Plant Growth and Development

Plant Growth and Development

• Development encompasses growth and differentiation from seed germination to senescence.
• Plant growth is influenced by both intrinsic (internal) and extrinsic (external) factors.

Growth

• Growth is defined as an irreversible increase in size of cells, organs or the whole organism
• It involves metabolic processes; anabolism and catabolism, requiring energy.
• Plant growth is unique due to the presence of meristems, allowing continuous growth.
• Meristem cells can divide and self-perpetuate, whereas their products differentiate and contribute to the plant body.
• Primary growth is caused by root and shoot apical meristems, leading to elongation, especially along the plant axis.
• Secondary growth, seen in dicots and gymnosperms via vascular and cork cambium, increases girth.

Measurable Growth

• Growth is measured indirectly by parameters like fresh weight, dry weight, length, area, volume, and cell number because it's difficult to measure protoplasm increase directly.
• A maize root apical meristem produces over 17,500 new cells per hour, while watermelon cells expand up to 350,000 times in size.

Phases of Growth

• Growth is divided into meristematic, elongation, and maturation phases, observable at root tips.
• The meristematic phase involves cells with rich protoplasm and large nuclei that divide constantly. These cells have thin, primary cell walls with plasmodesmata.
• Elongation follows the meristematic zone, marked by increased vacuolation, cell enlargement, and new cell wall deposition.
• Maturation is further from the apex, where maximal size, wall thickening, and protoplasmic modifications occur.

Growth Rates

• Growth rate is the increase in growth per unit time, which can be expressed mathematically.
• Arithmetic growth involves one daughter cell continuing to divide while the other differentiates, exemplified by constant root elongation where a linear curve represents organ length over time (Lₜ = L₀ + rt).
• Geometric growth shows an initial slow lag phase followed by rapid exponential increase, where both progeny cells divide until nutrient limitation leads to a stationary phase, illustrated by a sigmoid or S-curve (W₁ = W₀e^(rt)).

Growth Conditions

• Essential conditions for optimum growth in plants include adequate water, oxygen, nutrients, and an optimal temperature range.
• Water is essential for cell enlargement and turgidity, aiding extension, thus linking to water status
• Water acts as a medium for enzymatic activities
• Oxygen fuels metabolic energy for growth
• Nutrients, both macro and micro, help in protoplasm synthesis serve as source of energy
• Light and gravity also influence growth.

Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation occurs as cells from meristems mature into specific functional cells, developing structural changes like thick cell walls and protoplasmic changes in tracheary elements.
• Dedifferentiation enables differentiated cells to regain the ability to divide under certain conditions, such as parenchyma cells forming interfascicular and cork cambium.
• Redifferentiation is when dedifferentiated cells lose their ability to divide again to perform specific functions.
• Plant are open to differentiation because the meristems give rise to cells/tissues with differing structures when mature.
• The location of a cell also affects the final cell structure because the cells further from the root apical meristems become cells of the root-cap, and the cells that move to the periphery mature as the epidermis.

Development

• All changes that an organism goes through during its lifetime, from seed germination to senescence.
• Plants demonstrate plasticity by forming different structures under various environmental conditions or life phases.
• An example is heterophylly, where the shapes of leaves are different when produced in the air versus in the water.

Plant Growth Regulators (PGRs)

• PGRs include auxins, gibberellins, cytokinins, abscisic acid, and ethylene, which control differentiation and developmental events.
• Based on function, Plant growth regulators are divided into: plant growth promoters and plant growth inhibitors

Characteristics of Plant Growth Regulators (PGRs)

• Plant growth regulators are simple molecules which are chemically diverse
• Plant growth regulators could be terpenes, gases or indole based
• Auxins, gibberellins, and cytokinins promote growth.
• Abscisic acid inhibits growth.
• Ethylene can be in either group.

Discovery of Plant Growth Regulators

• Charles and Francis Darwin discovered that canary grass coleoptiles bend towards light (phototropism) due to a transmittable influence from the tip of the coleoptile, later identified as auxin.
• Kurosawa discovered that Gibberella fujikuroi filtrate caused "bakanae" (foolish seedling) disease in rice, leading to the identification of gibberellic acid.
• Skoog et al. found that callus proliferation in tobacco stems required auxins plus vascular tissue extracts, yeast extract, coconut milk, or DNA, which led to the discovery of kinetin by Miller et al.
• Three independent researchers identified three inhibitors(later named abscisic acid ABA)
• H.H. Cousins confirmed ethylene as a volatile substance released from ripened oranges

Auxins

• Auxins, including IAA and IBA (isolated from plants), as well as synthetic NAA and 2,4-D, promote rooting in stem cuttings, flowering in pineapples, prevent early fruit/leaf drop, and cause abscission of older leaves and fruits.
• Synthesis occurs mostly in growing apices of shoots, and roots where they then migrate to their areas of effect
• All auxins are used in agricultural and horticultural ways
• Apical dominance, where the apical bud inhibits axillary bud growth, can be reversed via decapitation, a method used in tea plantations and hedge-making.
• Auxins induce parthenocarpy; example is tomatoes; and act as herbicides, like 2,4-D for dicotyledonous weeds. Auxin also controls xylem differentiation and cell division.

Gibberellins

• Gibberellins include over 100 varieties, denoted as GA1, GA2, GA3 and so on, and are extracted from fungi and higher plants; all are acidic.
• Gibberellic acid (GA3) was the first to have been discovered and researched
• They increase axis length (grapes), elongate fruits (apple), and delay senescence, extending market periods.
• Applied to sugarcane stem to increase yield, and hastens maturity in juvenile conifers for seed production.
• Gibberellins promote bolting (internode elongation before flowering) in plants like beet and cabbages.

Cytokinins

• Cytokinins promote cytokinesis and were identified as kinetin (modified adenine) from autoclaved herring sperm DNA.
• Naturally derived cytokinins, like zeatin from corn-kernels and coconut milk, are used along with synthetic compounds for cell division.
• Cytokinins are synthesised in active cell division areas (root apices, shoot buds, young fruits) and encourage growth of new leaves, chloroplasts, lateral shoots, and adventitious shoots, while overcoming apical dominance and delaying leaf senescence through nutrient mobilisation.

Ethylene

• Ethylene is a gaseous hormone synthesized by tissues undergoing senescence and ripening; important in the ripening of fruits
• Influences of ethylene are horizontal growth of seedlings, apical hook formation in dicot seedlings.
• Ethylene is very effective in the process of fruit ripening and helps enhance respiration during that process which is called respiration climactic.
• Ethylene promotes root growth/hair formation for better absorption, and induces flowering synchronisation in pineapples/mangos.
• Ethylene is used widely in agriculture
• Ethephon, an aqueous solution, is used for fruit ripening in tomatoes and apples, and causes defoliation of cotton and cherries

Abscisic Acid

• Abscisic acid (ABA) regulates abscission and dormancy, acting as a general growth and metabolism inhibitor.
• It inhibits seed germination, stimulates stomatal closure, and increases stress tolerance, thus considered a stress hormone.
• ABA promotes seed development, maturation, and dormancy, aiding seeds to endure desiccation and poor growth conditions, often working against GAs.
• Plant growth regulators work through complimentary means, like hormones
• Plant growth regulators can be intrinsic however extrinsic also plays a major part
• Extrinsic factors are temperature or light