Chapter 25:Plant Responses And

Adaptations

Hormones and Plant Growth

• Plant cells send signals to one another that indicate when to divide, when not to divide, and when to develop into a new kind of cell

• When animals reach adulthood, they stop growing

• Even when plants reach adulthood, they keep growing

• Meristems allow plants to remain “forever young”

What do plants grow in response to?

• Light• Moisture• Temperature• Gravity

What is a plant hormone?

• A substance that is produced in one part of an organism and affects another part of the same individual

• Control a plant’s patterns of growth and development, and the plant’s responses to environmental conditions

Plant Hormones• To respond to a hormone, the target cell must

contain a receptor to which the hormone binds• If the receptor is present, the hormone can

influence the target cell by: -changing its metabolism

-affecting its growth rate-activating the transcription of

certain genesA single hormone may affect two different

tissues in different ways

Auxins

• Phototropism – tendency of a plant to grow toward a source of light

• Auxins are produced in the apical meristem and are transported downward into the rest of the plant

• Auxins stimulate cell elongation• Responsible for gravitropism– Gravitropism – response of a plant to the force of

gravityInvolved in the way roots grow around objects in

the soil

Auxins

• Regulate cell division in meristems• Apical dominance is the phenomenon in

which the closer a bud is to the stem’s tip, the more its growth is inhibited

• Compounds that mimic auxins are used as herbicides to kill weeds– Herbicides – compounds that are toxic to plants

Cytokinins

• Cytokinins – plant hormones that are produced in growing roots and in developing fruits and seeds

• Stimulate cell division and the growth of lateral buds

• Cause dormant seeds to sprout• Delay the aging of leaves • Inhibit elongation and cause cells to grow

thicker

Plant Hormones

• Stimulate cell elongation• Inhibit growth of lateral

buds

• Inhibit elongation • Stimulate lateral bud

growth

Auxins Cytokinins

BOTH: Determine how a plant grows

Gibberellins

• Gibbberellin – growth-promoting substance• Produce dramatic increases in size, especially

in stems and fruits• Produced by seed tissue • Responsible for rapid early growth of plants

Ethylene

• Ethylene – plant hormone that stimulates fruits to ripen

• Plants produce their own ethylene• In response to auxins, fruit tissues release

small amounts of ethylene, which then stimulates fruits to ripen

• Used to control the ripening process of fruit

25-2 plant responses

Plant tropisms include gravitropism, phototropism, and thigmotropism. Each of these responses demonstrate the ability of plants to respond to external stimuli

25-2 Plant ResponsesTropism= the response of plants to external

stimuli– Phototropism: response to light• Auxin controls it• Auxin=hormone• Maximizes plant exposur to sunlight

– Gravitropism: response to gravity• Auxin controls• Roots grow with gravity• Stem grows against gravity

More tropism!!! :D• Thigmotropism=Response to touch– Plants that are touched regularly may be

stunted in growth– Vines and climbing plants grow around

anything that they touch.

EVEN MORE Tropisms!!!:D• Hydrotropism: response to water• E.g. plant grows towards where water is

abundant• Chemotropism: response to chemicals• Thermotropism: response to temperature• Heliotropism: response to the position of

the sun

Rapid Response• Plant movements that are not tropisms– Tropisms involve plant growth– Tropisms DO NOT happen rapidly!!!

Ex. Venus fly trap

Photoperiodism

Photoperiodism: plants ability to respond to periods of light and darkness– Short-day plants: flower when days are short– Long-day plants: flower when days are long

Phytochromes are plant pigment that absorbs red light is responsible for photoperiodism.

Winter DormancyDormancy: is the period during which an organism’s

growth and activity decrease or stop

• Shorter days and colder temperatures reduces rate of photosynthesis

• Keeping the leaves would be costly in terms of water loss

• The leaves have little chance of surviving the winter

Leaf Abscission• Plant reduces auxin production, and

increases ethylene production• Leaf stops making chlorophyll• Remaining chlorophyll breaks down and

exposes accessory pigments• Nutrients are transported from the leaf to

the plant • Abscission Layer – seals off the leaf from

the rest of the plant

Plant Adaptations

By Jenny Song

Angiosperms can survive in many different locations.

How is this possible?

Through natural selection, plants have evolved different adaptations to live successfully in

each environment.

Adaptations are special features that allow a plant or animal to live in a particular place or habitat

Aquatic Plants Often live in MUD(contain little oxygen) To get enough oxygen, many aquatic pants

have air-filled spaces in their tissues. Oxygen diffuses through these spaces from

the leaves to the roots.

Example of an Aquatic Plant: Waterlillies

Salt-Tolerant Plants

Plants that grow in salt water or in very salty air near the ocean.

These plants have special cells that pump salt out of the plant tissues and onto the leaf surfaces.

Then, the rain washes off the salt. Ex. Lampranthus spectabilis (trailing iceplant)

Xerophytes (Desert Plants)

These plants often have extensive roots, reduced leaves, and thick stems that can store water.

Seeds of many desert plants can remain dormant for years.

These seeds will germinate only when enough moisture guarantees them a chance to survive.

Nutritional Specialists

Plants that grow in soil with little nutrients.Ex. Carnivorous plants and parasites

Carnivorous plants trap and digest insects to get nitrogen.

Parasites get water and nutrients directly from a host plant.

These plants HARM their host plants

Epiphytes

Plants that are not rooted in soil. They grow directly on the bodies of other

plants. Are NOT parasites. They gather their own moisture from the rain. They make their own food. Most live in Rain Forests.

Chemical Defenses Many plants produce chemicals that are

poisonous to the animals that eat them. Chemical defenses protect plants from

potential predators.

Digitalis (Foxglove)is

poisonous when eaten.