Chapter 32: Plant Responses

Stimulus and Response

Animal Stimuli:

Loud noise

Hunger

Pain

Animal Responses:

Movement

Production of Enzymes

Hormones and Feeding

Plant Stimuli:

Light

Gravity

Water

Plant Responses:

Growth

Flowering

Fruit Formation

Structures Required for Response

  • A chemical or hormonal system (present in plants and animals)

  • A nerve and sense organ system (animals only)

  • A method of movement, which includes growth along with muscular and skeletal system (animals only).

  • A defence or immune system.

Responses in Flowering Plants

Growth Regulation

  • Controlled by external and internal factors.

External Factors

Light

  • Provides energy needed for photosynthesis, in turn supplies energy rich molecules needed for growth.

  • Needed to produce chlorophyll, fully formed chloroplasts, normal sized leaves and strong stems.

Day Length

  • Plays a role in causing plants to flower.

  • Has a role in fruit and seem fromation, dormancy, leaf loss and germination.

Gravity

  • Can cause roots to grow down into the soil, while shoots grow upwards.

Temperature

  • Affects rate of enzyme activity.

Internal Factors

  • Plants produce growth regulators in meristematic regions e.g. root tip or shoot tip.

Tropism

  • A change in growth of a plant in response to an external stimulus.

  • Allows plants to obtain more favourable growth conditions.

Negative Tropism

When growth is away from the stimulus.

Positive Tropism

When growth is towards the stimulus.

Phototropism

  • Change in growth of a plant in response to light, usually from one direction.

Geotropism

  • Change in growth of a plant in response to gravity.

Thigmotropism

  • Change in growth of a plant in response to touch.

Hydrotropism

  • Change in growth of a plant in response to water.

Chemotropism

  • Change in growth of a plant in response to chemicals.

Growth Regulators

  • Chemical that controls the growth of a plant.

  • Can behave as growth promoters or growth inhibitors.

Growth Promoters

Auxins and Indolecetic Acid (IAA)

Is made in shoot tips, young leaves, and seeds. It moves down the stem by an unknown mechanism. Auxins cause stem and root growth and stimulate fruit formation.

Functions:

  • 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

  • Tropisms

  • Apical Dominance

    • If apical bud is intact, auxin produced in the tip will pass down the stem and inhibit lateral buds and any side branching.

    • If apical tip is removed side branches are allowed to develop.

  • Fruit Formation

    • IAA is made in developing seeds. It stimulates food to form in the fruit that surrounds the seeds.

  • Root Growth

    • At low concentrations, IAA causes roots to grow.

    • At high concentrations, IAA causes stems to grow.

Mechanism of Phototropism

Auxin loosens cell walls, which allows them to expand. Cell elongation is essential for normal growth and tropisms.

  1. IAA is produced in the meristems.

  2. If the stem is exposed to light from one side IAA will diffuses fown the shaded side.

  3. The concentration of IAA present in the shaded cells causes them to elongate more than the cells on the bright side of the stem.

  4. As a result of uneven elongation, the stem bends towards the light.

Growth Inhibitors

Chemical that causes reduction in growth of plants e.g. ethene, abscisic acid.

Ethene

  • Only growth regulator that is gas.

  • Made by plants in stem nodes, ripe fruits, and decaying leaves.

Functions:

  • Ripening fruits

  • Causing fruit colour to form, fruit flavour to develop and fruit tissues to soften.

  • Stimulating leaves to fall in autumn.

  • Ageing of plants.

  • Stimulating more ethene production.

Chemical Protective Features

Excessive heat may cause plant enzymes to lose shape and become denatured. This may harm or even kill plants.

  • Many plants form special heat-shock proteins once the temperature rise above about 40oC. They normally surround other proteins and help them to maintain their shape.

  • Also formed by animcals and micro-organisms that are subjected to high temperatures.

When a plant is infected by a micro-organism the plant is sometimes able to produce stress proteins. Some of these stress proteins are called phytoalexins.

Act by:

  • Damaging the micro-organisms by attacking their cell walls.

  • Stimulating the formation of specialised plant cell walls that prevent the spread of the micro-organism.

  • Stimulating nearby plant cells to respond to the micro-organism.