LC BIOLOGY- Plant Responses

Stimulus

anything that causes a reaction in an organism or in any of its parts

Response

the activity of a cell or organism as a result of a stimulus

Tropism

a change in the growth of a plant in response to an external stimulus

Positive Tropism

occurs when the growth is towards the stimulus

Negative Tropism

occurs when the growth is away from the stimulus

Phototropism

the change in growth of a plant in response to light, usually from one direction (i.e. unidirectional light)

Geotropism

the change of growth of a plant in response to gravity

Thigmotropism

a change of growth of a plant in response to touch

Hydrotropism

a change of growth in a plant in response to water

Chemotropism

a change in growth of a plant in response to chemicals

Coleoptile

the sheath around the leaves and shoot of grass seedlings, such as corn or oats

Growth Regulator

a chemical that controls the growth of a plant

Growth Promoter

a chemical that causes increased growth in plants

Auxins

an example of growth promoters in plants

Indoleacetic Acid

the best known auxin (IAA)

Auxin Production Sites

• meristematic tissue in the tips of shoots

• young leaves

• developing seeds

Functions of Auxin

• stimulating stem elongation

• stimulating root growth

• causing cells to form into different structures

• developing fruit

• inhibiting side branching in stems

• causing phototropism

• causing geotropism

Effects of IAA on Phototropism

• IAA is made in the tip of the stem

• diffuses down the stem

• causes stem cells to elongate

• diffuses down the shaded side of a stem

• cells on the shaded side elongate, causing the stem to bend towards light

Growth Inhibitor

a chemical that causes a reduction in growth of plants

Examples of Growth Inhibitors

ethene and abscisic acid

Ethene

a gas that ripens fruit, and causes ageing and leaf fall

Abscisic Acid

responds to stress in plants by closing stomata, forming bud scales, and inhibiting seed germination

Commercial Growth Regulators

• stimulate root formation in cuttings

• stimulate the formation of new plants in tissue culturing

• ripen bananas and other fruits

Plant Protective Methods

may be:

• anatomical or structural (e.g. epidermis, bark, cuticle, closure of stomata)

• chemical (heat-shock proteins, stress proteins)