Seed priming is a term used to describe any number of postharvest, presowing hydration treatments whose purpose is to improve seedling establishment (Taylor and Harman, 1990). Reports across a collection of species as diverse as sweet corn [Zea mays ‘Saccharata’ L. (Parera and Cantliffe, 1994)], tomato [Lycopersicon esculentum Mill (Alvarado and Bradford, 1989)], parsley [Petroselinum crispum (P. Mill.) Nyman ex A.W. Hill (Pill and Kilian, 2000)], and warm-season grasses (Bush et al., 2000) have documented that priming increases the rate of germination over a wide range of temperatures. Priming prevents the induction of high-temperature dormancy in both crisphead (Valdes et al., 1985) and romaine (Cantliffe et al., 1981) lettuce (Lactuca sativa L.) types. Other benefits of priming include increasing the effectiveness of beneficial organisms in preventing seedling diseases (Taylor et al., 1985) and eliminating seedborne bacteria from Brassica species (Hill, 1999).
A reported disadvantage to priming is shorter seed longevity. Sh-2 sweet corn seed primed at 20 °C exhibited a lower germination percentage after 3 months of storage than the nonprimed control seed (Chiu et al., 2002). Primed tomato seeds exhibited delayed germination and a lower mean germination when stored at 30 °C for 6 months as compared with the control (Argerich et al., 1989). In lettuce, any priming protocol that improved seed germination rate decreased longevity faster than the nonprimed control seed under controlled deterioration conditions (Tarquis and Bradford, 1992). Even under milder storage conditions (45 °C and 50% relative humidity), primed lettuce seeds exhibited slower and less synchronous germination than the nonprimed seeds in as little as 14 d (Hacisalihoglu et al., 1999).
The seed viability equation (Ellis and Roberts, 1980) used to describe orthodox seed longevity in relation to seed moisture content (MC) and temperature (T) is:
where ν is the probit of percent viability after a period of p (days) in storage at a given MC and T (°C), Ki is an index of the initial seed quality of the lot, and σ is defined as the standard deviation of individual seed lifespans and varies with storage conditions according to the following equation:
The constants KE, Cw, CH, and CQ are species constants used to quantify the relative effects of seed moisture (Cw) and temperature (CH and CQ) on longevity. The KE constant provides a measure of the sensitivity of a species to the effects of temperature and moisture on seed longevity (Roberts, 1986).
This equation has been shown to be valid for a wide range of plant species and storage conditions, including lettuce (Kraak and Vos, 1987). The advantage of this equation in predicting lettuce seed aging is that different temperature and MC values can be inserted to determine how v (probit of percent viability) varies with storage duration (p). Different reciprocal inputs (i.e., increased T along with decreased MC) can be selected that would predict the same rate of deterioration.
The purpose of this study was to test whether reciprocal T and MC environments predicted by the viability equation to result in the same rate of deterioration in nonprimed seeds would result in the same relative rate of deterioration, albeit a more rapid one, for primed lettuce seeds. The specific constants proposed by Kraak and Vos (1987) for predicting lettuce seed longevity were developed using nonprimed seeds. Because priming causes a shorter shelf life, the use of these constants in the viability equation would not be expected to predict accurately the longevity of primed lettuce seeds. However, deviations from this expectation might reveal what aspects of seed longevity are altered by priming.
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Sun, W.Q. , Liang, Y. , Huang, S. & Fu, J. 2003 Biopolymer volume change and water clustering function of primed Vigna radiata seeds Seed Sci. Res. 13 287 302
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