Chapter 32 Plant Growth and
Development
How Do Seeds Germinate?
• Germination is the resumption of growth after
a time of arrested embryonic development
• Environmental factors influence germination
– Spring rains provide the water amounts necessary
to swell and rupture the seed coat (taking in water
is imbibition)
– Oxygen moves in and allows the embryo to switch
to aerobic metabolism
– Increase temperatures and number of daylight
hours
Continue…
• Repeated cell divisions produce a seedling
with a primary root.
Genetic Programs, Environmental Cues
• Patterns of germination and development
have a heritable basis dictated by a plant’s
genes
• Early cell divisions may result in unequal
distribution of cytoplasm
– Cytoplasmic differences trigger variable gene
expression, which may results in variations in
hormone synthesis
– Even though all cells have the same genes, it is the
selective expression of those genes that results in
cell differentiation.
Growth and Development
• Growth and development are necessary for
plants to survive
– Growth is defined as an increase in the number,
size, and volume of cells
– Development is the emergence of specialized,
morphologically different body parts
Plant Hormones
• Plant hormones have central roles in the
selective gene expression underlying cell
differentiation and patterns of development.
Types of Plant Hormones
• Gibberellins: Promote stem elongation
– Help buds and seeds break dormancy and resume
growth in the spring.
– In some species, they influence the flowering
process.
• Cytokinins: stimulate cell division in root and
shoot meristems, where they are most
abundant; they are used commercially to
prolong the life of stored vegetables and cut
flowers
Grapes (Gibberellins)
With
Without
Continue (Plant Hormones)…
• Auxins: affect lengthening of stems and
coleoptiles (the protective cylinder that covers
and protects the tender leaves during
germination)
– May participate in growth responses to light and
gravity.
– Indoleacetic acid (IAA) is applied to fruit trees to
promote uniform flowering, set the fruit, and
encourage synchronous development of fruit.
– Synthetic auxins (such as 2,4-D) are used as
herbicides
Auxins Picture
Continue (Plant Hormones)…
• Abscisic Acid (ABA)inhibits cell growth, helps
prevent water loss (by promoting stomata
closure), and promoting seed and bud
dormancy.
• Ethylene stimulates the ripening of fruit and is
used commercially for this purpose.
• Other less well known hormones trigger
flowering and inhibit the growth of lateral
buds (apical dominance)
What are Tropisms?
• A plant tropisms is a growth response
– Evidenced by a turning of a root or shoot toward
or away from an environmental stimulus
– Hormones mediate the shifts in rates at which
different cells grow and elongate to cause the
overall response
Types of Tropisms
• Gravitropisms: is the growth response to
gravity– shoots grow up, roots grow down.
– Auxins, together with a growth—inhibiting
hormone, may play role in promoting, or
inhibiting, growth in strategic regions
– Statoliths, which are unbound starch grains in the
plastids, respond to gravity and may trigger the
redistribution of auxin
Roots moving down toward
gravity
Continue (Tropisms)…
• Phototropisms: is a growth response to light
– Bending toward the light is caused by elongation
of cells (auxin stimulation on the side of the palnt
not exposed to light).
– Flavoprotein, a pigment molecule probably plays a
role because of its capacity to absorb blue
wavelengths of light
Continue (Tropisms)…
• Thigmotropism is shift in growth triggered by
physical contact with surrounding objects.
– Prevalent in climbing vines and in the tendrils that
support some plants
– Auxin and ethylene may have roles in the
response
Response to Mechanical Stress
• Response to the mechanical stress of strong
winds explain why plants grown at higher
mountain elevations are more stubby than
their counterparts at lower elevations
• Human interventions such as shaking can
inhibit plant growth.
How Do Plants Known When to
Flower?
• Phytochrome: Alarm button for plants
– Biological Clocks are internal time-measuring
mechanisms that adjust daily and seasonal
patterns of growth, development, and
reproduction
• Phytochrome– a blue-green pigment, is alarm button
for some biological clocks in plants
• Phytochrome- can absorb both read and far-red
wavelengths with different results.
– When is the pigment activated?
– When is the pigment inactive?
Continue…
• Some plants activities occur regularly in cycles
of 24 hours (circadian rhythms) even when
environmental conditions remain constant
Flowering – A case of photoperiodism
• Photoperiodism is a biological response to a
change in relative length of daylight and
darkness in a 24-hour cycle; this resetting of
the biological clocks is necessary to make
seasonal adjustments
Continue…
• The flowering process is keyed to changes in
daylength throughout the year.
– Short-day plants: flower in late summer or early
autumn when daylength becomes shorter
• Example: Poinsettias
– Long-day plants: flower in the spring as daylength
becomes longer
• Example: Spinach
– Day-Neutral Plants: flower when they are mature
enough to do so
• Example: Roses
Senescence
• The dropping of leaves, flowers, fruits is called
abscission
• Senescence: is the sum total of the processes
leading to the death of plant parts or the
whole plant
– Decrease of daylight hours trigger the reduction of
auxin
– Cells in abscission zones produce ethylene which
causes cells to deposit suberin in their walls
Entering and Breaking Dormancy
• Dormancy occurs in autumn when daylight
shortens and growth stops in many trees and
nonwoody perennials– it will not resume until
spring
• Strong cues for dormancy include in short
days, cold nights, and dry, nitrogen-deficient
soil.
• Dormancy has great adaptive value in
preventing plant growth on occasional warm
autumn days only to be killed by later frost.
Vernalization
• Vernalization is the stimulation of flowering
only after plants have been exposed to lower
temperatures (winter).
• Deliberately exposing seeds to lower
temperature to stimulate flowering the next
season is common agricultural practice.
Breaking Dormancy
• Dormancy is broken by milder temperatures,
rains, and nutrients.
• It probably involves gibberellins and abscisiic
acid, and require exposure to specific periods
of low temperatures.