Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered.
# Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928) from coleoptile tip of Avena . Went also developed Avena curvature test for bioassay of auxin. Kogl and Haagen. Smit (1931) introduced the term auxin.
# Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins.
- (a) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.
The first naturally occuring auxin was isolated by Kogl and Haagen-Smit (1913) from human urine. It was identified as auxin-a (auxentriolic acid, C18H32O5). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid (4-chloro-IAA) etc., are some other natural auxins.
Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and not through the vascular tissues. Auxins help in the elongation of both roots and shoots. However, the optimum concentration for the two is quite different.
It is 10 ppm for stem and 0.0001 ppm for the root. Its translocation rate is 1–1.6 cm/hr. (In roots 0.1 to 0.2 cm/hr). Higher concentration of auxins show inhibitory effect on growth.
Natural auxins are of two types : free and bond auxins. The auxins which can easily be extracted are called free auxins, whereas auxins which are hard to extract and need the use of organic solvents are termed as bound auxins. The free form of auxin is active, while the bound auxin is inactive in growth. A dynamic equilibrium exists between these two forms.
- (b) Synthetic auxins : These are synthetic compounds which cause various physiological responses common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the weedicide, 2, 4, 5-T (2, 4, 5-trichlorophenoxy acetic acid), IBA (indole 3-butyric acid), NAA (naphthalene acetic acid, PAA (Phenyl acetic acid), IPA (Indole 3-propionic acid). IBA is both natural and synthetic auxin. Certain compounds inhibit action of auxin and compete with auxins for active sites are called antiauxins. e.g., PCIB (p- chlorophenoxy isobutyric acid), TIBA (2, 3, 5-tri iodobenzoic acid). TIBA is used in picking cotton bolls.
# Bioassay of Auxins : Testing of biological activity (growth) of a substance (auxin) by employing living material is called bioassay. Auxin bioassay is also quantitative test as it measures amount of effect in response to a particular concentration of auxin.
- (a) Avena coleoptile curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant growth by performing some experiments with the oat (Avena sativa) coleoptile.
• When the tips of the coleoptiles were removed, no growth took place.
• When the freshly cut coleoptiles were placed on agar blocks for a few hours (during this period auxin diffused into the agar block) and then the agar blocks were placed on the cut ends of the coleoptile, growth occurred.
• When the agar block with the diffused substance was placed laterally on the cut tip of the coleoptile, only that side of the coleoptile elongated resulting in a curvature.
- (b) Split pea stem curvature test : This test was also discovered by Went, 1934. Dark germinated seeds of pea are decapitated. About half an inch part of stem between 2nd and 3rd node is removed and split longitudinally. It is then floated on the test solution contained in a beaker. At first, negative curvature occurs due to water uptake. Then positive curvature occurs which is proportional to the log of the concentration of auxin.
These experiments indicated that some substance is synthesised in the coleoptile tip is translocated downward. He called this substance auxin.
- (c) Root growth inhibition test (Cress root inhibition test) : Sterilized seeds of cress are germinated over moist filter paper. Root lengths are measured. 50% of seedlings are placed in test solution while the rest are allowed to grow over the moist filter paper. Lengths of roots are measured after 48 hours. Seedlings placed in test solution show very little root growth while the roots of controlled seedlings show normal growth. The degree of root growth inhibition is proportional to auxin concentration.
# Functions of auxins : Auxins control several kinds of plant growth processes. These are as follows :
- (a) Cell elongation : Auxins promote elongations and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.
The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of microfibrils, synthesis of more wall materials, increased membrane permeability and respiration.
- (b) Apical dominance : In many plants, the apical bud grows and the lower axillary buds are suppressed. Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is checked by applying synthetic auxin (NAA).
- (c) Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced when the auxin content falls below a minimum. Addicot and Lynch (1951) put forward auxin gradient theory about abscission :
• No abscission if auxin content is high on the organ side.
• Abscission layer begins formation when auxin content becomes same on stem and organ sides.
• Abscission is favoured when auxin content is low on the organ side.
Premature drop of fruits such as apple, pear and citrus can be prevented to a great extent by spraying the trees with a dilute solution of IAA, NAA or some other auxin.
- (d) Weed control : Weeds are undesirable in a field with a crop. Weeds cause competition for water, mineral, light and space. This causes poor yield. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but 2, 4-D does not affect mature monocotyledonous plants.
- (e) Root differentiation : Many new plants are usually propogated by stem cutting e.g., Rose, Bougainvillea. If we dip the lower cut end of a cutting in dilute solution of auxins (specially IBA gives very good results) very soon large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
- (f) Parthenocarpy : It is the process of formation of fruits without fertilization. Such fruits are called as parthenocarpic fruits and are without seeds. Parthenocarpy can be induced by application of IAA in a paste form to the stigma of a flower or by spraying the flowers with a dilute solution of IAA. Banana, oranges and grapes are now-a-days grown parthenocarpically on commercial scale.
- (g) Control of lodging : In some plants when the crop is ripe and there is heavy rain accompanied by strong winds, the plants bends as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute solution of any auxin is sprayed upon young plants the possibility of bending of plants is reduced as the stem becomes stronger by the application of auxins.
- (h) Flowering : In pineapple, NAA promotes flowering. In lettuce, auxins help in delaying the flowering. In cotton plants, the use of auxins increases the cotton seeds production.
- (i) Differentiation of vascular tissues : Auxins induce the differentiation of xylem and phloem in intact plants and also in callus produced in vitro during tissue culture experiments.
- (j) Sex expression : The spray of auxins increases the number of female flowers in cucurbits. In maize application of NAA during the period of inflorescence differentiation can induce formation of hermaphrodite or female flowers in a male inflorescence.
Thus auxins cause femaleness in plants.
- (k) Healing : Healing of injury is effected through auxin induced division in the cells around the injured area. The chemical was formerly named traumatic acid or traumatin.
- (l) Nodule formation : In legumes, IAA is known to stimulate nodule formation.
- (m) Respiration : According to French and Beevers (1953) the auxin may increase the rate of respiration indirectly through increased supply of ADP by rapidly utilizing the ATP in the expanding cells.
Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered.
# Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928) from coleoptile tip of Avena . Went also developed Avena curvature test for bioassay of auxin. Kogl and Haagen. Smit (1931) introduced the term auxin.
# Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins.
- (a) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.
The first naturally occuring auxin was isolated by Kogl and Haagen-Smit (1913) from human urine. It was identified as auxin-a (auxentriolic acid, C18H32O5). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid (4-chloro-IAA) etc., are some other natural auxins.
Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and not through the vascular tissues. Auxins help in the elongation of both roots and shoots. However, the optimum concentration for the two is quite different.
It is 10 ppm for stem and 0.0001 ppm for the root. Its translocation rate is 1–1.6 cm/hr. (In roots 0.1 to 0.2 cm/hr). Higher concentration of auxins show inhibitory effect on growth.
Natural auxins are of two types : free and bond auxins. The auxins which can easily be extracted are called free auxins, whereas auxins which are hard to extract and need the use of organic solvents are termed as bound auxins. The free form of auxin is active, while the bound auxin is inactive in growth. A dynamic equilibrium exists between these two forms.
- (b) Synthetic auxins : These are synthetic compounds which cause various physiological responses common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the weedicide, 2, 4, 5-T (2, 4, 5-trichlorophenoxy acetic acid), IBA (indole 3-butyric acid), NAA (naphthalene acetic acid, PAA (Phenyl acetic acid), IPA (Indole 3-propionic acid). IBA is both natural and synthetic auxin. Certain compounds inhibit action of auxin and compete with auxins for active sites are called antiauxins. e.g., PCIB (p- chlorophenoxy isobutyric acid), TIBA (2, 3, 5-tri iodobenzoic acid). TIBA is used in picking cotton bolls.
# Bioassay of Auxins : Testing of biological activity (growth) of a substance (auxin) by employing living material is called bioassay. Auxin bioassay is also quantitative test as it measures amount of effect in response to a particular concentration of auxin.
- (a) Avena coleoptile curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant growth by performing some experiments with the oat (Avena sativa) coleoptile.
• When the tips of the coleoptiles were removed, no growth took place.
• When the freshly cut coleoptiles were placed on agar blocks for a few hours (during this period auxin diffused into the agar block) and then the agar blocks were placed on the cut ends of the coleoptile, growth occurred.
• When the agar block with the diffused substance was placed laterally on the cut tip of the coleoptile, only that side of the coleoptile elongated resulting in a curvature.
- (b) Split pea stem curvature test : This test was also discovered by Went, 1934. Dark germinated seeds of pea are decapitated. About half an inch part of stem between 2nd and 3rd node is removed and split longitudinally. It is then floated on the test solution contained in a beaker. At first, negative curvature occurs due to water uptake. Then positive curvature occurs which is proportional to the log of the concentration of auxin.
These experiments indicated that some substance is synthesised in the coleoptile tip is translocated downward. He called this substance auxin.
- (c) Root growth inhibition test (Cress root inhibition test) : Sterilized seeds of cress are germinated over moist filter paper. Root lengths are measured. 50% of seedlings are placed in test solution while the rest are allowed to grow over the moist filter paper. Lengths of roots are measured after 48 hours. Seedlings placed in test solution show very little root growth while the roots of controlled seedlings show normal growth. The degree of root growth inhibition is proportional to auxin concentration.
# Functions of auxins : Auxins control several kinds of plant growth processes. These are as follows :
- (a) Cell elongation : Auxins promote elongations and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.
The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of microfibrils, synthesis of more wall materials, increased membrane permeability and respiration.
- (b) Apical dominance : In many plants, the apical bud grows and the lower axillary buds are suppressed. Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is checked by applying synthetic auxin (NAA).
- (c) Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced when the auxin content falls below a minimum. Addicot and Lynch (1951) put forward auxin gradient theory about abscission :
• No abscission if auxin content is high on the organ side.
• Abscission layer begins formation when auxin content becomes same on stem and organ sides.
• Abscission is favoured when auxin content is low on the organ side.
Premature drop of fruits such as apple, pear and citrus can be prevented to a great extent by spraying the trees with a dilute solution of IAA, NAA or some other auxin.
- (d) Weed control : Weeds are undesirable in a field with a crop. Weeds cause competition for water, mineral, light and space. This causes poor yield. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but 2, 4-D does not affect mature monocotyledonous plants.
- (e) Root differentiation : Many new plants are usually propogated by stem cutting e.g., Rose, Bougainvillea. If we dip the lower cut end of a cutting in dilute solution of auxins (specially IBA gives very good results) very soon large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
- (f) Parthenocarpy : It is the process of formation of fruits without fertilization. Such fruits are called as parthenocarpic fruits and are without seeds. Parthenocarpy can be induced by application of IAA in a paste form to the stigma of a flower or by spraying the flowers with a dilute solution of IAA. Banana, oranges and grapes are now-a-days grown parthenocarpically on commercial scale.
- (g) Control of lodging : In some plants when the crop is ripe and there is heavy rain accompanied by strong winds, the plants bends as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute solution of any auxin is sprayed upon young plants the possibility of bending of plants is reduced as the stem becomes stronger by the application of auxins.
- (h) Flowering : In pineapple, NAA promotes flowering. In lettuce, auxins help in delaying the flowering. In cotton plants, the use of auxins increases the cotton seeds production.
- (i) Differentiation of vascular tissues : Auxins induce the differentiation of xylem and phloem in intact plants and also in callus produced in vitro during tissue culture experiments.
- (j) Sex expression : The spray of auxins increases the number of female flowers in cucurbits. In maize application of NAA during the period of inflorescence differentiation can induce formation of hermaphrodite or female flowers in a male inflorescence.
Thus auxins cause femaleness in plants.
- (k) Healing : Healing of injury is effected through auxin induced division in the cells around the injured area. The chemical was formerly named traumatic acid or traumatin.
- (l) Nodule formation : In legumes, IAA is known to stimulate nodule formation.
- (m) Respiration : According to French and Beevers (1953) the auxin may increase the rate of respiration indirectly through increased supply of ADP by rapidly utilizing the ATP in the expanding cells.