Growth and Development of Horticultural Crops (1+1)

Synthetic Auxins

• Auxin is a general term used to denote substances that promote the elongation of coleoptiles tissues, particularly when treated in the Avena coleoptiles test or in several other bioassay techniques. Indoleacetic acid is an auxin that occurs naturally in plants.
  
• Soon after the recognition of the importance of IAA as a plant hormone, compounds similar in structure were synthesized and tested for biological activity. Among the first compounds studied were substituted indoles, such as indole-3-propionic acid and indole-3-butyric acid. Both compounds are biologically active and commonly used as rooting hormones in horticultural work. Both have the same indole rings as IAA and a terminal carboxyl group but differ in their side chains. If longer side chains are added to the indole ring, the compounds generally lack biological activity. Certain species of plants, however, possess enzymes capable of shortening the side chains and will convert the compounds to a biologically active molecule.
  
• Compounds lacking the indole ring but retaining the acetic acid side chain present in IAA are also biologically active. Naphthaleneacetic acid is such a compound and it is used as a rooting hormone for certain plants. Another biologically active synthetic auxin lacking the indole ring is 2,4-dichlorophenoxyacetic acid. This compound, known as 2,4-D, is a potent auxin and is used as a weed killer. It is probably the most widely used of the synthetic auxins in commercial crop production. The carbamate compound was developed for use as a fungicide but was also found to have auxin activity. It lacks a ring structure but does possess an acetic acid side chain.