Lesson 8
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Lesson 8. Understanding Plant Growth Regulators. Next Generation Science/Common Core Standard Addressed.

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Lesson 8

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Lesson 8

Lesson 8

Understanding Plant Growth Regulators

Next generation science common core standard addressed

Next Generation Science/Common Core Standard Addressed.

  • HS‐LS1‐2. Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. [Clarification Statement: Emphasis is on functions at the organism system level such as nutrient uptake, water delivery, and organism movement in response to neural stimuli.

  • HS‐LS1‐3. Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis. [Clarification Statement: Examples of investigations could include heart rate response to exercise, stomate response to moisture and temperature, and root development in response to water levels.] [Assessment Boundary: Assessment does not include the cellular processes involved in the feedback mechanism.

Bell work

Bell Work!

  • 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.

    • What chemical changes so plants undergo?

    • Is it possible to control changes in plant life cycles? If yes, explain.

Bell work cont

Bell Work Cont.!



  • Abscisic acid

  • Apical dominance

  • Auxins

  • Cytokinins

  • Ethylene

  • Gibberellins

  • Phototropism

  • Plant hormones

  • Thigmotropism

Interest approach continued

Interest Approach 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?

What do plant growth regulators do

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

Lesson 8

  • 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 rather eat?

The ones on the left were treated with gibberellic acid to make them larger.

What are the functions of several plant hormones

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



  • 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

Effects of auxin on plants

Effects of Auxin on Plants

  • 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

This trunk should be a single stem; however, the terminal bud was removed forcing it to branch.

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  • 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.

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  • 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



  • 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.



  • 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

Effects of ethylene

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.



  • 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

Effects of gibberellins

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

Abscisic acid

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.

What are several commercial uses for plant growth regulators

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 better 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

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  • 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 once they get to market and to induce flowering in pineapple crops



  • What is a plant growth regulator called and what does it do?

  • What is the difference between a plant hormone and a synthetic hormone?

  • Where in a plant is auxin produced?

  • Explain how apical dominance works in a plant.

  • How is phototropism different from thigmotropism?

Summary continued

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.

The end

The End!

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