Bi 103 Notes: Chapter 26: Plant Responses to the Environment

I. Hormone Background

A. Definition: Chemicals produced in one location and transported to other locations where they exert an effect

B. Discovery of plant hormones

1. Charles and Francis Darwin: late 1800’s

2. Peter Boysen-Jensen: Early 1900’s

3. Fritz Went: 1920

C. Basic Plant Hormones

1. Auxins

2. Gibberellins

3. Cytokinins

4. Ethylene

5. Abscisic Acid

II. Hormonal regulation of plant life cycle

Stage I: Germination

A. Abscisic acid maintains seed dormancy

1. must be removed before germination can happen

B. Gibberellins stimulate germination

1. Initiate synthesis for enzymes to release E from endosperm/cotyledons

Stage 2: Seedling development

A. Auxin controls seedling orientation (aka which way is up?!)

1. phototropism: growth to/from light

2. gravitropism: grown with/against gravity

B. Shoot and root growth depends on localized [Auxin]

1. Auxin produced in shoot tip.

2. Shoots: higher [Auxin] stimulate cell elongation, lower [Auxin] inhibit.

3. Roots: higher [Auxin] inhibit cell elongation, lower [Auxin] stimulate

Stage 3: Maturation

A. Shoot Branch growth

1. Auxin inhibits growth of lateral buds (strongest inhibition near shoot tip)

2. Auxin in combination with Cytokinin stimulates lateral bud growth

a. cytokinin produced in roots.

B. Root brancing

1. Auxin stimulates root branching

a. stimulates pericycle cells to divide, producing branch roots.

Stage 4: Flowering: Timing of flowering has to relate to reliable environmental clue:

A. Day neutral plants: flower when physiologically ready

B. Long- night plants: Flower when nights longer than critical value.

C. Short-night plants: Flower when nights shorter than critical value.

D. Plants detect day length (photoperiod) with Phytochrome pigments.

E. Florigens (new hormone) stimulates flower production.

Stage 5: Fruit development

A. developing seeds (and pollination) produce auxin and or gibberellin

1. stimulate ovary to fruit

B. Mature seed releases surge of Auxin… it triggers ethylene release to ripen.

Stage 6: Senescence and Dormancy

A. Scenescence: Rapid aging of leaves, fruits, flowers, then abscission (drop)

1. Pre-senescence, Auxin/Cytokinin maintain leaves

2. As growth ceases, Auxin/cytokinin production drops.

3. Meanwhile, Ethylene released by ripened/ripeining fruits.

a. initiates enzymes in abscission layer to degrade cells, form “detachment” layer

4. Abscission (structure falls off!)

5. Bud dormancy in plant is maintained by abscisic acid

III. Chemical communication in plants

A. Tobacco plants:

1. Salicylic acid (protection)

2. Methyl salicylate (produced by infected plants, warns other plants)

B. Corn plants

1. Call in predators via chemical cues to attack chewing caterpillars

C. Rapid Response:

1. Venus Fly Traps.