Understanding Plant Growth Regulators

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.

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?

Vocabulary

Abscisic acid Apical dominance Auxins Cytokinins Ethylene Gibberellins

Phototropism Plant hormones Thigmotropism Geotropism

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

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

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.

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

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

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.

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.

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

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

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

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.

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

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

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?

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’

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

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?

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.