Longevity of seeds

A second set of seed ageing data was available

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Unformatted text preview: as calculated in 4-year increments (see Fig. 2). In a future paper, time courses for individual accessions will be analysed separately to provide information on intraspecific and harvest year variability. A second set of seed ageing data was available through an experiment initiated in 1977 by Dr Phillip Stanwood, which compares the longevity of 42 species stored at 5, 2 18 and 2 1968C [data for cryogenic storage are reported in Walters et al. (2004) and data for 2 188C are not presented]. Unlike the NPGS collection described above, seeds used in this experiment were commercially available varieties donated by seed companies or breeders. The experiment included 1–21 accessions per species (median ¼ 3 accessions per species), harvested in 1977 or 1978 (see Table 2). Seeds, with water contents ranging from 3.5 to 9.5% (depending on species), were stored in envelopes, plastic vials or cans at a constant temperature (58C), and germination was assessed 3 periodically. As with the NPGS dataset, germination data were averaged among accessions within a species for each monitoring time. Longevity parameters for each species in both datasets were calculated by solving for the coefficients of the Avrami equation using a least squares linear fit of the double logarithmic expression: ln½lnðN 0 =N ފ ¼ n½lnðtimeފ þ y0 , where n is the slope and y0 is the y-intercept. So that the double ln value of N0/N could be calculated, the constant N0 was assigned as 0.5 þ the maximum germination percentage of the averaged time course (usually the average initial germination percentage). Values for N were the average germination percentage for the time (t) in years (solid circles in Fig. 2). The coefficient f and the exponential factor n of the Avrami equation were calculated from the coefficients of the regression line ðf ¼ eð2y0 =nÞ and n ¼ slope). The time for germination to drop to 50% (P50) was determined by interpolating (if the final average germination percentage was , 50%) or extrapolating (if the final average germination percentage was . 50%) the Avrami equation, using the coefficients calculated from fitted time courses. For the few species with initial germination percentages , 70%, P50 was calculated as the time for germination to decrease to half the initial value. Values of P50 for species represented in this study were also calculated using the Ellis viability equations if coefficients were reported in the literature. The water content used for this calculation was the average water content for the species in the NPGS collection; the storage temperature was assigned as 58C; and the initial germination was assigned as the maximum germination percentage of the averaged time course of the NPGS collection (i.e. N0 2 0.5). Longevity in the genebank was also compared to longevity in the soil. Results from Duvel’s buried seed experiment (Toole and Brown, 1946) were used as a measure of seed persistence in the soil, expressed as the s...
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