crops. Priming or controlled hydration refers to conditioning seeds in an aerated solution with a high so-
lute content, which keeps the seed in a partially hydrated state [47]. Polyethylene glycol (PEG), an inert
compound, is often used, although some systems use salt solutions of various compositions (Table 4).
Primed seeds may be sown moist, dried, or even stored for later use.
Seeds treated with osmotic solutions ranging from 1.0 to about 2.0 mPa water potentials may
germinate more rapidly and uniformly under a wider range of temperatures than untreated seeds. The wa-
ter potential of the priming solution, priming temperature, and priming duration are all important [54] if
radicle elongation is to be prevented while at the same time allowing most other germination processes to
proceed.
Priming sometimes improves germination of aged seeds [55]. Reinvigoration of aged seeds during
the priming process is associated with partial reversal of some lipid peroxidation [56]. However, priming
of nonaged seeds ages them faster than untreated counterparts [57].
Osmoconditioning does not affect stage I water uptake (imbibition) because the priming solutions
have much higher water potentials than the water potential of the colloid-like seed tissue. However, the
stage II processes (active metabolism and hydrolysis) occur during the priming treatment. Thus, osmo-
conditioning enables the seed to absorb enough water to become metabolically active and accumulate re-
serves of sugars, amino acids, proteins, and other substances required for germination. The water poten-
tial of the priming solution is insufficient for visible germination to occur.
The primed seeds germinate uniformly and rapidly once the osmotic stress is relieved and the final
phase of seed hydration occurs. The water potential of the osmoconditioning solution varies among os-
motica and species (Table 4). Osmotic priming can substitute for the chilling requirement in certain
species [58].
A more recent improvement of the seed priming technique, referred to as matriconditioning, involves
the use of a protective gel or colloidal agent with a high water absorptive property instead of an osmotic
solution [59,60]. Matriconditioning may be better suited than osmoconditioning for treating large
amounts of seeds.
Seed priming, either osmoconditioning or matriconditioning, may be integrated with use of growth
regulators and with fluid drilling (pregerminated seeds suspended in a protective gel) to improve plant
emergence and performance under field conditions. Different gels are used and growth regulators, fertil-
izers, and pesticides are incorporated into the gels in attempts to increase the effectiveness of the tech-
nique.112 CHONG ET AL.TABLE 4 Examples of Successful Seed Osmoconditioning Treatments for Selected Species
Osmoconditioning Osmotica
Temp. Duration Amount (g per Estimated water
Species (C) (days) Chemical per kg of H 2 O) potential (mPa)a Reference
Beta vulgaris
Sugar beet 15 7 PEGb 8000 302 1.22 48
Daucus carota
Carrot 15 28 K 3 PO 4 KNO 3 21.65 1.5 49
20.6
Daucus carota
Carrot 15 14 K 2 HPO 4 KNO 3 18.28 1.69 49
21.1
Allium cepa
Onion 15 14 PEG 8000 342 1.55 50
Apium graveolens
Celery 15 14 PEG 8000 273 1.0 51
Lycopersicum esculentum
Tomato 15 14 K 2 HPO 4 KNO 3 15.67 1.0 49
11.92
Petroselinum crispum
Parsley 15 21 PEG 8000 296 1.17 52
aWater potential estimates for PEG 8000 from Ref. 53.
bPEG, polyethylene glycol.