Growth and development are the most important characteristics of living organisms. In plants, growth is the irreversible, permanent increase in the size of an organ, a part, or a cell, resulting from cell division and cell enlargement. Plants show unlimited growth because the meristematic tissues remain active throughout life. Unlike animals, which have a fixed body plan, plants continuously produce new organs throughout their lives, a property that makes them unique. Growth is accompanied by differentiation, in which cells become specialised to perform specific functions, and together these processes constitute development.
Development is the sequence of all events from the germination of the seed to the senescence and death of the plant. It includes growth, differentiation, maturation, flowering, fruiting and even programmed cell death. This chapter describes the phases of plant growth, the measurable parameters of growth, the concept of growth rates, and the roles of the plant growth regulators (hormones). It also covers photoperiodism, vernalisation and seed dormancy, which regulate the timing of flowering and germination.
Growth is defined as an irreversible permanent increase in the size of an organ, a part, or an individual cell. In plants, growth is accomplished by cell division (increase in cell number) followed by cell enlargement (increase in cell size), which requires turgor, energy from respiration, and the synthesis of new protoplasm. Plant growth is unique because it is open (indeterminate): plants keep growing throughout their lives because meristems remain active, whereas animal growth is closed (determinate) and stops after a certain age.
The period of growth in a plant or a cell is divided into three phases: - Meristematic phase (formative phase): The cells in the meristematic region divide actively and have dense cytoplasm, thin walls and prominent nuclei. They do not have vacuoles. - Elongation phase: The cells in the region just behind the meristem elongate, increasing their length. They develop large central vacuoles and thin walls. - Maturation phase: The cells in this phase attain their maximum size and undergo differentiation to become specialised cells with thick walls.
Plants have meristems that retain the capacity to divide throughout life, so plant growth is indeterminate. The root apical meristem and shoot apical meristem are responsible for primary growth (increase in length), while the lateral meristems (cambium) are responsible for secondary growth (increase in girth). The primary and secondary growths together constitute the total growth of the plant.
Growth is measured in terms of increase in length, area, volume, weight, or cell number, and is expressed as a growth rate. There are two types of growth rates: - Arithmetic growth: In arithmetic growth, after mitotic cell division, only one daughter cell continues to divide and the other differentiates and matures. The growth follows a linear pattern, expressed as Lt = L0 + rt, where Lt is the length at time t, L0 is the length at time zero, and r is the growth rate. An example is the elongation of roots at a constant rate. - Geometric growth: In geometric growth, the growth is rapid at first but slows down later because of nutrient limitation and competition for space. It follows a typical sigmoid curve, with three phases: lag phase (slow initial growth), log phase (exponential growth) and stationary phase (growth stops). Most living systems show geometric growth, expressed mathematically as W1 = W0 e^(rt).
Differentiation is the process by which cells derived from the meristem take up a permanent shape, size and function. For example, cells of the meristem differentiate into tracheary elements (xylem vessels and tracheids), sieve tubes and other specialised cells. Differentiation involves changes in the cell walls, loss of protoplasm and deposition of lignin and other substances.
Dedifferentiation is the process by which living, differentiated cells that have lost their ability to divide regain the capacity to divide under certain conditions. For example, the inter-fascicular cambium and cork cambium arise from fully differentiated parenchyma cells, which dedifferentiate and resume cell division.
Redifferentiation is the maturation of the cells produced by dedifferentiated meristems. For example, the secondary xylem and cork cells produced by the cambium and cork cambium mature into their final differentiated state. Redifferentiation is the process in which the cells of the dedifferentiated tissue mature to perform a specific function.
Development is the sum total of growth and differentiation. A plant shows different phases of development: vegetative, reproductive and senescence. Development in plants is under the control of intrinsic (genetic) factors and extrinsic (environmental) factors such as light, temperature, water and nutrients. The transition from the vegetative to the reproductive phase involves the flowering of the plant, which is regulated by photoperiod and temperature.
Plant growth regulators are small, simple molecules of diverse chemical composition that regulate plant growth and development. They are broadly divided into two groups: growth promoters (auxins, gibberellins and cytokinins) and growth inhibitors (abscisic acid and ethylene). The discovery of the plant hormones began with Charles Darwin and his son Francis, who showed that the coleoptile of canary grass bent towards light, indicating that the stimulus of light was detected at the tip. This led to the discovery of auxin.
Auxins were the first plant hormones to be discovered, from the tip of the coleoptile. The naturally occurring auxin is indole-3-acetic acid (IAA). Auxins promote cell elongation, apical dominance, root formation in cuttings, and they are used in agriculture to prevent premature fruit drop, to produce seedless fruits (parthenocarpy, e.g., in pineapple), and as weed killers (2,4-D).
Gibberellins were first discovered in Japan from a fungus called Gibberella fujikuroi, which causes the "foolish seedling" (bakanae) disease of rice. Gibberellins promote stem elongation, fruit growth, seed germination and flowering in some plants. They are used in agriculture to increase the length of grape stalks and to produce malting in barley.
Cytokinins were discovered from the coconut milk and herring sperm DNA. They promote cell division (cytokinesis), delay senescence (ageing), and help in the formation of new leaves and chloroplasts. Kinetin was the first cytokinin discovered, and benzylaminopurine and zeatin are natural cytokinins.
Ethylene is a gaseous plant hormone involved in fruit ripening, senescence and abscission. It promotes the ripening of fruits, such as tomatoes and apples. It also breaks seed and bud dormancy, and is used to initiate flowering in mango and to synchronise fruit set in pineapple.
Abscisic acid is a plant growth inhibitor, also called the stress hormone. It promotes seed dormancy, induces stomatal closure during water stress, and promotes abscission and dormancy of buds. ABA is present in various parts of plants and is the antithesis of the growth promoters.
Photoperiodism is the response of plants to the relative lengths of daylight and darkness. The role of the photoperiod in flowering was discovered by Garner and Allard, who found that the flowering of tobacco plants (a variety called Maryland Mammoth) depends on the length of the day. Based on their response to the photoperiod, plants are classified into three categories: - Short day plants (SDP): Flower when the day length is shorter than a critical photoperiod, e.g., tobacco, soybean. - Long day plants (LDP): Flower when the day length is longer than a critical photoperiod, e.g., radish, spinach. - Day neutral plants (DNP): Flower irrespective of the photoperiod, e.g., tomato, cucumber.
Vernalisation is the process by which certain plants require a period of low temperature (cold treatment) to initiate flowering. This promotes early flowering in biennial plants like cabbage, sugar beet and carrot, and in winter varieties of wheat and rye. Vernalisation prevents premature reproductive development late in the growing season, and it is a qualitative or quantitative requirement depending on the species.
Seed dormancy is the state in which seeds fail to germinate even under favourable conditions such as adequate moisture, temperature and light. Dormancy is caused by hard seed coats, the presence of germination inhibitors, and immature embryos. Dormancy can be broken by: - Chilling treatment (stratification) - Scarification (mechanical abrasion of the seed coat) - Treatment with gibberellins - Exposure to light or treatment with chemicals
| Phase | Event | Parameter |
|---|---|---|
| Meristematic | Active cell division | Cell number |
| Elongation | Cell enlargement | Cell length |
| Maturation | Differentiation | Cell specialisation |
| Hormone | Type | Major Function | Example of Use |
|---|---|---|---|
| Auxin (IAA) | Promoter | Cell elongation, apical dominance | Parthenocarpy, weed control |
| Gibberellins | Promoter | Stem elongation, seed germination | Grape stalk lengthening |
| Cytokinins | Promoter | Cell division, delay senescence | Leaf greening |
| Ethylene | Promoter/Inhibitor | Fruit ripening | Ripening of tomatoes |
| Abscisic acid | Inhibitor | Seed dormancy, stomatal closure | Stress hormone |
| Term | Meaning |
|---|---|
| Arithmetic growth | Linear growth, e.g., root elongation |
| Geometric growth | Sigmoid curve, lag-log-stationary |
| Photoperiodism | Response to day length for flowering |
| Vernalisation | Cold treatment for flowering |
| Seed dormancy | Failure to germinate under favourable conditions |
Plant growth and development represent the dynamic, lifelong processes that shape every plant from a germinating seed to a mature organism. Growth occurs through the coordinated activities of the meristematic, elongation and maturation phases, and is measured through arithmetic and geometric growth rates. Differentiation, dedifferentiation and redifferentiation explain how cells specialise and how plants regenerate new tissues. The plant growth regulators, from the growth-promoting auxins, gibberellins and cytokinins to the growth-inhibiting abscisic acid and the ripening gas ethylene, coordinate every phase of the plant's life. Photoperiodism and vernalisation link flowering to the environment, while seed dormancy protects the embryo until conditions are right. Together, these mechanisms enable plants to time their growth and reproduction optimally, forming a complete picture of plant physiology.