Plant organism (plant)

Autotrophy is the ability to synthesize organic compounds from inorganic substances independently, using light energy. This process, called oxygenic photosynthesis, occurs in chloroplasts with the help of the pigment chlorophyll (Graham et al. 2014). Plants assimilate carbon dioxide from the air and water from the soil, converting them into carbohydrates while releasing molecular oxygen. Due to autotrophy, plants occupy the position of primary producers in ecosystems, providing organic matter to all heterotrophs (Serebryakova et al. 2006). A few plants (e.g., parasitic species — dodder Cuscuta, broomrape Orobanche) have lost chlorophyll and switched to heterotrophic nutrition, but they descended from photosynthetic ancestors and retain the basic features of plant organization (Mauseth 2017).

Most plants lead an attached (sedentary) life — their vegetative body is fixed to the substrate by roots or rhizoids (Bidlack & Jansky 2021). Immobility is a direct consequence of autotrophic nutrition: plants do not need to actively search for food, because solar energy and mineral substances are available from the environment. Sedentary lifestyle led to the development of modular organization (see Section 4), the capacity for unlimited growth, and specialized mechanisms for diaspore (spore, seed, fruit) dispersal by wind, water, or animals (Simpson 2019).

Unlike animals, which grow to a certain age and then cease growing (“closed” growth), plants retain the ability to grow throughout their life (“open” growth) (Evert 2006). Unlimited growth is ensured by the presence of meristems — formative tissues whose cells retain the capacity for division. Apical meristems (at the tips of shoots and roots) provide growth in length, while lateral meristems (vascular cambium and cork cambium) provide thickening of stems and roots (Serebryakova et al. 2006). Thanks to open growth, plants can reach enormous sizes (giant sequoia — up to 100 m in height) and live for thousands of years, annually adding new shoots, leaves, and roots (Beck 2010).

Polarity is the structural and functional difference between opposite poles of the plant organism (Serebryakova et al. 2006). It manifests itself in the division of the body into shoot (orthotropic, growing toward light) and root (geotropic, growing downward) systems. At the cellular level, polarity is expressed in the uneven distribution of organelles: the nucleus is often located in the peripheral cytoplasm, the vacuole is displaced toward the base, and the endoplasmic reticulum is elongated along the axis (Evert 2006). Polarity is driven by polar transport of auxins — from the shoot apex to the roots — which determines the basipetal direction of tissue differentiation and organ regeneration: roots form on the morphologically lower end of a cutting, while shoots form on the upper end (Mauseth 2017).

Totipotency is the property of any living plant cell that retains a complete set of the genome to realize its full developmental potential, i.e., to give rise to an entire plant under appropriate conditions (Evert 2006). Unlike animal cells, which differentiate irreversibly, many differentiated plant cells (e.g., parenchyma cells) are capable of dedifferentiation and return to a meristematic state. Totipotency underlies vegetative propagation, regeneration of damaged parts, and in vitro tissue culture methods (Bidlack & Jansky 2021). Thanks to this property, genetically identical plants (microclonal propagation) can be obtained from a single somatic cell clone, which is widely used in agronomy and biotechnology (Mauseth 2017).

Plant cells are surrounded by a rigid cell wall located outside the plasma membrane. The main structural component of the wall is cellulose (40–80% of dry mass depending on cell type and wall age) (Evert 2006). Cellulose microfibrils are embedded in a matrix of hemicelluloses, pectins, and glycoproteins. Many cells develop a secondary wall containing lignin — a polymer that provides mechanical strength and hydrophobicity (Beck 2010). The cell wall performs supportive, protective, and transport-barrier functions; its presence generates turgor pressure and limits phagocytosis, so plants absorb nutrients only by adsorption (osmotrophically) (Graham et al. 2014). The cell walls of adjacent cells are cemented by a middle lamella (composed of pectin) and communicate through plasmodesmata, which enable intercellular transport (Serebryakova et al. 2006).

The vacuolar system is a critical component of the plant cell, occupying up to 90% of the protoplast volume in mature cells. The large central vacuole is bounded by the tonoplast and filled with cell sap — an aqueous solution of sugars, organic acids, salts, pigments (anthocyanins), and secondary metabolites (Evert 2006). Functions of vacuoles include: maintenance of turgor pressure (via osmotic water uptake), regulation of intracellular pH, storage of reserve substances, accumulation of waste products and secondary metabolites (including defensive compounds — alkaloids, tannins) (Mauseth 2017). The vacuole also participates in lysosomal degradation of macromolecules and organelles, as it contains hydrolytic enzymes (Bidlack & Jansky 2021).

Plastids are organelles characteristic exclusively of plants and photosynthetic eukaryotes (algae). They possess their own DNA, multiply by division, and develop from small undifferentiated precursors — proplastids (Evert 2006). Three main types of plastids are distinguished:

The presence of plastids is one of the most reliable diagnostic features distinguishing plants from animals, fungi, and most protists (Simpson 2019).

Despite the superficial similarity of some forms (e.g., sessile animals such as sponges, or fungi producing fruiting bodies), plants differ fundamentally from representatives of the Kingdoms Animalia and Fungi in a number of key traits: type of nutrition, cell structure, growth pattern, presence of specific organelles, and chemical composition of the cell wall (Evert 2006; Mauseth 2017). The main differences are summarized in Table 1.

At the molecular level, biopolymers are studied — nucleic acids (DNA, RNA), proteins, lipids, polysaccharides (cellulose, starch, pectins) and low-molecular-weight compounds (amino acids, sugars, phytohormones, secondary metabolites). The elementary structures of this level are macromolecules, while elementary phenomena include their synthesis, modification, degradation, as well as DNA replication and transcription (Bidlack & Jansky 2021). It is at the molecular level that hereditary information, which determines all traits of the plant organism, is encoded (Simpson 2019).

The subcellular level encompasses intracellular structures — organelles, as well as the cytoskeleton, plasmalemma, tonoplast, and cell wall. Key elementary structures: the nucleus (with nuclear envelope, chromatin, and nucleolus), mitochondria, plastids (chloroplasts, chromoplasts, leucoplasts), endoplasmic reticulum, dictyosomes (Golgi apparatus), ribosomes, microbodies (peroxisomes, glyoxysomes), microtubules, and microfilaments (Evert 2006). Elementary phenomena at this level include cellular respiration (in mitochondria), photosynthesis (in chloroplasts), synthesis and transport of macromolecules, and cyclosis (cytoplasmic streaming) (Beck 2010). The study of the subcellular level became possible with electron microscopy (Graham et al. 2014).

Main article: Plant cell

The cell is the elementary structural and functional unit of life. The plant cell is characterized by the presence of a cellulose cell wall, a large central vacuole, and plastids (Serebryakova et al. 2006). Elementary phenomena at the cellular level include metabolism, growth, cell division (mitosis, meiosis), differentiation, aging, and programmed cell death (apoptosis) (Evert 2006). Plant cells are interconnected by plasmodesmata, forming the symplast — a continuous system of protoplasts (Mauseth 2017). The cellular level is the foundation for understanding tissue organization, since tissues consist of cells of one or several types.

Main article: Plant tissues

A tissue is a group of cells similar in origin, structure, and function. In plants, the following tissues are distinguished: meristems (formative tissues), dermal tissues (epidermis, periderm), ground tissues (parenchyma, collenchyma, sclerenchyma), vascular tissues (xylem, phloem), and secretory tissues (secretory cells and cavities) (Evert 2006; Serebryakova et al. 2006). Elementary phenomena at the tissue level include histogenesis (tissue formation from meristems), functional specialization of cells, regulation of substance transport, and mechanical support. Tissues form organs, so the tissue level occupies an intermediate position between the cellular and organ levels (Beck 2010).

An organ is a part of the plant body that has a definite shape, structure, position, and performs one or more specific functions. Vegetative organs: root, stem, leaf. Reproductive organs: flower (in angiosperms), fruit, seed, as well as cones (in gymnosperms) and sporangia (in spore plants) (Simpson 2019). Elementary phenomena at the organ level include organogenesis (formation of organs from meristems), interaction of organs within the whole organism (transport of assimilates and water, hormonal regulation), and metamorphosis (modifications of leaves, roots, shoots in connection with new functions) (Mauseth 2017). The organ level is the main object of plant morphology.

At the organismal level, the plant organism is considered as an integrated system developing from the zygote to natural death. Elementary structures are individual plants (individuals); elementary phenomena are ontogeny (individual development), including embryogenesis, seed germination, vegetative growth, flowering, fruiting, and senescence (Evert 2006). In plants, due to open growth and modular organization, the lifespan of an individual can range from a few weeks (ephemerals) to several thousand years (giant sequoia, long-lived pine) (Bidlack & Jansky 2021). At the organismal level, properties such as polarity, totipotency, and regenerative capacity are manifested.

Individual plants of the same species inhabiting a certain territory and partially or completely isolated from other such groups form a population. A set of populations capable of interbreeding and producing fertile offspring constitutes a species (Simpson 2019). Elementary structures at the population-species level are populations; elementary phenomena are changes in the gene pool under the influence of mutations, recombinations, genetic drift, and natural selection (microevolution) (Serebryakova et al. 2006). The species is the basic taxonomic category, and the population-species level is the object of study of systematics, population genetics, and biogeography (Yakovlev et al. 2004).

In addition to those listed, biogeocenotic and biosphere levels are also distinguished in the structure of living matter, but they go beyond the individual plant organism and belong to ecology and the study of the biosphere (Serebryakova et al. 2006; Graham et al. 2014). Sequential consideration of organization levels allows a systemic understanding of the structure, vital activity, and evolution of the plant organism.

In higher plants, a module is an elementary structural unit of the shoot system, comprising a node with one or more leaves and an axillary bud, as well as the corresponding internode. In most angiosperms, the shoot consists of sequentially repeating metameres (Serebryakova et al. 2006; Mauseth 2017). Due to the activity of the apical meristem, new modules are formed continuously throughout the life of the plant, providing open growth (see Section 2.1). The root system also has a modular organization, but root modules (segments with lateral roots) are less clearly expressed than shoot modules (Beck 2010).

Modular organization provides the plant with several advantages:

Despite modular discreteness, the plant represents an integrated system in which all parts are in complex interaction. Integrity is maintained by several mechanisms:

Thanks to these mechanisms, the plant maintains relative homeostasis over a wide range of environmental conditions. However, unlike animals, plant homeostasis is achieved mainly through osmotic and hormonal regulation rather than neurohumoral control (Beck 2010).

The modular organization and integrity of the plant organism have important practical implications for agricultural production:

Thus, the plant represents a dynamic modular system in which the discreteness of metamere structure is combined with a high degree of integration through a developed conducting network, hormonal signals, and symplastic connections. This feature distinguishes plants from animals and is key to understanding the production process in agroecosystems (Serebryakova et al. 2006; Beck 2010).

Morphology describes the external structure of the plant, its organs, and their metamorphoses. In vascular plants, the vegetative body is divided into a shoot and a root system. The shoot, in turn, consists of a stem (axial part) and leaves (lateral organs). The point of leaf attachment to the stem is called the node, and the stem segment between two adjacent nodes is the internode (Bidlack & Jansky 2021; Mauseth 2017).

Anatomy studies the internal organization of plants at the tissue and cellular levels. In the body of a vascular plant, three main tissue systems are distinguished (Evert 2006; Beck 2010):

In ontogeny, primary and secondary structures of organs are distinguished (Evert 2006; Mauseth 2017).

The primary body is formed from apical meristems and consists of primary tissues: epidermis, primary cortex, primary xylem and phloem, and in the stem also the pith. Primary structure is characteristic of all young organs and persists in herbaceous plants throughout their life (Serebryakova et al. 2006).

The secondary body arises from the activity of lateral meristems — the vascular cambium and the cork cambium (phellogen). The vascular cambium deposits secondary xylem (wood) to the inside and secondary phloem to the outside, providing thickening of stems and roots. The cork cambium forms cork (phellem), which replaces the epidermis. Secondary structure is characteristic of woody and shrubby forms, as well as some perennial herbs (Bidlack & Jansky 2021; Beck 2010).

Knowledge of the external (morphological) and internal (anatomical) structure of plants is of fundamental importance for agronomic practice and agricultural production (Bidlack & Jansky 2021; Serebryakova et al. 2006). The main areas of applied use are given below.

Identification and diagnosis of crop and weed plants. Morphological traits (leaf shape, inflorescence type, flower and fruit structure) and anatomical characteristics (features of epidermis structure, stomatal type and arrangement, presence of sclereids, crystal morphology) allow accurate species and cultivar identification. This is necessary for monitoring field weed infestation, quarantine control, and cultivar identification of seed material (Simpson 2019).

Assessment of adaptation to environmental conditions. Anatomical features reflect plant adaptation to environmental factors. For example, xeromorphic traits (thick cuticle, sunken stomata, well-developed sclerenchyma) indicate drought tolerance (Evert 2006). The presence of aerenchyma (air-conducting tissue) in roots and stems is a sign of tolerance to waterlogging and flooding (Mauseth 2017). Anatomical screening allows the selection of breeding material with desirable adaptive properties (Beck 2010).

Diagnosis of diseases and injuries. Microscopic analysis of tissues helps identify fungal, bacterial, and viral pathogens by characteristic structural changes (e.g., mycelium inside vessels, necrosis, cell hypertrophy) (Graham et al. 2014). Anatomical methods are also used to assess the extent of damage caused by pests, herbicides, and frost (Bidlack & Jansky 2021).

Quality control and authenticity of plant raw materials. In pharmacognosy and the food industry, microscopic (anatomical) features serve as criteria for the authenticity of medicinal plant raw materials (e.g., presence of secretory cavities, type of pubescence, stomatal apparatus structure). This allows the detection of adulterants and counterfeits (Serebryakova et al. 2006).

Cultivar development and certification. For many agricultural crops (fruit trees, cereals, vegetables), morphological traits (crown shape, fruit colour and size, leaf venation) are included in official cultivar descriptors (distinguishing characteristics) and are used for registration and patenting (Simpson 2019). Anatomical features (wood structure, seed coat anatomy) sometimes serve as additional diagnostic markers (Evert 2006).

Development of agronomic practices. Understanding the anatomy of shoots and roots provides a scientific basis for pruning methods (bud location, branching type), hilling (formation of adventitious roots), tillage and mulching (preservation of the root absorption zone) (Mauseth 2017). Morphological knowledge of phenological stages (growth and development phases) underlies the precise timing of irrigation, fertilization, and harvest operations (Beck 2010).

Thus, plant morphology and anatomy constitute a necessary theoretical foundation for solving a wide range of practical agronomic problems — from plant recognition to growth management and protection (Bidlack & Jansky 2021; Serebryakova et al. 2006).

Systematics is the scientific discipline concerned with the description, identification, nomenclature, and classification of plants (Simpson 2019). In a broad sense, systematics includes:

The main goal of modern systematics is to construct a natural (phylogenetic) classification that reflects true relationships and common descent of taxa (Simpson 2019). In contrast to artificial systems (e.g., the Linnaean system based on a few arbitrarily selected characters), natural classification uses the maximum possible number of traits — from morphology to DNA sequences (Graham et al. 2014).

The hierarchical classification of plants includes the following main ranks (from highest to lowest) (Bidlack & Jansky 2021; Simpson 2019):

Kingdom (Regnum) → Division (Divisio) → Class (Classis) → Order (Ordo) → Family (Familia) → Genus (Genus) → Species (Species).

When necessary, intermediate categories are used (subspecies, variety, form). The species is the basic structural unit of classification; it is a set of populations capable of interbreeding to produce fertile offspring, sharing common morphophysiological traits, and occupying a definite range (Simpson 2019).

The scientific name of each species consists of two words (binominal) — the genus name and the specific epithet (Bidlack & Jansky 2021). For example, Triticum aestivum — bread wheat; Solanum tuberosum — potato. Binomial nomenclature was introduced by Carl Linnaeus in 1753 in his work Species Plantarum and has since been mandatory for all botanical publications (Serebryakova et al. 2006). Genus and species names are written in italics; the first letter of the genus is capitalized, the specific epithet is written in lower case.

According to current concepts, plants (Plantae) belong to the domain Eukarya (Eukaryota) and are subdivided into several divisions (Graham et al. 2014; Simpson 2019). Depending on the classification, the following groups are distinguished:

Classical methods of systematics are based on the analysis of morphological, anatomical, embryological, palynological (pollen structure), and cytogenetic characters (Serebryakova et al. 2006). Modern methods include:

Thanks to molecular data, many traditional classifications have been revised. For example, the former group “dicotyledons” turned out to be paraphyletic, and now basal angiosperms, magnoliids, eudicots, and monocots are distinguished (Simpson 2019; Mauseth 2017). This process of refining phylogeny continues.

Knowledge of systematics is necessary for:

Thus, plant systematics serves as the foundation for all botanical and agronomic disciplines, providing a common language and reference system for the study of plant diversity (Simpson 2019).

Photosynthesis is the key process by which plants, using the pigment chlorophyll and other components of the photosynthetic chain, convert the energy of sunlight into the chemical energy of organic compounds (Bidlack & Jansky 2021). The summary equation of photosynthesis is:

Photosynthesis occurs in chloroplasts and includes two phases:

Many plants (maize, sugarcane, millet) employ the C_4 photosynthetic pathway, which allows efficient carbon dioxide fixation under high temperatures and drought conditions. In succulents and some other plants (cacti, Crassulaceae), CAM photosynthesis (crassulacean acid metabolism) occurs, in which stomata open at night, carbon dioxide fixation takes place in the dark, and the products are used during the day (Mauseth 2017).

Respiration is the set of oxidative processes by which organic substances (primarily carbohydrates) are broken down into carbon dioxide and water, releasing energy stored in the form of ATP (Evert 2006). The overall equation of respiration is the reverse of photosynthesis:

A distinction is made between aerobic respiration (involving oxygen, occurring in mitochondria, including glycolysis, the Krebs cycle, and oxidative phosphorylation) and anaerobic respiration (fermentation, occurring in the cytoplasm without oxygen, yielding much less ATP) (Bidlack & Jansky 2021). Respiration provides energy for all vital processes of the plant — synthesis of substances, growth, transport, maintenance of ion gradients, etc. (Serebryakova et al. 2006).

Water relations include water uptake by roots, transport through xylem, transpiration (evaporation from leaves), and water exudation as droplets (guttation) (Beck 2010). The main driving force of upward water transport is transpirational pull, created by evaporation of water from stomata. In addition, root pressure and cohesion forces between water molecules play important roles (Evert 2006). Water is necessary for photosynthesis, maintenance of turgor, transport of minerals, and leaf cooling. Regulation of water balance is achieved through stomatal control, cuticle development, and root system architecture (Mauseth 2017).

Plants absorb mineral substances from the soil in ionic form (nitrates, phosphates, potassium, calcium, magnesium, sulfates, micronutrients) (Bidlack & Jansky 2021). Main functions of elements:

In leguminous plants, symbiotic nitrogen-fixing bacteria (rhizobia) convert atmospheric molecular nitrogen into ammonia available to the plant (Mauseth 2017). Mycorrhizal fungi improve phosphorus and water uptake (Evert 2006).

Transport of substances in plants occurs via two main pathways (Beck 2010):

In addition, cells are interconnected via plasmodesmata, forming the symplast, through which ions and small molecules move (Evert 2006).

Growth is the irreversible increase in size and mass of the organism, occurring through cell division and expansion (Serebryakova et al. 2006). Development refers to qualitative changes in structure and function, including differentiation of cells, tissues and organs, transition to flowering and fruiting, and senescence (Bidlack & Jansky 2021).

The main stages of plant ontogeny are:

Growth and development are regulated by internal (genetic) programmes and external factors (light, temperature, humidity, photoperiod). Phytohormones play a crucial role (see next subsection).

Phytohormones are low-molecular-weight organic compounds produced in some parts of the plant and transported to others, where they regulate growth, differentiation, and stress responses (Bidlack & Jansky 2021). Major groups:

Understanding physiological processes underlies most agronomic practices and breeding programmes:

Thus, plant physiology provides the scientific basis for increasing yield, product quality, and resistance of crop plants to adverse environmental factors (Bidlack & Jansky 2021). A detailed treatment of each physiological process will be presented in the corresponding block of articles on plant physiology.

Abiotic factors are components of the non-living environment that influence plant vital activity (Bidlack & Jansky 2021). These include light, temperature, water, air, mineral nutrition, topography, and other physicochemical properties of the environment.

Biotic factors are the effects on plants of other living organisms: animals (herbivores, pollinators, seed dispersers), plants (competition, parasitism, allelopathy), fungi, bacteria, and viruses (Graham et al. 2014).

In the course of evolution, plants have developed diverse adaptations (morphological, anatomical, physiological, biochemical) that enable survival in specific conditions (Mauseth 2017). Examples:

Knowledge of environmental factors and adaptations has direct practical importance for agriculture:

Thus, plant ecology provides the scientific basis for adaptive farming, increasing agroecosystem productivity, and enhancing plant resistance to adverse factors (Bidlack & Jansky 2021). Ecological patterns and their agronomic applications will be discussed in more detail in the relevant section.

Since the plant is an integrated self-regulating system (Section 4), changes in its habitat conditions directly affect physiological processes (Section 7) and, ultimately, yield (Serebryakova et al. 2006).

Agronomy uses knowledge of morphology (Section 5) and modular organisation (Section 4) to manage plant growth and development:

Phytohormones and their synthetic analogues (growth regulators) allow targeted influence on physiological processes (Section 7.7) (Bidlack & Jansky 2021).

Genetic-level plant management is the most fundamental way to alter plant properties (Simpson 2019).

Modern crop production aims at integrated crop management, which combines all the above approaches while taking into account economic and environmental constraints (Bidlack & Jansky 2021).

Thus, practical management of the plant draws on all aspects of modern botanical science — from molecular biology to agroecology. The integration of this knowledge makes it possible to sustainably increase crop productivity, improve product quality, and reduce negative environmental impacts (Bidlack & Jansky 2021; Beck 2010).