Overview
This lecture covers how hormones regulate plant growth in plants, focusing on the five major plant hormones, their functions, tropisms, and processes like senescence and dormancy.
Plant Hormones Overview
- Plant growth is controlled by chemical messengers called hormones.
- Five major plant hormones regulate most plant growth: auxin, cytokinins, gibberellins, ethylene, and abscisic acid.
- Each hormone influences different aspects of growth or stress response.
Major Plant Hormones and Functions
- Auxin stimulates cell elongation and is key in apical dominance and phototropism. Made in shoot apical meristem (leaves and fruit). Diffusion between parenchyma cells associated with vascular tissue. Primary growth
- Cytokinins promote cell division and work with auxin to balance growth. Made in root apical meristem. (Secondary growth, width and girth). in xylem
- Gibberellins promote seed germination, stem growth, and fruit development. Developing seeds. Vertical growth to enlongading inner nodes) in xylem and phloem
- Ethylene regulates fruit ripening and promotes senescence (aging). Only gas.
- Abscisic acid induces dormancy and inhibits growth in response to stress. Mature leaves, freezes stress. In xylem and phloem. Makes leaves break and fall.
Apical Dominance & Roles of Hormones
- Apical dominance is when the main shoot inhibits growth of lateral buds, mainly controlled by auxin.
- Cytokinins promote lateral bud growth and can counteract auxin’s effects.
- The balance between auxin and cytokinins determines overall plant shape.
Plant Responses to Environmental Stimuli
- Plants respond to light (phototropism), gravity (gravitropism), and touch (thigmotropism).
- Phototropism: plants grow toward light, mainly regulated by auxin redistribution.
- Gravitropism: plant roots grow downward and shoots upward, using statoliths to sense gravity.
- Thigmotropism: plants change growth direction in response to touch (e.g., vines wrapping).
Senescence and Dormancy
- Senescence is the aging and death of plant parts, accelerated by ethylene.
- Dormancy is a survival strategy where growth temporarily stops, promoted by abscisic acid.
Key Terms & Definitions
- Hormone — chemical messenger affecting plant growth and development.
- Auxin — hormone stimulating cell elongation and controlling apical dominance. (Response to stim)
- Cytokinins — hormones that promote cell division and bud growth.
- Gibberellins — hormones that stimulate seed germination and stem growth.
- Ethylene — gaseous hormone involved in fruit ripening and aging.
- Abscisic acid — hormone that induces dormancy and inhibits growth under stress.
- Phototropism — plant growth response to light direction.
- Gravitropism — plant growth response to gravity.
- Thigmotropism — plant growth response to touch.
- Senescence — aging process leading to death of plant tissues.
- Dormancy — period of suspended growth and metabolic activity. Slows down growth in winter.
Action Items / Next Steps
- Review the roles and interactions of the five major plant hormones.
- Practice explaining how plants respond to light, gravity, and touch.
- Complete any assigned Mastering Concepts questions for Chapter 24.