1. Understanding Plant Growth Regulators

2. Interest Approach
Let’s talk for a moment about yourself. What are some differences between your present body and the one you had as a child?
What kinds of chemicals were involved in those changes?
These chemicals are called hormones; for example, estrogen, testosterone, adrenaline, etc.

3. Continued
Plants also experience growth changes with the help of hormones
Sometimes these changes are more dramatic than what you experienced!
Take a look at this plant to the right which fell over the night before. What happened to the plant?

4. Vocabulary Abscisic acid Apical dominance Auxins Cytokinins Ethylene
Gibberellins
Phototropism
Plant hormones
Thigmotropism
Geotropism

5. Student Objectives 1. Describe the work of plant growth regulators
2. Explain the functions of several plant hormones
3. List several commercial uses for plant growth regulators

6. What Do Plant Growth Regulators Do?
Plant growth regulators are chemicals which affect the plant in many complex ways
They can control such activities as cell division and differentiation, root and shoot growth, flowering and ripening
Plant growth regulators which are made by the plant are called plant hormones
They are moved around the plant in very low concentrations

7. Some growth regulators are man-made, or synthetic
They can be applied to plants to obtain a wide variety of changes
These changes often make the plant better or more saleable!
Which grapes would you probably buy?
The ones on the right were treated with gibberellic acid to make them larger.

8. What Are the Functions of Several Plant Hormones?
Several hormones are made by tissues of the plant
Each hormone has many different jobs, thus they are very much like human hormones
There are five groups of plant regulators: auxins, cytokinins, ethylene, gibberellins and abscisic acid

9. Auxins
This growth hormone is produced by the tip, or apical meristem, of the stem
It causes the stem cells to elongate and divide
They also flow down from the tip of the stem, preventing lateral buds from sprouting until needed
There are three major affects caused by auxins on the plant

10. Effects of Auxin on Plants
This trunk should be a single stem; however, the terminal bud was removed forcing it to branch.
1. Auxin causes the tip of the middle stem to grow at a faster rate
This is known as apical dominance
Apical dominance is why many conifers have a pyramid shape
It can be overcome by cutting off the dominant or terminal stem, losing the source of auxin

11. 2. Auxins are responsible for allowing a plant stem to grow toward the sun
This is known as phototropism
Sunlight slowly breaks down auxin; When the side not exposed to the sun grows faster, the stem bends towards the light
The left side of the plant was exposed to the sun. The right side had shade and grew faster.

12. 3. Auxins allow a plant to respond to the touch of a person or other object
This is known as thigmotropism
The repeated touch of an object causes less auxin to remain on that side of the stem
When the auxin side starts to grow faster, the plant grows towards the object and ultimately wraps around it

13. Cytokinins
These are hormones that are mostly responsible for cell division and differentiation
They are produced in the root tips in seeds
They tend to travel up the stem
In tissue culture, cell division or root growth can be encouraged by adjusting hormones in the agar. If given higher levels of auxins in the agar, roots are produced. If given higher levels of cytokinins, shoots multiply.
Courtesy of McGraw Hill Publishers

14. Ethylene This is a gas that affects the plant like a hormone
It is produced in ripening fruit and dying plant material
It stimulates flowering in some plants and causes other fruits and flowers to ripen more quickly and evenly

15. Effects of Ethylene
Ethylene gas is why fruit will ripen faster in a paper bag, than on the counter
The bag helps to concentrate the gas in a specific area
Ethylene has a negative effect on cut flowers & foliages
It causes them to age more quickly, reducing their useful life
In this experiment, two holly twigs were placed under separate jars for a week. At the same time, an apple was placed in the second jar. The ethylene given off by the apple caused the holly to lose its leaves and die.

16. Gibberellins
These hormones cause the internode of a stem to elongate and cell division to occur
They are produced in the stems, roots and young leaves
Gibberellins are commonly used on commercially grown dessert grapes to spread the fruits out and cause them to be bigger
Flowering of plants and the breaking off of seed dormancy can also be achieved by adding gibberellins

17. Effects of Gibberellins
The cabbage plants on the left were grown on their own
The plants on the right were given gibberellic acid once a week for eight weeks
Notice the long stems and flowers at the top of the plants on the right
They are a result of this hormone

18. Abscisic Acid This hormone inhibits (prevents) growth
It is found in seeds which are dormant and in dying leaves
It also appears to help a plant prepare its buds for winter
The dormant bud on the left has a high concentration of abscisic acid. As spring nears, the acid level drops and the bud begins to develop a new shoot.

19. What are Several Commercial Uses for Plant Growth Regulators?
Synthetic growth regulators are very useful for commercial plant crops
They can save money, time and can lead to a more sellable crop
There are at least three commercial uses of regulators:
1. Growth regulators are routinely sprayed on crops such as poinsettias, Easter lilies and mums to reduce size and make a shorter, bushier and more attractive plant
Products such as A-rest, B-nine, Cycocel and Florel are commonly used
Prevents greenhouse plants from getting ‘leggy’

20. 2. Growth regulators are commonly used to help plants root more completely
These are often sold as a powder under the names Rootone and Hormodin
3. Ethylene gas is used commercially to ripen bananas, tomatoes once they get to market; and to induce flowering in pineapple crops

21. Summary What is a plant growth regulator called and what does it do?
Where in a plant is auxin produced?
Explain how apical dominance works in a plant.
How is phototropism different from thigmotropism?

22. Summary continued What do cytokinins control?
How does ethylene effect plants?
What does gibberellin cause in a plant?
What does abscisic acid control?
Name one way that hormones are used commercially to control plants.