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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