Longevity of seeds

Avrami kinetics which describe co operative reactions

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Unformatted text preview: hese viability equations, we needed an independent method that has parameters with physical meaning that can be applied to different storage conditions. Avrami kinetics, which describe co-operative reactions based on visco-elastic properties (Avrami, 1941), have been used recently to describe ageing kinetics in seeds (Walters, 1998; Walters et al., 2004). We have used the Johnson–Mehl –Avrami form of the equation to calculate time coefficients (Williams et al., 1993): lnðN0 =NÞ ¼ ðt=fÞn where t is storage time, and N0/N is the reciprocal of percentage germination. The coefficients f and n describe the shape of the sigmoidal curve, with the abruptness at which germination declines increasing as n increases above 1. When n ¼ 1, the Avrami equation assumes the form of a typical first –order equation. The time coefficient, f, can be treated as an Arrhenius function of temperature (Williams et al., 1993; Walters et al., 2004). For this paper, deterioration time courses of seeds of 276 species stored for more than 30 years were fitted to Avrami equations, and the calculated time for seed germination to decrease to 50% was used to summarize the longevity of the species. Longevity of seedbank-stored seeds Materials and methods Germination data from 276 species of seed accessions, stored at the USDA National Center for Genetic Resources Preservation (formerly National Seed Storage Laboratory; NSSL), are reported here. Data were retrieved by querying the Genetic Resource Information Network (GRIN) database of the USDA National Plant Germplasm System (NPGS) for accessions from particular genera or species that were harvested between 1934 and 1975, and had initial viability greater than 75% (with the exception of seeds from Anethum graveolens, Rheum spp., Zinnia haageana, Lesquerella palmeri, Elymus drobovii and E. hystix, which had poor initial seed quality) (see Table 1). The number of accessions within a species that fit these criteria ranged from 1 (usually for wild relatives of domesticated species) to 8220 (large numbers of accessions were available for important crop species), with a median of three accessions per species (see Table 1). Accessions are from breeding lines, land races and wild populations collected from around the world, where they presumably represent the genetic diversity of an agronomically important plant species. Most of the stored seeds were from plants grown at government plant introduction sites within the US and harvested between 1963 and 1968. Upon receipt at NSSL, seed water content was adjusted to 4–8%, and seeds were placed in screw–cap metal cans, but transferred years later to foil –laminate bags. Seeds were stored at 58C until 1978, when the temperature of the NSSL storage vaults was reduced to 2 188C. Certified seed analysts monitored viability of seed accessions periodically (AOSA, 2003). To obtain an ageing time course that was characteristic for the species, germination data for all accessions of a species were pooled, and the average germination after various storage times w...
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