CHAPTER IX
Stages of Growth and Development
VEGETATIVE GROWTH AND
DEVELOPMENT
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Shoot and Root Systems
The root system and the shoot system tend
to maintain a balance:
– As the top of the plant grows larger and larger, the
leaf area increases and water loss through
transpiration increases. This increased water loss is
made up by water absorption from an increasing
water system.
– The enlarging shoot system also requires greater
amounts of mineral that are absorbed by the
increasing root system
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Definitions and Measurements
• Growth can be measured as increases in fresh
weight or dry weight, or in volume, length,
height, or surface area.
• Plant growth is a product of living cells, with all
their myriad ‫ هائل‬metabolic processes.
•
• We generally think of growth as an irreversible
increase in volume or dry weight.
• Plant growth: size increase by cell division and
enlargement, including synthesis of new cellular
material and organization of subcellular
organelles.
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Science, 4th edition
Plant shoot growth can be classified as:
1. Determinate growth: after a certain period of
vegetative growth, flower bud clusters form at the shoot
terminals so that most shoot elongation stops.
2. Indeterminate growth: bear the flower clusters laterally
along the stems in the axils of the leaves so that the
shoot terminals remain vegetative and the shoot continues to grow until it is stopped by senescence or some
environmental influence. Example: grapevines.
•
The determinate, bush-type plants produce much less
vegetative growth than do the indeterminate type.
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Shoot Growth Patterns:
• Annuals, which are herbaceous (nonwoody) plants,
complete their life cycle (seed to seed) in one growing
season.
• Biennials, which are herbaceous plants, require two
growing seasons (not necessarily two years) to
complete their life cycle (seed to seed). Example:
cabbage
• Perennials, which are herbaceous or woody plants,
lives for more than two growing seasons.
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Annual Plants
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Biennial Plants
• Stem growth is limited during
the first growing season.
• The plants remain alive but
dormant through the winter.
• Exposure to chilling
temperatures triggers
hormonal changes leading to
stem elongation, flowering,
fruit formation, and seed set
during the second growing
season.
** Most annual and biennial
plants flower and fruit only
once before dying.
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Perennial Plants
In herbaceous perennials, the roots and shoots can remain alive
indefinitely but the shoot system may be killed by frosts in coldwinter regions or by senescence-inducing factors.
Shoot growth resumes each spring from latent or adventitious buds
at the crown of the plant (Figure 9-5)
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In woody perennials, both the shoot and root system
remain alive indefinitely.
• Shoot growth of
temperate zone
plants takes place
annually during
the growing
season, as
indicated by
Figure 9-6,
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Root Growth Patterns
In deciduous woody perennials, root growth peaks in the
spring and again in late summer or early autumn.
Reasons:
1. The spring flush of root growth results from the
accumulated foods stored in the tree the previous year.
2. When this source is depleted, root growth slows
3. Following gradual accumulation of carbohydrates from
photosynthesis through the summer, root growth again
increases in the autumn.
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Genetic Factors Affecting Plant Growth
and Development
• The organism developing by cell division and elongation from
the fertilized egg—the zygote—in every case is under the
genetic control of the genes inherited from the parents at the
time of fertilization.
• The genes direct the form and shape of the organism.
• At any given time, some of the organism's genes are
transcriptionally active, while others are silent. The control of
gene activity depends on :
1. The cell type,
2. Environmental conditions or,
3. The particular stage of development.
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Environmental Factors Influencing Plant
Growth and Development
• Light
The sun is the source of energy for photosynthesis and
other plant processes,
• Light quality
Wavelengths of 400 to 700 nm are commonly referred to
as visible light or photosynthetic active radiation (PAR)
(the most important to life on earth). Highest PAR is at
680 nm = red and then at 480 nm=blue
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Photomorphogenesis describes several highly
integrated processes:
1. Seed germination in light most seeds are sensitive .
2. De-etiolation (the greening of young seedlings when
they emerge from the soil),
3. Stem growth in plants competing for light with other
plants.
• Most photomorphogenesis responses are regulated
by the phytochrome pigment system
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Phototropism
• Phototropism is a photomorphogenic response
of plants to the direction of light.
• A blue light receptor called phototropin is
responsible for sensing the direction of light.
• The bending in positive phototropic responses is
due to increased cell growth on the side away from
the light source. It is believed that the plant
hormone auxin accumulates on the shaded side,
promoting cell expansion
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Photoperiodism
Photoperiodism is the photomorphogenic response to
seasonal variations in the amount of daylight.
Numerous aspects of plant growth and development are
controlled by photoperiod including:
1. Flowering (which will be discussed in more detail later
in this chapter);
2. Induction of bud dormancy in woody species
3. The formation of vegetative propagules such as
bulbs, tubers, corms, and runners (stolons).
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• All photoperiodically controlled processes can be
categorized into three basic response types:
1. Long-day plants (LDPs);
2. Short-day plants (SDPs); and
3. Day-neutral plants (DNPs); which are
photoperiodically neutral.
• The designation as a long- or short-day plant is not
based on the absolute length of the day, but rather if the
photoperiodically controlled process is induced only at
daylengths longer or shorter than specific daylength,
called the critical day length (CDL).
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• Long-day plants : A plant with photoperiodically
controlled process that is induced only when the
day length is longer than the CDL.
• Short-day plants A plant with photoperiodically
controlled process that is induced only when the
day length is shorter than the CDL.
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No direct relationship exists between the response type and the
absolute length of the CDL.
For example:
- Red clover is an LDP with a CDL of twelve hours,
Garden chrysanthemum, which is an SDP with a CDL of fifteen
hours,
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Temperature
• All plants have optimal temperatures for maximum
vegetative growth and flowering.
• Most temperate-region plants grow between temperatures
of 4°C and 50°C, but these are generally the limits of plant
growth.
- The high temperatures destroy the protoplasm of
most cells;
- At the low temperatures, most plants just fail to grow
because of a lack of cell activity and burning by frost .
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• Low, non-freezing temperatures (0°C to 10°C) are
sometimes used by plants as cues to coordinate growth
and development with the changing seasons.
• Examples of cold-induced processes include:
1. Seed germination.
Some seeds require a period of time during which the
seeds are imbibed at low temperatures (stratification)
before germination is possible.
2. Flowering.
The cold induction of flowering is called vernalization.
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3. Dormancy breakage.
The duration required for complete loss of dormancy is
called the chilling requirement.
4. Acquisition of cold and freeze tolerance.
For many herbaceous perennials, the low, nonfreezing
temperatures (0°C to 10°C) that frequently occur during
autumn nights induce the physiological processes
responsible for the ability to survive the freezing
temperatures of winter=hardening.
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Water
Most growing plants contain about 90 percent water.
– It is stored in various plant tissues and
– It is used as one of the raw materials for photosynthesis.
•
When the light from the sun is strong, the leaves tend
to lose large quantities of water by transpiration and,
additionally, the soil loses water by evaporation.
•
Substantially more soil water is lost from transpiration
by plants than by evaporation from the soil.
•
The total soil water loss by both means is called
evapotranspiration.
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If more water is lost through the stomata than the roots can
supply, the plant wilts. Wilted plants eventually die if
they cannot recover enough soil water to regain their
turgidity.


Herbaceous plants may wilt slightly at midday or later on a
bright sunny day but they usually recover during the night =
mid day wilting.
Deciduous fruit plants often fail to recover from this water loss.
•
While the plant is wilted, the stomates are closed,
cutting off the intake of CO2 for photosynthesis, and
thus reducing carbohydrate manufacture.
•
The quality of soil water, as determined by the
quantity of minerals and salts dissolved in the water, is
very important to the growth of plants.
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Gases
• The two gases most important to the growth of green
plants are
1. Oxygen (O2) and
2. Carbon dioxide (CO2).
• Carbon dioxide is the third most abundant gas in the
atmosphere:
 Nitrogen is approximately 78%
 O2 is approximately 21%
 CO2 is approximately 0.035%
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• Plants do most of the work to keep our atmosphere in a
favorable balance.
• However: Atmospheric CO2 rose ‫ تراجع‬20 to 30 ppm
during the 1900s. This elevation is believed to be
enough to contribute significantly to global warming.
• Global warming results from atmospheric gases,
especially CO2, which trap heat at the earth's
surface. The phenomenon is sometimes referred
to as the greenhouse effect.
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Phase Change: Juvenility, Maturation,
Senescence
A newly emerged seedling pass through:
• Embryonic growth
• Juvenility
• Phase change (transition phase)
• Maturity or adult phase;
• Senescence and
• Death.
• The juvenile phase is characterized by the inability to
reproduce sexually; (cannot flower).
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The duration of the juvenile phase varies from a week or
two up to thirty or forty years in some tree species.
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Aging and Senescence
• The life spans of the different kinds of flowering plants
differ greatly, ranging from a few months to thousands of
years.
• Olive trees with
huge trunks found in
the eastern
Mediterranean area
are believed to be
several thousand
years old (see Fig.
9-9).
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• Senescence is considered to be a terminal,
irreversible deteriorative change in living
organisms, leading to cellular and tissue
breakdown and death of
1. Annual plants (population senescence) and of
2. Individual plants (whole plant senescence),
3. Leaves, seeds, flowers, or fruits (organ
senescence).
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Senescence is usually considered to be due to:
1. Inherent physiological changes in the plant, or
2. Pathogenic attack or
3. Environmental stress.
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REPRODUCTIVE GROWTH AND
DEVELOPMENT
• Fruit and seed production involves several
phases:
1. Flower induction and initiation
2. Flower differentiation and development
3. Pollination
4. Fertilization
5. Fruit set and seed formation
6. Growth and maturation of fruit and seed
7. Fruit senescence
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Flower Induction and Initiation
• In some species, the formation of flowers is
influenced by:
• Daylength (photoperiodic effect) and/or
• Low temperatures (vernalization),
• Flowering in most trees have neither response to
daylength nor cold temperatures.
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Photoperiodism (Daylength)
• Figure 9-10
demonstrates how
a flash of light (or
night break) of
sufficient intensity
or duration
• Inhibits flowering
of a short-day
plant (long-night
plant)
• Induce flowering
of a long-day
plant (LDP). DMA: Chapter 9 Hartmann's Plant
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This information has been useful to commercial
chrysanthemum growers who grow these short-day plants
on a year-round schedule
• When they want the young plants to reach a size
adequate for flowering, the growers use fluorescent
lamps over the chrysanthemum plants (near midnight),
each night for one to four hours, depending on time of
year and latitude. This night break inhibits flowering until
the plants reach the desired height.
• Conversely, when the natural daylight of summer is too
long for chrysanthemum plants to flower, they cover the
plants of proper size with black cloth or plastic each
evening about 6 P.M. and remove it in the morning at
about 8 A.M. This shortens the plant's day (lengthens the
night) enough to induce and fully develop the flowers.
.
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Pollination
• Pollination is defined as the transfer of pollen from an
anther to a stigma.
• The anther and stigma may be in:
– Same flower (self-pollination),
– Different flowers on the same plant (self-pollination),
– Different flowers on different plants of the same
cultivar (self-pollination),
– Different flowers on plants of different cultivars
(cross-pollination).
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• If a plant is self-fertile, it
produces fruit and seed
with its own pollen,
without the transfer of
pollen from another
cultivar.
• If it is self-sterile, it
cannot set fruit and seed
with its own pollen, but
instead requires pollen
from another cultivar.
• Often this is due to
incompatibility, where a
plant's own pollen will not
grow through the style
into its embryo sac (see
Figs. 9-12 and 9-13).
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Parthenocarpy
• Parthenocarpy is the formation of fruit
without the stimulation of pollination and
fertilization. Washington Navel orange,
and many fig cultivars.
(Not all seedless fruits are parthenocarpic)
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Fertilization
• In the angiosperms the pollen tube grows
through the micropyle opening in the ovule into
the embryo sac and discharges two sperm
nuclei (IN each):
1. One unites with the egg (IN) to form the zygote (2N), which
will become the embryo and eventually the new plant.
2. The other sperm nucleus unites with the two polar nuclei
(IN each) in the embryo sac to form the endosperm (3N),
which develops into food storage tissue.
• This process is termed double fertilization.
• The elapsed time between pollination and
fertilization in most angiosperms is about 24 to
48 hours.
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THE LIFE CYCLE OF A CORN PLANT (A
MONOCOT)
• When a corn seed is planted in moist soil, it
imbibes (absorbs) water from the soil.
• Germination begins with the emergence of the
radicle (the primary root) and the plumule (the
primary shoot).
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• The radicle grows
downward through a
protective sheath, the
coleorhiza, from which the
primary root develops and
the secondary roots
branch.
• Adventitious roots (roots
other than those that develop
from the radicle) grow from the
shoot axis just at or above the
soil surface (Fig. 8-2):
• The emerging plumule is
protected by a sheathlike
leaf, the coleoptile, that
envelops the main stem as
it grows upward through
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soil.
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• Female flowers, known as
pistillate flowers or ears,
appear at the base (axil) of one
or more sheath leaves.
• Male flowers, known as
staminate flowers or tassels,
develop at the top of the plant.
Figure 8-3 shows both kinds of
flowers.
• Blown by the wind, pollen
grains from the tassels fall on
and pollinate the long pistillate
filaments (silks) and
subsequently fertilize the
ovaries, which become the
individual corn kernels borne on
a stalk (cob).
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• Each ovary develops into a fruit, called a caryopsis, that
encloses the true seed.
• After the kernels mature and dry, the fruits (containing
the seeds) are harvested and stored over the winter.
• The seeds can be sown when weather conditions are
favorable for germination, and the life cycle repeats
itself.
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THE LIFE CYCLE OF A BEAN PLANT (A
DICOT)
• The radicle grows
downward and the hook of
the bean, known as the
hypocotyl, emerges above
the soil, carrying the two
cotyledons with it.= epigeal
growth
• Between the cotyledons lies
a growing point (apical or
shoot meristem) flanked
by two opposite primary
foliage leaves.
• The stem region just above
the cotyledons and the first
trifoliate leaves is called the
epicotyl (Fig. 8-4).
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• The shoot apical meristem rapidly produces two
trifoliate leaves opposite each other on the stem.
• The plant's green leaves are now capable of
manufacturing food for future growth of the seedling.
• Flowers begin to develop in the axils of about the
fourth set of leaves and in each succeeding set.
• These flowers are self-pollinated; thus, fruits (pods)
develop as long as environmental conditions are
favorable.
• The seeds mature and dry within the pod, and they
can be sown at once to produce another generation
of bean plants
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The difference in emergence
• The difference in emergence of the growing
points of beans and corn from beneath the soil
affects the tolerance of each crop to light frosts.
• A late frost would be more likely to severely
damage a newly emerged bean seedling
(epigeal) than a newly emerged corn seedling
because the growing point of corn is below the
soil (hypogeal) and protected.
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PLANT GROWTH REGULATORS
• Plant hormone is a natural substance
(produced by the plant itself) that acts to
control plant activities.
• Plant growth regulators, include plant
hormones and other, nonnutrient
chemicals not found naturally in plants but
that, when applied to plants, influence their
growth and development.
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• There are five traditionally recognized
groups of natural plant hormones:
Plant Hormone Rule
1 Auxins
Promote both cell division and cell growth
2 Cytokinins
Promote cell division
3 Gibberellins
Involve in regulating cell elongation
4 Abscisic acid
Inhibit cell division
5 Ethylene
Control fruit ripening
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