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BASIC HORTICULTURE – NOTES
WEEK #7
SEEDS AND SEED GERMINATION
SEEDS AND SEED GERMINATION
FORMATION OF SEEDS
 SEED production begins in the OVARY with the UNION of
the POLLEN SPERM NUCLEUS and EGG in the OVULE
o FERTILIZATION > ZYGOTE (group of
undifferentiated cells) > EMBRYO (miniature plant)
o APOMIXIS - Occasionally SEEDS are produce with
out FERTILIZATION
 OVULE TISSUE develops to form SEED
 CLONE of parent plant
 SIMULTANEOULY the second POLLEN SPERM nuclei
joins with two haploid nuclei in OVULE to form
ENDOSPERM
o ENDOSPERM nourishes the ZYGOTE
 MONOCOTS - Eventually ENDOSPERM is
main FOOD RESOURCE for an EMBRYO
 DICOTS - COTYLEDONS main FOOD
SOURCE for an EMBRYO
 SURROUNDING the SEED is the SEED COAT
o TOUGH LAYER for PROTECTION
o PREVENTS WATER LOSS
MORPHOLOGY of SEEDS (Show Overhead)
 SEED COAT – tough outer layer
o PROTECTION of embryo
o PREVENTION of water loss
 Mature seed DRIES until about 2.5 % WATER
and then SHED from plant
 PROTECTS from TOTALLY DRYING OUT and
FREEZING temps
Example: Peanut (thin), Coconut (thick)
 HILUM
o SCAR left behind where OVULE (SEED) attached to
OVARY wall
Example: Like belly button is attachment point to
placenta
 MICROPYLE
o Scar left behind where POLLEN TUBE entered
OVULE
ANATOMY of a SEED
 EMBRYO
o MINIATURE PLANT surrounded by FOOD
SOURCE
o EPICOTYL - develops into STEMS, LEAVES, and
FLOWERS
o HYPOCOTYL - the STEM / ROOT transition area
Example: in BEAN the rounded stem that breaks
through the soil surface
o RADICLE - the EMBYONIC ROOT that forms
PRIMARY ROOT
 FOOD SOURCE (FATS, PROTEINS, STARCHES)
o DICOTS – 2 COTYLEDONS for food storage
o MONOCOTS – 1 small COTYLEDON, most food
stored in ENDOSPERM
GERMINATION
 GERMINATION is the INITIATION of ACTIVE
GROWTH by the EMBRYO
 STEPS to GROWTH
o
o
o
o
SEED IMBIBES water
SEED COAT BURSTS as more water is absorbed
Allows O2 in for RESPIRATION
EMBRYO produce HORMONES which ACTIVATE
aleurone CELLS which produce DIGESTIVE
ENZYMES that CONVERT FOOD STORES
 PROTEINS, FAT, STARCHES > SUGARS + O2
> ENERGY
o ENERGY is used to produce new CELLS and
TISSUES for GROWTH and DEVELOPMENT of
EMBRYO
 RADICLE emerges from SEED COAT to grow
downward
 HYPOCOTYL and EPICOTYL pushed upward to
break the SOIL SURFACE and become the
SEEDLING
 ALL SEEDS need to have enough FOOD STORAGE to
reach the LIGHT before it’s gone.
o SMALL SEEDS must be planted SHALLOW
o LARGE SEED can be planted DEEPER, which allows
it to absorb MORE WATER and not dry out
o Usually 3X’s DIAMETER good depth
CONDITIONS for GERMINATION
 3 REQUIREMENTS for GERMINATION
o Seed must be VIABLE or ALIVE
o FAVORABLE ENVIRONMENTAL CONDITIONS
o FAVORABLE INTERNAL CONDITIONS
 VIABILITY
o Ability to GERMINATE is individual to SPECIES and
ENVIRONMENTAL HABITAT
 1-3 years is NORMAL for most SEED
o VIABILITY can be prolonged by:
 COOL temperatures
 LOW HUMIDITY
 LOW O2
Example: Seed storage in a sealed jar with paper
envelopes or silica gel in refrigerator works well
 FAVORABLE ENVIRONMENTAL CONDITIONS
H2O, O2, TEMPERATURE, and sometimes LIGHT
o H2O
 SOFTENS seed coat
 SWELLS TISSUES to rupture seed coat
 ACTIVATES HORMONES and ENZYMES to
aid respiration
 AMOUNT of H2O needed varies from SPECIES
to SPECIES and the NATIVE ENVIRONMENT
that the plant is ADAPTED to
Example: Rice needs saturated soil to germinate
o O2
 Needed for RESPIRATION
 GROWING MEDIUM or SOIL must have O2 in
pores
 AMOUNT of O2 needed varies with SPECIES
and HABITAT plant adapted to
Example: Plants growing in dry, sandy soil need
more O2
Example: Marshy soil plants require less O2
o TEMPERATURE
 affects all CHEMICAL reactions
 H20 absorption
 Movement of HORMONES and FOOD
 ENZYMES reactions
 CELL DIVISION and GROWTH
 RANGE of TEMPERATURE needed varies with
SPECIES
Example: Hardy temperature climate seeds can
germinate at lower temperatures than tender
climate seeds
 INSURES that a plant won’t GERMINATE
and be KILLED by frost
o LIGHT
 SOME plants must be EXPOSED to LIGHT to
GERMINATE
 Usually SMALL SEED
 Usually THIN COATED SEED
 Assures seed near SURFACE of soil and not
BURIED too deep
 Assures enough and right kind of LIGHT to
GROW and PROSPER
Example: Lettuce, Celery, Weeds
 SOME seed must have COMPLETE DARKNESS
to GERMINATE
 Usually LARGE SEED
 Require more WATER ABSORPTION
Example: Garden Peas
 SEED must absorb WATER before it responds to
LIGHT or DARK
 LIGHT is absorbed in seed by PIGMENT called
PHYTOCHROME
 Exists in two forms – P 660 & P 730 nm
 AMOUNT of P 660 or P 730 vacillates with
TYPE of LIGHT PRESENT
o P 660 with LIGHT (red 660) > P 730
 Higher P 730 concentrations
INDUCES GERMINATION
o P 660 with DARK (far red 730) <
P730
 Higher P 660 concentrations
INHIBITS GERMINATION
 Seed SENSES light present by
CONCENTRATION of P 660 and P 730
PRESENT within cells
Example: Under canopy of trees – Red (660)
is absorbed by leaves, more Far Red (730)
light available, which produces more P 660
and inhibits germination in LIGHT requiring
seed
 FAVORABLE INTERNAL CONDITIONS
o If seed is VIABLE and it has FAVORABLE
ENVIRONMENTAL CONDITIONS, but there is LACK
of GERMINATION, we call the seed DORMANT due to
UNFAVORABLE INTERNAL CONDITIONS
o INTERNAL FACTORS causing DORMANCY
 PHYSICAL BARRIERS
 PHYSIOLOGICAL BARRIERS
 CHEMICAL BARRIERS
 PHYSICAL BARRIERS with SEED COAT
 SEED COAT impermeable to H20, O2, or
both
 SEED COAT resists EXPANSION
 SCARIFICATION (or SCARIFY seed coat)
to make small opening or wear down seed
coat
o MECHANICAL
 NICK seed coat with file
 SANDPAPER in drum
 GRAVEL in drum
o CHEMICALLY
 TEMPERATURE
 HOT WATER to soften
 HIGH TEMPS
 SULFURIC ACID
 SIMULATES NATURE
o FREEZING, THAWING, SHIFTING
soils
o FIRE, HOT DESERT areas
o DECOMPOSITION by bacteria / fungi
o DIGESTION by animals
 PHYSIOLOGICAL BARRIERS
 IMMATURE EMBRYO - seed SHED
EARLY
o Needs TIME to ripen before germination
Example: Orchids, Pines, Ranunculas
 MATURE EMBRYO - needs further
ripening called AFTER RIPENING
o STRATIFICATION - application of
MOISTURE and LOW TEMPS
 Or alternate between low and high
temps with moisture
 MIMICS NATURE’S SEASONS
Example: moisten seed and put in
refrigerator to induce germination
 CHEMICAL BARRIERS
 CHEMICAL INHIBITORS in seed coat
o LEACH OUT with WATER
 CHEMICAL INHIBITORS in PULP of
FRUIT
o SOAK and REMOVE pulp, dry seed
Example: Tomatoes, Strawberries
SURVIVAL SIGNIFICANCE of SEED DORMANCY
 DORMANCY ensures that seeds GERMINATE DURING
CONDITIONS that favor MAXIMUM SURVIVAL of the
SEEDLINGS and SPECIES
Example: Desert plants – chemical inhibitors in seed coat,
only germinate after hard rains wash away, ensures enough
water to germinate and grow
Example: Chaparral plants and some Pines – fire allows
germination. Gets rid of competition, adds nutrients, more
sun
 SOME SEEDS of the same SPECIES can VARY in
LENGTH of DORMANCY
o ADVANTAGE to WILD, NATIVE seed and WEEDS
o Allows them to germinate over many seasons
Example: Weeds – “ONE year’s seedin’ is SEVEN
years weedin’.”