class 11 biology

CHAPTER  15: PLANT GROWTH AND DEVELOPMENT

Growth:

• It is a characteristic of living beings in which an irreversible permanent increase in size of an organ or its parts occur or an increase in the size of a cell.

Types of Growth Rate

• Growth rate can be defined as the increase in growth per unit time.
• Plants show two types of growth—Arithmetic and Geometric—according to the increase shown by the growth rate.
• Arithmetic growth - Only one daughter cell continues to divide while others differentiate or mature. Example − root elongating at a constant rate.

o    Geometric Growth - Initial growth is slow (lag phase), followed by a rapid increase in growth (log/exponential phase), and followed by a phase where growth slows down (stationary phase). Example − all cells, tissues and organs show this type of growth

Conditions for Growth

• Include: water, oxygen, nutrients and temperature.

Differentiation, Dedifferentiation and Redifferentiation:

• Differentiation
• In this process, cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.

• Dedifferentiation
• Process in which living differentiated cells regain their capacity to divide
• Redifferentiation
• Process in which differentiated cells that have lost their ability to divide are reformed from dedifferentiated cells and have the ability to perform specific functions.

 Development:

·         Development – changes in the life cycle.

·         Plasticity – different kinds of structure in response to environment or phases of life.

·         Eg. Heteropylly in cotton and coriander.  In these plants, leaves have different shapes based on the phase of life cycle as well as the habitat.

·         Development can also be termed as − growth + differentiation

• Development is controlled by intrinsic as well as extrinsic factors.
• Intrinsic − Genetic factors and plant growth regulators
• Extrinsic − light, temperature, water, oxygen, etc.

Plant Growth Regulators / Phytohormones:

Classification based on their nature of action:

·         Plant growth promoters. –  Auxins, Gibberellins and Cytokinins.

·         Plant growth  inhibitors - Absissic acid  (ABA)

• Ethylene may fit in either of the two groups, but is largely an inhibitor.

Types of phytohormones:

·         Auxins

·         Gibberellins

·         Cytokinins

·         Ethylene

·         Abscisic acid

Auxins

Discovery :– auxins were discovered by Charles Darwin and Francis Darwin.

Isolation :– they were isolated from tips of coleoptiles of oat seedlings by F.W.Went as IAA and IBA.

Effects: –

·         Initiate rooting in stem cuttings, plant propagation.

·         Promote flowering, prevent fruit anf leaf drop.

·         Promote abscission of older mature leaves.

Uses:-

·         Induce parthenocarpy

·         Widely used as herbicides (2,4 – D)

·         To kill dicotyledonous weeds

·         Prepare weed free lawns.

·         Controls xylem differentiation and helps in cell division

Gibberellins

Discovery : E. Kurosawa identified gibbereilins present in a fungal pathogen Gibberella fujikuroi

Isolation: Infected rice seedlings when treated with sterile filtrates of fungus

Effects:

·         GA’S are  acidic.

·         Increase in length, cause fruits to elongate and improve its shape.

·         Delay senescence, extend the market period.

·         GA3 used to speed up malting  process in brewing

Uses:

·         Spraying sugarcane crop with this

·         Increases length of stem

·         Fastens maturity period.

·         Promotes bolting

Cytokinins

Discovery : Skoog and Miller

Isolation:  Crystallized it promoting active substance named it kinetin from coconut milk, corn – kernels.

Effects: 

·         They are synthesized where rapid cell division takes place

·         Produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.

Uses:  

·         Help overcome apical dominance

·         Promote nutrient mobilization which helps in the delay of leaf senescence

Ethylene (gaseous hormone):

Discovery : Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored un ripened bananas

Effects:

·         Promotes senescence and abscission

·         Highly effective in fruit ripening

·         Enhances the respiration rate

·         Breaks seed and bud dormancy

·         Initiates germination in peanut seeds.

·         Sprouting  potato tubers, promotes  root growth root hair formation

Uses:

·         Used to initiate flowering, for synchronizing fruit, induces flowering, regulates physiological processes.

·         Hastens fruit ripening, accelerates abscission and Promotes female flowers.

Abscisic Acid (ABA):

Discovery: Researchers.

Isolation: 3 kinds of inhibitors - Inhibitor – B, abscission ll & dormin.

Effects:

·         Regulates abscission dormancy

·         ABA stimulates the closure of stomata

·         Increases tolerance, seed development

·         Maturation, dormancy, withstand desiccation

Uses: 

·         There are no. of events in a plant

·         Where more than one PGR interact to affect that event, example - Dormancy in seeds / buds abscission, senescence, apical dominance.

Photoperiodism

• It is the response of plants to periods of day/night
• Some plants require periodic exposure to light to induce flowering. Duration of dark period is equally important for flowering.

Long Day Plants − Plants that require exposure to light for a period exceeding critical duration to induce flowering.

Short Day Plants − Plants that require exposure to light for a period less than this critical period to induce flowering.

Day Neutral Plants − Plants where there is no correlation between exposure to light duration and induction of flowering.

Vernalization

• It is the phenomenon of dependence of flowering on exposure to low temperature.
• Example − Biennial plants
  These are monocarpic plants that flower and then die in second season. Some examples are sugar beet, cabbage, carrot, etc.

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