Plant Hormones and Growth Response Mechanisms
Plant Hormones and Their Functions
Auxins and Indoleacetic Acid
Auxins are chemical hormones produced in the stems and apical meristems. Roots produce a substance called indoleacetic acid (IAA). Their primary function is to provide the foundation for plant cell division and the stimulation of root growth. They are also responsible for various tropisms.
Gibberellins and Seed Germination
Gibberellins promote stem growth and the induction of seed germination.
Cytokinins and Abscisic Acid
Cytokinins are plant hormones produced in the root apical meristem, as well as in fruits and embryonic cells. They are often composed of zeatin. Abscisic acid (ABA) is a plant hormone synthesized in tissues; it is a potent plant growth inhibitor involved in the regulation of responses such as fruit and leaf abscission and water stress.
Ethylene and Fruit Ripening
Ethylene is a plant hormone formed by an unsaturated hydrocarbon. Its function is to accelerate the maturation process of fruits. Ripe fruit produces ethylene, and this, in turn, stimulates the production of more ethylene in surrounding fruits.
Plant Growth Action and Tropisms
In the action in plants, seeds are activated by hormones, creating gibberellins for growth and auxins for root development. Afterward, as the plant grows, root growth is regulated by the plant hormone cytokinin. Roots typically grow in response to gravity, while stems grow toward the light.
Understanding Tropisms
- Tropism: The mechanism by which plants develop their stems toward the sun and their roots in the direction of gravity.
- Phototropism: The orientation of plant organs in response to light.
- Geotropism: The orientation of plant growth in the direction of the Earth's gravity.
- Chemotropism: A tropism that occurs in response to chemical substances dissolved in water that reach all parts of the plant.
- Haptotropism: A type of tropism caused by sensitivity to contact stimuli.
Nastic Movements in Plants
Nasties are mechanisms where there is no relationship between the direction of movement and the position of the stimulus.
- Photonasty: A type of nastic movement triggered by the intensity of light.
- Thermonasty: Movements due to variations in temperature.
- Haptonasty: A nastic movement caused by physical contact.
- Seismonasty: A type of nastic movement where a mechanical shock leads to a variation in turgor.
Turgor: The process of water crossing from a higher concentration to a lower concentration through a semi-permeable membrane.
Plant Defense Mechanisms
- Small Plants: Production of toxic alkaloids.
- Large Plants: Use of lignin or tannins to deter injury.
- Stems and Trunks: Development of thorns or spines as a defense against animals.
- Pine Trees: Leaves release toxic substances onto the ground to deter animals, insects, and bacteria.