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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 curveﬁtting 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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