Regulation of Plant Growth
38
Interacting Factors in Plant Development
• The development of a plant is regulated by four
factors:
 Plants sense and respond to environmental cues.
 Plants use receptors, such as photoreceptors,
which absorb light, to sense some environmental
cues.
 Chemical messages, or hormones, mediate the
effects of the environmental cues.
 Enzymes, which are encoded by the plant’s
genome, catalyze the biochemical reactions of
development.
38
An Overview of Plant Development
• As plants develop, environmental cues,
photoreceptors, and hormones affect three
fundamental processes:
 Cell division
 Cell expansion
 Cell differentiation
• 5 major classes of hormones in plants mediate
growth and development.
38
Interacting Factors in Plant Development
• Auxins promote the elongation of
cells in parts of the plant.
• Both phototropism (growth
toward light) and gravitropism
(directional growth with respect to
gravity) is controled by auxins.
Phototropism: Plants
grow toward light.
• Light and gravity are important
environmental cues in plants.
Gravitropism: stems grow
up and roots down.
38
Interacting Factors in Plant Development
• Hormones called Gibberellins
control:
 Germination of seeds and
sprouting of buds
 Elongation of stems
 Stimulation of flowering
 Development of fruit
38
Interacting Factors in Plant Development
• Photoreceptors are protein
molecules that absorb light.
• Light is an environmental cue that
acts directly on the photoreceptor.
• Onset of winter dormancy is
controlled by the length of the light.
• Other environmental cues are light
color, intensity, duration, and
temperature.
• Light regulates many aspects of
plant development.
38
An Overview of Plant Development
• Seed dormancy must be broken for a plant embryo to
begin developing.
• As a seed begins to germinate it first takes up water.
• The embryo uses the seed reserves by digesting stored
polymers into monomers.
• Germination is completed when the embryonic root
emerges from the seed coat.
• At this point, the plant is called a seedling.
Figure 38.1 Patterns of Early Shoot Development (Part 1)
Figure 38.1 Patterns of Early Shoot Development (Part 2)
38
An Overview of Plant Development
• Some plants flower when they reach a certain
size or age; others flower during certain times of
the year.
• The latter have photoreceptors in the leaves that
measure the length of night.
• When it is time for the plant to flower, a signal
must be transmitted from the leaves to the place
where flowers will form.
• Following fertilization a fruit develops and ripens
under hormonal control.
38
An Overview of Plant Development
• Perennials continue to grow year after year, but some go
into dormancy for part of the year.
• A hormone helps maintain this dormancy.
• In some species, leaves senesce and fall at the end of the
growing season.
• This process is regulated by the interactions of hormones.
38
Ending Seed Dormancy and
Beginning Germination
• Seed dormancy may last for weeks, months,
years, or even centuries.
• The principal mechanisms of dormancy are:
 Exclusion of water and/or oxygen by the seed
coat
 Mechanical restraint of the embryo
 Chemical inhibition of embryo development
38
Ending Seed Dormancy and
Beginning Germination
• Seed dormancy must be
broken before the embryo can
germinate. There are many
mechanisms for breaking
dormancy.
• The seed coat may be
abraded, broken, or chemically
weakened to allow
germination.
• The activity of microorganisms
in soil probably help soften
seed coats.
A developing
runner bean.
Figure 38.2 Fire and Seed Germination
Fire releases
mechanical
restraint in some
species, or melts
waterproof waxy
coatings.
Scorching by fire
may break down
chemical
inhibitors.
Seeds that
must be
scorched by
fire avoid
competition
with other
plants by
germinating
only in areas
cleared by
fires.
Figure 38.3 Leaching of Germination Inhibitors
Prolonged soaking
in water can leach
chemical
inhibitors to break
seed dormancy.
38
Ending Seed Dormancy and
Beginning Germination
• Seed dormancy ensures that plants survive
unfavorable conditions, germinate when conditions
are likely to be favorable, and aids in plant dispersal.
• Some seeds require exposure to cold, ensuring that
they don’t germinate until spring brings favorable
conditions.
• Some seeds will not germinate until a certain
amount of time has passed, which prevents
germination while the seeds are still attached to the
parent plant.
38
Ending Seed Dormancy and
Beginning Germination
• The first step in seed germination uptake of water.
• The interior of the seed has a negative water potential,
and water causes the seeds to swell substantially.
• Water uptake activates certain enzymes, and proteins
are synthesized.
38
Ending Seed Dormancy and
Beginning Germination
• Reserves stored in the endosperm provide energy and
materials until the leaves begin photosynthesis.
• The energy storage molecules include starch, fats or oils,
and proteins. These giant molecules must be digested to
monomers by enzymes.