Transcript Plant Responses

Pg. 139-145
What are plant responses?
• A stimulus is anything in the environment that
causes a response in an organism. The response
often involves movement either toward the
stimulus or away from the stimulus.
• A stimulus may come from outside (external) or
inside (internal) the organism.
What are plant responses?
• Internal stimuli are usually chemicals produced
by organisms.
• Many of these chemicals are hormones.
Hormones are substances made in one part of
an organism for use somewhere else in the
organism.
What are plant responses?
• Many different
chemicals are known to
act as hormones in
plants.
• Plants respond to
external stimuli such as
touch, light, and
gravity.
• Some responses are
rapid. Other plant
responses are slower
because they involve
changes in growth.
Tropisms
• Some responses of a plant to an external
stimuli are called tropisms.
• A tropism is a response of a plant to an
external stimulus.
• For example, plants might grow toward a
stimulus a positive tropism or away from a
stimulus a negative tropism.
Touch
• One stimulus that can result in a change in a
plant’s growth is touch.
• When some plants touch a solid object, they
respond by growing faster on one side of its
stem than on the other side.
• As a result the stem bends and twists around
any object it touches.
Light
• Light is an important stimulus to plants.
• When a plant responds to light, the cells on
the side of the plant opposite the light get
longer than the cells facing the light.
• Because of this uneven growth, the plant bends
towards the light.
Light
• When a plant grows toward light it is called a
positive response to light, or positive
phototropism.
Gravity
• The downward
growth of plant
roots is a positive
response to
gravity.
• A stem growing
upward is a
negative response
to gravity.
Plant Hormones
• Hormones control the changes in growth that
result from tropisms and affect other plant
growth.
Ethylene
• Ethylene is produced in cells of ripening fruit,
which stimulates the ripening process.
• Helps fruit
ripen
Auxin
• Auxin is a type of
plant hormone that
causes plant stems
and leaves to grow
toward the light.
• When light shines on
a plant from one
side, the auxin moves
to the shaded side of
the stem where it
causes a change in
growth.
Auxin
• Development of many parts of the plant,
including flowers, roots, and fruit, is
stimulated by auxins.
• Because auxins are so important in plant
development, synthetic auxins have been
developed for use in agriculture.
Gibberellins and Cytokinins
• Gibberellins are chemical
substances that were
isolated first from a
fungus.
• The fungus infects the
stems of plants and
causes them to grow too
tall.
Gibberellins and Cytokinins
• Gibberellins can be mixed
with water and sprayed on
plants and seeds to
stimulate plant stems to
grow and seeds to
germinate.
Gibberellins and Cytokinins
• Cytokinins also cause rapid growth.
• Cytokinins promote growth by causing faster
cell divisions.
Abscisic Acid
• Abscisic acid is the substance that keeps seeds
from sprouting and buds from developing
during the winter.
• This plant hormone
also causes stomata to
close and helps plants
respond to water loss
on hot summer days.
Photoperiods
• A plant’s response to the number of hours of
daylight and darkness it receives daily is
photoperiodism.
• The hours of daylight and darkness vary with
the seasons.
• These changes in lengths of daylight and
darkness affect plant growth.
Darkness and Flowers
• Generally,
plants that
require less
than 10 to 12
hours of
darkness to
flower are
called longday plants.
Darkness and Flowers
• Plants that
need 12 or
more hours
of darkness to
flower are
called shortday plants.
Day-Neutral Plants
• Day-neutral plants have no specific
photoperiod, and the flowering process can
begin within a range of hours of darkness.
• In nature,
photoperiodism
affects where
flowering plants can
grow and produce
flowers and fruit.