Plant Growth and Development

In this chapter: characteristics and measurement of growth; phases of growth; arithmetic and geometric growth; conditions for growth; differentiation, dedifferentiation, redifferentiation; plasticity; auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Growth

Growth is an irreversible, permanent increase in size of an organ, its parts or a cell, accompanied by metabolic processes. Plant growth is generally indeterminate (open form of growth), because meristems keep dividing throughout life.

• Primary growth: by root and shoot apical meristems (elongation along the axis). Secondary growth: by lateral meristems (vascular and cork cambium) in dicots and gymnosperms (increase in girth).
• Growth is measured as increase in fresh or dry weight, length, area, volume or cell number. A single maize root apical meristem can produce over 17,500 new cells per hour; cells in a watermelon may increase in size by up to 350,000 times. Growth of a pollen tube is measured as increase in length.

Phases of growth

A root or shoot tip shows three zones. There is the meristematic zone (constantly dividing cells with large nuclei, thin primary walls and abundant plasmodesmata), the zone of elongation (cell enlargement, vacuolation and new wall deposition), and the zone of maturation (cells reach maximum size in wall thickening and protoplasmic modifications).

Growth rates

In the geometric equation, W₁ = final size, W₀ = initial size, r = growth rate (relative growth rate, a measure of the ability to produce new material, called the efficiency index), t = time, e = base of natural logarithms.

Absolute growth rate is the total growth per unit time. Relative growth rate is growth per unit time per unit initial size. Two leaves may show the same absolute growth, but the smaller one has the higher relative growth rate.

Conditions for growth

The conditions are water (for cell enlargement through turgidity, and as a medium for enzymes), oxygen (for metabolic energy), nutrients (macro and micro), an optimum temperature, light and gravity.

Differentiation and development

• Differentiation: cells from meristems mature to perform specific functions (e.g. tracheary elements lose protoplasm and develop strong, lignified walls).
• Dedifferentiation: differentiated living cells regain the ability to divide. Examples: formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
• Redifferentiation: cells produced by dedifferentiated tissue lose the capacity to divide again and mature to perform specific functions.
• Plant differentiation is open: the same meristem can produce different kinds of cells depending on position.
• Plasticity: plants follow different pathways in response to environment or life phase. Example: heterophylly, different leaf shapes in juvenile and mature plants (cotton, coriander, larkspur), or in air and water (buttercup).

Plant growth regulators (PGRs)

PGRs are small, simple molecules of diverse chemical nature. Promoters: auxins, gibberellins, cytokinins. Inhibitor: abscisic acid. Ethylene can act either way but is largely an inhibitor.

How they were discovered

Auxins

Natural: IAA (indole-3-acetic acid) and IBA (indole butyric acid). Synthetic: NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid).

• Initiate rooting in stem cuttings, widely used in plant propagation.
• Promote flowering, e.g. in pineapple.
• Prevent early fruit and leaf drop, but promote abscission of older, mature leaves and fruits.
• Apical dominance: the growing apical bud inhibits growth of lateral buds. Removing the shoot tip (decapitation) makes plants bushy; used in tea plantations and hedge-making.
• Induce parthenocarpy, e.g. in tomato.
• Used as herbicides: 2,4-D kills dicot weeds but does not affect mature monocots; used for weed-free lawns.
• Control xylem differentiation and help in cell division.

Gibberellins

More than 100 are known (GA₁, GA₂, GA₃...). GA₃ was the first discovered and is the most studied. All are acidic.

• Increase axis length: used to elongate grape stalks.
• Make fruits like apple elongate and improve their shape.
• Delay senescence, so fruits can be left on the tree longer to extend the market period.
• GA₃ speeds up malting in the brewing industry.
• Spraying sugarcane increases stem length, raising yield by as much as 20 tonnes per acre.
• Hasten maturity of juvenile conifers, leading to early seed production.
• Promote bolting (internode elongation just before flowering) in beet, cabbage and other rosette plants.

Cytokinins

Kinetin (a modified form of adenine, a purine) was discovered from autoclaved herring sperm DNA; it does not occur naturally in plants. Natural cytokinins include zeatin, from corn kernels and coconut milk. They are synthesised where cell division is rapid: root apices, developing shoot buds, young fruits.

• Help produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoots.
• Help overcome apical dominance.
• Promote nutrient mobilisation, which delays leaf senescence.

Ethylene

Ethylene is a simple gaseous PGR, made in large amounts by tissues undergoing senescence and by ripening fruits.

• Causes horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
• Promotes senescence and abscission of leaves and flowers.
• Promotes fruit ripening and increases the rate of respiration during ripening: the respiratory climacteric.
• Breaks seed and bud dormancy; initiates germination in peanut seeds and sprouting of potato tubers.
• Promotes rapid internode and petiole elongation in deep-water rice, keeping leaves above water.
• Promotes root growth and root hair formation, increasing absorption surface.
• Initiates flowering and synchronises fruit-set in pineapple; induces flowering in mango.
• Ethephon, the most widely used source of ethylene in agriculture, is absorbed and slowly releases ethylene. It hastens ripening in tomato and apple, accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut), and promotes female flowers in cucumber, raising yield.

Abscisic acid (ABA)

• A general plant growth inhibitor and inhibitor of metabolism; inhibits seed germination.
• Stimulates closure of stomata and increases tolerance to stresses, so it is called the stress hormone.
• Important in seed development, maturation and dormancy, helping seeds withstand desiccation.
• Acts in most situations as an antagonist to gibberellins.

Common traps: (1) Auxins cause apical dominance; cytokinins overcome it. (2) Bolting = gibberellin; female flowers in cucumber = ethylene (ethephon). (3) Bakanae disease is caused by a fungus, Gibberella fujikuroi. (4) Kinetin is not naturally present in plants; zeatin is. (5) Interfascicular cambium formation is dedifferentiation.

NEET focus

• Arithmetic vs geometric growth equations and curves; relative growth rate.
• Dedifferentiation examples; heterophylly examples.
• PGR discoveries (who, from what).
• Functions and agricultural uses of each PGR, which often come as match-the-column questions.

Practice questions