Chapter 13: Plant Growth And Development

• Ordered sequence: A zygote develops into a plant with roots, leaves, branches, flowers, fruits and seeds; eventually, it dies.
• Differentiation: Cells descended from one zygote develop different structures and functions.
• Germination: Seed growth begins when environmental conditions are favourable.
• Dormancy/rest: In unfavourable conditions, seeds enter suspended growth; favourable conditions restart metabolism and growth.
• Control: Plant development is regulated by intrinsic (internal) and extrinsic (external) factors.

• Metabolism and energy: Growth is generally accompanied by anabolic and catabolic processes that use energy.
• Example: Expansion of a leaf is growth.

• Indeterminate growth: Plants can grow throughout life because their meristems retain the capacity to divide.
• Meristem cells: They divide and self-perpetuate; their products usually lose the ability to divide and form the plant body.
• Open growth: Meristem activity continually adds new cells to the plant body.
• Apical meristems: Root and shoot apical meristems cause primary growth, mainly elongating the plant along its axis.
• Lateral meristems: Vascular cambium and cork cambium appear later in dicots and gymnosperms and increase the girth of active organs.

• Cellular basis: Growth mainly reflects an increase in protoplasm, which is difficult to measure directly.
• Measurements: Growth can be estimated by changes in fresh weight, dry weight, length, area, volume or cell number.
• Cell number: One maize root apical meristem can produce more than 17,500 new cells per hour.
• Cell size: A watermelon cell may increase in size up to 350,000 times.
• Examples: Pollen-tube growth is measured by length; growth of a dorsiventral leaf is measured by surface area.

Arithmetic-growth symbols

• \(L_t\): Length at time \(t\); \(L_0\): length at time zero.
• \(r\): Growth rate, or elongation per unit time.

Geometrical-growth symbols and curve

• \(W_1\): Final size (weight, height or number); \(W_0\): initial size.
• \(r\): Growth rate; \(t\): time of growth; \(e\): base of natural logarithms.
• Typical curve: The sigmoid (S-shaped) curve has a lag phase, an exponential phase and a stationary phase.
• Stationary phase: Growth slows when resources, such as nutrients, become limited.

Examples and applications

• Woody dicots: Secondary xylem, secondary phloem and cork are examples of tissues that mature through redifferentiation.
• Plant tissue culture: Parenchyma cells induced to divide under controlled laboratory conditions undergo dedifferentiation.

• Chemical forms: PGRs include indole compounds (IAA), adenine derivatives (kinetin), carotenoid derivatives (ABA), terpenes (GA₃) and gases such as ethylene (\(C_2H_4\)).
• Other names: They are also called plant growth substances, plant hormones or phytohormones.
• Growth promoters: Auxins, gibberellins and cytokinins promote cell division and enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation.
• Growth inhibitors: ABA contributes to biotic and abiotic stress responses and growth-inhibiting processes such as dormancy and abscission.
• Ethylene: It can fit either group, but is mainly a growth inhibitor.

Key discoveries

• Auxin: Charles Darwin and Francis Darwin observed that canary-grass coleoptiles bend towards one-sided light. They concluded that the tip sends a signal causing the bending; F. W. Went later isolated auxin from oat coleoptile tips.
• Gibberellin: Rice seedlings with “bakanae” (foolish seedling) disease were linked to the fungus *Gibberella fujikuroi*. In 1926, E. Kurosawa found that sterile fungal filtrates caused the symptoms; the active substances were later identified as gibberellic acids.
• Cytokinin: F. Skoog and colleagues found that tobacco-stem callus proliferated with auxin only when the nutrient medium also contained certain extracts or DNA. In 1955, Miller and colleagues identified and crystallised the active cell-division-promoting substance, named kinetin.
• Abscisic acid: In the mid-1960s, three inhibitors— inhibitor-B, abscission II and dormin—were purified and characterised; they proved chemically identical and were named abscisic acid (ABA).
• Ethylene: In 1910, H. H. Cousins confirmed that ripened oranges release a volatile substance that hastens ripening of stored unripe bananas. This substance was later identified as ethylene.

Types and effects

• Natural auxins: IAA (indole-3-acetic acid) and IBA (indole butyric acid) occur in plants; auxin was first isolated from human urine.
• Synthetic auxins: NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are widely used in agriculture and horticulture.
• Propagation: Auxins stimulate root initiation on stem cuttings.
• Flowering and fruiting: They can promote flowering, for example in pineapples, and induce parthenocarpy, for example in tomatoes.
• Abscission: They prevent early dropping of leaves and fruits, but promote abscission of older mature leaves and fruits.
• Other roles: Auxins help control xylem differentiation and promote cell division.

• Decapitation: Removing the shoot tip removes apical dominance, allowing lateral buds to grow.
• Applications: Decapitation is used in tea plantations and hedge-making to encourage branching.
• Weed control: 2,4-D kills dicot weeds but does not affect mature monocot plants; it is used to make weed-free lawns.

Key effects and uses

• Types: More than 100 gibberellins are known from fungi and higher plants; they are named GA₁, GA₂, GA₃, and so on. GA₃ was among the first discovered and is extensively studied.
• Axis elongation: GAs increase axis length; this is used to lengthen grape stalks and improve apple shape by elongating the fruit.
• Delayed senescence: GAs delay ageing, allowing fruits to remain on trees longer and extending their market period.
• Industry: GA₃ speeds up malting in the brewing industry.
• Crop yield: Gibberellin spraying elongates sugarcane stems, where sugar is stored, and can increase yield by as much as 20 tonnes per acre.
• Early reproduction: Spraying juvenile conifers with GAs hastens maturity and leads to earlier seed production.
• Bolting: GAs promote bolting—internode elongation just before flowering—in beet, cabbage and other rosette plants.

Discovery, synthesis and effects

• Kinetin: Identified from autoclaved herring sperm DNA; it is a modified adenine (a purine) and does not occur naturally in plants.
• Zeatin: A naturally occurring cytokinin isolated from corn kernels and coconut milk; natural and synthetic cytokinins are now known.
• Synthesis sites: Natural cytokinins are produced in regions of rapid cell division, including root apices, developing shoot buds and young fruits.
• Growth effects: Promote new leaves, chloroplast development, lateral shoot growth and adventitious shoot formation; they also help overcome apical dominance.
• Senescence: Promote nutrient mobilisation, which delays leaf senescence.

Major effects of ethylene

• Seedlings: Causes horizontal growth, axis swelling and apical-hook formation in dicot seedlings.
• Senescence and abscission: Promotes ageing and shedding of plant organs, especially leaves and flowers.
• Fruit ripening: Highly effective in ripening fruits and increases their respiration rate; this rise is called the respiratory climacteric.
• Dormancy and sprouting: Breaks seed and bud dormancy; initiates germination in peanut seeds and sprouting in potato tubers.
• Growth in water: Promotes rapid internode and petiole elongation in deep-water rice, helping upper shoots and leaves remain above water.
• Roots: Promotes root growth and root-hair formation, increasing the surface available for absorption.
• Flowering and fruit set: Used to induce flowering and synchronise fruit set in pineapple; it also induces flowering in mango.

Agricultural uses

• Ethephon: The most widely used source of ethylene; its aqueous solution is absorbed and transported through the plant, then releases ethylene slowly.
• Ripening: Hastens ripening in tomatoes and apples.
• Abscission and thinning: Accelerates shedding of flowers and fruits; used for thinning cotton, cherry and walnut crops.
• Cucumbers: Promotes female flowers, which can increase yield.

Functions of ABA

• Growth inhibition: Acts as a general growth inhibitor and inhibits plant metabolism and seed germination.
• Water conservation: Stimulates stomatal closure and increases tolerance to different kinds of stress; hence it is called the stress hormone.
• Seeds: Supports seed development and maturation, and induces dormancy.
• Dormancy benefit: Dormant seeds can withstand desiccation and other conditions unfavourable for growth.
• Hormone interaction: In most situations, ABA acts antagonistically to gibberellins (GAs).

Plant growth regulators and other controls

• Five principal PGR groups: Auxins, gibberellins, cytokinins, abscisic acid and ethylene.
• Multiple roles: One or more PGRs can influence every phase of plant growth, differentiation and development.
• Interactions: PGRs may act individually or together; their effects can be synergistic (working together) or antagonistic (opposing one another).
• Shared processes: Dormancy, abscission, senescence and apical dominance can be influenced by more than one PGR.
• Control factors: PGRs are one form of intrinsic control, alongside genomic control; external factors also influence plant development.
• Extrinsic factors: Light and temperature can affect development through PGRs; other influences include nutrition, oxygen status and gravity. Related events include flowering, vernalisation, dormancy, seed germination and plant movements.