GROWTH
- Plant growth continues throughout the life due to the meristems.
- The growth where new cells are always added to the plant body by the meristem is called open form of growth.
- Primary growth: Due to root apical meristem & shoot apical meristem. It causes the elongation of the plants.
- Secondary growth: Due to lateral meristems, vascular cambium & cork-cambium. It causes increase in the girth of organs.
Phases of Growth
3 phases: meristematic, elongation & maturation.
· Meristematic phase: Occurs in the meristems at root apex & shoot apex. Cells have rich protoplasm and large nuclei.
· Elongation phase: Occurs in cells proximal to the meristematic zone. Cells have increased vacuolation, size and new cell wall deposition.
· Maturation phase: Occurs in the cells further away from the apex. Cells attain maximal size.
Conditions (essential elements) for Growth: Water, Oxygen, Nutrients, Temperature, Light & gravity.
Growth Rates
1. Arithmetic growth:
In this, only one daughter cell continues to divide while the other differentiates & matures. On plotting the length of the organ against time, a linear curve is obtained.
2. Geometrical growth:
Here, both daughter cells continue mitotic cell division.
In most systems, the initial growth is slow (lag phase), then it increases rapidly (log or exponential phase).
If nutrient supply is limited, the growth slows down leading to a stationary phase.
On plotting the parameter of growth against time, we get a sigmoid (S) curve. It is a characteristic of living organism growing in a natural environment.
DIFFERENTIATION, DEDIFFERENTIATION & REDIFFERENTIATION
- Differentiation: The process in which cells in meristems and cambium differentiate and mature to perform specific functions. The capacity of cell division is lost.
- Under certain conditions, living differentiated cells regain the capacity of division. This is called dedifferentiation. E.g. formation of meristems (interfascicular cambium & cork cambium) from differentiated parenchyma cells.
- The dedifferentiated cells can divide and produce cells that again lose the capacity to divide but mature to perform specific functions. It is called redifferentiation.
DEVELOPMENT
- It is a process that includes all changes from seed germination to senescence.
- It is the sum of growth and differentiation.
- Heterophylly due to phases of life: E.g. In cotton, coriander and larkspur, the leaves of the juvenile plants and mature plants are different in shape.
- Heterophylly due to environment: E.g. Difference in shapes of leaves produced in air and water (e.g. buttercup).
Factors controlling the development:
- Intrinsic factors: Include intracellular (genetic) or intercellular factors (such as plant growth regulators).
- Extrinsic factors: Include light, temperature, water, oxygen, nutrition, etc.
PLANT GROWTH REGULATORS (PGR / PLANT HORMONES / PHYTOHORMONES)
Simple molecules that regulate plant growth. 2 groups:
- Plant growth promoters: For growth promoting activities like cell division & enlargement, tropic growth, pattern formation, flowering, fruiting & seed formation. E.g. auxins, gibberellins and cytokinins.
- Plant growth inhibiters: For growth inhibiting activities like dormancy & abscission. Respond to wounds & stresses of biotic and abiotic origin. E.g. abscisic acid & ethylene.
1. Auxins
Charles Darwin & Francis Darwin studied phototropism in the coleoptiles of canary grass and concluded that the tip of coleoptile cause the bending of the entire coleoptile.
F.W. Went isolated Auxin from tips of coleoptiles of oat.
Types of Auxins:
o Natural: E.g. Indole-3-acetic acid (IAA) and indole butyric acid (IBA).
o Synthetic: E.g. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic).
Functions of auxins:
o Initiate rooting in stem cuttings for plant propagation.
o Promote flowering. E.g. in pineapples.
o Prevent fruit and leaf drop at early stages.
o Promote the abscission of older leaves and fruits.
o Induce parthenocarpy. E.g., in tomatoes.
o Used as herbicides. E.g. 2, 4-D is used to kill dicot weeds.
In higher plants, the growing apical bud inhibits the growth of lateral (axillary) buds. It is known as apical dominance.
2. Gibberellins
These are acidic PGR.
Gibberellic acid (GA3 or Terpenes) is one of the first discovered gibberellins.
Functions:
- They cause an increase in length of axis. So they are used to increase the length of grapes stalks.
- To elongate and improve the shape of fruits such as apple.
- They delay senescence. So the fruits can be left on the tree to extend the market period.
- Spraying gibberellins increases length of sugarcane stem.
- For bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.
3. Cytokinins
- F. Skoog observed that from the internodal segments of tobacco stems, the callus proliferated only if the nutrients medium was supplemented with extracts of vascular tissues, yeast extract, coconut milk or DNA.
- Skoog & Miller later identified and crystallized the active substance and termed as kinetin.
- Cytokinins were discovered as kinetin from the herring sperm DNA.
- Zeatin (from corn-kernels and coconut milk) is the natural substances with cytokinin-like activities.
Functions:
- Play a role in cytokinesis.
- Help to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
- Help overcome the apical dominance.
- Helps in the delay of leaf senescence.
4. Ethylene (C2H4)
A simple gaseous PGR discovered by Cousins.
Functions:
- Influences horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
- Promotes senescence and abscission of plant organs.
- Promotes fruit ripening. It enhances respiration rate during fruit ripening. This is called respiratory climactic.
- Breaks seed and bud dormancy.
- Promotes root growth and root hair formation.
- Used to initiate flowering in pineapples & mango.
- It is widely used in agriculture.
- The most widely used source of ethylene is ethephon.
5. Abscisic acid (ABA)
ABA is the derivatives of carotenoids.
Functions:
- Inhibitor of plant growth and metabolism.
- Inhibits seed germination.
- Stimulates the closure of stomata.
- Increases the tolerance of plants to various stresses. Therefore, it is also called the stress hormone.
- For seed development, maturation & dormancy.
Factors influencing the action of PGR
- Intrinsic factor: Genomic control.
- Extrinsic factors: Light and Temperature.
ROLE OF LIGHT AND TEMPERATURE ON FLOWERING
1. PHOTOPERIODISM
It is the response of plants to periods of day/night.
Based on light duration, plants are 3 groups:- Long day plants: They need exposure to light for a period exceeding a critical duration for flowering.
- Short day plants: They need exposure to light for a period less than the critical duration.
- Day-neutral plants: They have no correlation between exposure to light duration and induction of flowering.
2. VERNALISATION
It is the phenomenon in which some plants depend on exposure to low temperature for flowering.
Examples for vernalisation:
1. Some food plants, wheat, barley & rye have two varieties:
- Spring varieties: These are planted in the spring and come to flower before the end of the growing season.
- Winter varieties: They are planted in autumn. They germinate, and over winter come out as small seedlings, resume growth in the spring.
SEED DORMANCY
- Certain seeds fail to germinate even under favourable external conditions. Such seeds are in dormancy.
- Dormancy is caused by endogenous conditions within the seed. E.g. Hard seed coat; chemical inhibitors such as ABA, phenolic acids, para-ascorbic acid; and immature embryos.
- Dormancy can be overcome naturally and artificially. E.g.
- Breaking of seed coat barrier: By mechanical abrasions or vigorous shaking.
- Removing inhibitory substances: By subjecting the seeds to chilling conditions or by application of certain chemicals like gibberellic acid and nitrates.
- Changing the environmental conditions such as light and temperature.