PLANT GROWTH AND DEVELOPMENT

Chapter overview: plant growth and development

Plant development is the orderly sequence through which a zygote gives rise to roots, stems, leaves, flowers, fruits and seeds. It includes growth and differentiation, and is controlled by intrinsic factors such as genes and plant growth regulators (PGRs), as well as external factors such as light, temperature, water, oxygen and nutrition.

Chapter map Growth -> phases and rates -> differentiation -> plasticity -> five plant growth regulators and their applications.

Seed germination starts when conditions become favourable. In unfavourable conditions, seeds enter a period of suspended growth or rest; metabolic activity and growth resume when conditions return.

13.1 Growth: definition and indeterminate nature

Growth is an irreversible, permanent increase in the size of an organ, its parts, or an individual cell. It involves metabolic activity and an increase in protoplasmic material; swelling of a piece of wood in water is therefore not growth.

Plants retain the capacity for unlimited growth because meristems keep dividing and self-perpetuating. New cells are continually added to the plant body, giving plants an open form of growth.

Meristem	Contribution
Root and shoot apical meristems	Primary growth: elongation of the plant axis.
Intercalary meristem	Also contributes to elongation in suitable plant parts.
Vascular cambium and cork cambium	Lateral meristems in dicots and gymnosperms; cause secondary growth or increase in girth.

Measuring growth Since protoplasm is difficult to measure directly, growth is measured as increase in fresh/dry weight, length, area, volume or cell number. A pollen tube is measured by length, while a dorsiventral leaf is measured by surface area.

13.1.3 Phases of growth

Growth at the root and shoot tips occurs in three sequential phases: meristematic, elongation and maturation.

Phase	Location and features
Meristematic	At root and shoot apices. Cells divide continuously; they are rich in protoplasm, have large conspicuous nuclei, thin primary cellulosic walls and abundant plasmodesmatal connections.
Elongation	Immediately proximal to the meristematic region. Cells enlarge, become more vacuolated and deposit new cell wall material.
Maturation	Further away from the apex. Cells attain maximal size and undergo wall thickening and protoplasmic modification to acquire their final functions.

Growth rates and growth curves

Growth rate is the increase in growth per unit time. It can be arithmetic or geometrical.

Type	Cell division pattern	Graph / expression
Arithmetic growth	After mitosis, only one daughter cell continues to divide; the other differentiates and matures.	Linear curve. L_t = L₀ + rt
Geometrical growth	Both daughter cells retain the ability to divide. Growth begins slowly, then becomes rapid and exponential.	W₁ = W₀e^rt
Sigmoid growth	With limited nutrients, geometrical growth slows and reaches a stationary phase.	S-shaped curve with lag, log/exponential and stationary phases; typical of living systems in natural conditions.

In W₁ = W₀e^rt, W₁ is final size, W₀ is initial size, r is relative growth rate and t is time. Relative growth rate is also an efficiency index for production of new plant material.

Absolute vs relative growth rate Absolute growth rate compares total growth per unit time. Relative growth rate expresses growth per unit time on a common basis, such as per unit initial area or weight. Equal absolute increases can therefore have different relative rates.

Conditions required for growth

Water, oxygen and nutrients are essential. Water permits cell enlargement by maintaining turgidity and provides a medium for enzyme activity. Oxygen releases metabolic energy, while macro- and micronutrients are needed for protoplasm synthesis and energy supply.

Each plant has an optimum temperature range. Light and gravity are also environmental signals that influence specific stages of growth and development.

13.2 Differentiation, dedifferentiation and redifferentiation

Term	Meaning and example
Differentiation	Cells from meristems mature and become structurally specialised for particular functions. A tracheary element loses protoplasm and develops strong lignocellulosic secondary walls for water transport.
Dedifferentiation	Living differentiated cells regain the capacity to divide under certain conditions; differentiated parenchyma may form interfascicular cambium or cork cambium.
Redifferentiation	Cells produced by a dedifferentiated meristem lose the capacity to divide and mature again to perform specialised functions.

Both growth and differentiation in plants are open. The final form of a cell also depends on position: cells away from a root apical meristem may become root-cap cells, whereas peripheral cells may become epidermis.

13.3 Development and plasticity

Development includes all changes in the plant's life cycle, from seed germination to senescence. Broadly, it is the sum of growth and differentiation.

Plants can follow different developmental pathways in response to their environment or stage of life. This ability is called plasticity. Heterophylly is its example: juvenile and mature leaves differ in cotton, coriander and larkspur; in buttercup, leaves formed in air differ from those formed in water.

Control of development Intrinsic factors include genetic control and intercellular chemicals such as PGRs. Extrinsic factors include light, temperature, water, oxygen and nutrition. They act together rather than independently.

13.4 Plant growth regulators: overview and discovery

PGRs are small, simple molecules with diverse chemical compositions. They include indole compounds (IAA), adenine derivatives (kinetin), carotenoid derivatives (ABA), terpenes (GA₃) and the gas ethylene (C₂H₄).

PGR	Discovery clue	General role
Auxin	Darwin and Francis Darwin showed that coleoptile tips transmit the influence causing phototropic bending; F.W. Went isolated auxin from oat coleoptile tips.	Predominantly growth-promoting.
Gibberellin	Bakanae (foolish seedling) disease of rice caused by Gibberella fujikuroi; fungal filtrate produced symptoms.	Growth-promoting.
Cytokinin	Skoog's work on tobacco callus; Miller et al. identified kinetin, a cytokinesis-promoting substance.	Cell division and growth-promoting.
Ethylene	H.H. Cousins observed a volatile substance from ripe oranges hastened banana ripening.	Gaseous regulator, largely inhibitory but with diverse roles.
ABA	Inhibitor-B, abscission II and dormin were shown to be the same chemical.	Growth-inhibiting and stress responses.

Auxins, gibberellins and cytokinins are classed as growth promoters. ABA is an inhibitor involved in dormancy and abscission. PGR actions may be complementary, antagonistic, synergistic or individualistic.

Auxins, gibberellins and cytokinins

PGR	Major physiological effects	Uses / examples
Auxins	Initiate roots in stem cuttings; promote flowering in pineapple; control xylem differentiation and assist cell division. Apical bud auxin inhibits lateral buds (apical dominance). Prevent early fruit and leaf drop, but promote abscission of older mature leaves and fruits.	IAA and IBA are natural; NAA and 2,4-D are synthetic. Decapitation removes apical dominance, useful in tea plantations and hedge-making. Induce parthenocarpy in tomato; 2,4-D kills dicot weeds without affecting mature monocots.
Gibberellins	Increase axis length, delay senescence, promote bolting in rosette plants and hasten maturity in juvenile conifers.	GA₃ lengthens grape stalks, improves apple shape, speeds malting in brewing, and increases sugarcane stem length and yield.
Cytokinins	Promote cytokinesis, formation of new leaves and chloroplasts, lateral shoot growth and adventitious shoots. Overcome apical dominance, mobilise nutrients and delay leaf senescence.	Natural cytokinins occur at rapidly dividing regions such as root apices, developing buds and young fruits. Zeatin was isolated from corn kernels; kinetin is not naturally found in plants.

Ethylene and abscisic acid

PGR	Major physiological effects	Uses / examples
Ethylene	Promotes senescence and abscission; ripens fruits and causes respiratory climacteric. Breaks seed and bud dormancy; promotes peanut germination and potato sprouting. Promotes root growth, root hairs, and rapid petiole/internode elongation in deep-water rice.	Initiates flowering and synchronises fruit-set in pineapple; induces mango flowering. Ethephon releases ethylene slowly: hastens tomato and apple ripening, accelerates abscission for thinning, and promotes female flowers in cucumber.
Abscisic acid (ABA)	General growth and metabolic inhibitor. Inhibits seed germination, closes stomata and increases tolerance to stresses; also regulates seed development, maturation and dormancy.	Called the stress hormone. By inducing dormancy, ABA helps seeds withstand desiccation and other adverse conditions. It generally antagonises gibberellins.

Fast recall Rooting in cuttings - auxin; rapid fruit ripening - ethylene; delay leaf senescence - cytokinin; axillary-bud growth - cytokinin / removal of apical auxin; bolting - gibberellin; immediate stomatal closure - ABA.

NCERT revision prompts

Define growth, meristem, growth rate, differentiation, dedifferentiation, redifferentiation and development.
Why is plant growth called open or indeterminate?
Compare arithmetic, geometrical and sigmoid growth curves.
Differentiate absolute and relative growth rate.
Describe the three phases of growth at a root tip.
Explain plasticity and heterophylly with examples.
List the five main groups of PGRs and compare their major effects.
Why is ABA called a stress hormone?
State one agricultural or horticultural use each of auxin, gibberellin, cytokinin and ethylene.