Auxins are weakly acidic growth hormones having an unsaturated ring structure. They are capable of promoting cell elongation, especially of shoots.
Charles Darwin (1880) discovered the existence of plant growth hormones while working on canary grass. He demonstrated bending of grass coleoptile towards unilateral source of light. Decapitating coleoptile or covering the tip with black cap resulted in loss of sensitivity of the plant towards light. Replacing the decapitated coleoptile or removing the cap results in bending.
Boysen-Jensen (1910-1913) observed that coleoptiles with decapitated tips did not bend towards light. Boysen-Jensen showed that normal bending occurred if the tip was cut off and replaced with an intervening block of gelatin. It was, thus, evident that the influence was due to chemical diffusing through the block of gelatin.
Paal (1914-1919) demonstrated that coleoptile would bend even in dark after certain treatments. He sliced off the coleoptile tip and replaced it eccentrically, causing the shoot to stagnate on the side that received its chemical influence.
W. Went (1928) placed several decapitated tips of Avena coleoptile over a thin block of agar-agar. The coleoptile was allowed to stay on agar block for some time. The block was then cut into small pieces and each piece was placed eccentrically on the cut end of coleoptile.
Even in darkness, characteristic bending was observed. This clearly demonstrated that the substance responsible for growth was synthesized on apex and translocated downwards . Went named this chemical influence as auxin.
Kogl (1931) isolated auxin. Auxins are weak organic acids. The naturally occurring auxins are indole 3 acetic acid, indole 3 pyruvic acid, indole 3 acetal dehyde etc.
The synthetic auxins are indole 3 butyric acid, alpha and beta naphthalene acetic acid, phenol acetic acid, 2,4 dichlorophenoxy acetic acid etc.
Auxins are synthesized continuously in the shoot apices and young leaves. Auxin is mainly located in young actively growing tissues like root apices, shoot apices, lateral meristem and enlarging leaves.
Translocation of auxin in plants:
It has been demonstrated that upward movement of auxin from root to shoot takes place through xylem and downward movement through phloem i.e., from shoot to root. This basipetal (from apex to base) movement is called polar transport.
Bioassay of auxins:
In the study of biologically active substances several methods are used to determine their presence and effects. All such studies in which a living test material is used are known as bioassays.
Avena coleoptile curvature test is one of the most widely used bioassay for auxin activity.The procedure for Avena coleoptile curvature test involves following steps:
- When the seedlings attain a height of 15-30 mm, 1mm tip of the coleoptile is removed.
- The tips are now placed on agar blocks for few hours. During this period the auxins diffuse out of these tips into the agar.
- The auxin containing agar block is now placed on one side of the decapitated stump of the Avena coleoptile. The auxin from the agar block diffuses down through the coleoptile along the side to which the ‘auxin agar block’ is placed.
An agar block without auxin is placed on another decapitated coleoptile. This serves as control.
- Within hours the coleoptile with the ‘auxin agar block’ bends on the side opposite to which the agar block is placed. The curvature occurs due to more rapid growth of the side on which ‘auxin agar block’ is placed. The amount of auxin in the agar block is directly proportional to the degree of curvature.
Physiological effects of auxin:
- Cell elongation and longitudinal growth: Auxins promote cell elongation in stem and coleoptile. High auxin concentration promotes cell elongation but the same concentration inhibits root growth.
- Cell division in the cambium:It helps in the initiation and promotion of cell division in cambium
- Cell division and tissue culture: Initiation and promotion of cell division brought about by auxin is very useful in tissue culture for the formation of callus.
- Root formation: It helps in the formation of adventitious and lateral roots.
- Apical dominance: It inhibits the development of lateral buds in plants thereby promoting apical dominance.
- Prevention of abscission layer: Auxins in the cell prevents the formation of abscission layer which may otherwise result in the fall of premature leaf, flower and fruit from the stem.
- Parthenocarpy: External application of auxin in flower induces parthenocarpy and causes development of seedless fruits.
- Initiation of flowering: Spraying of dilute concentration of auxins initiates flowering. High concentration of auxin, however, inhibits flowering. Auxins helps in delaying flower formation in lettuce.
- Eradication of weeds: Some synthetic auxins acts as herbicides. The best known herbicide is 2,4-D.
- Dormancy: Treatment of potato tuber with indole butyric acid, alpha napthalene acetic acid and malic hydrazine inhibits the sprouting of lateral buds. These treatments permit storage of potato tubers upto 3 years.