Agriculture Reference
In-Depth Information
2005). After priming treatments seeds can be dried and still maintain most of
the benefits of priming. Dried, primed seed can be sown using normal seed
drills. Onion and leek seeds have been thoroughly investigated in research and
development studies on priming, and the technique is widely applied to
commercial leek seeds to improve the rate and uniformity of emergence of
direct-sown crops.
Two fundamental approaches to controlling seed hydration have been
used in priming. First, the seeds have been suspended in a medium with a water
potential below that which allows radicle emergence but that allows sufficient
hydration to progress germination up to this point. Secondly, water vapour has
been slowly dosed to bulks of seed to hydrate them to the same level directly.
Oxygenated solutions of polyethylene glycol (PEG) of molecular weight (MW)
6000, with the PEG concentration adjusted to give an osmotic potential of -1.5
MPa, have been used in bioreactors for large-scale priming of onion and leek
seeds (Bujalski
et al.
, 1989; Nienow
et al.
, 1991). Other solutes can also be used
(Gray
et al.
, 1991a).
An alternative hydration medium is a granular, solid material of water
content adjusted to give a water potential -1.5 MPa into which the seeds are
mixed; this is termed solid matrix priming (Taylor
et al.
, 1988). Various clay
and silicaceous minerals have been used as the solid material.
The second approach to hydration is more direct, in that water vapour is
injected into a slowly rotating horizontal drum of seeds in order to hydrate them
progressively over 24 h to a water potential suitable for priming (i.e. -1.5 MPa for
leeks and onions) (Rowse, 1996); this is termed 'drum priming'. The required
amount of hydration is first determined by equilibrating small quantities of seed on
a semi-permeable membrane enclosing a PEG solution of osmotic potential -1.5
Mpa, and then determining the water content of the seed by oven drying. Drum
priming has the advantage for large-scale application that it does not produce a
large bulk of osmotic solution or solid matrix that requires disposal after use. Drum
priming is now widely used for commercial priming of leek and onion seeds in
Britain (Rowse
et al.
, 1999). Both osmotic and drum priming give the same
advancement of seed germination and seedling emergence (Gray
et al.
, 1990a).
Primed seed has accelerated germination and a reduced spread of germi-
nation compared with natural seed (Fig. 4.8a). The differences in germination
rate and its variability are most evident at suboptimal temperatures and water
potentials (see Table 6.2). Finch-Savage and Steckel (1994) described experi-
ments where primed leek seeds sown into a moist seedbed were able to emerge
rapidly and uniformly irrespective of subsequent irrigation, whereas natural seed
was slower and required irrigation either regularly or at the optimum time
following sowing (see Fig. 4.14) for satisfactory emergence. The primed seeds
were able to germinate and emerge before the soil had dried sufficiently to prevent
growth.
Maximum advancement of onion and leek seeds by priming occurs when
they are hydrated to a water content of about 45% of fresh weight. It takes
