Story Title: Gibberellins – Plant Growth Hormones
Author: Leo Wright
The power of gibberellins to accelerate growth, and to induce or promote flowering, continues to fascinate both amateur botanists and commercial flower growers. One gibberellin is gibberellic acid, a natural hormone that can be readily extracted from common plants.
Auxins, cytokinins and gibberellins are the principle growth-promoting hormones found in plants. All three control, stimulate, inhibit or alter a plant’s development to one degree or another, depending upon the external environment. Auxins tend to promote rooting, leaf and fruit retention and directional growth; and cytokinins promote active cell mitosis, ion transport and general plant vigour. Gibberellins are noted as the most powerful of the growth promotors because they , increase internode spacing, induce and promote flowering in many plants, and modify the flower sex expression in some plants.
Investigations in Japan in the 1920’s of the pathogenic rice fungus Gibberella fujikuroi, which caused rice plants to grow abnormally tall, led to the eventual isolation from the fungus of several types of gibberellins or growth-promoting hormones, including Gibberellic Acid (GA-3).
Gibberellins are well known to promote uniform growth through cell enlargement. They cause plants to grow tall and elongated, with light green leaves, and also stimulate seed germination and other growth phenomena such as early flower formation.
Flower Induction and Promotion
In many plants flower formation is governed by internal factors; in other plants it is controlled by precise environmental conditions. Some plants initiate flowering after having undergone exposure to a period of cold. In nature, these cold-requiring plants usually flower in spring or early summer, after having been exposed to the cold temperatures of winter.
In other plants, flower formation depends upon day length or photoperiod. Basically, there are two principal photoperiodic plants – “long-day” plants which flower when the day length exceeds a certain minimal value which may vary from one plant to another, and “short-day” plants which exhibit the opposite behaviour, flowering in relatively short days when the photoperiod remains below a certain maximal duration.
Under these conditions, long-day plants flower in summer when the days are longer, and short-day plants flower in autumn and winter when the day length drops below the critical maximum.
Then there are plants that are described as “dual-day length” plants, where they stay vegetative if grown on continuous long day or continuous short day, but flower if exposed either first to long then short days (“long-short-day” plants), or vice versa (“short-long-day” plants). Most cold-requiring plants also have dual environmental requirement, flowering if the low-temperature treatment is followed by a long-day regime.
The phenomenon of cold requirement with regard to flower formation is called “vernalization”, and that of day length control as “photoperiodism”. The conditions conducive and nonconducive to flower formation in a given plant type have been termed “inductive” and “noninductive”, and exposure of cold-requiring and photoperiodic plants to inductive temperatures and photoperiods are called “thermo-induction” and “photo-induction” respectively. In cold-requiring and photoperiodic plants alike, the need for induction may be absolute, whereby the plant will fail to form flowers altogether unless given inductive treatment; or it may be facultative whereby flowering will ultimately occur without induction, although with greater or lesser delay.
The use of gibberellins for cold-requiring and long-day plants can induce or promote flowering to one degree or another. Typical gibberellin responses include larger blooms, stem elongation, flower stalk elongation, and in some cases earlier flowering, which are all desirable elements to commercial flower growers.
When gibberellic acid is sprayed on gardenia or geranium flowers, there is a 25% -50% increase in flower size. The treatment is used at the rate of 5 mg/L (5ppm) at the time of first colour appearance.
The flowering of cyclamens can be accelerated by 4-5 weeks with a single spray of gibberellic acid, at the rate of 50 mg/L (50ppm), 60-75 days prior to the projected flowerdate (Widmer et al. 1974). Higher concentrations will result in adversely tall and weak flower stems. More recently, Lyons and Widmer (1983) suggest applying 15 gms/L (15ppm) of gibberellic acid to the crown of the plant below the leaves, 150 days after seed is sown.
Gibberellins are popular with commercial growers to replace the cold treatment or long night treatment of plants such as azaleas to induce or force flowering. Standard cultivation techniques require flower-bud induction with about six weeks of long-night treatment. Once flower buds are established, a temperature of 7°C (45°F) or lower is required for six weeks to ensure flower bud development. After this, flowers are forced into bloom in 4-6 weeks. However, a weekly spray treatment of gibberellic acid for five weeks, at a concentration of 1000 gms/L (1000ppm), will result in earlier flowering and larger blossoms. The five consecutive weekly sprays should commence when flower buds are well developed after the short-day treatment.
Hydrangeas, another cold-requiring plant, also respond favourably to gibberellic acid. Using the same five-weekly treatment, the concentration should be reduced to 5-50gms/L (5-50ppm) to ensure earlier flowering and larger blooms.
Gibberellic acid can also be used to delay flowering and to stimulate rapid growth in plants such as geraniums and fuchsia. The treatment requires weekly sprays at the rate of 250gms/L (250ppm) for four weeks.
According to Carlson (1982), gibberellic acid can also be used to produce tree-type geraniums and fuchsia when applied at the rate of 250gms/L (250ppm) two weeks after potting, then once weekly for five weeks.
It should be noted here that the precise function of applied gibberellins to flower formation is not entirely clear since all plants react differently to treatments, and in many cases gibberellins do not promote flower formation.
Flower sex expression can be modified in some plants by treating seedlings with several growth-regulating substances. With the exception of gibberellin, these substances tend to reduce the number or suppress the development of staminate flowers, and increase the number or accelerate the development of pistillate flowers. In contrast, in the case of cucumbers, gibberellins increase the number of staminate flowers on monoecious cucumbers (plants that have the stamens and the pistils in separate flowers on the same plant), and result in the formation of staminate flowers on gynoecious (female) cucumbers which would otherwise only produce pistillate flowers.
The ultimate effect of a chemical on sex expression would be a complete reversal of flower sex. To validate a flower sex reversal one would have to replace the intial staminate stage with pistillate flowers, or the pistillate stage with staminate flowers in monoecious plants. It has been found that gibberellins will increase the number of staminate flowers in monoecious cucumbers, resulting in the formation of staminate flowers on gynoecious cucumbers which would otherwise only produce pistillate flowers.
Extracting Gibberellic Acid
Although several types of gibberellin are found in plants as natural hormones, Gibberellic Acid (GA-3) is the best known. While it is a natural product of the Asian fungus that destroys rice, growth-promoting substances that are either identical with, or closely related to, gibberellic acid can also be found in common plants such as cucumber, rock melon (cantaloupe), corn, peas and beans, and it can be readily extracted in crude form by amateur botanist.
Edward Pinto, a student at St Peter’s Preparatory School in Jersey City, developed a simple and inexpensive procedure for extracting gibberellic acid from common plants, which was reported in American Scientific ( August 1967). As sources of materials, he used the seeds of fresh cantaloupe (rockmelon), fresh wild cucumber, and the dry seeds of corn, peas and three species of bean – pencil rod, lupine and pinto. The cantaloupe and cucumber seeds were dried at room temperature and chopped into particles about 3mm in diameter. The procedure used 200 grams of finely chopped seeds which were soaked for seven days in a solution of acetone (10 parts by volume), isopropyl alcohol (5 parts), ethyl alcohol (2 parts), and distilled water (5 parts), to give a total volume of 110 millilitres. The solution was then poured off and the seed particles rinsed with 40 millilitres of a solution consisting of equal parts of acetone and isopropyl alcohol. The rinsing solution was then added to the first solution, and heated to a temperature of 45°C (113°F)
WARNING: it should be noted that the solution is highly flammable and must not be exposed to an open flame. The heating procedure was continued until the residue evaporated to the consistency of thin tar and was almost dry. The residue was then taken and mixed with 100 millilitres of distilled water and ethyl acetate.
According to Pinto, a key factor to extracting gibberellic acid is to raise the pH of the water to about pH8 (slightly alkaline) – at this pH the gibberellins are soluble in water. The pH was achieved by adding potassium hydroxide, or concentrated pH lower to the solution. The mixture was then shaken for two minutes, and the water drawn off and mixed with another 100 millilitres of ethyl acetate. This procedure was carried out a total of three times.
Now the water was made acidic (pH3) by the addition of hydrochloric acid – at this pH the gibberellins are soluble in ethyl acetate. The solution of acidic water was added to 100 millilitres of ethyl acetate. The water was drawn off and the procedure repeated twice more, after which the ethyl acetate solution was dried to a paste. The tarlike mass was then mixed with about 8 grams of lanolin. The lanolin paste is the final product, and it is applied to plants as a thin coat to the upper surface of each mature leaf, taking care not to damage the plant.
The role of plant hormones is complicated biologically and biochemically, and even today their roles are not fully understood. What works for one plant does not necessarily follow for another. In most cases it is which will signal a homonal response. When applied externally, hormones will influence the organisation of the internal chemistry of the plant cell, and the interaction among cells, but the degree of interaction will still depend upon the plant specie, the stage of plant development and the external environment.