Growth and Development of Horticultural Crops (1+1)

Introduction- Plant Growth Hormones

• Most of the physiological activities and growth in plants are regulated by the action and interaction of some chemical substances in them called as hormones and by certain naturally occurring inhibitors e.g., phenols, flavonols and abscisic acid.
  
• To distinguish the plant hormones from the animal hormones they are termed as phytohormones. According to Pincus and Thimann (1948), a plant hormone is defined as “organic substance produced naturally in the higher plants, controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts.”
  
• These phytohormones have also been termed as growth hormones, growth promoting substances, growth substances, growth factors, growth regulators etc., by various workers and defined accordingly. The auxins were the first hormones to be discovered in plants and at one time considered to be the only naturally occurring plant growth hormones. Since then besides other less important hormones, two important groups of chemical substances having profound influence on the regulation of growth and development in plants have been discovered which are also considered as natural plant growth hormones. They are gibberellins and cytokinins.
  
• Beside these, ethylene and absicisic acid (ABA) and more recently brassinosteroids have also acquired status of natural plant growth hormones.