Agriculture Reference
In-Depth Information
X
water
X
air (oxygen)
X
temperature suitable for the specifi c plant (see
Table 5.1).
Also, for some, an exposure to light, or, for others, an
absence of light (see p. 67). However, there are many
plants that do not allow their viable seed to germinate
until the seed has had its 'dormancy' broken. Typically
dormancy ensures that the seed does not germinate
until favourable germination conditions occur in the
following spring.
Overcoming dormancy
can be
problematic for amateurs and professionals alike; the
commonest types of seed dormancy with the usual
methods of overcoming them are given in Chapter 5.
Anemone nemorosa
,
Calendula
and
Ranunculus
. This
sort of seed germinates best if sown as soon as it
ripens.
For most species such as carrot (
Daucus carota
)
,
French bean, kidney bean, haricot bean, (
Phaseolus
vulgaris
),
Nigella damascena
,
Lobelia erinus
and
Lolium perenne
the seed can be stored dry and
cool (but not in a plastic bag) and then sown at an
appropriate time. Such seed can be harvested,
cleaned and stored following these guidelines
ensuring that
labelling
is in place from start to finish.
See the companion website: www.routledge.com/cw/
adams.
Dried and cleaned seed can be stored in a range
of packaging such as paper bags or plastic boxes.
These should be labelled and kept in a dry and cool
environment; silica gels can be used to help ensure
dryness in the packaging.
The ability of seeds to germinate depends greatly on
their storage factors including:
X
moisture content of seed when put in store
X
temperature in store
X
length of time held in store.
Purchasing seeds
, especially vegetable and flower
seeds, has the advantage of convenience and the
protection of the regulations (Plant Varieties and
Seeds Act 1964). A check of the date should always
be made to ensure that the seeds are from the last
seed harvest. The seeds are usually supplied in foil
packets. Once opened the seeds deteriorate rapidly
so should be sown immediately but, so long as they
are kept dry and cold in a resealed packet, most seeds
will remain viable for a year and some, often the larger
seeds, for many years (see p. 66).
There are difficulties when it comes to seeds from
trees or shrubs because there are fewer regulations to
protect the buyer. In the preparation of seeds for sale,
the drying process used often increases the dormancy
effect (harder coats), adversely affects the energy
reserves and damages the embryo, so reducing seed
viability (see p. 129).
11
Dormancy
is the condition when viable seed
fails to germinate even when all germination
requirements are met.
Once the viable seed is able to germinate, it takes
up water leading to processes resulting in the
emergence of the seedling
(see p. 66):
X
increasing respiration rate
X
breakdown of food store to release energy
X
rapid cell division
X
splitting of the seed coat (testa)
X
emergence of the radicle.
This is followed by the development of the plumule
which reaches the surface by one of two main ways:
epigeal or hypogeal germination (see Figure 5.4).
Harvesting and storing seed
Most organisms including plants cannot survive
drying out, but many seeds are an exception,
which makes them readily storable (e.g.
Phaseolus
vulgaris
and
Nigella damascena
). Once dry, these
orthodox seeds
age slowly and the ageing process
is slowed even more if they are kept cold as well as
dry; sometimes they remain viable for hundreds of
years. However, some seed is intolerant of drying
and cannot be frozen, so storage has to be in cool,
moist conditions and then only for short periods
of time. This
recalcitrant seed
(also known as
'unorthodox') includes
Quercus robur
(oak),
Aesculus
hippocastanum
(horse chestnut),
Castanea sativa
(sweet chestnut), willow, elm, avocado, mango,
rubber, cocoa and many climax species.
Normally immature dry seed will not germinate, but
there are some plants where it is collected when
well developed but still immature and green such as
Growing seeds in containers
The ideal conditions for raising plants from seed can
be achieved in a protected environment such as a
glasshouse or cheaper alternatives such as polythene
tunnels or cold frames.
Containers
Most seeds grown in protected culture are
sown into
containers
(Figure 11.3):
