Agriculture Reference
In-Depth Information
Table 5.1
Optimum germination temperature ranges
Seed
Plant type
Temperature range
°
C
Tulip (
Tulipa
)
Mediterranean bulb
10-15
Busy Lizzie (
Impatiens
)
Summer bedding
20-25
Pansy (
Viola
)
Winter bedding
13-16
Sea holly (
Eryngium
)
Sun-tolerant herbaceous
20-25
Solomon's seal (
Polygonatum
)
Shade-tolerant herbaceous
10-15
Grasses
Turf grasses
10-12
Bottlebrush (
Callistemon
)
Sun-tolerant shrub
20-25
Spotted laurel (
Aucuba
)
Shade-tolerant shrub
15-18
Carrot (
Daucus carota
)
Temperate vegetable
10-30
Tomatoes (
Solanum lycopersicum
)
Tropical vegetable
15-30
Aubergine (
Solanum melongena
)
Tropical vegetable
25-30
Seed dormancy
The seedling
As soon as the seed
germinates
, the plant is
vulnerable to damage from cold or drought. Seeds,
therefore, often have
dormancy
mechanisms
which prevent germination occurring when poor
growing conditions prevail, as in the winter in
temperate climates. Dormant seeds are unable to
germinate even though water, oxygen and the correct
temperature are given to them (unlike quiescent
seeds) and they use a variety of mechanisms to delay
germination until conditions become more favourable.
For example, the seed coat may prevent water and
oxygen entering or may be so hard that the embryo is
unable to penetrate it, as in many leguminous plants
such as
Lathyrus odoratus
(sweet pea). Many seeds
(e.g. in
Malus
spp. (apple)) have chemical inhibitors in
their seed coat and embryo which prevent germination
while others are shed with immature embryos
which need a period of time to develop fully before
germination can commence (e.g. in
Fraxinus excelsior
(ash)).
In the wild, such dormancy mechanisms are gradually
overcome through the abrasive action of the soil and
exposure to cold temperature cycles and freezing
and thawing which soften and break down the seed
coat. Inhibitors are washed out by rain or are broken
down chemically, and embryos can mature during a
period of dormancy. Artificial methods of breaking
dormancy mimic these conditions - for example, by
scarification
,
physically damaging the seed coat by
nicking or scratching it, as in
Lathyrus sativus
and
Paeonia
, or by
soaking
,
as in
Camellia.
Stratification
,
in which soaked seeds are stored in warm or cold
temperatures in a moist environment, is also used as
in
Lupinus
,
Aconitum
and
Euonymus.
See 'Seed Dormancy' on the companion website.
The emergence of the plumule above the growing
medium is usually the first occasion that the seedling
is subjected to light. This stimulus prevents rapid
extension of the stem so that it becomes thicker
and stronger, the leaves unfold and become green
in response to light, which enables the seedling to
photosynthesize and so support itself. Seedlings
which are deprived of light show
etiolated growth
with elongated internodes, few leaves or branches
and no chlorophyll. At this stage the seedling is still
very susceptible to attack from pests and damping-off
diseases. The cotyledons are often the first part of the
seed to develop and they may emerge from the testa
and remain in the soil, as in
Prunus persica
(peach)
and
Vicia faba
(broad bean) (
hypogeal
germination),
or be carried with the testa into the air, above the
soil where the cotyledons then expand (
epigeal
germination), as in
Solanum lycopersicum
(tomatoes),
Prunus avium
(cherry) and
Phaseolus vulgaris
(French
bean) (Figure 5.4)
Hypogeal
germination occurs when the
cotyledon(s) remain below the ground inside
the testa.
Epigeal
germination occurs when the
cotyledon(s) emerge above the ground, initially
enclosed in the testa.
Cotyledons in epigeal germination may be called '
seed
leaves
' when they emerge and they usually look quite
different from '
true leaves
'. They turn green and
contribute initially to photosynthesis in the seedling
but the true leaves very quickly unfold and take over
this function (Figure 5.5)
Once the food store in the cotyledons and/or
endosperm has been exhausted, the seedling
