Introduction Auxins are weak organic compounds having an unsaturated ring structure and capable of promoting cell elongation, especially of shoot and roots in low concentration. Auxin is the first plant hormone to be discovered and are perhaps the most important group of phytohormones.
History of Auxins The discovery of auxins was starting from the classical work of Charles Darwin (1880) while working on Canary grass (Phlaris canariensis). Darwin observed that when unilateral light was given to the tip of grass coleoptile, it exhibited positive phototrophic curvature and when the tip was removed it fails to cause curvature. The tip also failed to show curvature when it was covered by a tin foil cap. On this basis, he predicted that the tip contained a growth substance which was transmitted to the lower portion where it causes curvature. Boysen- Jensen (1910-1913) found that if a transverse slit was caused in the coleoptile of Avena on the dark side and a piece of mica was inserted into the slit, no phototropic response took place. On the other hand, there was a phototropic response (curvature) if the slit and the piece of mica were on the illuminated side. Based on these results they conclude that a substance migrates down the dark side promoting growth curvature towards the light. Paras (1919) found that when the excised tip is replaced on one side of Avena coleoptile stump, accelerated growth beneath the tip, resulting in curvature. F.W. Went (1928) working in Avena coleoptile and succeed in isolating the growth substance. He cut coleoptile tips and placed them on agar blocks. After a particular time, he cut the agar block into small cubes, discarded the coleoptile tips and placed the agar cubes eccentrically on decapitated coleoptile stumps for two hours in dark and found that curvature was produced. He concluded that some substances are responsible for unilateral growth. This substance is synthesized in coleoptile tip and it translocated downwards. He found that the hormone diffused into the agar is diffused into coleoptile and caused growth and curvature like a coleoptile tip. He called this substance as auxin. In his, another experiment, Went separated two agar blocks by a mica plate. He kept the oat coleoptile on te agar block so that half-half portion of the coleoptile remain on the two blocks. When the coleoptile was illuminated, he observed that 65% of auxin from the coleoptile was diffused into the block of unilluminated side. Remaining 35% auxin was found diffused into the illuminated side agar block.
Distribution of Auxins in plants Auxins are commonly found in plants. Auxins are mostly concentrated on growing apices like coleoptile tips, floral and shoot buds, root apices and leaf apices. The distribution of auxins in the plant body was studied in detail by Thimann (1934) in etiolated Avena coleoptile. He proved successfully that auxins are concentrated on root and shoot tips. Thimann and Skoog observed a decreasing order auxin concentration in apical buds, young leaves and mature leaves.
Apical buds > Young leaves > Mature leaves
Physiological effects of Auxins The physiological effects of auxins are given below:
Kogl and Haagen Smit (1931) have isolated some auxins which are auxin-a, auxin-b, heteroauxin. These are called natural auxins. Some auxins have also been synthesized chemically which function like natural auxins. These are called synthetic auxins. Some synthetic auxins are P.A.A., P.O.A, 2,4-D, 2,4,5-T, etc.
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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