Plant Responses to Internal and External EnvironmentChapter 39

Review of Basic Plant Parts

root

shoot

apex

These are the parts of the plant we will be concerned with today

flowers

Hormonal Control•Hormone binds

to receptor

•Triggers second messengers

•Cell responses (greening, flowering, etc) take place

Plant Hormones•Auxins

Produced in embryo, apical meristemsStimulates root growth, stem elongation

•CytokininsMade in root, transported to other parts Stimulates cell division, plant growth

•Gibberelins:Made in meristematic tisssues Promotes bud/seed germination, stem

elongation

Plants exposed to gibberellins

Normal dwarf plant

Dwarf plant

treatedwith

gibberellin

Elongation of Cells

1. Proton pumps transport auxin produced in apical meristem into cell2. Proton pumps move H+ ions into cell wall matrix3. Acidity in cell wall increases so that cross bridges in cellulose breaks4. Cell wall weakens, water enters and cellulose fibers stretch5. Cell elongates, and cell produces more cytoplasm and organelles

Control of Apical Dominance

•Apical dominance: plant will grow tall with evenly distributed lateral growthAuxin produced by

terminal bud inhibits growth of lateral buds

Control of Apical Dominance

•When terminal bud trimmed: Cytokinins

produced by roots stimulates axillary bud growth

Axillary buds grow, plant gets bushier

Root growth•Auxins:

stimulate root branching

•Cytokinins: inhibit root branching

Tropic responses•Tropisms: plant

responds to environmental stimulus and movesPhototropism GravitropismThigmotropism

Phototropism•Plants move

toward the light when exposed to it

•Positive: toward light

Phototropism

•Light shines directly on plant tip: plant grows straight up since auxin concentration is high

Phototropism•When light is at angle,

auxin accumulates on shaded side

•This stimulates cell elongation

• Increased chemical messenger on sunny side inhibits cell elongation

•Result: Stem grows toward light source

Gravitropism

•Plants move toward or away from gravity’s pull

Gravitropism: Roots•Starch grains

settle to bottom of root cells

•Causes migration of Ca+2 ions

•Auxin moves laterally

Gravitropism: Roots• 1. Low auxin conc.

stimulates cell elongation

• 2. High auxin conc. inhibits cell elongation

• Result = root curves downward

• 3. Low auxin conc. inhibits cell elongation

• 4. High auxin conc. stimulates cell elongation

• Result = shoot curves upward

Thigmotropism

•Plants curl around objects or react to being touchedMimosa treesVenus fly traps Tendrils of vines

Photoperiodism•Plants use light

to determine when to flower, germinate seeds, break bud dormancy

Control of Flowering Key Concepts

•Phytochromes: pigments that regulate plant response to light

•Critical night length: the minimum amount of night time required for regulation of plant growth

Short-day plants

light

darkCritical night

length

If light exceeds or interrupts critical night length, no

flowering happens.

Long-day plants

light

darkCritical night

length

Night can exceed critical night length or be interrupted,

flowering can still happen.

Control of Fruit Ripening

•Ethylene gas: not really a hormone, but plays a role in fruit ripening

•Hastens fruit ripening

•“One bad apple spoils the bunch”

Plant Defenses•Plants produce

volatile chemicals in response to predation: secondary compounds

•Also, antioxidants to protect against UV rays