Ch 28: External Factors and Plant Growth

Tropism

Movement or growth responses to relative direction of external stimuli

Which hormon is responisble for most tropic responses?

Auxin

Positive vs negative tropism

positive = response towards stimulus

negative = response away stimulus

Phototropism

Response to light

How does phototropism work?

Blue light triggers auxin to stimulate cell elongation on the shaded side

What is gravitropism and how does it work

Response to gravity

Auxin on stem causes bend up

auxin on root causes bend down

Starch-statolith hypothesis

Amyloplasts in roots sense gravity, fall down to root cap

Hydrotropism

Movement of root to water

Thighmotropism

Movement in response to touch (tendrils)

Nastic Movements

Plant movements in response to an external stimulus, with direction of movement unrelated to direction of stimulus

Nutaton

Twisting, winding movement of plants

Sleep Movements

Up and down movement of leaves in response to light

What is responsible for sleep movements

Turgor pressure in leaf petioles or lead blades responsible for control of circadian rhythym

Thighmonastic Movement

Sudden change in turgor pressure in pulvini (carnivorous plants)

Solar Tracking

Leaves and flowers orient towards sun via pulvini

Photoperiodism

Response of organisms toa change in the proportions of light & dark in 24 hrs cycle, plants measure darkness not daylight!

Short-Day Plants photoperiodism vs Long-day plants vs day-neutral plants

Short=Flower when light period is shorter

Long=Flower when light period is longer

Neutral = flower w/out respect to day length

Chemical basis of photoperiodism

Phytochrome is a blue pigment which abosrbs 2 types of light:

PR: absorbs 660 nm, converts to PFr (not-active)

PFr: abosrbs 730nm, converts to PR (active, causes a response)

Eiolation

Increased stem elongation, poor leaf development, and lack of chlorophyll due to lack of light