Plant hormones, also known as phytohormones, are natural chemical substances that regulate and coordinate various processes of plant growth and development. They act in very small amounts and can either promote or inhibit specific activities in plants, such as cell division, elongation, flowering, fruiting, and responses to environmental stimuli.
Here’s how the major plant hormones regulate growth:
• Auxins:
Auxins are mainly produced in the shoot tips and young leaves. They promote cell elongation, root initiation, and differentiation of vascular tissues. Auxins also control apical dominance, where the main shoot grows more strongly than side branches. They help in phototropism (growth towards light) and gravitropism (growth in response to gravity).
• Gibberellins (GAs):
Gibberellins stimulate stem elongation, seed germination, and flowering. They break seed dormancy and promote the growth of fruits and leaves. For example, gibberellins are used to produce seedless grapes and increase the size of fruits.
• Cytokinins:
Cytokinins promote cell division (cytokinesis) and delay leaf senescence (aging). They work with auxins to control the balance between root and shoot growth. High cytokinin levels favor shoot formation, while low levels favor root development.
• Ethylene:
Ethylene is a gaseous hormone that regulates fruit ripening, leaf abscission (falling of leaves), and flower wilting. It also helps plants respond to stress, such as mechanical injury or flooding. In fruits like bananas and tomatoes, ethylene triggers the ripening process.
• Abscisic Acid (ABA):
ABA acts mainly as a growth inhibitor. It helps plants survive under stress by inducing stomatal closure during drought to reduce water loss. It also promotes seed dormancy, preventing germination until conditions are favorable.
• Brassinosteroids:
These hormones promote cell expansion, vascular differentiation, and seed germination. They also help improve plant resistance to environmental stresses such as cold or drought.
• Jasmonates and Salicylic Acid:
These hormones play important roles in defense responses against pests, pathogens, and wounding. They also influence flowering and senescence.
In summary, plant hormones work together in a balanced and coordinated way to regulate all aspects of plant growth—from seed germination to flowering and fruit formation. Their combined actions ensure that plants grow properly, adapt to environmental changes, and complete their life cycles efficiently.