Longevity of seeds

G in seeds of pisum sativum fabaceae germination

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Unformatted text preview: s. None the less, the hypothesis that species have characteristic ageing rates is supported by the time period during which percent germination remains clustered at high levels [e.g. in seeds of Pisum sativum (Fabaceae) germination remained high for about 30 years], and by the time in which most of the accessions show very low germination [e.g. most Arachis hypogaea (Fabaceae) accessions gave , 5% germination after 35 years of storage] (Fig. 2). The time required to reduce germination to 50% (P50) ranged among species from , 13 years [Gossypium lobatum (Malvaceae), Guizotia abyssinica (Asteraceae) and Bromus sitchensis (Poaceae)] to an extrapolated estimate of . 450 years [Trifolium campestre (Fabaceae), Gossypium bickii (Malvaceae), Phleum sp. (Poaceae), Solanum scabrum (nigrum) (Solanaceae) and Vigna radiata (Fabaceae)], and the median P50 was 54 years (Table 1), with a mode between 15 and 30 years (Fig. 1C) There was no significant relationship between initial germination percentage and calculated P50 (r 2 ¼ 0.03). Obviously, P50 values that are greater than the experimental storage duration are less certain because they were derived through extrapolation. However, after a 12–15% reduction in germination occurs, calculated Avrami parameters are fairly consistent when additional time course data are added to the curve-fitting routine (analyses not shown), suggesting a relatively reliable prediction of P50 with only partial deterioration. Values of P50 calculated from time courses with less than 30 years of storage, and less than 5–6% reduction of germination, are particularly suspect and are likely to be overestimates. Significant reductions in germination were only apparent after 30 years of storage in many species [e.g. Raphanus sativus (Brassicaceae), Pisum sativum (Fabaceae), Melilotus alba (Fabaceae) and B. vulgaris (Chenopodiaceae), Fig. 2], which explains the paucity of reliable estimates of seed longevity under refrigeration, since these conditions were rarely used before the 1960s. Avrami parameters describe the shape of the calculated time course and the rapidity with which germination is reduced. For example, the exponential factor for Melilotus alba is 1 (Table 1), resulting in a typical exponential decay curve (Fig. 2). As exponential factors increase to 1.6 (R. sativus), 1.7 [C. melo (Cucurbitaceae)], 2.0 (P. sativum), 3.1 (B. vulgaris) and 4.4 (A. hypogaea), there is a sharper downturn of germination in the cataclysmic decay phase of the time course. Generally, P50 decreases as the exponential factor (slope in the regression analyses) increases and as f decreases ðf ¼ eð2y0 =nÞ Þ. The average value for the exponential factor among species stored at NCGRP was 1.8. Values reported in another study for different accessions of lettuce (Lactuca sativa) seeds stored between 2 1 and 108C ranged from 2 to 3 (Walters et al., 2004), which is consistent with the current findings, considering that lettuce seeds tend to age more rapidly than those of most other spe...
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