Plant Responses to Internal & External Signals

Chapter 39

Tropism

A plant growth response from hormones that results in the plant growing either toward or away from a stimulus

Hormones – chemical messengers that coordinate the different parts of a multicellular organism

Types of tropisms: Phototropism – growth toward or away from light Gravitropism – growth toward or away from a

gravitational source Thigmotropism – growth toward or away from a touch

Vines grow toward a support and then grow toward (around) that support

Phototropism video

Gravitropism Video

Thigmotropism Video

Positive v. Negative Tropisms Positive is growth toward a stimulus

Positive Phototropism – growth toward light Positive Gravitropism – growth toward a

gravitational source Positive Thigmotropism – growth toward a

tactile stimulus

Negative is growth away from a stimulus

Replace the above terms’ “Positive” with “Negative” and then replace “toward” with “away”

Hormones

Auxins Stimulate cell elongation (remember that

zone?) Cause proton pumps to activate Lower pH

weakening of cell wall turgor pressure expands & elongates cell wall

High concentrations of synthetic auxins KILL certain plants, usually weeds

Synthetically produced auxins in high concentration = Herbicides

How do you make a …?

Cytokinins Stimulate cell division or cytokinesis Proper ratio of auxins & cytokinins cell

division & differentiation Gibberellins

Work with auxins to stimulate stem elongation Loosen cell walls allowing cellular expansion

stem expansion Signal the seed to cease dormancy and

germinate Many dwarf plant varieties have non-functional

gibberellins

Hormones (Page 3)

Abscisic acid SLOWS Growth Antagonistic to previously mentioned hormones

Promotes seed dormancy, but gibberellins cease it

Causes stomata to close to conserve water Ethylene

Gas Plays crucial role in programmed cell death

(apoptosis) Promotes ripening of fruit Ripening in one fruit ripening in other fruits

Positive feedback mechanism = rapid ripening of fruit

One bad apple does spoil the lot.

Plants respond to Light

Plants can detect presence, direction, intensity, and wavelength of light

Red & Blue wavelengths are most important

Red light is sensed by phytoreceptors Blue light is most important for

phototropisms and light-induced opening of stomata

Sensed by photoreceptors

Phytochromes

Photoreceptors for red light (mainly) Exist in 2 isomer forms which can switch

forms depending on wavelength available

Phytochrome form triggers plant’s developmental responses to light

Responsible for Circadian Rhythms 24 hour cycle & not paced by environmental

variables Chickens & Summer in Nome, Alaska

Photoperiodism

Physiological response to a photoperiod (relative lengths of day and night)

Night is always the more important of the 2!! But we (humans) focus on the daylight, we’re

dumb. Controls Flowering Short-day plants

Require a period of continuous darkness exceeding a critical measure in order to flower

Early Spring or Fall flowering AP Biology students call them “Long-Night”

plants

Besides Short-Day plants,

Long-Day Plants Flower only if a period of continuous darkness

is less than a critical value Flower in Late Spring or early Summer Considered “short-night” plants

Day-Neutral Plants No length of continual darkness is needed for

flowering

Plants respond to things other than light

Gravitropism – growth toward or away from light

Auxin plays key role If root is plased horizontally, then gravity

causes an accumulation of auxins in root’s (radicle’s) lower side

Remember HIGH [auxin] = inhibition of growth, so

lower side = no growth Upper side = growth Allows the root to grow down into the ground

Auxin Accumulation

Plants respond to other things… Drought

Stomata close Leaves will cease growth Leaves roll into shape that reduces

transpiration Deep roots (where H2O is) will speed their

growth, but shallow roots will stop growing Predators

Thorns, chemicals, distasteful compounds Some plants even attract parasites First layer of defense - Epidermis

Coordinated Plant-Parasite Defense