Chapter 24

Plant Hormones and

Tropisms

HormoneSubstance produced in one part of an

organsim that affects another part of

that same organism.

Plant Hormone •Chemical substance

•Controls plant’s:

• patterns of growth & development

• responses to environmental

conditions

Target Cell Portion of an organism affected by a

hormone

• Produced in apical meristem

• Transported downward to rest

of plant

• Stimulate cell elongation

Auxins

The above diagram shows

how auxin production in a

plant causes it to grow

toward sunlight.

Auxins

Auxin and

Phototropism

• Light hits one side of stem

• Higher concentration of

auxins develops in shaded

part of the stem

• Change in concentration

stimulates cells in the dark

side to elongate

• Result: stem bends away

from the shaded side and

toward the light

How does auxin affect plant

growth in the presence of gravity?

• Built up on the lower sides of roots and

stems but slows growth

• Result: roots grow downward

How does auxin affect plant

height or width?

• As a stem grows in length, it produces lateral buds

• Lateral bud gives rise to side branches on the side of stem

• Growth at lateral buds is inhibited by auxin, which is on the stem’s tip

• The closer a lateral bud is to stem’s tip, the more it is inhibited (apical dominance)

• Produced in growing roots

and developing fruits & seeds

• Stimulate cell division and

growth of lateral buds

• Cause dormant seeds to

sprout

• Delays aging of leaves

• Inhibit elongation and cause

cells to grow thicker

As a result of cytokinin the

top carrot is 2x bigger than

the other.

cytokinins

• Produce increase in size in stem and fruit

• Responsible for rapid early growth in many plants

How do gibberellins affect plant

growth?

Notice the difference in sizes of the fruit bunches and plants

• Trees suffer leaf loss

• Ethylene stimulates fruits to ripen

• Use to treat fruit picked before ripening to ripen them

when being handled for shipment

How does ethylene gas affect

plants and how is it used on a

fruit?

Homeostasis

• Plant hormones have opposing effect!

– Auxins (made in meristem – stimulate root

growth) VS. Cytokinins (made in roots/fruits –

stimulate leaf/tip growth)

– Gibberelins (growth of stems and fruit) VS.

Absicisic Acid (halt growth until conditions

improve)

Tropism response of a plant to an

environmental stimulus

Phototropism

Thigmotropism

Gravitropism

tendency for plants to grow

towards light

response of plants to touch

tendency of a plant to grow in a

direction in response to force of

gravity

Tradescantia- A plant that

shows growth response to

gravity upward when

young and downward

when older.

Notice how the trunk

of the tree to the right

grows up away from

gravity.

What Is Photoperiodism?Photoperiodism is the response of plants to periods of light and darkness.

Plants such as chrysanthemums and poinsettias flower when days are short.

Long Day Plants

Plants such as spinach and irises flower when days are long.

• It is a plant pigment that is held responsible for photoperiodism.

• Absorbs red light

• Activates a number of signaling pathways within plant cells