Plant growth and development are complex processes influenced by a variety of factors, including genetics, hormones, and environmental conditions. This chapter explores the key aspects of plant growth, including:
Growth
- Primary growth: Occurs at the tips of stems and roots, leading to an increase in length. It is driven by apical meristems.
Development
- Differentiation: The process by which cells specialize to perform specific functions.
- Morphogenesis: The formation of structures and organs.
Plant Hormones
- Auxins: Promote cell elongation, root development, and phototropism (bending towards light).
- Gibberellins: Promote cell elongation, seed germination, and flowering.
- Cytokinins: Promote cell division, shoot growth, and delay senescence (aging).
- Abscisic Acid (ABA): Promotes seed dormancy, stomatal closure, and stress responses.
- Ethylene: Promotes fruit ripening, senescence, and abscission (shedding of leaves or fruits).
Environmental Factors
- Light: Affects plant growth, development, and flowering.
- Temperature: Influences metabolic processes and growth rates.
- Water: Essential for plant growth and development.
- Nutrients: Plants require various nutrients, including nitrogen, phosphorus, and potassium, for growth.
Plant Movements
- Tropisms: Growth responses to environmental stimuli, such as phototropism (bending towards light) and geotropism (bending in response to gravity).
- Nastic movements: Non-directional movements, such as the folding of leaves in response to touch or changes in light.
Exercise
1. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate
Ans :
Growth
- Definition: An irreversible increase in size and mass due to an increase in the number and/or size of cells.
- Types: Primary growth (increase in length) and secondary growth (increase in girth).
Differentiation
- Definition: The process by which cells acquire specialized structures and functions.
- Example: A meristem cell differentiating into a xylem or phloem cell.
Development
- Definition: The process of growth and differentiation that leads to the formation of a mature organism.
- Stages: Embryonic development, juvenile stage, reproductive stage, and senescence.
Dedifferentiation
- Definition: The process by which mature cells revert to a less specialized state, regaining the ability to divide.
- Example: A phloem cell dedifferentiating into a cambium cell.
Redifferentiation
- Definition: The process by which dedifferentiated cells re-differentiate into a different cell type.
- Example: A cambium cell re-differentiating into a xylem or phloem cell.
Determinate Growth
- Definition: Growth that ceases after reaching a certain size or stage of development.
- Example: Most animals and some plants.
Meristem
- Definition: A region of actively dividing cells in plants.
- Types: Apical meristem (for primary growth), lateral meristem (for secondary growth), intercalary meristem (for growth in certain plants).
Growth Rate
- Definition: The rate at which an organism increases in size or mass over time.
- Factors: Influenced by genetics, environmental conditions, and hormonal factors.
2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Ans :
Growth: Increase in size and mass due to cell division and enlargement.
Development: Formation of structures and organs.
Factors: Influenced by genetics, hormones, and environment.
Types: Primary (length) and secondary (girth).
Meristems: Regions of actively dividing cells.
Hormones: Auxins, gibberellins, cytokinins, ABA, ethylene.
Environmental Factors: Light, temperature, water, nutrients.
Movements: Tropisms (directional) and nasties (non-directional).
3. Describe briefly:
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
Ans :
(a) Arithmetic Growth
- Definition: A type of growth where the rate of increase remains constant over time.
- Example: A plant growing at a constant rate of 1 cm per day.
- Graph: A linear line on a graph.
(b) Geometric Growth
- Definition: A type of growth where the rate of increase accelerates over time.
- Example: Bacterial growth, where the population doubles with each generation.
- Graph: A J-shaped curve on a graph.
(c) Sigmoid Growth Curve
- Definition: A growth curve that shows an initial exponential growth phase followed by a plateau.
- Example: Population growth of a species in a limited environment.
- Graph: An S-shaped curve on a graph.
(d) Absolute and Relative Growth Rates
- Absolute Growth Rate: The actual increase in size or mass per unit time.
- Relative Growth Rate: The rate of increase in size or mass relative to the initial size or mass.
4. List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.
Ans:
Five Main Groups of Natural Plant Growth Regulators
- Auxins
- Gibberellins
- Cytokinins
- Abscisic Acid (ABA)
- Ethylene
Note: Auxins
Discovery:
- Discovered by Charles Darwin and his son Francis in the late 19th century, who observed the bending of coleoptiles towards light (phototropism).
- The active substance was later isolated and identified as indole-3-acetic acid (IAA).
Physiological Functions:
- Cell elongation: Auxins promote cell elongation in stems and roots.
- Apical dominance: Auxins produced in the apical bud inhibit the growth of lateral buds, promoting a single, main stem.
- Phototropism: Auxins redistribute to the shaded side of a stem, causing it to bend towards the light.
- Root formation: Auxins can induce the formation of adventitious roots on stems or cuttings.
Agricultural/Horticultural Applications:
- Rooting of cuttings: Auxins are used to promote root formation in plant cuttings, facilitating vegetative propagation.
- Weed control: Synthetic auxins can be used as herbicides to control weeds by causing excessive growth and death.
- Fruit development: Auxins can promote fruit set and development in some plants.
- Parthenocarpy: Auxins can induce the development of seedless fruits without fertilization.
- Delaying senescence: Auxins can delay the aging and senescence of plant tissues.
5. Why is abscisic acid also known as stress hormone?
Ans : Abscisic acid (ABA) is often referred to as the stress hormone because it plays a crucial role in plant responses to various environmental stresses.
6. Both growth and differentiation in higher plants are open’. Comment.
Ans :
Growth:
- Primary growth: Occurs throughout the life of a plant, driven by apical meristems.
- Secondary growth: Can continue for many years in woody plants, driven by lateral meristems.
Differentiation:
- Continuous process: Cells can differentiate into various cell types at different stages of development.
- Plasticity: Plants can exhibit plasticity, meaning they can change their development in response to environmental cues.
Openness of growth and differentiation is a key characteristic of higher plants, allowing them to adapt to changing environmental conditions and exhibit a wide range of growth forms.
7. ‘Both a short day plant and a long day plant can produce can flower in a given place’. Explain.
Ans : Yes, both short-day plants and long-day plants can flower simultaneously in a given place.
The flowering response of plants is influenced by the length of day and night, a phenomenon known as photoperiodism. However, the specific day length requirements for flowering can vary widely among different plant species.
8. Which one of the plant growth regulators would you use if you are asked to:
(a) induce rooting in a twig
(b) quickly ripen a fruit
(c) delay leaf senescence
(d) induce growth in axillary buds
(e) ‘bolt’ a rosette plant
(f) induce immediate stomatal closure in leaves.
Ans :
(a) Induce rooting in a twig
- Auxin: Indole-3-acetic acid (IAA) is a common auxin used to promote root formation in cuttings.
(b) Quickly ripen a fruit
- Ethylene: This gas stimulates fruit ripening, often used commercially to ripen fruits like tomatoes and bananas.
(c) Delay leaf senescence
- Cytokinins: These promote cell division and delay aging, which can help prevent leaf senescence.
(d) Induce growth in axillary buds
- Cytokinins: Cytokinins can counteract the inhibitory effects of auxins, promoting the growth of axillary buds and reducing apical dominance.
(e) “Bolt” a rosette plant
- Gibberellins: These hormones promote stem elongation, which is essential for bolting (the transition from a rosette stage to a flowering stage) in many plants.
(f) Induce immediate stomatal closure in leaves
- Abscisic acid (ABA): ABA is a stress hormone that promotes stomatal closure in response to water stress or other environmental cues.
9. Would a defoliated plant respond to photoperiodic cycle? Why?
Ans :
Yes, a defoliated plant can still respond to the photoperiodic cycle.
While leaves are the primary site of photosynthesis and perceive light signals, other parts of the plant, such as stems and buds, can also detect changes in day length. These organs contain spe
10. What would be expected to happen if:
(a) GA3 is applied to rice seedlings
(b) dividing cells stop differentiating
(c) a rotten fruit gets mixed with unripe fruits
(d) you forget to add cytokinin to the culture medium.
Ans :
(a) Increased height: GA3 is a gibberellin that promotes stem elongation. Applying GA3 to rice seedlings would likely result in taller plants.
(b) Abnormal development: Differentiation is essential for the formation of specialized tissues and organs. If dividing cells stop differentiating, the plant would likely have abnormal structures and impaired functions.
(c) Ethylene release: Rotten fruits release ethylene gas, which is a natural plant hormone that promotes fruit ripening. Mixing a rotten fruit with unripe fruits could accelerate the ripening process of the unripe ones.
(d) Reduced cell division and growth: Cytokinins promote cell division and growth. If cytokinin is absent from the culture medium, plant cells may exhibit reduced growth and development.
