Auxins

There are two main types of auxins. These are synthetic auxins and natural auxins.

Synthetic auxins- are not naturally occurring but produced in Laboratories by man.They promote growth but also used as weed killers. E.g. 2, 4-D (2, 4- dichlorophenoxyacetic acid), that kills dicot leaves.

Natural auxins- are naturally found in plants.
e.g. IAA (Indole acetic acid):

• IAA major functions are promotion of elongation of stems and growth
• Formation of adventitious roots
• Inhibition of leaf abscission
• Promotion of cell division
• Inducement of ethylene production and
• Lateral bud dormancy.

Auxins stimulate formation of roots of stem cuttings.

Gibberellins

• Promotes bolting, i.e. sudden elongation of internodes, delays senescence.
• Induces parthenocarpy.
• Elongation of the stem and reverses dwarfism.
• activate cell division and cell elongation.

Cytokinins

• Helps in overcoming apical dominance induced by auxins.
• Stimulate the formation of chloroplast in leaves.
• Promotes nutrient mobilization and delay leaf senescence.

Ethylene

• It hastens the ripening of fruits.
• Controls epinasty of leaves.
• Breaks seed and bud dormancy.
• Induces root growth and root hair formation.

Abscisic acid

It is a growth-inhibiting hormone. ABAs act as an antagonist to GAs. It inhibits plant metabolism and regulates abscission and dormancy.

• Induces abscission of leaves and fruits.
• Inhibits seed germination.
• Induces senescence in leaves.
• Stimulates closure of stomata.

Primary and Secondary Growth in Plants

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem.