Plant Growth and Development
Definitions & Growth Models
1. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
- Growth: An irreversible permanent increase in size of an organ or its parts or even of an individual cell.
- Differentiation: The process in which cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions.
- Development: It is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence. (Growth + Differentiation).
- Dedifferentiation: The process where differentiated living cells, which have lost the capacity to divide, regain the capacity of division under certain conditions (e.g., formation of interfascicular cambium).
- Redifferentiation: The process where dedifferentiated cells again lose the capacity to divide and mature to perform specific functions (e.g., secondary cortex, secondary xylem).
- Determinate Growth: Growth where the plant organs (like leaves and flowers) grow to a certain size and then stop.
- Meristem: Specialized regions of active cell division in plants.
- Growth Rate: The increased growth per unit time.
2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Growth in plants occurs at cellular, tissue, and organ levels, and manifests in various ways depending on the part of the plant.
- Some parts grow by increasing length (e.g., roots, stems).
- Some grow by increasing surface area (e.g., leaves).
- Some grow by increasing volume/weight (e.g., fruits).
- Some grow by increasing cell number (e.g., maize root apical meristem).
Therefore, a single parameter (like length alone) cannot capture the complete picture of growth for the entire plant life cycle.
3. Describe Arithmetic growth, Geometric growth, Sigmoid curve, and Growth rates.
(a) Arithmetic Growth: Only one daughter cell continues to divide while the other differentiates and matures. The rate of growth is constant.
(Linear Curve)
(b) Geometric Growth: Both daughter cells retain the ability to divide. Growth is slow initially (lag phase) and then increases rapidly (exponential phase).
(Exponential Curve)
(c) Sigmoid Growth Curve: The characteristic growth curve of living organisms growing in a natural environment. It consists of Lag phase, Log (Exponential) phase, and Stationary phase.
[Image of Sigmoid growth curve](d) Absolute vs Relative Growth Rate:
- Absolute: Measurement and comparison of total growth per unit time.
- Relative: Growth of the given system per unit time expressed on a common basis (e.g., per unit initial parameter).
Plant Growth Regulators (PGRs)
4. List five main groups of PGRs. Write a note on one (Auxin).
Five Groups: Auxins, Gibberellins, Cytokinins, Ethylene, Abscisic Acid (ABA).
Discovery: First isolated from human urine. Charles Darwin and Francis Darwin observed phototropism in canary grass coleoptiles.
Physiological Functions:
- Initiate rooting in stem cuttings.
- Promote flowering (e.g., pineapples).
- Prevent fruit and leaf drop at early stages.
- Apical Dominance: Inhibit the growth of lateral buds.
5. Why is abscisic acid (ABA) also known as stress hormone?
Abscisic acid (ABA) stimulates the closure of stomata during conditions of water stress (drought). By closing stomata, it reduces transpiration and helps the plant conserve water. Because it increases the tolerance of plants to various kinds of stresses, it is called the stress hormone.
6. ‘Both growth and differentiation in higher plants are open’. Comment.
- Growth is Open: Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems. New cells are constantly being added.
- Differentiation is Open: The same apical meristem cells can differentiate into different types of tissues (xylem, phloem, parenchyma) depending on their location and timing. Furthermore, differentiated cells can dedifferentiate and redifferentiate.
7. ‘Both a short day plant and a long day plant can produce flowers simultaneously in a given place’. Explain.
Flowering depends on the critical photoperiod.
- Short Day Plant (SDP): Flowers when day length is shorter than critical period.
- Long Day Plant (LDP): Flowers when day length is longer than critical period.
If the day length in a given place is such that it is shorter than the critical period of the SDP but longer than the critical period of the LDP (assuming their critical periods differ significantly), or if we artificially manipulate light, simultaneous flowering is possible. More practically, if their critical requirements overlap at a specific duration (e.g., 12 hours) accessible in that location, they can flower together.
8. Which PGR would you use for the following?
| Goal | PGR to Use |
|---|---|
| (a) Induce rooting in a twig | Auxins |
| (b) Quickly ripen a fruit | Ethylene |
| (c) Delay leaf senescence | Cytokinins |
| (d) Induce growth in axillary buds | Cytokinins |
| (e) ‘Bolt’ a rosette plant | Gibberellins |
| (f) Induce stomatal closure | Abscisic Acid (ABA) |
Photoperiodism & Scenarios
9. Would a defoliated plant respond to photoperiodic cycle? Why?
No.
The site of perception of light/dark duration (photoperiod) is the leaves. It is hypothesized that a hormonal substance (florigen) migrates from leaves to shoot apices to induce flowering. Without leaves, there is no site to perceive the light signal, so the plant cannot respond.
10. What would be expected to happen if…
- (a) $GA_3$ is applied to rice seedlings: They will show excessive elongation (similar to Bakane/Foolish seedling disease) and may become tall and sterile.
- (b) Dividing cells stop differentiating: The plant would grow into a mass of undifferentiated cells (callus or tumor) without forming specific organs like roots or shoots.
- (c) A rotten fruit gets mixed with unripe fruits: The rotten fruit releases Ethylene gas, which accelerates the ripening process of the surrounding unripe fruits.
- (d) You forget to add cytokinin to the culture medium: Cell division will not occur or will be very slow. Shoot formation (morphogenesis) will not take place even if auxins are present (since high cytokinin:auxin ratio is needed for shoot formation).