. Define phytohormones. Explain the role of three plant growth promoters and one plant growth inhibitor

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Ans: Phytohormones are the specific organic chemical substances, produced in very minute quantities, and they have a marked effect on certain physiological processes. The three most important growth promoters are auxins, gibberellins and cytokinins, while abscisic acid is a growth inhibitor. Their roles are as follows

Role of auxins

Auxins play an important role in cell elongation during growth. They promote the growth of stems towards light (phototropism) and against force of gravity (geotropism).  

Role of gibberellins

They stimulate stem elongation, induce flowering and promote seed germination in some higher plants. 

Role of cytokinins

They mainly promote division of cytoplasm (cytokinesis) during cell division and help in breaking dormancy.  

Role of abscisic acid

It reverses the growth-promoting effects. Its function is opposite to that of auxins, gibberellins and cytokinins. It causes dormancy of seeds, tubers and bulbs. It promotes leaf and fruit fall.                     

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