1. Explain the role of auxins in plant growth and development.
Answer: Auxins are a group of plant hormones that play a central role in the regulation of plant growth and development. They are primarily responsible for promoting cell elongation, particularly in stems and roots, thus contributing to overall plant growth. Auxins are synthesized in the apical meristem of shoots and are distributed down the stem, where they help elongate cells. They also influence processes such as phototropism (growth in response to light) and gravitropism (growth in response to gravity), where auxins accumulate on the shaded side of a stem or the lower side of a root, causing elongation on those sides and leading to bending toward light or gravity. Auxins also regulate the formation of lateral roots and buds, maintaining apical dominance, where the main shoot grows more vigorously than lateral shoots.
2. Describe the mechanism of action of cytokinins in plants.
Answer: Cytokinins are plant hormones primarily involved in stimulating cell division, shoot formation, and delaying senescence (aging) in plants. Cytokinins are mainly synthesized in the roots and then transported to other parts of the plant where they regulate various developmental processes. One of their key functions is to promote cell division in the presence of auxins, especially in the shoot apical meristem. They also influence the differentiation of plant cells into specific tissues. Cytokinins delay leaf senescence by promoting chlorophyll retention and maintaining active photosynthesis. Additionally, cytokinins promote the growth of lateral buds, counteracting apical dominance caused by auxins. These hormones are also involved in the regulation of plant responses to environmental stresses, such as drought and high salinity.
3. What is the role of gibberellins in seed germination?
Answer: Gibberellins are a group of plant hormones that play a crucial role in promoting seed germination. During seed dormancy, gibberellins activate enzymes such as amylase that break down stored food reserves in the seed, providing energy for the developing embryo. The presence of gibberellins signals the seed to begin the process of germination. This hormone promotes the elongation of the embryo, allowing it to break through the seed coat and emerge from the soil. Gibberellins also stimulate cell division and expansion in the young seedling, aiding in the overall growth of the plant. In some plants, gibberellins are used commercially to break seed dormancy and enhance germination rates.
4. How does abscisic acid (ABA) help plants cope with environmental stress?
Answer: Abscisic acid (ABA) is a plant hormone that plays a critical role in the plant’s response to environmental stress, particularly during drought and water scarcity. ABA helps regulate the closure of stomata (pores on the leaf surface), reducing water loss through transpiration. This is a key mechanism for conserving water during periods of drought. ABA also induces the production of stress-related proteins that help protect the plant from damage caused by water deficiency, extreme temperatures, or other abiotic stresses. Additionally, ABA regulates seed dormancy, ensuring that seeds do not germinate under unfavorable conditions. During stress, ABA also triggers changes in gene expression that help the plant survive by adjusting its metabolic and physiological processes.
5. Discuss the role of ethylene in fruit ripening and senescence.
Answer: Ethylene is a gaseous plant hormone that plays a central role in regulating the processes of fruit ripening, flower wilting, and leaf senescence. Ethylene promotes the ripening of climacteric fruits (e.g., bananas, tomatoes, apples), which undergo a sudden increase in respiration as they ripen. Ethylene stimulates the production of enzymes that break down cell wall components, leading to softening of the fruit, as well as the degradation of chlorophyll, resulting in color changes such as from green to red or yellow. Besides fruit ripening, ethylene also plays a role in leaf senescence (aging) by promoting the degradation of chlorophyll and other cellular components, causing the plant to shed its leaves. In addition, ethylene is involved in processes such as seedling growth, root formation, and flower opening.
6. What is the relationship between auxins and cytokinins in plant growth?
Answer: Auxins and cytokinins have a synergistic relationship in regulating plant growth. Auxins primarily promote cell elongation, while cytokinins stimulate cell division. Together, they coordinate the growth of plant tissues by balancing cell division and elongation. The interaction between auxins and cytokinins is particularly important in the formation of roots and shoots. For example, in the presence of high auxin levels and low cytokinin levels, root formation is promoted, while the reverse (high cytokinin and low auxin levels) promotes shoot development. The balance between these hormones also plays a key role in apical dominance, where the primary shoot suppresses the growth of lateral buds, with auxins favoring apical dominance and cytokinins promoting lateral bud growth when auxin levels are lower.
7. Explain the process of phototropism and how auxins are involved.
Answer: Phototropism is the process by which plants grow in response to light. It is mainly observed in plant shoots, which bend toward a light source. The mechanism behind phototropism involves the uneven distribution of auxins in the plant tissues. When a plant is exposed to unilateral light, auxins accumulate on the shaded side of the stem. This results in the elongation of cells on the shaded side, causing the stem to bend toward the light source. The redistribution of auxins ensures that the plant maximizes its exposure to light, which is essential for photosynthesis. Phototropism is an example of how plants adapt to their environment using hormonal regulation to optimize their growth.
8. Describe the role of gibberellins in stem elongation and flowering.
Answer: Gibberellins are important regulators of stem elongation and flowering in plants. In terms of stem elongation, gibberellins promote the expansion of cells, resulting in increased length of the internodes (the segments between leaves on the stem). This is particularly important for plants that grow in dense environments, as elongating the stem helps the plant reach above surrounding vegetation for better access to light. Gibberellins also play a crucial role in flowering, especially in plants that require specific conditions such as long days or vernalization (cold exposure) for flower induction. In some plants, gibberellins are used to promote early flowering, particularly in ornamental plants and crops like strawberries. They also help in the development of flowers by influencing the transition from vegetative growth to reproductive growth.
9. How does auxin affect root development in plants?
Answer: Auxins play a critical role in the development of roots, particularly in the formation of lateral roots and root initiation. When a plant is exposed to auxins, the hormone stimulates the growth and differentiation of root cells. At higher concentrations, auxins promote the development of lateral roots, which are essential for anchoring the plant in the soil and increasing water and nutrient absorption. Auxins also regulate the growth of adventitious roots (roots that develop from non-root tissues). In addition to root initiation, auxins influence root architecture, ensuring that the root system is well-developed and able to support the plant’s growth. The balance between auxins and other hormones, such as cytokinins, determines whether the plant develops more roots or shoots.
10. Discuss the role of cytokinins in delaying leaf senescence.
Answer: Cytokinins are known to delay leaf senescence, which is the process of aging and deterioration that ultimately leads to leaf drop. The hormone helps maintain chlorophyll levels and sustain the photosynthetic activity of leaves for longer periods. Cytokinins promote cell division and protein synthesis, which helps in maintaining the structural integrity and function of chloroplasts, where photosynthesis takes place. By delaying senescence, cytokinins enable plants to continue producing energy through photosynthesis, thereby enhancing growth and survival. This function is particularly important in agricultural crops, as it can lead to extended periods of productive leaf function, thus improving crop yield and quality.
11. What is the function of ethylene in plant responses to stress?
Answer: Ethylene plays a significant role in plant responses to various environmental stresses, including mechanical stress, pathogen attack, and wounding. Ethylene is produced as a response to stress conditions and helps the plant adapt by activating specific defense mechanisms. For example, during wounding or pathogen attack, ethylene can induce the production of defense-related proteins and promote the synthesis of secondary metabolites that help protect the plant. Ethylene also mediates the plant’s response to mechanical stress, such as bending or compression, by promoting cell expansion and enabling the plant to adapt to the stress. Moreover, ethylene is involved in the plant’s response to water stress by regulating stomatal closure, which reduces water loss during drought.
12. How do gibberellins influence fruit development?
Answer: Gibberellins play an essential role in fruit development by promoting cell division, elongation, and differentiation in the fruit tissues. They influence the growth of both the fruit’s size and its shape. In some plants, gibberellins are used to promote fruit set in seedless varieties, as they help initiate fruit development in the absence of fertilization. Gibberellins also promote the development of the fruit’s internal tissues, ensuring that they expand and mature properly. In certain fruits, gibberellins are applied to extend the harvesting period and improve fruit size and quality. Additionally, gibberellins help regulate the transition from the immature to the mature fruit stage, making them important for ensuring uniform ripening.
13. Discuss how the balance between auxins and cytokinins affects plant tissue culture.
Answer: In plant tissue culture, the balance between auxins and cytokinins is critical for regulating the development of tissues and organs. Auxins promote the formation of roots, while cytokinins are involved in shoot formation. The concentration of these hormones in the culture medium determines whether the plant cells will differentiate into roots, shoots, or callus tissue. High auxin to cytokinin ratios generally promote root development, while high cytokinin to auxin ratios favor shoot formation. Equal concentrations of both hormones can lead to the formation of undifferentiated callus tissue. This hormonal manipulation allows researchers to control and direct the growth of plant tissues in vitro, which is vital for plant propagation, genetic modification, and conservation.
14. What is the role of auxins in apical dominance?
Answer: Auxins are central to the process of apical dominance, which is the phenomenon where the main shoot of a plant grows more vigorously than lateral buds. This occurs because auxins are produced in the apical meristem (the tip) of the main shoot and are transported downward through the plant. High concentrations of auxins in the apical bud inhibit the growth of lateral buds, ensuring that the plant’s energy is focused on growing taller and reaching for light. If the apical bud is removed, auxin levels decrease in the remaining stem, and lateral buds are no longer inhibited, allowing them to grow. This process ensures that the plant maintains an optimal growth pattern, especially when light availability is limited.
15. Explain the role of abscisic acid in seed dormancy.
Answer: Abscisic acid (ABA) plays a crucial role in regulating seed dormancy. During seed development, ABA promotes dormancy by preventing premature germination. ABA inhibits the enzymes necessary for seedling growth, such as those involved in the breakdown of stored food reserves. It also affects the seed’s sensitivity to environmental signals like temperature and moisture. ABA helps maintain dormancy until environmental conditions are favorable for germination, such as when there is sufficient water and appropriate temperatures. Once environmental conditions become favorable, the levels of ABA decrease, allowing other hormones like gibberellins to promote seed germination.
16. What are the commercial applications of auxins in agriculture?
Answer: Auxins have several commercial applications in agriculture, particularly in the areas of rooting, plant propagation, and fruit production. One of the most common uses of auxins is in rooting hormones, where they are applied to cuttings to promote the development of roots, allowing plants to be propagated vegetatively. Auxins are also used in tissue culture for the propagation of plants, as they help regulate the formation of roots and shoots. Additionally, auxins can be used to induce fruit set in certain crops, particularly in the absence of pollination, ensuring that flowers develop into fruit. They are also used to control the growth patterns of plants, such as in the production of uniform plant height in ornamental plants and crops.
17. How do cytokinins regulate plant cell division?
Answer: Cytokinins promote plant cell division by stimulating the expression of genes involved in the cell cycle, particularly those responsible for DNA replication and cell wall formation. They influence the transition from the G1 phase of the cell cycle to the S phase, thereby initiating DNA synthesis and cell division. Cytokinins also regulate the formation of new shoots and lateral buds by stimulating cell division in the shoot apical meristem. This hormone’s ability to stimulate cell division makes it essential for plant growth and development, particularly in the formation of new tissues and organs. By balancing cell division and differentiation, cytokinins contribute to the overall development of the plant.
18. What role does ethylene play in the regulation of leaf abscission?
Answer: Ethylene plays a key role in the process of leaf abscission, which is the shedding of leaves from a plant. As leaves age or are exposed to environmental stress, ethylene is produced in the petiole (the stalk that attaches the leaf to the stem). Ethylene promotes the breakdown of cell walls in the abscission zone, the area where the leaf is attached to the stem, making it easier for the leaf to separate. Ethylene also induces the production of enzymes, such as cellulase and pectinase, which degrade the structural components of the cell wall, leading to the leaf’s detachment. This process is an important adaptation for plants to conserve resources during periods of stress, such as drought or cold.
19. How do gibberellins contribute to plant height and overall growth?
Answer: Gibberellins contribute to plant height by promoting the elongation of cells in the stem, particularly in the internodes (the sections between the nodes). This cell elongation increases the overall length of the stem, contributing to greater plant height. Gibberellins also influence the development of other plant structures, such as leaves and flowers, by regulating cell division and expansion. The application of gibberellins in certain plants can overcome dwarfism caused by genetic factors or environmental stress, leading to more vigorous growth. As a result, gibberellins are commonly used in agriculture and horticulture to increase the height of crops like rice, wheat, and barley, which can lead to higher yields.
20. Describe the interaction between auxins and cytokinins in the formation of plant tissues.
Answer: The interaction between auxins and cytokinins is critical for the formation of plant tissues, particularly in the context of tissue culture. Auxins and cytokinins work together to regulate the differentiation of plant cells into specific tissues. High concentrations of auxins promote root formation, while high concentrations of cytokinins stimulate shoot formation. When both hormones are present in the right balance, they help in the formation of undifferentiated callus tissue. The balance between auxins and cytokinins determines whether the cells will differentiate into roots, shoots, or other tissues. This hormonal regulation is vital for plant growth and regeneration, particularly in tissue culture techniques used for plant propagation and genetic modification.
