Question 1

Q1. Name any five plant growth regulators.

Accepted Answer

The five major classes of plant growth regulators (PGRs) or plant hormones are:

1.  **Auxins**
2.  **Gibberellins (GAs)**
3.  **Cytokinins**
4.  **Ethylene**
5.  **Abscisic Acid (ABA)**

Question 2

Q2. 'Plasticity' in plant growth is that plants follow different pathways in response to environment. Elaborate.

Accepted Answer

### Plant Plasticity

**Definition:**
Plant plasticity refers to the ability of plants to form different kinds of structures in response to different phases of life or different environmental conditions. It is the capacity of a plant to alter its morphology, physiology, and development in response to the environment.

**Elaboration:**
Plants exhibit plasticity in various aspects, such as the formation of leaves, flowers, or even the overall growth habit. This adaptability allows them to optimize their survival and reproduction in diverse and changing environments. The developmental pathways followed by plants are not rigidly fixed but can be modified by internal (developmental stage) and external (environmental) cues.

**Examples of Plasticity:**

1.  **Heterophylly due to developmental stages:**
    *   In plants like cotton, coriander, and larkspur, the leaves produced in the juvenile phase are different in shape from those produced in the mature phase. This change in leaf morphology is an example of developmental plasticity, where the plant's internal developmental program dictates the change.

2.  **Heterophylly due to environmental factors:**
    *   A classic example is the buttercup (Ranunculus aquatilis). The leaves produced in air (terrestrial habitat) are typically lobed or dissected, while the leaves produced in water (aquatic habitat) are highly dissected and ribbon-like. This difference in leaf shape is a direct response to the aquatic environment, which requires different adaptations for gas exchange, light capture, and water flow.

**Significance:**
Plasticity is crucial for plants as it enables them to:
*   **Adapt to varying conditions:** Plants, being sessile, cannot move away from unfavorable conditions. Plasticity allows them to adjust their growth and development to cope with stresses like drought, nutrient deficiency, or changes in light availability.
*   **Optimize resource utilization:** By altering their form, plants can efficiently capture resources (e.g., light, water, nutrients) available in their specific microenvironment.
*   **Enhance survival and reproduction:** Ultimately, plasticity contributes to the plant's ability to survive, grow, and reproduce successfully in a dynamic environment.

In essence, plasticity highlights that plant development is not a predetermined, linear process but a flexible one, constantly modulated by both intrinsic factors and extrinsic environmental signals.

Question 3

Q3. Describe briefly the physiological effects of plant growth regulators.

Accepted Answer

Plant Growth Regulators (PGRs) are small, simple molecules of diverse chemical composition, which regulate plant growth and development. They are broadly classified into two groups based on their functions: plant growth promoters and plant growth inhibitors. Here are their brief physiological effects:

### I. Plant Growth Promoters
These are involved in growth-promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation.

1.  **Auxins (e.g., IAA, IBA, NAA, 2,4-D)**
    *   **Cell Elongation:** Promote cell enlargement, especially in stems and coleoptiles.
    *   **Apical Dominance:** The apical bud inhibits the growth of lateral buds.
    *   **Root Initiation:** Promote rooting in stem cuttings.
    *   **Parthenocarpy:** Induce fruit development without fertilization (e.g., in tomatoes).
    *   **Abscission Prevention:** Prevent premature fall of leaves and fruits at early stages.
    *   **Xylem Differentiation:** Help in xylem differentiation.
    *   **Herbicide:** 2,4-D (2,4-dichlorophenoxyacetic acid) is a synthetic auxin used as a weedicide to kill dicot weeds.

2.  **Gibberellins (GAs) (e.g., GA₃)**
    *   **Stem Elongation:** Cause a marked increase in the length of the axis, especially in genetically dwarf plants (e.g., pea, maize).
    *   **Bolting:** Induce sudden elongation of internodes just prior to flowering in rosette plants (e.g., cabbage, beet).
    *   **Seed Germination:** Break seed dormancy and promote germination.
    *   **Fruit Growth:** Promote fruit growth (e.g., apple elongation).
    *   **Malting Process:** Speed up the malting process in brewing industry.
    *   **Juvenility:** Delay senescence.

3.  **Cytokinins (e.g., Kinetin, Zeatin)**
    *   **Cell Division:** Primarily promote cell division (cytokinesis).
    *   **Lateral Shoot Growth:** Help to overcome apical dominance and promote the growth of lateral buds.
    *   **Chloroplast Development:** Promote chloroplast formation in leaves.
    *   **Delay Senescence:** Delay the aging of leaves (Richmond-Lang effect).
    *   **Nutrient Mobilization:** Help in the mobilization of nutrients.

### II. Plant Growth Inhibitors
These are involved in growth-inhibiting activities like dormancy and abscission.

4.  **Abscisic Acid (ABA)**
    *   **Dormancy Induction:** Induces dormancy in seeds and buds.
    *   **Stress Hormone:** Increases the tolerance of plants to various kinds of stresses, hence called the 'stress hormone'.
    *   **Stomatal Closure:** Promotes the closure of stomata during water stress.
    *   **Abscission:** Promotes abscission of leaves and fruits.
    *   **Antagonistic to GAs:** Acts as an antagonist to gibberellins in seed germination.

### III. Both Promoter and Inhibitor (Gaseous PGR)

5.  **Ethylene (C₂H₄)**
    *   **Fruit Ripening:** Promotes fruit ripening (climacteric fruits).
    *   **Senescence and Abscission:** Promotes senescence and abscission of leaves and flowers.
    *   **Apical Hook Formation:** Causes horizontal growth of seedlings and swelling of the axis in dicot seedlings.
    *   **Root Growth and Root Hair Formation:** Promotes root growth and root hair formation, increasing the absorption surface area.
    *   **Flowering:** Induces flowering in pineapple and mango.
    *   **Breaking Dormancy:** Breaks seed and bud dormancy.

Question 4

Q4. What is photoperiodism and its significance?

Accepted Answer

### What is Photoperiodism?

**Definition:**
Photoperiodism is the physiological response of plants to the relative lengths of day (light) and night (dark) periods. It is a crucial environmental cue that regulates various developmental processes in plants, most notably flowering.

**Mechanism:**
It is not the absolute length of the day or night that is critical, but rather the duration of the dark period. Plants perceive the photoperiodic stimulus through their leaves, and this perception is mediated by a photoreceptor pigment called **phytochrome**. The signal is then transmitted to the apical meristem to induce flowering.

**Classification of Plants based on Photoperiodism:**
Based on their response to photoperiod, plants are classified into three main categories:

1.  **Long Day Plants (LDPs):** These plants require exposure to light for a period longer than a critical duration (critical photoperiod) to flower. They flower when the day length is long and the night length is short (e.g., wheat, oat, radish, spinach).
2.  **Short Day Plants (SDPs):** These plants require exposure to light for a period shorter than a critical duration to flower. They flower when the day length is short and the night length is long (e.g., tobacco, chrysanthemum, soybean, rice).
3.  **Day Neutral Plants (DNPs):** These plants flower irrespective of the photoperiod. They are not dependent on the duration of light or dark periods for flowering (e.g., tomato, maize, cucumber).

### Significance of Photoperiodism

Photoperiodism plays a vital role in the life cycle of plants, ensuring that key developmental events occur at the most favorable time of the year. Its significance includes:

1.  **Timing of Flowering:** The most significant role is in regulating the timing of flowering. By flowering at specific times, plants can ensure successful pollination, seed development, and dispersal, optimizing their reproductive success. For example, LDPs flower in spring/early summer, while SDPs flower in late summer/autumn.
2.  **Adaptation to Seasons:** It allows plants to adapt to seasonal changes. For instance, seed germination, vegetative growth, and dormancy are also influenced by photoperiod, ensuring that these processes occur when environmental conditions are most suitable.
3.  **Geographical Distribution:** Photoperiodism influences the geographical distribution of plant species. Plants adapted to specific day lengths will thrive in regions where those conditions prevail.
4.  **Agricultural Importance:** Farmers and horticulturists utilize the knowledge of photoperiodism to manipulate flowering and fruiting times for better crop yield and off-season production. For example, providing artificial light to SDPs or shading LDPs can induce flowering out of season.
5.  **Vegetative Growth and Dormancy:** Besides flowering, photoperiod also controls other aspects like the onset of dormancy in buds and seeds, leaf fall (abscission), and tuberization, ensuring plant survival during unfavorable seasons.

Question 5

Q5. Long day plants and short day plants.

Accepted Answer

Long day plants (LDPs) and short day plants (SDPs) are categories of plants classified based on their photoperiodic response, specifically their requirement for light and dark periods to induce flowering.

### Long Day Plants (LDPs)

*   **Definition:** These are plants that require exposure to light for a period longer than a specific critical duration (critical photoperiod) to induce flowering. They flower when the day length is long and the night length is short.
*   **Critical Factor:** For LDPs, the *dark period must be shorter than a critical dark period*.
*   **Flowering Season:** Typically flower during late spring or early summer when day lengths are naturally long.
*   **Examples:** Wheat, Oat, Radish, Spinach, Sugar beet, Barley.

### Short Day Plants (SDPs)

*   **Definition:** These are plants that require exposure to light for a period shorter than a specific critical duration (critical photoperiod) to induce flowering. They flower when the day length is short and the night length is long.
*   **Critical Factor:** For SDPs, the *dark period must be longer than a critical dark period*.
*   **Flowering Season:** Typically flower during late summer, autumn, or winter when day lengths are naturally short.
*   **Examples:** Tobacco, Chrysanthemum, Soybean, Rice, Xanthium (cocklebur), Dahlia.

**Note:** It is the duration of the continuous dark period, not the light period, that is critical for flowering in both LDPs and SDPs. A flash of light during the critical dark period can inhibit flowering in SDPs and promote it in LDPs.

Question 6

Q6. What are the various phases of growth?

Accepted Answer

Plant growth is generally indeterminate and occurs in three distinct phases: the meristematic phase, the elongation phase, and the maturation phase. These phases represent a continuous process, with cells progressively moving from one phase to the next.

Question 7

Q7. Differentiate between growth and development.

Accepted Answer

Growth and development are two closely related but distinct processes in plants. Growth is a quantitative aspect, primarily referring to an irreversible increase in size, while development is a broader, qualitative concept encompassing all changes an organism undergoes throughout its life cycle.

Question 8

Q8. Describe briefly the physiological effects of plant growth regulators.

Accepted Answer

Plant Growth Regulators (PGRs), also known as plant hormones or phytohormones, are small, simple molecules of diverse chemical composition, which regulate plant growth and development. They are broadly classified into five major groups based on their functions: auxins, gibberellins, cytokinins, ethylene, and abscisic acid (ABA). Each group has distinct physiological effects.

Question 9

Q9. List five characteristics of plant hormones.

Accepted Answer

Plant hormones, also known as Plant Growth Regulators (PGRs) or phytohormones, possess several distinct characteristics that enable them to regulate plant growth and development effectively.

Question 10

Q10. Name a gaseous PGR.

Accepted Answer

Ethylene is the only gaseous plant growth regulator (PGR).

Question 11

Q11. What is the significance of photoperiodism?

Accepted Answer

Photoperiodism is the physiological response of plants to the relative lengths of day and night (photoperiod). It plays a crucial role in regulating various developmental processes, most notably flowering, in many plant species. Its significance can be understood as follows:

1.  **Regulation of Flowering:** The primary significance of photoperiodism is its role in controlling the timing of flowering. Plants are categorized into Short-Day Plants (SDPs), Long-Day Plants (LDPs), and Day-Neutral Plants (DNPs) based on their photoperiodic requirement for flowering. This ensures that plants flower at an optimal time of the year, maximizing successful pollination, seed formation, and dispersal, which are critical for species survival.
2.  **Adaptation to Environment:** Photoperiodism allows plants to adapt to specific environmental conditions prevalent at different times of the year. For example, flowering in spring (LDPs) or autumn (SDPs) ensures that favorable conditions (temperature, water availability, pollinator activity) are met for reproduction.
3.  **Geographical Distribution:** It influences the geographical distribution of plant species. Plants with specific photoperiodic requirements can only thrive in regions where those conditions are met, thus limiting their range.
4.  **Other Physiological Responses:** Besides flowering, photoperiodism also influences other physiological processes such as:
    *   **Tuberization:** Formation of tubers in plants like potato.
    *   **Bulbing:** Formation of bulbs in plants like onion.
    *   **Dormancy:** Induction and breaking of dormancy in buds and seeds.
    *   **Leaf Abscission:** Shedding of leaves in deciduous trees.
    *   **Vegetative Growth:** Regulation of overall plant growth patterns.

In essence, photoperiodism acts as an environmental cue, allowing plants to synchronize their life cycle events with the most favorable seasonal conditions, thereby enhancing their reproductive success and survival.

Question 12

Q12. Why is abscisic acid also known as stress hormone?

Accepted Answer

Abscisic acid (ABA) is widely known as the 'stress hormone' in plants due to its crucial role in mediating plant responses to various environmental stresses and inducing dormancy. It helps plants cope with adverse conditions by initiating protective mechanisms. Here's why:

1.  **Drought Stress:** Under drought conditions, ABA levels increase significantly. It promotes the closure of stomata, which are tiny pores on leaves responsible for gas exchange and transpiration (water loss). By closing stomata, ABA reduces water loss from the plant, thereby conserving water and preventing wilting. This is its most well-known anti-stress function.
2.  **Salinity Stress:** High salt concentrations in the soil can also induce ABA production. ABA helps plants tolerate salinity by regulating ion transport and maintaining osmotic balance.
3.  **Cold Stress:** ABA plays a role in increasing plant tolerance to cold and freezing temperatures by inducing the expression of cold-responsive genes and promoting the accumulation of cryoprotectants.
4.  **Induction of Dormancy:** ABA promotes dormancy in seeds and buds. Seed dormancy is a vital adaptation that prevents premature germination under unfavorable conditions, ensuring that seeds germinate only when conditions are optimal for seedling survival. Similarly, bud dormancy protects delicate meristematic tissues during harsh winters.
5.  **Promotes Abscission:** While ethylene is the primary hormone for abscission (shedding of leaves, fruits, flowers), ABA can also promote it, especially under stress conditions, allowing the plant to shed damaged or non-essential parts to conserve resources.

In summary, ABA acts as a key signaling molecule that enables plants to perceive and respond to environmental stressors by initiating adaptive physiological changes, thus earning its designation as the 'stress hormone'.

Question 13

Q13. 'Both short-day plants and long-day plants can flower simultaneously in a given place'. Explain.

Accepted Answer

The statement 'Both short-day plants and long-day plants can flower simultaneously in a given place' is indeed correct and can be explained by understanding the concept of critical photoperiod and the specific requirements of different plant species.

### 1. Understanding Photoperiodism
Photoperiodism is the physiological response of plants to the relative lengths of day and night. It is a crucial environmental cue that regulates flowering in many plant species. Plants are classified into:
*   **Short-Day Plants (SDPs):** These plants require a photoperiod *shorter* than a specific critical photoperiod to flower. More accurately, they require a continuous dark period *longer* than a critical dark period.
*   **Long-Day Plants (LDPs):** These plants require a photoperiod *longer* than a specific critical photoperiod to flower. More accurately, they require a continuous dark period *shorter* than a critical dark period.
*   **Day-Neutral Plants (DNPs):** These plants flower irrespective of the photoperiod.

### 2. The Concept of Critical Photoperiod
The key to understanding simultaneous flowering lies in the 'critical photoperiod'. It is important to note that the critical photoperiod is not a universal value but is **specific to each plant species**. For example:
*   An SDP might have a critical photoperiod of 14 hours. This means it will flower if the day length is *less than* 14 hours (or night length is *more than* 10 hours).
*   An LDP might have a critical photoperiod of 10 hours. This means it will flower if the day length is *more than* 10 hours (or night length is *less than* 14 hours).

### 3. Simultaneous Flowering Explained
Consider a geographical location where the day length is, for instance, **12 hours** (and thus night length is 12 hours) during a particular season (e.g., spring or autumn). In this scenario:

*   **For an SDP:** If an SDP species has a critical photoperiod of, say, **14 hours**, then a 12-hour day length (which is *shorter* than its 14-hour critical photoperiod) will satisfy its requirement for flowering. Thus, this SDP will flower.
*   **For an LDP:** Simultaneously, if an LDP species has a critical photoperiod of, say, **10 hours**, then a 12-hour day length (which is *longer* than its 10-hour critical photoperiod) will satisfy its requirement for flowering. Thus, this LDP will also flower.

Therefore, at the same time and in the same place, a specific day length can be simultaneously *shorter* than the critical photoperiod of one SDP species and *longer* than the critical photoperiod of another LDP species. This allows both types of plants to initiate flowering and bloom concurrently.

### Conclusion
The ability of both short-day and long-day plants to flower simultaneously in a given location is a testament to the diversity of critical photoperiods among different plant species. It highlights that the classification into SDP or LDP is relative to each plant's specific critical photoperiod, not to an absolute day length. This adaptation ensures that different plant species can optimize their reproductive timing within the same environment, utilizing the same seasonal cues for their unique developmental needs.

Chapter 15: Plant Growth and Development