Chapter 39-control Systems In Plants

Auxin is a plant hormone that plays a crucial role in plant growth and development. It promotes elongation by stimulating cell expansion and division in the stem, leading to increased height and overall growth of the plant. This hormone is responsible for tropisms, such as phototropism and gravitropism, which enable plants to respond and adapt to their environment. Auxin also regulates the development of roots, leaves, and flowers. Therefore, the correct answer is "promotes elongation."

Thigmotropism is the correct answer. Thigmotropism is a plant's response to touch, where it reacts by becoming flaccid or curling around a pole. Gravitropism is the response to gravity, statoliths are specialized cells that help plants perceive gravity, and geotropism is the response to the Earth's gravitational pull.

Plants shed leaves regularly as a part of their natural process called apoptosis or programmed cell death. This process is prompted by the hormone ethylene. Ethylene plays a crucial role in regulating various plant processes, including leaf senescence and abscission. It acts as a signal for the plant to initiate the shedding of older or damaged leaves. Ethylene triggers the activation of enzymes that break down cell walls, leading to the detachment and eventual shedding of the leaves.

Abscisic acid is a plant hormone that plays a crucial role in seed dormancy. It inhibits seed germination by preventing the synthesis of enzymes required for germination. This hormone helps seeds survive unfavorable conditions such as drought or extreme temperatures by maintaining their dormancy until conditions are suitable for growth. Therefore, the primary purpose of abscisic acid in seeds is to induce and maintain dormancy.

When apical dominance is removed from a plant, it means that the apical bud, which is responsible for inhibiting the growth of lateral buds, is no longer dominant. As a result, the lateral buds are able to grow and develop into branches, causing the plant to grow outward rather than upward. This leads to a bushier and more spreading growth pattern in the plant.

Ethylene is a plant hormone that plays a role in various physiological processes. It promotes abscission, which is the shedding of leaves, flowers, or fruits. It also induces leaf loss, particularly during senescence. Additionally, ethylene can oppose the effects of auxin, another plant hormone. However, ethylene does not directly stimulate cuticle production. The cuticle is a waxy layer on the surface of leaves that helps prevent water loss, but ethylene does not have a direct influence on its production.

Auxin causes elongation by activating expansins to loosen H bonds between cellulose fibers. Expansins are a group of proteins that disrupt the hydrogen bonds between cellulose microfibrils in the cell wall. This loosening of the cell wall allows for increased flexibility and expansion of the cell, leading to elongation. By activating expansins, auxin promotes cell growth and elongation in plants.

Phytochrome is the receptor protein for greening. It is a pigment found in plants that is responsible for detecting light. When phytochrome absorbs red light, it undergoes a conformational change, which triggers various physiological responses in plants, including the promotion of greening. Therefore, phytochrome is the correct answer as it plays a crucial role in the greening process of plants.

Far red absorbing phytochrome is blue-green in color and influences germination.

Gibberelins are hormones produced by plants that help break seed dormancy. These hormones stimulate seed germination and promote the growth of stems and leaves. They are responsible for regulating various plant processes, including seed development, flowering, and fruit ripening. Gibberelins also play a role in overcoming environmental conditions that inhibit seed germination, such as cold or drought. Therefore, gibberelins are the correct answer to the question.

Short day plants flower in late summer and fall because they require a longer period of darkness or shorter days in order to initiate the flowering process. These plants are sensitive to the length of daylight and require a certain amount of darkness to trigger flowering. In late summer and fall, the days become shorter and the nights become longer, providing the necessary conditions for short day plants to flower.