1 Apply Concepts Using a houseplant, a marker and
a sunny windowsill, describe how you might
measure the plant’s response to light
2 Review Summarize plant responses to seasonal
changes
3 Explain Which type of plant- short or long day- is
likely to bloom in the summer- explain
Design an Experiment Design a controlled
experiment to find out how a garden store owner
could determine what light conditions are needed
for a particular flowering plant to bloom
CH 24 PLANT REPRODUCTION AND RESPONSE
24.1 Plant Hormones
Hormones

Chemical signals produced by living organisms that
affect
 Growth,

Activity, and Development of cells and tissues
Coordinate responses to the environment.

Hormone-producing cells in
a mature flower release
hormones that travel into
flower buds and inhibit
development.

Once the mature flower is
done blooming, production
of the inhibiting hormone
will decline, and the flower
bud can then begin its
bloom.

Target cell
 Portion
of an organism affected by a particular
hormone

Hormone receptors
 Where
hormone molecules bind.


Hormones may affect roots differently than stems or
flowers and the effects may change as receptors are
added or removed
All depends on the receptors.


Why does grass bend toward light
Darwins hypothesized that the tip produces a
substance that regulates cell growth.
Auxins


Stimulate cell elongation and the growth of new
roots
Produced in the shoot apical meristem and
transported to the rest of the plant.
Auxins and Cell Elongation



Light hits one side of the shoot
Auxins collect in shaded part of shoot
Stimulates cells on dark side to lengthen
 Causes
shoot to bend away from shaded side.



Auxins also regulate cell division in meristems
Growth at lateral buds is inhibited by auxins
Apical dominance
 Lateral
buds near the apex grow more slowly than
those near the base of the plant
 Due to more auxin at apex.
Cytokinins




Produce effects opposite of auxins
Auxins stimulate new roots and inhibit growth of
new shoot tips
Cytokinins stimulate growth of new shoot tips and
inhibit new roots
Produced in growing roots and in developing fruits
and seeds.
Gibberellins



Promote germination of plants
May cause dramatic increases in size, particularly
in stems and fruits
Found in the meristems of shoots, roots, and seed
embryos.
Abscisic Acid


Inhibits cell division, thereby halting growth
Stops seed growth when seed development is
complete
 Causes


seed to go dormant
Interacts with gibberellins to control seed
dormancy
Found in the terminal buds and seeds.
Ethylene



Stimulates fruits to ripen
Released from fruit
Causes plants to seal off and drop organs that are
no longer needed (flower petals).
Tropisms

Plant response to environmental stimuli such as
light, gravity, and touch.
Phototropism


Tendency of a plant to grow
toward a light source
Changes in the concentration
of auxins.
Gravitropism


Response of a plant to
gravity
Auxins migrate to the
lower sides of horizontal
roots and stems.
Thigmotropism



Response of a plant to touch
Vines and climbing plants wrap
around objects
Grape vines have tendrils that
emerge from leaf base that wrap
around objects.
Rapid Movements

Mimosa will fold in leaves when touched
 Osmotic

pressure
Venus flytrap
 Insect
triggers sensory cells on the inside of the leaf,
send electric signals, leaf snaps shut due to osmotic
pressure and cell wall expansion.
Photoperiod



Plant response to the relative lengths of light and
darkness
Major factor in the timing of seasonal activities
such as flowering and growth
Due to phytochrome pigment.
Winter Dormancy


Phytochrome regulates the changes that prepare
plants for winter dormancy
Deciduous plants
 Turn
off photosynthetic pathways
 Transport materials from leaves to roots
 Seal off leaves from the rest of the plant.
Leaf Loss






Phytochrome absorbs less light as days shorten
Auxin production drops
Ethylene increases
Chlorophyll breaks down
Carotenoids (yellow and orange) become visible
Anthocyanin (red) are made.


Meristems produce thick, waxy scales that form a
protective layer around new leaf buds
Xylem and phloem tissues pump themselves full of
ions and organic compounds
 Prevents
tree’s sap from freezing.
Carrot cells were grown
in varying auxin
concentrations
1. At what auxin
concentration are the stems
stimulated to grow the most
2. How is the growth of the
roots affected by the auxin concentration at which
stems grow the most
3. If you were a carrot farmer, what concentration of
auxin should you apply to your fields to produce the
largest carrots