# **Growth Responses in Plants** This section covers how plants respond to their environment through growth, focusing on the roles of chemical regulators (hormones) and responses to external stimuli like light, gravity, and temperature. ### **9.7.1 Plant Growth Regulators** Plants coordinate their growth and development using chemical messengers called **plant hormones** or **growth regulators**. They are synthesized in specific locations and transported to sites of action to induce cellular changes like division, enlargement, or maturation. #### **Growth Promoters** * ***Auxins (e.g., Indole-3-acetic acid, IAA)*** * The first plant hormone discovered. * **Synthesis Sites:** Shoot tips, young leaves, embryos, developing flowers and seeds. * **Primary Function:** Promotes *cell elongation*, which is crucial for stem elongation and overall plant height. * ***Gibberellins (e.g., GA3)*** * Over 75 types have been identified. * **Synthesis Sites:** Abundant in seeds and young shoots. * **Primary Function:** Control stem elongation by stimulating both *cell division* and *cell elongation*. * ***Cytokinins (e.g., Zeatin)*** * Named for their role in *cell division* (cytokinesis). * **Synthesis Sites:** Areas of active cell division like root tips, seeds, fruits, and leaves. * **Primary Function:** Promote cell division. The ratio of cytokinin to auxin can determine organogenesis (shoot vs. root formation). #### **Growth Inhibitors** * ***Abscisic Acid (ABA)*** * **Synthesis Sites:** Synthesized in plastids, particularly during stress conditions. * **Primary Function:** A general *growth inhibitor*. It promotes seed dormancy and causes stomata to close in response to water stress. * ***Ethene (Ethylene)*** * A simple, gaseous hormone produced from an amino acid. * **Synthesis Sites:** Produced in most tissues, especially when stressed. * **Primary Function:** Stimulates *fruit ripening* and initiates *abscission* (the shedding of leaves and fruits). #### **Comparison of Plant Growth Regulators** | Part Affected | Auxins | Gibberellins | Cytokinins | Abscisic Acid (ABA) | Ethene | | :--- | :--- | :--- | :--- | :--- | :--- | | **Stem** | - Promote cell enlargement & division in cambium. | - Promote cell enlargement & leaf growth (with auxin). | - Promote cell division in apical meristem & cambium.
- Promote leaf growth. | - Inhibit growth during stress (drought, salinity). | - Inhibit growth during stress. | | **Root** | - Low conc: promotes growth.
- High conc: inhibits growth.
- Promotes growth from cuttings. | Nil | - Inhibit primary root growth.
- Promote lateral root growth. | Nil | - Inhibit root growth. | | **Floral buds** | - Promote bud initiation. | - Promote bud initiation. | - Promote bud initiation.
- Promote lateral bud growth.
- Break bud dormancy. | - Promote bud dormancy. | - Promote bud initiation. | | **Flowering** | Nil | - Promote in long-day plants.
- Inhibit in short-day plants.
- Antagonistic to ABA. | Nil | - Promote in short-day plants.
- Inhibit in long-day plants.
- Antagonistic to gibberellin. | - Promote in pineapple. | | **Apical Dominance** | Promote | Enhance the action of auxin | Inhibit | Nil | Nil | | **Fruit Growth** | Promote | Promote | Promote | Inhibits growth/Promotes abscission | Promote ripening | | **Parthenocarpy** | Rarely promote | Promote | Rarely promote | Nil | Nil | | **Leaf Senescence** | Delay | Delay | Delay | Promote | Promote | | **Seed Dormancy** | Nil | Break | Break | Promote | Break in some species | | **Abscission** | Inhibit (in young leaves) | Inhibit | Inhibit | Promote | Promote | | **Stomatal Opening**| Nil | Nil | Promote | Inhibit (causes closure) | Nil | ### **9.7.2 Growth Movements in Plants (Tropic Movements)** Plants exhibit movements by differential growth in response to stimuli. These are known as **tropisms**. - **Phototropism:** Growth in response to a *light* source. - *Positive Phototropism:* Shoots grow towards light. - *Negative Phototropism:* Roots grow away from light. - **Geotropism (Gravitropism):** Growth in response to the force of *gravity*. - *Positive Geotropism:* Roots grow downwards. - *Negative Geotropism:* Shoots grow upwards. - **Thigmotropism:** Growth in response to *touch*. Seen in climbing plants that wrap around supportive structures. - **Chemotropism:** Growth in response to a *chemical* stimulus. For example, fungal hyphae growing towards nutrients. ### **9.7.3 Photoperiodism** This is the response of plants, especially in flowering, to the relative lengths of day and night. #### **Classification of Plants** - ***Short-day plants:*** Flower when the photoperiod is *less than* a certain critical length. They are actually *long-night* plants. - *Examples:* Chrysanthemum, tobacco, cocklebur. - ***Long-day plants:*** Flower when the photoperiod *exceeds* a certain critical length. They are actually *short-night* plants. - *Examples:* Spinach, cabbage, henbane. - ***Day-neutral plants:*** Flowering is not affected by day length. - *Examples:* Tomato, rose, bean. #### **Mechanism of Photoperiodism** The mechanism involves a photoreceptor pigment called **phytochrome**. - **Phytochrome Forms:** Exists in two interconvertible forms: 1. **P_R (P₆₆₀):** Absorbs red light (wavelength ~660 nm). This is the inactive form. 2. **P_FR (P₇₃₀):** Absorbs far-red light (wavelength ~730 nm). This is the biologically *active* form that triggers responses. - **Interconversion:** - During the **day** (presence of red light), P_R is converted to P_FR. - During the **night** (darkness or far-red light), P_FR slowly reverts back to P_R. - **Flowering Stimulus:** The ratio of P_FR to P_R at the end of the night acts as a biological clock and determines if flowering will occur. - **Short-day (long-night) plants** require a *low* P_FR/P_R ratio to flower. A long, uninterrupted night allows most P_FR to convert back to P_R. - **Long-day (short-night) plants** require a *high* P_FR/P_R ratio to flower. A short night means less P_FR converts back to P_R, leaving a high concentration of the active form. - **Florigen:** The correct phytochrome ratio is believed to trigger the production of a flowering hormone, hypothesized as **florigen**, which moves to the buds to initiate flower formation. ### **9.7.4 Vernalization** Vernalization is the promotion of flowering through exposure to a prolonged period of **low temperature**. - **Stimulus:** Cold temperatures, typically between 0°C to 10°C. - **Reception:** The stimulus is received by the shoot apex or the embryo of a seed. - **Significance:** It is a requirement for many **biennial plants** (e.g., winter wheat), which need to experience a cold winter before they can flower in their second year. - **Mechanism:** Low temperatures are believed to stimulate the production of a hormone called *vernalin*, which is now thought to be a type of **gibberellin**.