Priming
could be defined as controlling the hydration level within
seeds so that the metabolic activity necessary for germination
can occur but radicle emergence is prevented. Different
physiological activities within the seed occur
 at different
moisture levels (Leopold and Vertucci, 1989; Taylor, 1997).
The last physiological activity in the germination process is
radicle emergence. The initiation of radicle emergence
requires a high seed water content. By limiting seed water
content, all the metabolic steps necessary for germination can
occur without the irreversible act of radicle emergence. Prior
to radicle emergence, the seed is considered desiccation
tolerant, thus the primed seed moisture content can be
decreased by drying. After drying, primed seeds can be stored
untill time of sowing.
Several different priming
methods have been reported to be used commercially. Among
them, liquid or osmotic priming and solid matrix priming
appear to have the greatest following (Khan et al., 1991).
However, the actual techniques and procedures commercially
used in seed priming are proprietary.
The benefits of seed priming
have been well documented in previous review articles
(Bradford, 1986; Khan, 1992; Taylor et al., 1998). For
practical purposes, seeds are primed for the following
reasons:
- to overcome or alleviate
phytochrome-induced dormancy in lettuce and celery,
- to decrease the time
necessary for germination and for subsequent emergence to
occur,
- to improve the stand
uniformity in order to facilitate production management
and enhance uniformity at harvest.
One of the primary benefits of
priming has been the extension of the temperature range at
which a seed can germinate (Valdes and Bradford, 1987; Ellis
and Butcher, 1988). The mechanisms associated with priming
have not yet been fully delineated. Several review articles
have done an excellent job in describing the current state of
knowledge (Taylor and Harman, 1990; Khan, 1992). From a
practical standpoint, priming enables seeds of several species
to germinate and emerge at supra-optimal temperatures. Priming
has also alleviated secondary dormancy mechanisms that can be
imposed if exposure to supra-optimal temperatures lasts too
long (Valdes et al., 1985) or in photo-sensitive lettuce
varieties.
 The other benefit of priming
has been to increase the rate of germination at any particular
temperature. On a practical level, primed seeds emerge from
the soil faster and often more uniformly than non-primed seeds
because of limited adverse environmental exposure. Priming
accomplishes this important development by shortening the lag
or metabolic phase (or phase II in the triphasic water uptake
pattern, Bewley and Black, 1978) in the germination process.
The metabolic phase occurs just after seeds are fully imbibed
and just prior to radicle emergence. Since seeds have already
gone through this phase during priming, germination times in
the field can be reduced by approximately 50% upon subsequent
rehydration. The increase in emergence speed and field
uniformity demonstrated with primed seeds have many practical
benefits:
- emergence occurs before soil
crusting becomes fully detrimental,
- crops can compete more
effectively with weeds, and
- increased control can be exercised
over water usage and scheduling.
Lastly, priming has been
commercially used to eliminate or greatly reduce the amount of
seed-borne fungi and bacteria. Organisms such as Xanthomonas
campestris in Brassica seeds and Septoria in
celery have been shown to be eliminated within seed lots as a
by-product of priming (Mel Bachman, personal communication).
In the case of Xanthomonas campestris in Brassica
sp., zero infection in 50,000 seeds is commonly reported. The
mechanisms responsible for eradication may be linked to the
water potentials that seeds are exposed to during priming,
differential sensitivity to priming salts, and/or differential
sensitivity to oxygen concentrations.
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