Here’s a structured summary addressing each of your points regarding plant growth, hormones, and responses to environmental factors:
1. Etiolation vs. De-Etiolation
Etiolation: This is the process by which a plant develops in the absence of light, characterized by elongated stems, small or absent leaves, and pale color due to lack of chlorophyll. This adaptation helps the plant to reach light sources more quickly.
De-Etiolation: When a plant is exposed to light after a period of etiolation, it undergoes de-etiolation, leading to the development of true leaves, the production of chlorophyll, and the cessation of stem elongation. This process converts the plant into its photosynthetically active form.
2. Six Classes of Plant Hormones
- Auxins: Promote cell elongation, coordinate responses to light and gravity (phototropism and gravitropism).
- Gibberellins: Stimulate stem elongation, seed germination, and flowering.
- Cytokinins: Promote cell division, delay leaf senescence, and affect apical dominance.
- Ethylene: A gaseous hormone that regulates fruit ripening, leaf abscission, and response to stress.
- Abscisic Acid (ABA): Involved in stress responses, particularly in closing stomata during drought.
- Brassinosteroids: Promote cell expansion and differentiation, and are important for overall growth.
3. Phytochrome Photoreversibility
Phytochromes are light receptors that exist in two forms: Pr (inactive) and Pfr (active). When exposed to red light, Pr is converted to Pfr, and it can promote processes like germination in seeds (e.g., lettuce). In turn, far-red light converts Pfr back to Pr. This photoreversibility allows plants to sense light quality and quantity, ensuring that germination occurs under optimal conditions.
4. Light and Biological Clocks
Plants use light exposure to synchronize internal biological processes with the external environment. The presence of light at certain times influences the circadian rhythms of plants, aligning processes such as flowering and leaf movements with optimal conditions for growth and reproduction.
5. Photoperiodism in Plants
- Short-Day Plants: Flower when the light period is shorter than a critical length.
- Long-Day Plants: Flower when the light period exceeds a certain critical length.
- Day-Neutral Plants: Flower regardless of light duration. These names can be misleading because the actual requirement for flowering may not solely depend on day length but also on other factors such as temperature and light intensity.
6. Tropisms
- Gravitropism: Growth response to gravity (roots grow downwards and stems grow upwards).
- Thigmotropism: Growth response to touch; plants may wrap around structures (e.g., vines).
- Thigmomorphogenesis: Changes in plant growth in response to mechanical stimulation, such as wind or touch.
7. Responses to Environmental Stresses
- Drought: Plants may close stomata, alter root growth, and produce stress hormones like ABA.
- Flooding: Some plants develop aerenchyma (air channels) to transport oxygen.
- Salt Stress: Plants accumulate solutes (osmotic adjustment) or develop salt glands to excrete excess salts.
- Heat Stress: Plants may produce heat shock proteins to protect against denaturation.
- Cold Stress: Can induce changes in membrane composition and production of antifreeze proteins.
8. Responses to Biotic Stress
Plants respond to biotic stress (like herbivory or disease) through:
- Production of secondary metabolites (e.g., alkaloids, terpenes) that deter herbivores.
- Activation of defense genes leading to systemic acquired resistance (SAR).
- Recruitment of beneficial organisms (e.g., predators of herbivores) through volatile organic compounds.
This structured summary provides a clear view of how plants respond to their environment and the hormones that regulate their growth and development.