Download Horticulture #11 - Horticulture Science Overview

Horticulture Science
Lesson 15
Propagating Plants Sexually
Interest Approach
Bring to class several varieties of blooming plants such
as geraniums and petunias. If plants are not available,
use a flowering plant seed catalog that contains color
pictures of plant varieties. Ask students about the
significance of variety. How does it happen? How has
genetic variation impacted our world food resources?
Show students a variety of bean seeds. What do the
seeds have in common? What is different about them?
How does germination unlock the “secret” within each
seed?
Student Learning
Objectives
•Discuss the importance of sexual
propagation of plants.
•Describe the process of seed
germination.
Student Learning
Objectives
•Describe the factors involved in
planting seeds for transplanting.
•Explain how to successfully direct
seed outdoors.
Terms
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direct seeding
germination
indirect seeding
medium
planting date
scarification
stratification
turgor
viability
vigor
How does the ability to sexually produce
seeds benefit a plant?
• Many important horticultural crops
are grown from seed, including
bedding plants, perennials,
vegetables, and woody plants.
• Most of the flower and plant
varieties used in the floriculture
industry today are the result of
cross-pollination by plant breeders.
How does the ability to sexually produce
seeds benefit a plant?
• Plant breeders select plants for
outstanding characteristics, such as
flower color and disease resistance.
• Then, they collect pollen from one
plant and transfer it to the flower of
another.
• Their hope is to have the outstanding
characteristics expressed in the
offspring.
• This process is called hybridization.
How does the ability to sexually produce
seeds benefit a plant?
• The offspring of two plants of the
same or related species that differ
genetically is said to be a hybrid.
• The advantage of hybridizing is that
desired traits of different parent
plants can be combined in the
offspring.
How does the ability to sexually produce
seeds benefit a plant?
• In the landscape/nursery
industry, many trees and
shrubs are produced from
seed.
• Seeds are collected from trees
and shrubs that exhibit
outstanding characteristics.
• These are sown and grown
for sale.
How does the ability to sexually produce
seeds benefit a plant?
• Sowing vast numbers of seeds also allows breeders to
select exceptional offspring.
• The exceptional plants are propagated asexually to
maintain genetic makeup.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• The beginning of plant growth from seed is called
germination.
• Germination is a
complex process that
begins when favorable
conditions exist for the
survival of the
developing seedling
plant.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• The environmental conditions important for germination
include moisture, temperature, oxygen, and light or darkness.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• The first important step in germination is usually the
absorption of water.
• Water enters the seed by diffusion.
• The moisture triggers several important processes.
• Water also causes the
seed to expand and
creates pressure within
the seed.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• This pressure, known as turgor, causes the seed coat
to rupture.
• The primary root, or radicle, emerges through the
rupture and forms the root system of the plant.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Water also stimulates the production of plant
hormones that begin the process of converting stored
food into energy for the developing embryo.
• In addition, plant hormones cause cells of the radicle
to divide, allowing it to emerge from the seed to
absorb water and nutrients for the embryo.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Seeds germinate at a wide range of temperatures.
• Some plants require very warm germination
temperatures, and other plants must have cool
temperatures.
• The temperature range is from 32° to 104°F, with the
optimum range for most seeds being between 65° and
80°F.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• All seeds need oxygen to germinate.
• The oxygen is necessary for the cellular
respiration.
• Through cellular respiration, chemical energy
held in the starches is released for the growth
and development of the embryo.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Without sufficient oxygen, the
embryo cannot develop.
• Wet soil that prevents oxygen
from reaching the seed can cause
the death of an embryo or
seedling.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Seeds of some plants need exposure to light before
they will germinate.
• Some plant seeds require darkness.
• Other plants are unaffected by light or darkness.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Some plants produce seeds that germinate
immediately; others produce seeds that remain
dormant, perhaps for years, until the conditions
are likely to result in the survival of the seedlings.
• Internal seed mechanisms are sometimes
responsible for preventing seed germination even
when environmental factors appear optimal for
germination.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• One mechanism is stratification, which is a required
period of cold temperature before germination.
• This is especially important for plants that produce
their fruit in the fall.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• In a natural setting, these types of plants receive the
cold period during the winter months.
• If the seeds of these plants germinated immediately,
they would die with the onset of cold winter
temperatures.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Scarification is a mechanism that prevents
germination until the seed coat has been physically
broken down.
• A tough, protective seed coat prevents diffusion of
both water and oxygen into the seed.
• The seed coat must be broken before germination can
begin.
What is germination and what conditions
are necessary for
germination to occur in a seed?
• Plants have developed many interesting methods to
accomplish this task.
• The seed coat can be damaged or broken by the acid
produced in an animal’s stomach, by soil
microorganisms, by repeated freezing and thawing,
by mechanical stress from being ground in the
gizzard of birds, and by being stepped upon or
chewed.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• Seeds can be planted indoors until they grow into
seedlings that are then transplanted into larger
containers or to a permanent location outdoors.
• This is called
indirect seeding.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• The quality of seed used is very important.
• Seed quality includes both viability and vigor.
– Viability is the ability of seeds to germinate under
optimum conditions.
• Quality seed should have high viability, with a
germination rate of more than 75 percent.
– Vigor is the ability of seeds to germinate under less
than optimal conditions and still produce healthy
seedlings.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• Seeds are stored in cool, dry conditions to maintain
seed viability and vigor.
• Most seed is stored at temperatures around 40°F and
at a relative humidity between 20 and 40 percent.
• No matter how well seed
is stored, its quality
decreases over time.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• Seed is tested for percent
germination and labeled for the
consumer.
• Many state and federal laws
regulate the shipment and sale of
seed.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• Seeds can be planted indoors to increase the length of
the growing season, to increase production, and for
economy of space.
– 1. Seeds may be planted in flats or containers of
germinating medium.
– Medium is the material in which plants are grown.
– Start with clean containers with drainage holes.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
– 2. A good germinating medium is fine textured, has
good water-holding capabilities, is well aerated, and
provides support for the growing plant.
– 3. Sow the seed in rows to reduce the spread of
disease.
• When using flats, the rows should be about 2 inches
apart.
• Plant the seeds close together at the approved depth and,
if recommended, cover the seeds with medium.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
– 4. Label each flat with the seed variety and the date the
seed was sown.
– 5. Maintain the proper temperature.
• The recommended temperature for most seed
germination is between 65° and 80°F.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• 6. Water seed flats lightly from the top using a
sprayer or mister.
• Cover seed flats with poly or glass to maintain
moisture level and humidity.
• Remove the covering once the seeds start to
germinate.
• 7. Since developing seedlings require higher amounts
of oxygen, avoid overwatering the seed flats.
• Over-watering and lack of proper drainage reduce the
oxygen level by filling the air spaces with water.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• Seedlings should be transplanted when the first true
leaves are fully developed.
• 1. When transplanting, care should be used in
handling seedlings.
• Seedlings should be held only by their leaves and not
by their stems.
What factors should be considered to
successfully grow seeds indoors from
seedlings to mature plants?
• 2. Make a hole with a dibble, stick, or forefinger in
the new location.
• Place the seedling in the hole at a depth slightly
below the former depth.
• 3. After transplanting, the seedlings should be
watered and placed in the shade to help prevent
wilting.
• In a few days, move the plants to receive proper light
intensity and water them to promote good root
growth.
What factors determine the success of
plants grown from the
direct seeding method?
• Many flowers, vegetables, and grass seeds are planted
directly into the soil outdoors in their permanent
location.
• In the greenhouse, seeds are sometimes planted right
into the finish pot.
• These methods are called direct seeding.
• Factors that should be considered when direct seeding
are site selection, seedbed preparation, planting date,
planting depth and spacing, and care of the seedlings.
What factors determine the success of
plants grown from the
direct seeding method?
• The site should have sufficient light for the plants to
grow.
• The seedbed should be prepared so that it is loose and
fine textured, allowing for adequate moisture and
aeration.
What factors determine the success of
plants grown from the
direct seeding method?
• The planting date is the date to plant seeds.
• It is determined by the soil temperature required by
the seeds for successful germination.
• Planting date is also
influenced by the time
of maturity, harvest
dates of vegetables, and
peak bloom dates of
flowers.
What factors determine the success of
plants grown from the
direct seeding method?
• Seeds should be sown at recommended planting
depths and spacing.
• A general rule, if the planting depth is unknown, is to
plant seeds at a depth of three to four times their
greatest thickness.
• Spacing between seeds is also important to prevent
overcrowding.
What factors determine the success of
plants grown from the
direct seeding method?
• The seedbed should be kept uniformly moist until the
seedlings are established.
• It is also important to control weeds that compete for
light, nutrients, and water.
Review/Summary
•How does the ability to sexually
produce seeds benefit a plant?
•What is germination and what
conditions are necessary for
germination to occur in a seed?
Review/Summary
•What factors should be
considered to successfully grow
seeds indoors from seedlings to
mature plants?
Review/Summary
•What factors determine the
success of plants grown from the
direct seeding method?