Plant hormones are chemical signals that help a plant coordinate growth, development, and survival. Unlike animals, plants cannot move away from shade, drought, or damage, so they adjust how roots, stems, leaves, flowers, and fruits grow. A tiny amount of hormone can change cell division, cell elongation, seed dormancy, or fruit ripening.
Understanding these signals helps explain plant shape, crop yield, and responses to environmental stress.
The five major plant hormones are auxin, gibberellin, cytokinin, ethylene, and abscisic acid. They do not act alone, because a plant response usually depends on the balance between several hormones in a tissue. Auxin can guide bending toward light, gibberellin can stimulate stem growth and seed germination, cytokinin can promote cell division, ethylene can ripen fruit, and abscisic acid can help close stomata during drought.
Farmers and scientists use hormone knowledge to root cuttings, control fruit ripening, improve germination, and manage stress responses.
Key Facts
- Auxin promotes cell elongation in shoots and helps control phototropism, gravitropism, apical dominance, and root formation.
- Gibberellin stimulates stem elongation, seed germination, and enzyme production in germinating seeds.
- Cytokinin promotes cell division and can delay leaf aging, especially when balanced with auxin.
- Ethylene is a gas hormone that promotes fruit ripening, leaf drop, and some stress responses.
- Abscisic acid, or ABA, promotes seed dormancy and helps close stomata during water stress.
- Plant response depends on hormone balance, not just hormone amount, such as a high auxin to cytokinin ratio favoring root formation.
Vocabulary
- Auxin
- A plant hormone that promotes cell elongation and helps control directional growth such as bending toward light.
- Gibberellin
- A plant hormone that stimulates stem elongation, seed germination, and growth after dormancy.
- Cytokinin
- A plant hormone that promotes cell division and helps regulate shoot growth and leaf aging.
- Ethylene
- A gaseous plant hormone that promotes fruit ripening, leaf abscission, and responses to stress or injury.
- Abscisic acid
- A plant hormone that helps plants survive drought by closing stomata and maintaining seed dormancy.
Common Mistakes to Avoid
- Thinking each hormone has only one job is wrong because plant hormones often affect several processes depending on tissue, timing, and concentration.
- Assuming more hormone always means more growth is wrong because high concentrations can inhibit growth or trigger different responses.
- Confusing ethylene with a liquid nutrient is wrong because ethylene is a gas hormone that can spread through air and trigger ripening in nearby fruit.
- Ignoring hormone balance is wrong because many plant responses depend on ratios, such as auxin and cytokinin together controlling root and shoot development.
Practice Questions
- 1 A scientist treats 40 pea seedlings with gibberellin and 40 untreated seedlings as a control. After 7 days, the treated seedlings average 18 cm tall and the controls average 12 cm tall. What is the average increase in height caused by gibberellin?
- 2 A fruit storage room contains 120 apples. If 30% of the apples begin producing high levels of ethylene, how many apples are producing high ethylene levels?
- 3 A plant cutting placed in growth medium develops many roots but few shoots. Based on hormone balance, what does this suggest about the relative levels of auxin and cytokinin, and why?