Plant Hormones and Responses

What are the 5 major plant hormones?

Auxin, cytokinin, gibberellin, ABA, and ethylene.

Main function of auxin

Cell elongation, apical dominance, root growth, and tropisms.

Main function of cytokinin

Cell division and lateral branching.

Main function of gibberellin

Stem elongation, seed germination, and fruit growth.

Main function of ABA

Seed dormancy, drought response, and stomatal closure.

Main function of ethylene

Fruit ripening, leaf abscission, and triple response.

How do auxin and cytokinin affect branching?

Auxin suppresses lateral buds; cytokinin promotes them.

What is apical dominance?

The shoot tip suppresses lateral branch growth.

ABA vs. gibberellin in seeds

ABA maintains dormancy; gibberellin promotes germination.

Which hormone helps plants tolerate drought?

ABA.

How does ABA help during drought?

It causes stomata to close and reduce water loss.

What is the triple response?

Reduced stem elongation, stem thickening, and horizontal growth.

Which hormone causes the triple response?

Ethylene.

Which hormone promotes fruit ripening?

Ethylene.

Which hormone promotes leaf abscission?

Ethylene.

What is phototropism?

Growth in response to the direction of light.

How does a shoot bend toward light?

Auxin moves to the shaded side, where cells elongate more.

Which light is important for phototropism?

Blue light.

What do blue-light receptors control?

Phototropism, stomatal opening, and other light responses.

What are phytochromes?

Photoreceptors that detect red and far-red light.

What are the two forms of phytochrome?

Pr and Pfr.

What does red light do to phytochrome?

Converts Pr to active Pfr.

What does far-red light do to phytochrome?

Converts Pfr to Pr.

Which phytochrome form is generally active?

Pfr.

How does red light usually affect seed germination?

More Pfr is produced, which often promotes germination.

Why does shade contain more far-red light?

Leaves absorb red light and transmit/reflect more far-red.

How do plants respond to shade?

They often elongate their stems to reach stronger light.

What is gravitropism?

Growth in response to gravity.

Positive vs. negative gravitropism

Positive = toward gravity; negative = away from gravity.

Roots show what gravitropism?

Positive gravitropism.

Shoots show what gravitropism?

Negative gravitropism.

How do plants detect gravity?

Statoliths settle in cells and indicate the direction of gravity.

What problem does flooding cause?

Low oxygen around roots.

Flooding adaptations

Air spaces, adventitious roots, and rapid stem growth.

What problem does drought cause?

Water loss, loss of turgor, and decreased photosynthesis.

Drought adaptations

Stomatal closure, thick cuticle, deep roots, small leaves, water storage.

What are heat-shock proteins?

Proteins that protect/refold other proteins damaged by heat.