Plant Growth
Growth Hormones
The term 'hormone' was coined first by Starling (1906) in animal physiology. The internal factors that influence growth are called growth hormones or growth regulators as they inhibit, promote or modify the growth. Growth promoters are auxins, gibberellins (GA) and cytokinins (CK). Growth inhibitors in plants are ethylene and abscissic acid (ABA). All phytohormones are growth regulators.
According to Thimann and Pincus (1948) "Plant hormones are organic substances produced naturally in higher plants affecting growth or other physiological functions at a site remote from its place of production and active in very minute (optimum) amount". Hormones are transported through phloem parenchyma (Phillips 1971).
F. W. Went in 1931, used this term first. Auxin was isolated from urine of a person suffering from Pellagra (Kogl and H. Smit 1931). In plants, it is synthesized in growing tips or meristematic regions of plants from where it is transported to other plant parts. The most common and important natural auxin is Indole-3-acetic acid (IAA). Tryptophan is the primary precursor of IAA in plants. It is the first hormone to be discovered in plants and is primarily responsible for cell elongation. It shows polar transport - Basipetal transport in stem. Now synthetic auxins like IBA (Indole butric acid), NAA (Naphthalene acetic acid), 2, 4-D pichloro (Phenoxy acetic acid), etc. are used.
The auxin is the first hormone to be discovered in plants. Discovery of auxins dates back to 19th century when Charles Darwin (1886) was studying tropism in plants. He exposed canary grass coleoptile to unilateral light. He concluded that a growth stimulus is developed in the coleoptile tip and transmitted downwards to the growth zone. This has caused bending of the tip towards light.
The Danish plant physiologist Boysen - Jensen (1910) cut off the colepotile and inserted thin plate of gelatin beween the tip and the cut stump. He observed that coleoptiles tip still bends towards unilateral light.
Paal (1919) cut off the tip of colepotile and replaced it asymmetrically on the cut coleoptile stump. He observed that the colepotile tip bent away from the side bearing tip even in dark. F.W. Went (1928) successfully isolated natural auxin from Avena coleoptile tips. He cut off the tip and placed them on small agar blocks. Then after certain period of time placed the agar blocks asymmetrically on cut coleoptile stump that caused bending. He demonstrated the presence of substance which could diffuse into agar blocks. Went named this substance as auxin.
The primary effect is cell enlargment. In most of the higher plants, growing apical bud inhibits the growth of lateral buds. This is called as apical dominance. Auxin stimulates growth of stem and root. Auxin induces multiplication of cells, hence used in tissue culture experiments to produce callus. It stimulates formation of lateral and adventitious roots. These are marketed as synthyetic herbicides. e.g. 2, 4-D (2,4 dichlorophenoxy acetic acid). It kills dicot weeds without affecting monocot crop plants.
The seedless fruits like orange, lemon, grapes, banana etc. are produced by application of auxin (i.e. induced parthenocarpy). Auxins promote cell division in cambium and also cause early differentiation of xylem and phloem. It promotes root elongation in low concentration and shooting at higher concentration. It also hastens early rooting in propagation by 'cutting'. Foliar spray of NAA and 2,4-D induces flowering in litchi and pineapple. Likewise, it prevents premature fruit drop in apples, pear and oranges, and also prevents formation of abscission layer. Auxins play a role in elongation of cell. It is known to increase rate of respiration. Auxins break dormancy in seed and promote quick germination.
It is another growth promoting hormone and is abundant in root tip and developing seeds. It shows non-polar transport through vascular tissue. Gibberellins were first isolated from the fungus Gibberella fujikuroi by Japanese scientist Kurosawa (1926). He observed that when rice plant was infected by fungus Gibberella fujikuroi, it shows extensive stem elongation called 'bakane disease'. The crystalline form of Gibberellins were isolated by Yabuta and Sumiki (1938) from the fungus culture. They named it as gibberellin. It is synthesized in young leaves, seeds, roots and stem tips. These are synthesized from mevalonic acid. More than 150 chemical types are known so far. GA3 is most common and biologically active form. Chemically it contains a gibbeane ring - a cyclic diterpene with four isoprene units.
Dormancy of bud can be broken by gibberellin treatment. It can promote seed germination in cereals like barley and wheat by synthesizing hydrolysing enzyme amylase to produce sugar. The most striking effect of it, is the elongation of stem where internodes increase in length. It also promotes bolting i.e. elongation of internodes just prior to flowering in plants those with rosette habit e.g. beet, cabbage. It causes parthenocarpy in tomato, apple and pear, and flowering in long day plants. It is used to increase the fruit size and bunch length of grapes. When gibberellins are applied on genetically dwarf plants like maize, the stem rapidly elongates and acquires the height of normal tall varities of maize. Application of gibberellins overcomes the requirement of vernalization. Usually, it inhibits growth of root, delays senescence and prevents abscission. It also breaks dormancy of seed and hastens germination. Application of gibberellin causes production of male flowers on female plant.
It is another growth hormone that promotes cell division. Letham coined the term cytokinin. The first cytokinin was discovered by Skoog and Miller (1954) during investigation of nutritional requirements of callus tissue culture of Nicotiana tabacum (Tobacco).They observed that the callus proliferated when the nutrient medium was supplemented with coconut milk and degraded sample of DNA (obtained from herring sperm). They named it as kinetin. Chemically kinins are 6-furfuryl amino purine. First natural cytokinin was obtained from unripe maize grains by Letham et al. It is known as Zeatin. 6-benzyl adenine is a synthetic cytokinin hormone. Seven different types of cytokinins are recorded from plants. Natural cytokinins are also reported from plants like Banana flowers, apple and tomato fruits, coconut milk, etc.
Besides cell division, it also promotes cell enlargment. High cytokinin promotes shooting. A low ratio of cytokinin to auxin induces root development but a high ratio causes buds and shoot to develop. Cytokinin and auxin ratio and their interactions controls morphogenic differentiation. It promotes the growth of lateral buds and controls apical dominance by cell division. It delays the senescence or ageing and abscission processes in plant organs. This was reported by Richmond and Lang (1957). Formation of interfasicular cambium and expansion of cells are other functions. It also breaks dormancy and promotes the germination of seeds. Cytokinin reverses apical dominance effect. It induces RNA synthesis and formation of interfascicular cambium.
It is the only gaseous growth regulator. Denny (1924) reported ethylene is effective in fruit ripening. Gane (1934) established that plants naturally synthesize ethylene. Crocker (1930) proposed that ethylene is the plant hormone responsible for fruit ripening. It is a simple gaseous hydrocarbon with essential role in the fruit ripening. The most widely used compound as a source of ethylene is ethephon. It is synthesized in roots, shoot apical meristem, ripening fruits etc.
It promotes ripening of fruits like bananas, apples and mangoes. It stimulates initiation of lateral roots in plants and breaks the dormancy of bud and seed. It accelerates the abscission activity in leaves, flowers and fruits by forming of abscission layer. Ethylene inhibits the growth of lateral buds and causes apical dominance and retards flowering. It is associated with the enhancement of process of senescence of plants organs. It inhibits flowering in most of the plants except pineapple. It causes epinasty (drooping) of leaves and flowers. It increases activity of chlorophyllase enzyme causing degreening effect in banana and Citrus fruits.
It is a natural growth inhibiting hormone. Carns and Addicott (1961-65) observed that the shedding of cotton balls was due to a chemical substance abscisin I and II. Wareing (1963) isolated a substance from buds of Acer that can induce bud dormancy and named it dormin. These two identical chemical substances were given the common name abscissic acid. It is synthesized in leaves, fruits, roots, seeds etc. Chemically, it is a 15-carbon sesquiterpenoid and is synthsized from mevalonic acid.
It promotes abscission of leaves and induces dormancy in many plants. It controls the dormancy in buds and seeds by inhibiting growth processes. It accelerates the senescence of leaves, flowers and fruits. It inhibits and delays cell division and cell elongation and suppresses cambium activity by inhibiting mitosis in vascular cambium. ABA could cause efflux of k+ ions from the guard cells and result in closure of stomata. So, it is known as an antitranspirant. It acts as a stress hormone by inducing the plant to bear the adverse environmental conditions. It inhibits flowering in long day plants but stimulates flowering in short day plants.
