Chapter 5 - Growth Regulators
Contents - Class Homepage
Role in Plant Growth and Development
Auxins Gibberellins Cytokinin Ethylene Abscisic Acid

When you have successfully completed this section you will:

1.  Know the five major classes of tree growth regulators.

2.   Be able to describe the general functions of each class of tree growth regulators and know some practical uses for tree growth regulators in forestry.

Although photosynthesis supplies the carbon and respiration supplies the energy for plant growth, a group of chemicals produced by plants known as plant growth regulators control the growth and development of trees. These chemicals act on plant processes at very low concentrations. Often they are produced at one location and transported to another where they exert their influence; however, they may also act on the same tissue in which they are produced.

Before discussing the various groups of plant growth regulators the terms Growth and development should be defined. Growth refers to an increase in size by cell division and enlargement. Development is a term used to refer to differentiation of cells into the various tissues and organs ( e.g. leaf versus flower). Plant growth regulators influence both growth and development.

Growth regulators play a very large part in the development of plants. The roles of individual regulators are described below, but please remember that the issue is often cloudy. Apical dominance, for example, may be influenced by the interaction of several growth regulators.

Auxins play a role in stem elongation and apical dominance. Gibberellins are important in elongation, bolting and flowering. Cytokinin is known to influence cell division, cell and organ enlargement, and the delay of senescence in flowers, vegetables and fruits. One of its most pronounced physiological effects of ethylene is abscission or the shedding of a plant part. It appears that an interaction between auxin and ethylene is responsible for control of abscission. Likely ABA is involved in dormancy induction.

Growth regulators can also be useful tools, and commercial formulations are available to slow stem elongation, promote rooting, and to promote flowering (among other things).

Back to top AUXINS
Auxins were the first group of growth regulators to be discovered. In the late 1800’s Charles Darwin and his son Francis observed that young grass seedlings would always bend towards the light. They discovered that if the growing tip of the grass plant was covered they would not bend towards the light. Around the same time, similar findings with roots bending down only when their tips were left attached were observed by a horticulturist by the name of Ciesielski. Both the Darwin's and Ciesielski reasoned that a signal of some sort was being transported from the plant tip resulting in curvature some distances below the tip. Later it was discovered that these earlier researchers were correct and that auxin produced in the growing tips was the signal responsible for this curvature. Today it is known that auxins, primarily IAA, play a major role in plant cell elongation and tropisms (movements in plants in response to unidirectional signals, for example bending towards light).

Auxins have a wide variety of effects on plants and the effects change with concentration, the chemical form present, the presence of other growth regulators and even the growth stage of the plant. Therefore it is difficult to generalize or summarize the basic effects of auxin. However, there are some well established effects which will be discussed briefly.

A major source of auxin are rapidly growing shoot and root tips in plants. It is auxin produced in these tissues when transported to lower portions of the stem promotes cell elongation. When shoot tips (the source of the auxin) are removed, growth is slowed. If auxin is applied to the severed shoot tip elongation and growth resumes. Only at very low concentrations (10 -5 to 10 -6 M) does auxin stimulate cell elongation.

Auxins are also known to play a role in apical dominance. In most plants, the growing apical tip inhibits the growth of lateral buds. This is known as apical dominance. If the terminal or apical tip of the plant is removed lateral buds will elongate and grow. It is further known that if auxin is applied to the severed shoot tip, the lateral buds will remain suppressed. However, it is unlikely that auxin acts alone in controlling apical dominance. Research also indicates that nutrition of the lateral buds, another growth regulator called cytokinin or other identified substances may interact with auxin to maintain apical dominance. Whatever the case, it is clear that auxin is either directly or indirectly is involved in apical dominance.

Related to apical dominance is the relationship auxin plays in controlling sprouting along the trunks of trees. Some species of trees sprout along the stem when they lose their tops or are cut down. These sprouts arise from dormant buds just under the surface of the bark. Normally, auxin supply from the actively growing tops inhibits these buds from sprouting.

Auxins are also involved in sprouts which form along the upper portion of stems when trees are bent over by ice, snow, or wind.

Researchers have found when artificial auxin is supplied to cut stems no sprouts develop. When the auxin is removed the stems sprout. Again auxin may not be acting alone but it clearly has a role. Similarly many hardwood trees will develop epicormic sprouts along their trunks when they are suddenly exposed to increased light. Researchers believe that the sudden exposure to light may break down auxin levels which in turn allows dormant buds along the stems to grow.

Auxin is involved in phototropism or the bending of young stems towards the light. It is believed that auxin in the plant tip occurs in higher concentrations on the shaded side which is then transported down causing cells on the shaded side to elongate more. This results in the plant bending towards the light. A similar effect also occurs in the roots. The unequal distribution of auxin to the upper root surface of a horizontal root results in a bending downward or what is called a gravitropism (a movement in response to gravity).

Auxins, primarily synthetic ones, have been used commercially in the plant industry for many years. One of the most well known uses of auxin is for the rooting of cuttings for plant propagation. Shoot tips of many plant species when dipped or coated with small amounts of auxin develop roots more quickly and in higher numbers. Most commercially available rooting powders take advantage of this effect. Many herbicides are also synthetic auxins. When applied at higher (but still relatively low) concentrations auxins cause abnormal leaf curling and eventually plant death. Auxins have also been used by horticulturist in the development of parthenocarpic fruit. This is the production of fruit without fertilization which results in seedless fruit.

These rooted cuttings of slash pine were produced by dipping cut shoots in auxins.

Gibberellins are a large group of related chemical compounds (over 80 have been identified) with a wide range of effects. They were first identified by Japanese scientists (in 1930’s) who were investigating a rice disease known as bakanae (or foolish seedling). The disease was found to be caused by a fungus which when it infects rice seedlings causes them to grow abnormally tall, fall over and produce no seed. It was discovered that a group of chemicals secreted by the fungus was responsible for the symptoms. These chemicals were named gibberellins after the name of the fungus Gibberella fujikuroi. It was not until after World War II that this information reached the west and a large amount of research began on the effects of gibberellins.

Probably the most conspicuous and well known effect of gibberellin is stem elongation. Applications of gibberellin to genetically dwarf plants is known to greatly increase their growth to the point where they actually appear normal. Gibberellins have little effect when they are applied to normal plants. Related to this stem elongation effect is the influence gibberellins have on bolting and flowering. Many plants will naturally stay small and only produce leaves under short days. Later in the growing season as the days get longer the plants will eventually elongate rapidly (bolt) and typically flower. Examples of this include spinach and cabbage. Application of gibberellins will often cause plants to bolt and flower even when the days are short. Based on these and other observations researchers believe that gibberellins play a major role in controlling stem elongation in plants. Gibberellins are also involved in flower formation and fruit development and are used by grape growers to increase cluster size and by apple and pear growers to improve size, color and fruit quality.

Chemicals have also been found that inhibit gibberellin production. These "anti-gibberellin" compounds have been found to retard growth in many types of plants. These types of chemicals are used to stunt trees under power lines and rights-of-ways to reduce the number of times the trees must be top clipped. They are also used in greenhouse production to control the growth of many species of flowers.

Back to top CYTOKININS
Cytokinins were discovered by scientists looking for ways to stimulate plant cell division. A scientist by the name of Folke Skoog at the University of Wisconsin was looking for a substance that would promote the growth of tobacco pith cells. While working with autoclaved (pressure-heated) herring sperm DNA he discovered a substance called Kinetin which stimulated cell growth. Although this substance did not occur naturally in any plants it led researchers to look for similar substance in plants. Shortly after the discovery of kinetin, the first naturally occurring cytokinin called zeatin was discovered in corn kernels. many types of cytokinins are now known to occur in woody plants. They are produced in meristematic regions. The roots in particular appear to be a source of cytokinins and from there they are transported throughout the plant in the xylem.

As mentioned a major effect of cytokinin is cell division. They are also involved in cell and organ enlargement, and delay senescence in flowers, vegetables and fruits. They may also play a role along with Auxins in regulating apical dominance. Recall that in many plants when the terminal shoot is removed lateral buds will often develop and elongate. This is believed to be due to the loss of auxin from the apical meristem. In a similar fashion, artificially applied cytokinins will often stimulate dormant lateral buds to develop even in the presence of an intact terminal bud. This has led researchers to conclude that it is a balance of cytokinin and auxin which controls apical dominance. Cytokinins promote cell division and lateral bud development, while auxins inhibit it.

A good example of this balance at work is in the shearing of Christmas trees. In order to have a dense, full tree, Christmas tree growers must shear or clip the trees. In the Pinus genera, each fascicle is actually a short shoot with a dormant lateral bud at its base. When the terminal bud is clipped (auxin supply interrupted) many of these dormant fascicular buds develop and the branch density increases resulting in a full tree. Christmas tree growers have also experimented with applying an artificial cytokinin (Benzylaminopurine) which also causes the fascicular buds to develop since the balance is now in the favor of cytokin. This is illustrated on the next several pages.

The tip of this Scots pine twig was clipped, causing disruption of auxin and development of dormant fascicle buds.

Normal Steady State - A pine shoot where cytokinins (promoters) moving up in xylem are balanced by auxins (inhibitors) produced by the apical bud are in balance and dormant fascicular buds do not develop.

Top Clipped - When the top is clipped the auxins are removed.  Cytokinins promote the development of numerous fascicular buds.

Cytokinins Applied - An artificial application of cytokinins overwhelms the supply of auxins and promotes the development of fascicular buds.

This Scots pine tree produced dozens of lateral buds after being sprayed with cytokinins. The grower was attempting to fill in a large gap in this Christmas tree. However none of them elongated normally causing a very unnatural look.

Research at Virginia Tech has also determined that cytokinin can also be used to delay dormancy in Fraser fir seedlings. Fraser fir seedlings grow very slowly for 2 to 3 years and often are only a few inches tall at the end of this time. This slow growth is due to the rapid onset of dormancy in the terminal bud. By spraying cytokinins on these seedlings the plants maintain cell division and apical growth for long periods of time resulting in seedlings several inches tall in less than one year. The purple needles are a side effect of cytokinin application.

A "witches broom" in the crown of a hardwood tree can be seen to the right. This deformity of many short, small, twisted twigs is often caused by a pathogen which produces a cytokinin that causes massive bud proliferation.

Back to top ETHYLENE
In the nineteenth century when coal gas was used to illuminate street lamps it was observed that trees near the lamps often defoliated unnaturally. It was assumed that some component in the combustion gases was responsible. In 1901 a Russian graduate student identified the responsible gas as ethylene. Ethylene is a very simple molecule and is produced during many combustion reactions. Later scientists observing fruit ripening discovered that ethylene was produced naturally by plants. During ripening of fruits ethylene gas is formed which in turn can hasten ripening in nearby fruit. The age-old adage of "one bad apple spoils the bunch" literally is true and ethylene is the responsible agent.

Rotten apples produce ethylene.

Ethylene is now recognized as a major plant growth regulator. One of its most pronounced physiological effects is abscission or the shedding of a plant part. It appears that an interaction between auxin and ethylene is responsible for control of abscission. During the growing season auxin levels remain high in leaves and other organs. Later in the season auxin supply diminishes and the level of ethylene increases. The reduce auxin along with the increased ethylene results in the formation of an abscission layer, which is a thin region where the cells slowly break down forming a thin line which eventually breaks and shedding occurs. Increased ethylene during the growing season can trigger premature abscission and the application of auxin can prevent abscission.

Fruit ripening is also a major physiological effect of ethylene. Addition of ethylene gas to fruit will accelerate ripening. Similarly if ethylene is removed, fruits will remain unripened. Fruit growers will take advantage of this fact to control the ripening of fruit during transport to markets. Over-ripened fruit produces large amounts of ethylene and if kept nearby other fruit will cause them to ripen and rot quickly.

Ethylene production in young forest tree seedlings can result in reduced vigor or in a sense "over ripening". Forest tree seedlings are often held in cold storage for long periods of time. If ethylene concentrations build up, seedling vigor is reduced and later survival when planted is poor. Tree seedlings should never be stored along with apples or other fruits since the fruit could produce enough ethylene to harm the seedlings. Plants are also known to produce what is called “stress ethylene”. Ethylene is known to be formed when cells are damaged or when plants are experiencing general stress due to any number of factors. The production of stress ethylene could contribute to declining vigor in cold stored seedlings.

Ethylene is also known to play a role in seed and bud dormancy, induction of roots, flowering, and stem elongation.

Back to top ABA
Abscisic acid was discovered by scientists working on dormancy in trees. A substance isolated from sycamore tree leaves, when applied to the actively growing tips, was found to stop growth and induce bud formation. A second group of scientists isolated a compound from cotton fruit that induce abscission. It was later determined that both chemicals were the same compound and it was named abscisic acid (ABA).

It is now known that ABA occurs in all plants and is produced in many locations within the plant. ABA was originally believed to play a major role in leaf abscission and dormancy. It is now known that ethylene is more directly involved with leaf abscission. Bud formation and dormancy is influenced by ABA but more recent experiments have clouded the issue. In some experiments applications of ABA will induce a resting bud. Other experiments however failed to get a dormancy response. Likely ABA is involved in dormancy induction, but it is probably the balance of inhibitors to growth such as ABA and growth-inducing regulators such as cytokinin and gibberellins that induce dormancy.

ABA is known to be a major factor in seed dormancy. Clear relationships between the level of ABA and dormancy have been established for seeds with high levels of ABA resulting in deeper dormancy. Applications of ABA will inhibit seed germination and maintain dormancy further supporting its role in seed dormancy. During the onset of seed germination cytokinin and gibberellins have been observed to increase suggesting again that a balance of growth regulators is the likely control of seed dormancy.

ABA also is related to the response to water stress. ABA builds up to very high levels during water stress which is believed to signal the closure of stomata. Even after water stress is relieved stomata in some plants do not immediately reopen which is believed to be due to residual ABA still in the leaf. Mutant plants with an inability to produce ABA have been found to wilt permanently and their stomata to never close. Applications of ABA to the leaves of well watered plants will result in stomatal closure.