Editorial Seed Science and Technology. Volume 49 Issue 1 (2021)
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Dickie (2021).
Seed Science and Technology, 49, 1, 73-80.
https://doi.org/10.15258/sst.2021.49.1.08
Editorial
Seed Science and Technology. Volume 49 Issue 1 (2021)
John B. Dickie
Collections Department, Royal Botanic Gardens Kew, Wakehurst Place, Ardingly, West Sussex, RH17 6TN, UK
(E-mail: j.dickie@kew.org)
Editorial
At the time of writing, in the UK and Northwest Europe the wood anemone (Anemone
nemorosa L., Ranunculaceae) is in full flower, as Spring gets under way. This common
herbaceous perennial of woodland and hedgerows is a possibly somewhat extreme
example of the challenges facing conservationists striving to preserve seeds of their native
species, in all their diversity, ex situ in conventional seed banks. Its seeds will survive
drying, but only after the achenes, still green when shed, are dispersed from the parent
plant. Ali et al. (2007) observed an initial increase in desiccation tolerance, followed
by a decline; and the proportion of seeds surviving desiccation and their increase in
subsequent longevity coincided with the growth and development of their embryos ex
planta. However, developmental arrest of the embryo was not required for the acquisition
of desiccation tolerance, and continued growth and development of the embryo resulted
in loss of desiccation tolerance, analogous to that seen in orthodox (sensu Roberts, 1973)
seeds upon radicle emergence. Consequently, the window of desiccation tolerance and
maximum potential longevity is comparatively short; and in nature occurs when seeds
are on or close to the soil surface. Though the seeds can be air-dried with care and stored
at sub-zero temperatures, high initial viabilities are difficult to achieve and their storage
lives are short (ten years or less under conventional seed bank conditions); throwing
doubt on the role of conventional seed banking as a means of long, or even medium-term
ex situ conservation for this species. Furthermore, while A. nemorosa seeds, like those
of many species from the Ranunculaceae, might be described as having morphological
or morphophysiological dormancy (Baskin and Baskin, 2014), the observed lack of
developmental arrest is somewhat at odds with most definitions of dormancy.
© 2021 John B. Dickie. This is an open access article distributed in accordance with the Creative Commons
Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build
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73
JOHN B. DICKIE
This overview of the articles in the current issue of Seed Science and Technology is
written very much from the perspective of the curation and management of all aspects of
the quality and availability for use of the seed collections of very diverse wild species.
The Millennium Seed Bank currently holds collections of around 41,000 wild species,
representing 6,140 genera and 344 seed-plant families; almost all collected from the wild
in 190 countries and territories worldwide; and covering all nine major biogeographical
regions and all 35 biodiversity hotspots (U. Liu, pers. comm.; see also Liu et al., 2018).
Whereas most crop seed banks focus on conserving intra specific genetic diversity in one,
or just a few domesticated species, wild species seed banks focus on higher level diversity,
which brings particular challenges (Hay and Probert, 2013; Walters, 2015; Dickie, 2018).
One such challenge is seed longevity, though that is also a problem for crop seed
banks. However, while regeneration of collections is almost routine in crop seed banks, it
is generally avoided in wild species banks, where reliance is placed on maximising initial
viability at collection and banking, together with optimising the storage environment.
While it is estimated that a majority of wild species bear orthodox seeds, even among
those of tropical moist forests (Wyse and Dickie, 2017), little or nothing is known of
the potential longevity in dry-cold storage for seeds the vast majority of species. Seed
storage longevity under the same conditions is known to vary considerably among and
within species (e.g. Colville and Pritchard, 2019) and indications from periodic viability
monitoring of the MSB’s older collections are that around 20% of the seed samples may
have useful storage lives, rather less than the many tens, if not hundreds of years expected
for most species (R. Davies, draft in preparation). Five of the seven papers in this issue
of Seed Science and Technology are concerned with various aspects of seed longevity or
its close associate, vigour.
The other major attribute of many wild species’ seeds, in contrast to many crops, is
the presence of one or more mechanisms for either ‘bet hedging’ or delaying germination
until the new seedling has the best chance of survival and growth; usually referred to
as various kinds of dormancy (e.g., Willis et al., 2014). Overcoming dormancy, both
discovering protocols for its removal and their application to promote germination of
all viable seeds during periodic viability monitoring demand substantial staff resource
in wild species seed banks. Around one-fifth, or more of the staff time dedicated to seed
collection curation at the Millennium Seed Bank is taken up by seed germination and its
application to periodic viability monitoring (Terry et al., 2002) Aspects of seed dormancy
and germination are the subject matter of the two remaining papers in this issue.
The perennial buffelgrass (Cenchrus ciliaris L., Poaceae) is nutritious, as well as
tolerant of drought; and is widely grown in tropical and sub-tropical arid rangelands.
However, its resilience traits also make it a significant invasive species. It is known to be
apomictic and its seeds have characteristically low germination when freshly collected,
relieved somewhat by post-harvest storage. Al-Soqueer et al. (2021) studied the variation
in germination responses among 12 buffelgrass genotypes in Saudi Arabia, both native and
introduced. They investigated the effects of air-dry, laboratory storage duration (12 and 24
months) and a range of five constant incubation temperatures (15, 20, 25, 30 and 35°C)
on germination. The observed variation in response among the different genotypes to the
different storage durations and germination temperature regimes indicated considerable
74
EDITORIAL: SEED SCIENCE AND TECHNOLOGY, VOLUME 49, ISSUE 1 (2021)
genetic variability among buffelgrass genotypes for seed germination, suggesting the
potential for further improvement by selection and breeding. While that variation may
indeed be predominantly genetic, it would be interesting to know the effect of extended
periods of laboratory storage on seed viability and whether there is any trade-off with
improved germinability, which was presumably due to dormancy loss.
Seeds of peanut (Arachis hypogaea L., Fabaceaea) cv. ‘Margenta’ (figure 1) are
reported to have germination as low as 10% at harvest. Interestingly, unlike many annual
crops, but in common with many wild species, especially perennials, peanut has an
indeterminate growth form, resulting in its pods showing varying levels of maturity at
harvest, a possible source of the low germination. Liew et al. (2021) investigated the
relation between maturity stages and seed quality, by considering the flowering pattern
and seed development in relation to the pattern of plant growth and development. The
flowering pattern was normally-distributed, spread over approximately 80 days. Plants
(A) (B)
(C)
Figure 1. Seed development in peanut cv. ‘Margenta’ was studied by Liew et al. (2021). (A) Pegs
penetrating the soil. (B) Seed harvest. (C) Flowering begins 25 days after sowing. Photos taken by and
used with permission of Xi Yun Liew.
75JOHN B. DICKIE
were uprooted and pods and seeds examined over a period from 24-114 days after
anthesis, along with germination capacity. Physiological maturity occurred at 94 DAA,
with germination at 32%, while the highest germination (66%) was recorded sometime
prior to physiological maturity, allowing the authors to make practical recommendations
for harvesting to ensure maximum germinability. While germinability of developed seeds
here is confounded with the onset of dormancy, this example highlights the need for care
in making seed collections from wild species with protracted flowering and seed ripening
periods. Ellis (2019) draws attention to the observed variations in timing of the onset
of maximum seed quality and subsequent seed storage longevity, even among annual
cereal crops, bred for uniformity of harvest date. Those variations in pattern and timing
can only be magnified massively in non-domesticated species growing in natural plant
communities.
As for wild species, seed longevity is unknown for many traditional vegetables,
with consequences for seed bank collection management. Li and van Zonneveld (2021)
assessed the comparative seed longevity of two nutrient-rich leafy vegetables, blood
amaranth (Amaranthus cruentus L.) and edible amaranth (A. tricolor L.) (figure 2)
by comparing them with two reference crops, crookneck squash (Cucurbita moschata
Duchesne) and soya bean (Glycine max L. Merr.). They used accelerated ageing (AA;
41.1°C and 89.9% relative humidity) for 33 days to generate seed survival curves and
calculate the time taken for seed viability to fall to 50% (p50). Seeds of the A. cruentus
and A. tricolor accessions had high p50 values, of 19.5 and 32.8 days respectively; whereas
those of the C. moschata and G. max accessions (at 8.2 and 4.9 days, respectively), were
considerably lower. By comparison with some other vegetable crops, it is suggested that
these leafy amaranths will have a high seed longevity and that properly dried seeds of
amaranth crops can be stored for long periods of time at 5°C in laminated aluminium
foil packets. However, this conclusion should perhaps be treated with some caution. Over
recent years, several authors have pointed out the potential weakness in assuming that the
main cause of death at the cellular level is the same under all seed storage conditions;
and that survival patterns under various rapid ageing conditions in the laboratory will be
the same as those for a given species stored dry and cold in medium- or long-term seed
storage facilities (e.g., Hay et al., 2018; Colville and Pritchard, 2019). However, it is
worth noting that Davies et al. (2020) found reasonable correspondence in the rankings of
comparative seed longevity under both conventional seed bank and laboratory accelerated
ageing conditions for a range of UK native tree species.
Alongside viability and related to it, seed vigour is another component of overall
seed quality; and it has been shown that hydro-priming can restore a certain level of
declining viability through its effect on vigour (e.g. Butler et al., 2009). Lin et al. (2021)
investigated the effects of adjuncts to hydro-priming on the the beneficial effect of
hydro-priming on aged seeds of two hybrid rice cultivars. They used Fe-Zn-NA chelate
(FeSO4+ZnSO4+niacinamide) as the adjunct; and when compared with untreated and
hydro-primed seeds, Fe-Zn-NA chelate priming significantly enhanced germination energy
(GE), germination percentage (GP), germination index (GI), vigour index (VI) and normal
seedling rate (NSR), and increased seedling shoot height (SH) and seedling dry weight
(DW) in both cultivars. However, while the Fe-Zn-NA chelate priming was an effective
76EDITORIAL: SEED SCIENCE AND TECHNOLOGY, VOLUME 49, ISSUE 1 (2021)
method to improve the vigour and viability of hybrid rice aged seeds, they conclude that
more work is needed to establish the duration of the beneficial effect of priming over a
range of storage temperatures.
(A)
(B)
(C)
Figure 2. Seed longevity of genebank accessions of (A) Amaranth cruentus and (B, C) A. tricolor was
studied by Li and van Zonneveld (2021). Photos taken by Tien-hor Wu and used with permission of the
World Vegetable Center.
77JOHN B. DICKIE
Though applications are probably species-specific and difficult to calibrate for a diverse
range of wild species, the release of volatile compounds is correlated with decline in seed
vigour and viability. In certain circumstances head space analysis promises to be useful in
the rapid early detection of viability loss in store seed samples, potentially saving valuable
staff time (e.g. Colville et al. 2012). Kucukhuseyin et al. (2021) measured ethanol release
in relation to seed vigour, as measured by seedling emergence and controlled deterioration
tolerance in multiple seed lots of six vegetable species. They found that ethanol release
was highly negatively correlated with controlled deterioration tolerance in radish, with
seedling emergence in watermelon and to both traits in pepper; but not with any variable
for aubergine and leek. While there is a relation between ethanol release and seed vigour,
their results show it to be quite species-dependent.
Native to eastern Asia, Glehnia littoralis Fr. Schmidt ex Miq. is a member of the
Apiaceae and is used in Chinese traditional medicine. In common with many members
of the carrot family, its seeds, or mericarps, are reported to have morphophysiological
dormancy, commonly broken by cold stratification. Shao et al. (2021) studied the physio
logical and biochemical characteristics of dormancy release in seeds of this species,
by subjecting them to cold stratification (4°C) for periods ranging from 0 to 120 days.
Dormancy release was complete after 120 days; and they observed strong positive
correlation with anti-oxidant activity, H2O2 accumulation and total soluble sugars, while
coumarins decreased significantly over the same period. While those associations appear
strong, more work will be needed to establish direct causal links.
Also in the Apiaceae, the germination of dill seeds (Anethum graveolens L.) is the
subject of research by Bukharov et al. (2021). They studied the thermo-sensitivity of dill
seed germination to temporary (1-5 days) exposures to high temperature (40°C), for seeds
from both primary and secondary umbels. The growth of the embryo had a significant
effect on seed germination (r = 0.976; P < 0.001). Whether the apparent greater sensitivity
to heat stress in seeds from secondary umbels was in fact due to loss of vigour and/
or viability, or whether it could have been due to the induction of secondary dormancy
was not resolved; but subsequent germinability at ‘normal’ temperature (20°C) was less
affected in seeds collected from primary umbels, whose embryos were more developed
than those from secondary umbels. Presumably, the physiological age of the seeds (days
after anthesis or pollination) was less than in those from primary umbels. This study again
highlights the potential for heterogeneity in quality in single bulk seed lots collected from
populations of species with protracted flowering periods.
Whether primarily concerned with aspects of vigour or viability, or with germination
and release from dormancy, all seven papers in this issue of Seed Science and Technology
provide examples of sources of variability in seed quality. Variability and diversity set
big challenges, in both basic research and its application in ex situ conservation (see
Pritchard, 2020); and, as well as genetic variation, the effects of maternal environment are
increasingly recognised as an important component (e.g. Kochanek et al., 2010; Penfield
and MacGregor 2016). There appears to be a tension between adaptive responses to
the natural environment and the need in agriculture to control variability and maximise
production; (Finch-Savage and Bassel, 2016). Yet, deeper understanding of the sources
of variability underlying both seed dormancy and germination on the one hand, and their
78EDITORIAL: SEED SCIENCE AND TECHNOLOGY, VOLUME 49, ISSUE 1 (2021)
viability, vigour and survival on the other,
will enable more effective and efficient ex
situ conservation of wild species as well as
agricultural productivity. At the mechanistic
level, considerable progress is being made
towards fundamental understanding of the
observed variability at different scales (e.g.
Mitchell et al., 2017). Deeper investigation
of dry-cold biological systems will help to
explain the variation in storage longevity of
desiccation tolerant seeds (see Ballesteros
et al., 2020). The deteriorative chemical
reactions that are slowed down by dry-cold
storage involve reactive oxygen species and
oxidative stress; but the same or closely
related chemistry appears to be involved in
the removal of dormancy during warm-dry
after-ripening (e.g. see review by Cahtane
et al., 2017). While it may be in some
ways convenient to separate consideration
of germination and dormancy from seed
John Dickie pictured with some flowering wood
quality and survival, it seems likely that anemones. Picture credit: Kamini Dickie.
many underlying mechanisms at the
cellular, molecular and physico-chemical level are overlapping or shared (e.g. Nguyen
et al., 2012). Meanwhile, will ex situ conservation of Anemone nemorosa through con
ventional seed banking ever be a practical proposition?
References
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79JOHN B. DICKIE
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