Although plants may seem to be simple organisms, as they are made up of millions of cells they require some means for these cells to relate to each other in order to grow into their functional state. This is carried out by a number of chemicals or plant hormones (phytohormones) which are produced in the plant and which regulate processes within cells as well as being transported to other locations where they can have an effect. Most do not have a single action, but can be involved in a number of processes and act on each other to keep the plant functioning normally by stimulating or inhibiting their production.
Plant hormones influence the rate and direction in which the plant grows, the timing and formation of leaves, flowers and fruit, and whether a plant should be an annual or a perennial. They help the cells differentiate into the many types that make up a mature plant. For gardeners understanding how they work can improve the way we look after our plants, eg. when pruning, propogating with cuttings or germinating from seed.
The five main growth regulators are Abscisic Acid, Auxins, Cytokinins, Ethylene and Gibberellins, which perform most of the important influences, but there are some other substances which can have more specific effects that may only occur in certain plants or affect functions not directly involved in growth, such as Salicylic Acid which helps in the defense against pathogens.
The phytohormones are not fully understood so scientists are still finding out how they interact with each other and with other processes in plants.
Abscisic Acid ABA
Originally thought to be involved in the dropping of fruits and leaves, but has subsequently been found to have other roles. It is partially synthesized in the chlorplasts and production is increased during stresses such as freezing temperatures or drought. It is translocated in the xylem and phloem so it can move up and down within the plant. It has an inhibitory effect on bud growth and on the dormancy of seeds and buds. During winter buds and seeds would start to grow during milder periods were it not for the presence of ABA, so they are protected from later frosts. It is accumulated in seeds as they mature in the fruit and this prevents them from germinating immediately. As winter progresses the levels decrease and water entering seeds and buds reduces the amount of ABA to allow growth to begin in the spring. The level continues to decrease as the seedling develops, but as shoots begin to grow the ABA levels increase, slowing the growth of cells.
When plants are suffering from a lack of water, the leaves respond to signals from the roots by producing the precursors of ABA in the chloroplasts and these pass down to the roots in the phloem which release the completed hormone which moves back up to the leaves. In the guard cells that surround the stomata the balance of sodium and potassium ions is changed so that the cells lose turgidity and the stomata close, so reducing to amount of water loss.
Some of the known functions of ABA are:
Auxins
There are a number of compounds that are called Auxins, but they all have an influence on cell elongation similar to the effects of Indoleacetic Acid (IAA), the first growth regulator to be isolated. They also stimulate the production of other plant hormones and along with the cytokinins, exert control over stem and root growth and the production of flowers and fruit.
Charles Darwin experimented on the effect of light on plants and published his findings in his book 'The Power of Movement in Plants' in 1880. Studying the growth tips of canary grass seedlings, he described how they grew towards a light source and if the tip was covered this did not happen. However if the area behind the tip was covered the response was normal, so he concluded that the tip was producing a signal which was passed to lower tissues to cause the change in growth direction. Also by cutting off the tip the response to light was removed.
Indoleacetic Acid (IAA) was discovered in fermentation media by Leopold Salkowski in 1885 but it was a further 50 years before it was isolated from plants. Later experiments elaborated on Darwin's work and showed that the signal he postulated was caused by the production of auxins on the darker side of a shoot tip lit on one side, which travel down to lower tissues and cause them to grow at a faster rate; this forces the shoot to bend towards the light source. A similar result could be obtained by cutting off the tip of a shoot that had been lit from above, then replacing it to one side and it grew faster on that side. If the severed tip is placed on a block of agar gel it absorbs the auxin, then if the agar is put on the shoot, growth is resumed as the auxin moves down to the lower tissues. Again the direction of growth can be controlled by placing the block to one side and the amount of curvature is proportional to the amount of auxin in the agar.
Some of the known responses caused by Auxins are:
Therefore the Auxins are probably the most important plant hormones for gardeners to understand as knowing how they work can be useful for better plant care and cultivation. In high concentrations they are toxic to plants as they cause excessive growth. This is particularly the case for dicotyledons or broadleaved plants and less so on monocotyledons or grass-like plants, so this property has been applied to the development of synthetic Auxins as selective herbicides such as 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T a major ingredient of Agent Orange). The rooting compounds used when growing from cuttings contain Auxins such as 1-Naphthaleneacetic acid (NAA) and Indole-3-butyric acid (IBA).
Cytokinins
Cytokinins are compounds which promote cell division (cytokinesis). It was noticed that a compound found in the phloem of plants could stimulate cell division and that similar compounds were present in animals. The first one was isolated from herring sperm and named Kinetin due to this action. The first cytokinin isolated from plants was Zeatin which was extracted from corn (Zea mays). The highest concentration is found in areas of growth such as the meristem at the tip of shoots and roots, the cambium layer, young leaves and developing fruit.
There are a number of compounds which have similar action and there are about two hundred natural and synthetic cytokinins known at present. Their molecular structure resembles adenine the base pair of thymine in DNA and RNA. Cytokinin synthesis occurs with the addition of a side chain to the adenine molecule and the action of the enzyme cytokinin oxidase breaks it down to adenine again.
There is a close relationship between the levels of Auxins and Cytokinins and the ratio between their concentrations remains constant, so if one changes the other increases or decreases to a corresponding level.
The functions of the cytokinins depend on the type present and on the particular species where it is found, but the main responses are:
Ethylene
The ability of a gas to stimulate the ripening of fruit was known by ancient Egyptian and Chinese civilizations. Also in 1864 it was observed that leaks from street lighting would cause stunting, twisting and abnormal thickening of plant stems - a phenomenon known as the triple response. At around the turn of the 20th century the active agent was identified as Ethylene and in 1934 it was found that plants could synthesize it.
Ethylene in a simple hydrocarbon (C2H4) which can be synthesized by all plant cells. It is relatively insoluble in water and diffuses out of the cell so its hormonal effect depends on how much is produced.
It is now known that Ethylene is involved in how a plant responds to its environment as it develops and grows. As seeds germinate it helps the emerging shoot cope as it travels to the surface. In rapidly dividing cells production is high, especially in darkness, and in young seedlings this is faster than it is lost so it accumulates in the cell where it prevents elongation. This has an advantage in that it allows the stem to expand, so pushing obstacles out of the way of seedling shoots as they emerge.
Above ground the influence continues as it becomes involved with ripening, senescence of older tissues in leaves and their abscission as they are lost from the plant. Also the lateral movement of the wind induces greater ethylene production and this makes the stems more sturdy - part of the triple response.
For gardeners controlling the ripening of fruit can be important, especially in commercial production where the metabolic processes involved in the production of the gas within cells has been manipulated to reduce the amount formed to delay it. On the domestic level, one method to encourage it has been to put a ripe banana with unripe fruit to quicken the process as it releases ethylene, conversely bananas should not be placed with other fruit in the fruit-bowl.
The functions of Ethylene are:
Gibberellin (GA)
Gibberellins were first discovered when studying abnormal growth of rice plant seedlings when they were infected by a fungal disease called Bakanae. Around the end of the nineteenth century it was known that a fungus was causing the problem, but the actual compound being secreted was not isolated from Gibberella fujikuroi until 1934 and it was named Gibberellin in 1935. In the mid nineteen fifties it was found that gibberellin-like substances are found naturally in all higher plants and since then over 130 have been found in plants, fungi and bacteria. They are categorized by their structure which has a four or five ring system with 19 or 20 carbons and named GAn, where n = the order in which they were discovered, ie. GA1 was the first Gibberellin isolated.
They are synthesized in developing seeds and in the immature tissues of shoots with some evidence that leaves also produce some. When water is absorbed by a grain seed, production of GA starts and this travels to the aleurone layer which surrounds the endosperm where the food is stored. The aleurone layer then produces enzymes that break down the food reserves to be used by the germinating seed.
Chemicals used to stunt the growth of plants work by blocking the synthesis of Gibberellins - these are now banned by EU regulations as they were shown to damage embryos of experimental animals.
The functions of the Gibberellins depend on the type that is present and on the plant species:
Some Other Plant Hormones
Polyamides - affect mitosis and meiosis the stages during cell division when the chromosomes are replicated.
Strigolactones - inhibit the branching of shoots.
Plant Peptide Hormones - carry signals from cell-to-cell during growth and development, in defensive responses, during cell division and expansion, and the compatibility of pollen grains.
Karrikins - found in the smoke of burning plant material, they stimulate the germination of seeds of plants that originate in areas prone to fires.
Salicylic Acid - stimulates the production of chemicals that defend against pathogens. In human medicine it was extracted by early healers from willow bark and the active ingredient of Asprin is the synthetic derivative acetylsalicylic acid.
The Relevance to Gardening
The auxins and their synthetic analogues are used to promote root growth of cuttings. They are applied to the cut surface, with the usual addition of a fungicide to prevent infectin, where a callus forms and from which new roots develop. In laboratory conditions new plants can be produced from small sections of cells using growth hormones to induce multiplication and rooting of the plantlets; a process called micropropogation. They can also be used when grafting to help the formation of callus tissue which joins the cut surfaces.
An extract from Willow twigs can be prepared for use as a rooting promoter which also contains Salacylic Acid that will protect cuttings from infections. A method of brewing the mixture is given on the Gardening Recipes page.
During germination of seeds the ratio of ABA to GA is important. ABA controls the dormancy of the embryo and GA the germination, so the level of the former has to decrease while sensitivity to the latter increases. Also as the seed coat forms its thickness and form is affected by ABA so depending on the plant and environmental conditions levels of ABA can vary and this will influence when germination will eventually take place. Knowing how different seeds behave allows for better rates of germination or whether it will occur at all, eg. using stratification techniques where seeds are chilled to mimic the winter cold then warming of spring which stimulates the breaking of dormancy.
When pruning, knowing that removing the source of growth stimulating or inhibiting hormones will have the desired effect on the plant, should give a better result. So removing the tip of a shoot causes it to branch lower down - the phenomenom known as Apical Bud Dominance, due to the effects of Auxins. This is why hedging plants have to be trimmed regularly as they grow to produce a thick hedge, if not they will quickly reach the desired height, but lower down will be thin and see-through.
When training roses and fruit trees bending the shoots and branches to a more horizontal plane affects the hormone balance to encourage more side shoots and therefore fruit or flowers to be produced.
Back to Home Page
or use the floating menu at the top of the page to find other articles.
