin) (Ruiz-Medrano et al., 2001; Lucas et al., 2013). This opens the possibility for long‑distance signaling control of development and stress responses.

5.5. Role of aquaporins and ionic composition

Aquaporins (aquaglyceroporins) – channel proteins in membranes that facilitate diffusion of water and some small molecules (e.g., glycerol, NH₃, H₂O₂). In xylem parenchyma cells and in the endodermis, aquaporins (PIP, TIP, NIP) regulate the rate of water entry into and exit from vessels, and also participate in embolism refilling (Steudle, 2000; Brodersen et al., 2010; Maurel et al., 2008). In phloem, aquaporins are present in the plasma membrane of sieve elements and companion cells, modulating the osmotic response.
Ionic composition and ionic effect. Ions K⁺, Na^\+, Ca²⁺, Mg²⁺, present in xylem and phloem sap, alter the viscosity and structure of water, and also interact with hydrogels in pits, affecting hydraulic conductivity (Zwieniecki et al., 2001; Jansen et al., 2011). Calcium ions also serve as second messengers, regulating phloem loading/unloading and defense responses.

Thus, the physiological mechanisms of conducting tissue function represent a complex combination of passive (osmosis, tension, mass flow) and active (transporters, aquaporins, P‑protein, callose) processes that allow the plant to adapt to changing conditions and efficiently distribute resources.

6. Factors affecting the state

The functioning of conducting tissues depends on many external and internal factors. Disruption of water supply, deficiency of mineral elements, attacks by pathogens and pests can significantly reduce transport efficiency and, in severe cases, lead to plant death. Understanding these factors is the basis for developing agronomic practices that increase crop resilience.

6.1. Water deficit and xylem cavitation

Lack of water in the soil or increased transpiration (drought, dry winds) increases the tension in the xylem. When a critical threshold is exceeded (from −1.5 to −10 MPa depending on the species), cavitation occurs – rupture of the water column with formation of a gas‑vapor bubble. The bubble then expands, causing embolism – complete filling of the vessel or tracheid with gas, rendering it unable to conduct water (Tyree & Zimmermann, 2002; Evert, 2006).

Consequences: reduction in hydraulic conductivity (Kₓ) and, consequently, decreased photosynthesis (due to stomatal closure), growth inhibition, leaf drop, branch dieback. In agricultural crops, embolism can lead to significant yield loss.

Adaptive mechanisms:

Torus aspiration in conifers – when a pressure difference occurs, the torus shifts and closes the pit, localizing the embolism to a single tracheid (Evert, 2006; Hacke & Sperry, 2001).
Refilling in angiosperms – at night or when stress is relieved, some species (grape, laurel, poplar) can dissolve bubbles by releasing sugars from parenchyma cells into vessels, creating an osmotic gradient that draws water (Brodersen et al., 2010; Salleo et al., 2004). This process requires metabolic activity, aquaporins, and P‑protein‑mediated signaling (Secchi & Zwieniecki, 2016).
Hydraulic segmentation – in many woody plants, older vessels (deep wood) embolize first, while younger peripheral vessels retain conductivity (Zimmermann, 1983).

Agronomic significance: Irrigation according to growth phases, mulching, use of drought‑tolerant rootstocks and varieties with narrower vessels (less prone to cavitation) are the main practices for managing water regime (Sperry et al., 2006; Martinez-Vilalta et al., 2002).

6.2. Boron deficiency and disruption of phloem transport

Boron is an essential micronutrient critically important for plant growth and development. Its main role in conducting tissues is the stabilization of cell wall structure (pectins, rhamnogalacturonan II) and plasma membrane function (Brown et al., 2002; Marschner, 2012).

Effects of boron deficiency on phloem:

Destruction of companion cells and sieve plates, disruption of sucrose loading (Dell & Huang, 1997; Camacho-Cristóbal et al., 2008).
Death of growing points (roots, shoots) – precisely where sucrose transport is interrupted.
Reduced synthesis and transport of phytohormones (auxin) (Blevins & Lukaszewski, 1998).
In boron‑deficient plants, callose deposition in sieve tubes is observed, leading to their early obliteration (Evert, 2006).

Symptoms: stem distortion, brittle inflorescences, “hollow heart” in root crops (beet), empty grains in the ear (cereals), misshapen fruits (apple, tomato). Root system develops poorly, secondary thickening is suppressed (Evert, 2006; Marschner, 2012).

Agronomic management: Foliar boron application (at the bud‑bloom‑early fruit set stage) is a mandatory practice in intensive cultivation of rapeseed, sunflower, fruit crops, and sugar beet (Shorrocks, 1997). Boron is immobile in the phloem, so its deficiency manifests in young organs.

6.3. Damage to phloem by insects (aphids)

Insect phytophages, especially aphids, scale insects, whiteflies, specialize in feeding on phloem sap. They insert stylets into sieve tubes and, thanks to high pressure, suck out the sugary fluid. In response, plants mount a complex set of defense mechanisms:

Deposition of wound callose. When a sieve plate is damaged or at the feeding site, callose synthesis is sharply activated, quickly plugging the pores and isolating the damaged area (Evert, 2006; Furch et al., 2010). However, aphids secrete proteins in their saliva that suppress callose formation (Will et al., 2009; Zhang et al., 2015).
Aggregation of P‑protein. P‑protein (in some species – forisomes, in legumes) upon contact with calcium ions from the insect’s saliva polymerizes and forms clots that block the pores (Knoblauch & van Bel, 1998; Walker, 2000). This is a very rapid (seconds) response.
Release of phloem signals. Phloem damage triggers systemic defense: in undamaged parts of the plant, synthesis of chitinases, protease inhibitors, callose synthases, and other resistance mechanisms is induced (Walling, 2008; Rodriguez-Saona et al., 2010).

Agronomic measures: Aphid control includes not only insecticides but also the use of tolerant varieties (in which callose is deposited faster and more densely, and P‑protein is more active). In protected cultivation, biological control agents such as lacewings and aphidius wasps are used.

6.4. Blockage of vessels by pathogens and tyloses

Many vascular plant diseases (fusarium wilt, verticillium wilt, bacterial wilt) are caused by fungi and bacteria that enter the xylem through wounds or from roots and spread through vessels, partially blocking them with mycelium or bacterial mass. In response, the plant forms protective structures:

Tyloses – outgrowths of adjacent parenchyma cells that penetrate through pits into the vessel lumen and fill it. Tyloses often contain phenolic compounds, suberin, and callose, isolating the pathogen (Evert, 2006; Clerivet et al., 2000). Tyloses are actively formed in oak, elm, chestnut, and some fruit trees upon infection.
Lysigenous or schizogenous resin ducts in conifers – in response to damage, they begin to secrete resin that envelops and disinfects the wound (Chano et al., 2015; Nagy et al., 2000).
Formation of gels and gums – in many angiosperms (fruit trees, soybean), viscous polysaccharides are deposited in vessels, blocking further spread of phytopathogens (Rioux et al., 1998).

6.5. Freezing and thawing: damage to xylem

When xylem water freezes, dissolved gases are released as bubbles. Upon thawing, the bubbles either dissolve or expand, causing embolism. Repeated freeze‑thaw cycles are the main cause of winter embolism in temperate‑zone trees (Sperry et al., 1994; Tyree & Zimmermann, 2002). Most vulnerable are early‑spring wide vessels (in ring‑porous species – oak, ash). Conifers are more resistant due to narrow tracheids and torus aspiration.

Adaptations: In many perennial plants, embolization and obliteration of old vessels occurs in autumn before winter, and new vessels are formed in spring (especially in ring‑porous species). In addition, during the cold season, accumulation of sugars and proteins in xylem parenchyma lowers the freezing point (Evert, 2006; Zwieniecki & Holbrook, 2009).

6.6. Damage to cambium and conducting system by sunscald, frost cracks, and girdling

Damage to the cambium or phloem over a significant portion of the trunk (e.g., frost cracks, sunscald, mechanical injuries) can disrupt the connection between roots and crown. Girdling (removal of a ring of bark) completely interrupts the downward flow of assimilates, and in the absence of regeneration, the roots die. However, in some species (e.g., Pinus canariensis), active regeneration of conducting tissues is observed: from parenchyma cells associated with resin ducts, column‑like structures are formed that produce new vessels and sieve tubes, restoring transport (Chano et al., 2015). This phenomenon has significance for forestry and horticulture (regeneration after hail, windthrow).

6.7. Effect of heavy metals and salinity

Excess salts (NaCl) and heavy metals (Cd, Pb, Cu) cause damage to the endodermis, reduce water entry into the xylem, and also stimulate the formation of callose and tyloses, decreasing hydraulic conductivity. Under salinity, suberization of xylem parenchyma cells is often observed, hindering radial transport (Munns, 2002; Hossain & Komatsu, 2013). Tolerant halophytic varieties have narrower vessels and are capable of refilling after cavitation (Steudle, 2000; Munns & Tester, 2008).

Thus, the factors affecting the state of conducting tissues span the entire spectrum of abiotic and biotic stresses. Resistance to them is determined by anatomical structure (vessel diameter, pit type, density of tyloses), physiological flexibility (ability to refill, callose deposition), and genetically controlled repair mechanisms. Understanding these factors allows targeted management of the productive process – from selecting resistant varieties to optimizing irrigation and mineral nutrition regimes.

7. Practical management in agroecosystems

Knowledge of the anatomy and physiology of conducting tissues makes it possible to purposefully influence the productive process, increase plant resistance to stress, and improve crop quality. The main areas of practical application are given below.

7.1. Ratio of xylem to phloem in the cereal ear and grain set

In cereal crops (wheat, barley, rice), yield formation is determined not only by photosynthesis of the flag leaf but also by the efficiency of assimilate transport into the developing grain. An important role is played by the ratio of conducting tissues in the peduncle (uppermost internode) and in the glumes. The higher the proportion of phloem relative to xylem in the vascular bundles, the more active the sucrose loading and the higher the attracting ability (sink strength) of the ear – its ability to attract assimilates (Wang et al., 2014; Wei et al., 2017). High‑yielding varieties, as a rule, show more extensive phloem development and a greater number of sieve tubes per unit cross‑sectional area.

Agronomic conclusions:

Breeding for an increased number of sieve‑tube members and their diameter in the glume bundles raises the potential for grain set.
Nitrogen top‑dressing at heading stimulates cambial division in the vascular bundles of the upper internodes and increases phloem area (Wei et al., 2017).
Growth regulators (auxins, brassinosteroids) can modulate conducting tissue differentiation in reproductive organs (Fischer et al., 2019).

7.2. Girdling of grapevines and fruit crops

Column‑like structures that restored conducting connections after complete stem girdling
150 days after removal of a bark ring, column‑like structures that developed from parenchyma cells associated with resin ducts reached the lower edge of the wound and restored the continuity of xylem and phloem. Chano et al. (2015), figure 7, <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>.

Girdling (removal of a bark ring 2–5 mm wide) temporarily interrupts the downward flow of assimilates in the phloem but does not damage the xylem (water continues to ascend). This technique is widely used in viticulture and horticulture to:

Increase sugar content in berries and fruits before harvest (Gutiérrez‑Gamboa et al., 2019; Intrigliolo et al., 2018). Assimilates that would otherwise go to the roots accumulate in the crop.
Accelerate ripening and improve fruit color (in apple, cherry).
Regulate vegetative vigor of vigorous rootstocks.

Limitations: Girdling weakens the tree and may cause premature aging; it is not recommended to be done annually. After the wound heals over, transport is restored.

7.3. Control of parasitic flowering plants (dodder)

Broomrape (Orobanche, Phelipanche) and dodder (Cuscuta) are obligate parasites that, using haustoria, connect to the xylem and phloem of the host and extract water, minerals, and organic substances (Parker, 2012; Albert et al., 2019). After initial contact with the host, dodder germinates and forms a branched network that intercepts phloem sap.

Control methods based on knowledge of conducting tissues:

Systemic herbicides (glyphosate, imazapic) are transported through the phloem and accumulate in the actively growing points of the parasite (Dawson, 1989; Runyon et al., 2019). Application during the active growth phase of dodder (before flowering) is most effective.
Mowing the above‑ground mass before haustoria formation, while the parasite has not yet penetrated the conducting tissues – disrupts its connection with the host.
Trap crops: some crops (clover, alfalfa) release the substance cleverin, which stimulates broomrape seed germination (the parasite dies without a host) – this practice reduces the parasite’s seed bank in the soil.
Use of resistant varieties with dense bark that prevents haustorial penetration, or with reduced emission of attractants (e.g., low orobanchol content) (Rubiales et al., 2009).

7.4. Boron application at flowering and fruit set

Boron is a micronutrient critically important for the development of generative organs and phloem function. Its deficiency manifests during active flowering and fruit formation, when the demand for sucrose transport is especially high (especially in rapeseed, sunflower, apple, grape) (Marschner, 2012).

Practical recommendations:

Foliar boron application at the bud‑bloom‑immediately‑after‑bloom stages (Shorrocks, 1997). Boron is immobile in the phloem, so it must be supplied directly to the growing organs.
Rates: 100–300 g/ha of boron (as pure element) in the form of borax, boric acid, or chelates. Overdose must be avoided (boron is toxic).
For accumulator crops (rapeseed, sunflower), pre‑sowing soil boron application and top‑dressing at rosette, stem elongation, and flowering stages are necessary.
Boron effectiveness is enhanced when applied together with calcium (the preparation Calcium‑Boron on a chelate basis has proven successful), because they are synergistic in stabilizing the cell walls of phloem elements (Brown et al., 2002).

7.5. Transport system for future fertilizers and phytomedicines

Understanding the mechanisms of phloem loading opens up possibilities for targeted delivery of biologically active substances to the required plant organs (so‑called phloem‑loading strategy) (Lalonde et al., 2003; Jensen et al., 2016). Promising directions:

Creation of pro‑drugs (xylem‑phloem conjugates) that, after uptake by roots or leaves, enter the transport stream and accumulate in growing points, fruits, or roots (Liang et al., 2014; Hodge, 2004). For example, conjugates of glyphosate with amino acids or sugars that are actively transported by the phloem.
Design of controlled‑release fertilizers (e.g., nanoparticles coated with phloem‑active ligands) to minimize losses and reduce environmental pollution (Raliya et al., 2018; Chen et al., 2019).
Peptide and RNA systems for metabolic control: using phloem‑mobile signaling peptides and microRNAs to selectively activate resistance genes or delay ripening (Ruiz-Medrano et al., 2001; Lucas et al., 2013).
Biofortification: enriching seeds with iron, zinc, or iodine by injection into the xylem stream (phloem transport of these elements is usually limited) followed by accumulation in the grain (Cakmak, 2008).

7.6. Agronomic regulation of secondary growth (forestry, fruit growing)

In woody crops, management of the conducting system affects wood quality and yield:

Formative pruning removes large earlywood vessels, stimulating the development of new, narrower ones, which reduces the risk of winter embolism (Evert, 2006; Zwieniecki & Holbrook, 2009).
Irrigation according to phases of cambial activity (spring and early summer) ensures the formation of wide vessels, increasing hydraulic conductivity; during the dry period, restricting irrigation narrows vessels, increasing cavitation resistance (Fischer et al., 2019).
Use of retardants (chlormequat, paclobutrazol) increases the phloem/xylem ratio and improves assimilate flow to fruits (e.g., in apple) (Sala et al., 2019).

7.7. Resistance to pests and diseases

Breeding for resistance to sap‑sucking insects (aphids, leafhoppers) includes evaluation of the speed of callose deposition and P‑protein polymerization (Evert, 2006; Will et al., 2009). Genotypes with a rapid callose reaction and aggressive P‑protein are less susceptible.
Use of systemic resistance inducers (e.g., salicylic acid, methyl jasmonate) enhances protective responses of xylem and phloem, including tylosis deposition and synthesis of secondary metabolites (Walling, 2008).
Forest vaccination against vascular mycoses (Dutch elm disease, oak wilt) consists of injecting systemic fungicides (benzimidazoles, triazoles) into the xylem, which are transported with the upward flow and protect the entire crown (Kirisits & Schwanda, 2015).

Thus, practical management of the state of conducting tissues relies on fundamental knowledge of their anatomy, physiology, and ontogeny, and makes it possible to increase the productivity, resilience, and longevity of agrocenoses. Further development of technologies – from molecular breeding to nanodelivery of agrochemicals – will increasingly exploit the conducting system as a highway for targeted intervention in plants.

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