A Slideshow for Gr 12 Life Sciences students, focussing on aspects of plant responses to the environment. It is essentially about plant hormones - Auxins, gibberellins, Abscicic acids, etc. Also contains information on plant defense, geotropism and phototropism.

1. Module 2: Life Processes
Life Sciences
Matric Syllabus
Mind Action Series: Life Sciences
Textbook and Workbook

2. • Plants, like animals, respond to stimuli in their environment
as a safety measure to ensure their survival. Most plants
respond more slowly than animals by growing, either towards
or away from stimuli.
• This sort of response is called a tropism and it is brought
about by hormones, which are involved in the control of
many aspects of plant growth and development.
• Different plant hormones interact with one another to bring
about the necessary responses.
Plant Responses to the Environment

3. • Auxins are one of the most important groups of plant
hormones. Indoleacetic acid (IAA) is the most common,
naturally occurring auxin.
• Auxins (IAA) are made all the time by cells in the tip of the
shoot (in the apical meristem).
• The auxins then diffuse downwards towards the roots. Very
small amounts are also produced in the root tip. There is
therefore a gradient along the main axis of the plant, the
concentration being highest at the tip and lowest in the root.
Plant Responses to the Environment

4. • Promote growth
After mitosis occurs, auxins cause the cells to elongate, bringing about
growth. The cells then differentiate and develop into the various tissues
of the plant.
– Growth is inhibited if the concentration of the auxin is too high.
– The further the tissues are from the shoot tip, the more sensitive they
become to auxin concentration.
– Root elongation requires much less auxin than stems. Higher
concentrations will inhibit cell elongation in roots.
Plant Responses to the Environment

5. • Cause apical dominance
The high concentration of auxins as they move down from the apical
meristem, inhibits the growth of the lateral buds. If they develop, these
buds would compete with the apical tip for light and nutrients. This is
known as apical dominance.
– Lower down the plant, the auxin concentration decreases so these
lower lateral buds can produce shoots which grow into lateral
branches.
– If the apical meristem is removed, lateral buds will grow as they are
no longer inhibited by auxins and a bushier plant will develop.
• Cause tropic movements
External stimuli bring about tropic movements, e.g. phototropism and
geotropism as a result of auxins causing cell elongation on only one side
of the stem or root.
Plant Responses to the Environment

6. • Tropism = growth movement of part of a plant in response to
a stimulus.
• For plants to respond to a stimulus, there must be:
– a receptor to pick up the stimulus and
– an effector to respond to it.
• These two parts must communicate with one another and
this is brought about by hormones.
• Plants may respond:
– positively by growing towards the stimulus or
– negatively by growing away from it.
Plant Responses to the Environment

7. • Phototropism is a growth movement of a part of a plant in
response to the stimulus of light.
• When light shines onto a shoot from one side, the auxin at
the tip moves away from the light and concentrates in the
cells on the shady side. This makes these cells elongate more
and therefore grow faster than those on the bright side, so
the shoot bends towards the light.
• Shoots and stems are therefore positively phototropic – they
grow towards light.
• Roots are negatively phototropic – they grow away from light.
Plant Responses to the Environment

8. • Geotropism is a growth movement of part of a plant in response to
the gravitational pull of the earth.
• If a young seedling is placed horizontally in a dark chamber and left
to continue its development for a few days, the shoot will
eventually bend upwards and the root downwards.
• This is because gravity causes auxins to accumulate on the lower
side of the shoot and root.
– In the shoot, this high concentration results in more growth taking place on
the lower side than the upper, resulting in an upward curvature.
– In a root, growth on the lower side is inhibited by the increased
concentration of auxin. Cells on the top of the root where the auxin
concentration is less, elongate causing the root to grow downwards.
– Primary roots are therefore positively geotrophic. Main stems are negatively
geotrophic.
Plant Responses to the Environment

9. • Gibberellins are one of the largest groups of hormones.
Gibberellic Acid (GA) is an example of a gibberellin and it
performs the following functions:
– Its main function is to cause elongation of the main stem by causing the
internodes to lengthen.
– It stimulates seed germination. As the seed absorbs water, the embryo
produces GA. This sets in motion a chain of reactions which cause
stored starch in the endosperm of the seed to be broken down to
glucose. This diffuses into the embryo and is used for plant growth,
breaking the plant out of dormancy.
• Unlike auxins, gibberellins
– Stimulate the growth of side branches from lateral buds.
– Play no part in the bending of a shoot or tropic movements.
Plant Responses to the Environment

10. • The plant hormone abscisic acid, a growth inhibitor, helps
plants adapt to adverse environments. It:
– Causes stomata to close when soil water is insufficient to keep up
with transpiration, thus reducing the loss of water through
transpiration.
– Promotes dormancy in both buds and seeds so that seeds will not
germinate in unfavourable environments. It also helps convert the
apical meristem into a dormant bud to protect it from drying out
during the winter.
– Promotes abscission i.e. it hastens the process of leaves and fruits
falling in unfavourable conditions. This is a response to the onset of
winter in adverse conditions.
Plant Responses to the Environment

11. Plant hormones are used in the following ways:
•Commercial fruit growers spray plant hormones onto flowers to:
– Mimic fertilisation, creating seedless plants (parthenocopy)
– Promote development of fruit
– Prevent premature fruit drop
•Selective weed killers contain high amounts of auxins. They are
used on fields and lawns, where they:
– Kill dicotyledons (weeds)
– Do not harm monocotyledons (grasses)
•New plants are produced by plant growers, using root powder
containing auxins. It stimulates the development of adventitious
roots from stems.
Plant Responses to the Environment

12. As producers, plants must protect themselves from pathogens and
herbivores. Certain defences have evolved against these groups due
to the chemical substances, secondary metabolites.
•Constitutive defences are defences which are always present in the
plant species.
– Barriers – cuticles, bark, resin.
– Thorns, spines and prickles.
– Unpalatable species have evolved warning colouration,
– Toxins – poison ivy
– Silica and lignins – grind insects mandibles to slow down feeding
•Induced defences only occur when a plant is diseased or stressed.
– Premature leaf abscission
– Produce alkaloids, terpenoids or phenols: discourage consumption by
producing aromas or tastes.
Plant Responses to the Environment