Further Evidence That the Genebank Standards
for Drying Orthodox Seeds May Not Be Optimal
for Subsequent Seed Longevity
Katherine J. Whitehouse,1,* Olorunnisola F. Owoborode,2 Olufemi O. Adebayo,2
Olaniyi A. Oyatomi,3 Amudalat B. Olaniyan,2 Michael T. Abberton,3 and Fiona R. Hay1,**
Maximizing seed longevity is important for genebanks to efficiently manage their accessions, reducing the
frequency of costly regeneration cycles and the loss of genetic integrity. Research on rice seeds has shown that
subsequent longevity in air-dry storage can be improved by drying seeds, which are metabolically active at harvest
(moisture contents above a critical value close to 16.5%), for an initial period at a higher temperature (40C–60C)
than that currently recommended by the current genebank standards (5C–20C). The aim of this study was to test
whether similar benefits could be achieved in two legume species—cowpea and soya bean—by drying freshly
harvested seeds, from two separate harvests, at 40C and 35% relative humidity, for up to 8 days before
equilibrium drying in a drying room (17C and 15% relative humidity). Improvements in longevity were observed
in three of the four accessions of soya bean, with the greatest improvement generally occurring after the maximum
duration (8 days) at the higher temperature. However, of the five accessions of cowpea, only seeds of TVu-9698
and TVu-13209 from the first harvest, and of TVu-13193 from the second harvest, showed an improvement in
longevity compared with drying following the standard protocol. A negative effect of high-temperature drying was
also observed in one accession of cowpea, TVu-11980, but only in seeds harvested later in the season, 13 weeks
after planting. This research not only provides evidence of the potential benefits of drying orthodox seeds at an
alternative, higher, temperature instead of at the conventional lower temperature, before long-term storage, but
also raises awareness of how genebanks can improve the management of their accessions.
Keywords: seed longevity, genebank, seed drying, soya bean, cowpea
Introduction
The long-term preservation of the genetic diversityof orthodox species can be ensured by storing their
seeds at a low temperature (-20C) and moisture content
(MC) (3%–7%) in genebanks. Breeders rely on the genetic
resources to produce more high yielding varieties and/or
improve their resistance to a wider range of biotic/abiotic
stresses.1 Therefore, it is critical that genebanks effectively
manage accessions, by monitoring their viability at regular
intervals and regenerating them when germination falls.2–5
Recommendations for the management of genebank ac-
cessions emphasize the importance of initial seed drying
to extend the subsequent storage longevity.2,3,4,6 It has been
reported that there is an upper temperature limit for safe
drying, which varies between species and depends on the
MC of the seeds—as seeds dry, the maximum safe drying
temperature increases.7 Consequently, the values of safe
drying temperatures for seed drying reported in the literature
are not consistent. For example, it has been claimed that a
safe drying temperature for rice seeds is 48.9C and 60C
when seeds are at an MC >20% and <15%, respectively,8
1T.T. Chang Genetic Resources Center, International Rice Research Institute, Los Ban˜os, Philippines.
2Department of Agronomy, University of Ibadan, Ibadan, Nigeria.
3Genetic Resources Center, International Institute of Tropical Agriculture, Ibadan, Nigeria.
*Current address: Australian Grains Genebank, Agriculture Victoria Research, Department of Economic Development, Jobs, Transport
and Resources, Horsham, Australia.
**Current address: Department of Agroecology, Aarhus University, Flakkebjerg, Slagelse, Denmark.
ª Katherine J. Whitehouse et al., 2018; Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of
the Creative Commons License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
BIOPRESERVATION AND BIOBANKING
Volume 16, Number 5, 2018
Mary Ann Liebert, Inc.
DOI: 10.1089/bio.2018.0026
327
whereas onion seeds are particularly vulnerable, and are
recommended not to be dried at temperatures exceeding
20.1C when seeds are at an MC >20%.9 More generally, a
cooler limit of 45C and 35C has been suggested when
drying cereal and vegetable seeds, respectively.10
In 1994, The Genebank Standards were published and
recommended to dry seeds at 10C–20C and 10%–15%
relative humidity (RH).6 More recently, these drying stan-
dards were modified, combining a lower temperature (5C–
20C) and broader humidity (10%–25% RH) range.2 It is
important to note that these standards were developed based
on their suitability to dry mature seeds of a very diverse range
of species (all crops and wild relatives with orthodox seeds,
from across the globe) to a low MC for storage and, therefore,
are neither species specific nor dependant on initial seed
moisture. However, as mature seeds at high temperatures are
likely to be more sensitive to damage,7,11 especially during
the later stages of drying when evaporative cooling can no
longer suppress seed temperature,3 a low temperature com-
bined with a low humidity was adopted.2,6
Evidence from previous studies on rice, both cultivated12–15
and wild,16 suggests that these conditions, in particular the
lower drying temperature of the modified standards, are not
optimum for subsequent rice seed longevity when seeds are
harvested at an MC close to 16.5%.12–14 Similarly, research on
pea17 and soya bean18 has shown that the physiological
quality (vigor and viability) of seeds can be maintained fol-
lowing drying at 40C, but declines at temperatures ‡50C.
The aim of this study was to evaluate whether similar
benefits to longevity (as seen in rice) could be achieved in
seeds of cowpea and soya bean (two of the mandate crops
conserved at the International Institute of Tropical Agriculture
[IITA]) when they are dried for an initial period at a higher
temperature (40C) before equilibrium drying at a cooler
temperature (17C which complies with the current genebank
standards). Based on the response seen in rice,12–16 the aim
was to dry seeds as close to 45C as possible; however, due to
equipment constraints, a temperature of 40C was used.
Materials and Methods
Plant material
Seeds of four soya bean accessions (TGm-22, TGm-891,
TGm-1013, and TGm-1014) and five cowpea accessions
(TVu-8851,19 TVu-9698,20 TVu-11980,21 TVu-13193; and
TVu-13209), of varying seed coat thickness, were sampled
from the active collection of the IITA genebank. Seeds from
both crops were grown according to the normal regeneration
procedure at IITA, with standard production practices and
plant protection measures.22 Seeds were sown in two 100m2
plots on November 10, 2016, and harvested, when seeds had
reached an MC between 13% and 18% (*12 and 13 weeks
after planting), on two separate occasions on January 31, 2017
(Harvest A) and February 6, 2017 (Harvest B). For each of the
harvests, seeds were randomly sampled across the two plots.
Immediately after each harvest, a small subsample of pods,
from each accession, enough to fill the precalibrated AgriPro
6095 MC analyzer (SINAR), was hand-threshed and the
seed harvest MC (fresh weight basis) determined. The freshly
harvested pods from each accession were bagged in bulk and
transferred to the fumigation chamber (25C–27C and 50%
RH) where they were exposed to 57% aluminum phosphide
(Phostoxin) for*24 hours. Following fumigation, the MC of
a sample of seeds from threshed pods of each accession was
measured, as before, before discarding.
Seed drying
For each accession · harvest, with the exception of ac-
cession TVu-9698 from harvest B, the remaining fumigated
pods were divided into five (equating to the five drying
treatments) 1 kg (cowpea) or 1.4 kg (soya bean) samples and
placed into mesh bags. Due to the low yield of accession
TVu-9698, there were insufficient seeds to carry out two full
sets (harvests A and B) of drying treatments, and therefore,
only two out of the four high-temperature treatments (days 2
and 8 of drying), plus the drying room (DR) control, were
carried out for harvest B. A sample from each accession was
immediately placed into the DR and maintained at 17C/
15% RH for 7 days (intact pod drying) before measuring
seed MC, following the same method as before. Pods were
then threshed and the extracted seeds were given a second
24-hour cycle of fumigation before returning to the DR for
7 days, to reach equilibrium (seed drying). The remaining
four samples per accession (or two for accession TVu-9698
from harvest B), instead of immediate drying in the DR,
underwent an initial period of high-temperature drying in a
chamber maintained at 40C and 35% RH (intact pod dry-
ing). After 2, 4, 6 and 8 days, a sample was removed and
seed MC determined, as before. The remaining pods from
this sample were transferred to the DR for 7 days (intact pod
drying) before measuring the MC once again. The pods were
then threshed and seeds fumigated for 24 hours before final
drying in the DR (seed drying).
After all seed samples (control and high-temperature treat-
ments) had undergone a total of 14 days in the DR, the final
MC was determined. The seed lots were then sent for manual
cleaning where any empty, damaged, and/or diseased seeds
were removed. Clean seeds were then sealed inside labeled
aluminum foil packets (accession · harvest · treatment
combination) and stored in the medium-term storage facility
(5C) for 7 days, until the storage experiments began.
Seed storage
Seed lots (accession · harvest· treatment combination)
were removed from the medium-term storage facility and
equilibrated at room temperature before opening. Hard
coated seeds (accessions TVu-9698, TVu-11980, and TGm-
891) were scarified using a surgical blade to allow the up-
take of moisture. Each sample was then transferred to a
cloth bag and placed over water in a Percival incubator set at
25C until they had reached 60% RH (equating to 11.1%
and 9.0% MC for cowpea and soya bean, respectively
[calculated using the Seed Information Database; SID23]).
The MC was estimated daily by monitoring the change in
weight. Once the desired weight had been achieved, the
seeds were sealed inside aluminum foil packets and left to
equilibrate at room temperature for 24 hours to ensure even
moisture distribution.
The seeds from each packet were divided into either 19
(cowpea) or 18 (soya bean), 15 g subsamples and sealed
inside smaller aluminum foil packets. A subsample was
immediately used to estimate initial ability to germinate,
before experimental storage, and the remaining subsamples
328 WHITEHOUSE ET AL.
were placed in a Percival incubator at 45C for a maximum
of 42 and 72 days for cowpea and soya bean, respectively. A
sample was removed at regular intervals during storage for
germination testing.
Seed germination
The 60 seeds from each packet were subdivided for
sowing onto Seedburo K-22 germination paper and wetted
with distilled water, in plastic germination boxes or germi-
nation plates (20 seeds per box/plate). Cowpea seeds were
mixed with a small amount of Mancozeb powder (antifungal
agent) before being sown. Seeds were then transferred to an
incubation room at 25C–30C and 68% RH for 10 days
before the total number of germinated seeds was recorded.
Statistical analyses
For each accession · treatment combination, change in
the ability to germinate after different periods of experi-
mental storage was analyzed by probit analysis, using
GenStat for Windows, Version 18 (VSN International Ltd.,
Hemel Hempsted, United Kingdom), thereby fitting the
following viability equation24:
v¼Ki p=rð Þ, (1)
where v is the viability (NED) after p days in storage (45C/
60% RH), Ki is the initial viability (NED), and r (days) is
the standard deviation of the normal distribution of seed deaths
in time. The time for viability to decline to 50% ( p50) is a
product of Ki and r ( p50=Ki·r), and was used as a measure
of longevity. For those seed lots (accession·harvest·drying
treatment) that showed a reduced initial viability, the ‘‘control
mortality’’ parameter (‘‘immunity’’ in GenStat) was included
in the probit analysis to estimate the proportion of ‘‘non-
responding’’ seeds within the seed lot.25 Probit analysis was
carried out for all seed lots within an accession simulta-
neously, fitting the full model (different estimates for each
parameter) and reduced models (one or more parameter
constrained to the same value for different seed lots). An
approximate F-test was used to determine the best model.
Results
Seed drying
The harvest MC of four out of the five cowpea accessions,
with the exception of accession TVu-11980, was higher
when seeds were harvested later in the season (harvest B), at
13 weeks after planting, compared with when seeds were
harvested a week earlier (harvest A) (Table 1). The harvest
MC of accessions ranged between 10.7% and 13.3% in
harvest A and between 7.6% and 17.2% in harvest B. Ac-
cessions TVu-9698 and TVu-11980 showed the highest and
the lowest MC, respectively, in both harvests. All seed lots
lost moisture over time, irrespective of the timing of harvest
and/or drying treatment, and reached a final MC between
6% and 8.7% following 14 days in the DR. Generally, seed
lots that underwent an initial period of drying at 40C/35%
RH lost the most moisture during the first 2 days, followed
by a gradual decline until equilibrium was reached. Seed
lots from harvest A reached their lowest MC after 6 days of
drying at 40C/35% RH compared with seed lots from
harvest B, which reached their lowest MC after 8 days of
drying.
After the cowpea seeds were transferred to the DR, the
MC of most seed lots, although they fluctuated slightly,
generally reached their lowest recorded MC after 14 days
(Table 1). Furthermore, in general, seeds that were exposed
to an initial period of high-temperature drying reached a
lower final MC (after 14 days in the DR) compared with
when seeds were immediately placed in the DR. However,
seeds of accession TVu-11980 from both harvests and seeds
of accession TVu-13191 and TVu-13209 from harvest B
regained moisture in the DR, which led them to be at a
higher final MC than what was achieved after 8 days at 40C.
The harvest MC varied between accessions of soya bean,
with the accession with the largest seeds (TGm-1014)
showing the highest MC in both harvests (26.8% and 23.1%
in harvest A and harvest B, respectively) and the accession
with the hardest seed coat (TGm-891) showing the lowest
MC (11.8%), at least in harvest A (Table 1). With the ex-
ception of accession TGm-1013, seeds harvested earlier in
the season (harvest A), 12 weeks after planting, showed a
higher MC compared with seeds harvested a week later
(harvest B)—the opposite to what was observed in cowpea.
However, as in cowpea, soya bean seeds that were dried for
an initial period at 40C/35% RH lost the most moisture
during the first 2 days of drying and, as expected, seeds that
were at a higher MC at harvest took longer to reach their
lowest recorded MC (after 6 days of drying at 40C) com-
pared with lower MC seeds; they reached their lowest MC
after only 4 days. Furthermore, the seed lots from harvest B,
with the exception of accession TGm-22, appeared to regain
moisture after being transferred for final drying in the DR
compared with seed lots from harvest A, which all contin-
ued to dry in the DR, and reached their lowest recorded MC
after 14 days.
For all the soya bean accessions, from both harvests, seeds
that underwent an initial period of high-temperature drying
before being transferred for final drying in the DR reached a
lower final MC (after 14 days of drying in the DR) compared
with seeds that were immediately placed in the DR (Table 1).
However, in most seed lots, DR-dried seeds appeared to
regain moisture during drying in the DR, with their lowest
recorded MC generally being after only 7 days of drying.
Cowpea seed longevity
Seed longevity varied between accessions of cowpea and,
in some cases, between treatments within each accession
(Fig. 1; Table 2). Although dormancy was not observed,
there were many instances where drying, either in the DR or
at the higher temperature, reduced the proportion of re-
sponding seeds within the population (proportion of nonre-
sponders; Table 2). This asymmetry in the survival curve
did not appear to be linked to any particular drying treat-
ment. The longevity ( p50) of seeds when dried following the
standard protocol at IITA (DR) ranged between 17.2 and
23.7 days and between 18.6 and 25.7 days for harvests A
and B, respectively, with seeds harvested later in the season
generally showing the higher estimates in longevity for each
accession.
An improvement in longevity when drying seeds for an
initial period at 40C/35% RH compared with solely drying
in the DR was only seen in accessions TVu-9698 and TVu-
GENEBANK STANDARDS FOR DRYING ORTHODOX SEEDS 329
13209, from harvest A, which showed the greatest value
of p50 after 2 and 8 days, respectively, and in accession
TVu-13193, from harvest B, which showed the greatest
value of p50 after 6 days of drying, and was maintained
thereafter (Fig. 1; Table 2). The remaining accessions from
each harvest, other than accession TVu-11980 from har-
vest B, which showed an overall negative effect of high-
temperature drying on subsequent seed longevity, drying for
an initial period at 40C/35% RH did not improve the lon-
gevity compared with drying in the DR throughout (i.e., Ki
and s could be constrained [p < 0.05] for 1 or more of the
high-temperature treatments (2–8 days) and the DR). Out of
these six accessions · harvest, with the exception of TVu-
13193 from harvest A and TVu-9698 from harvest B where
there was no significant difference in the longevity between
any of the drying treatments, the response of seeds to high-
temperature drying was variable depending on the duration
of drying; generally, the seeds reached the same longevity,
as achieved when drying in the DR, after 2 or 4 days of
drying and declined thereafter (Table 2). Furthermore,
there did not appear to be any relationship between
the duration of high-temperature drying, which produced the
lowest estimates of p50, and its effect on Ki, r, and/or the
proportion of non-responders (Table 2). To conclude, on
average (7 out of the 10 seed samples tested), drying cowpea
seeds for an initial period at a high temperature did not
improve their subsequent longevity compared with drying
following the standard protocol at IITA.
Soya bean seed longevity
As observed in cowpea, the longevity of soya bean seeds
varied both between accessions and between harvests, with
higher estimates of DR p50 seen in seeds from harvest
A compared with seeds from harvest B (Fig. 2; Table 3).
The accessions showing the greatest longevity ( p50) fol-
lowing drying in the DR were TGm-1014 and TGm-1013
for harvests A and B, respectively. Differences in longevity
Table 1. Moisture Content (% Fresh Weight) of Seeds from Five Accessions of Cowpea
and Four Accessions of Soya Bean Harvested on January 31, 2017 (Harvest A), and February 6, 2017
(Harvest B), Which Were Either Immediately Dried After Harvest in the Genebank Drying Room
(17C/15% Relative Humidity) or Initially Dried at 40C/35% Relative Humidity for 2, 4, 6,
or 8 Days Before Equilibrium Drying (Total 14 Days) in the Drying Room
Cowpea
Harvest
TVu-8851 TVu-9698a TVu-11980a TVu-13193 TVu-13209
A B A B A B A B A B
Harvest MC 12.0 15.9 13.3 17.2 10.7 7.6 12.2 13.1 12.5 15.0
High-temperature drying
40C/35% RH (2d) 9.8 10.2 6.7 6.7 9.0 9.7 8.9 8.5 9.3 9.1
40C/35% RH (4d) 9.4 6.8 8.1 — 10.8 6.0 8.6 7.8 8.5 9.7
40C/35% RH (6d) 7.5 8.2 6.3 — 6.4 7.3 7.5 6.7 7.6 7.7
40C/35% RH (8d) 7.5 8.0 6.4 6.0 6.3 6.0 7.5 6.0 7.6 7.3
/DR (7d) 7.4 6.9 6.5 6.1 7.1 7.6 7.2 7.5 7.5 7.8
/DR (14d) 7.5 7.6 6.0 6.0 7.0 8.1 7.4 7.0 7.3 7.9
Control
DR (7d) 8.7 10.4 6.8 8.2 9.3 8.9 8.4 8.0 9.6 8.9
DR (14d) 8.0 7.3 6.0 7.4 8.7 7.5 8.6 7.1 7.8 8.2
Soya bean
Harvest
TGm-22 TGm-891a TGm-1013 TGm-1014
A B A B A B A B
Harvest MC 17.6 15.0 11.8 15.3 13.7 12.9 26.8 23.1
High-temperature drying
40C/35% RH (2d) 8.6 7.0 8.5 7.5 9.5 10.2 14.8 8.3
40C/35% RH (4d) 8.7 6.5 8.6 6.2 9.7 7.7 8.7 6.4
40C/35% RH (6d) 8.5 7.5 8.4 7.2 9.3 7.4 8.6 6.6
40C/35% RH (8d) 8.6 7.8 8.5 6.9 9.6 6.6 8.6 7.4
/DR (7d) 7.7 7.5 7.5 7.3 7.9 7.6 7.0 7.7
/DR (14d) 7.6 7.4 7.9 7.6 7.7 7.7 7.9 7.9
Control
DR (7d) 7.1 7.7 7.0 8.4 7.1 9.1 6.9 9.3
DR (14d) 8.5 8.0 8.4 8.1 8.1 8.1 8.3 7.5
The MC was determined, nondestructively, using a seed MC analyzer (SINAR AGRIPO). Due to the low yield of accession TVu-9698,
there were insufficient seeds from harvest B to carry out the full set of drying treatments. Therefore, only two out of the four high-
temperature treatments (2 and 8d), plus the DR control, were carried out.
aHard-coated seeds.
d, days; DR, drying room; MC, moisture content.
330 WHITEHOUSE ET AL.
(resulting from differences in Ki, r, or both (Table 3)) were
also observed, in some cases, among the different drying
treatments within accessions. Despite these differences, with
the exception of accession TGm-22, at least one of the high-
temperature treatments (period of drying at 40C/35% RH)
resulted in an improvement in longevity ( p50) compared
with drying in the DR. In all instances, with the exception of
accession TGm-1014 from harvest B, the highest estimates
of p50 were observed in seed lots dried for 8 days at 40C/
35% RH. Although drying at this higher temperature, for up
FIG. 1. Ability to germinate
when tested during experi-
mental storage at 45C and
60% RH for seeds of five ac-
cessions of cowpea harvested
on January 31, 2017 (Harvest
A), and February 6, 2017
(Harvest B), which were im-
mediately dried after harvest in
the DR (17C/15% RH) or
initially at 40C/35%RH for 2,
4, 6, or 8 days before equilib-
rium drying in the dryroom.
The viability model24 was fit-
ted to the data with or without
parameter constraints; the re-
sults shown are for the model
with the fewest parameters that
could be fitted without a sig-
nificant increase in the residual
deviance compared with the
best-fit model (Table 2). The
dashed lines correspond to
treatments that could be con-
strained to a single curve
( p> 0.05). For those seed lots
that showed a reduced initial
viability (<100%), an addi-
tional parameter was applied
to determine the proportion of
responding seeds within the
population.25 DR, dryroom;
RH, relative humidity.
GENEBANK STANDARDS FOR DRYING ORTHODOX SEEDS 331
Table 2. Results of Fitting the Viability Model for Samples of Cowpea Harvested on January 31, 2017
(Harvest A), and February 6, 2017 (Harvest B),Which Were Immediately Dried After Harvest in the Dryroom
(17C/15% Relative Humidity) or Initially at 40C/35% Relative Humidity for 2, 4, 6, or 8 Days
Before Equilibrium Drying in the Dryroom
Treatment Model
Proportion of
nonresponders
(s.e.)
Ki (s.e.)
(NED)
s-1 (s.e.)
(days-1)
p50 (s.e.)
(days)
Harvest A
TVu-8851 Ki and r constrained within
40C/35% RH (2d) and (4d)
/ DR and DR
40C/35% RH (2d)/ DR 0.050 (0.020) 2.20 (0.17) 0.10 (0.01) 21.8 (0.61)
40C/35% RH (4d)/ DR 0.050 (0.020) 2.20 (0.17) 0.10 (0.01) 21.8 (0.61)
40C/35% RH (6d)/ DR — 1.50 (0.12) 0.09 (0.01) 17.4 (0.73)
40C/35% RH (8d)/ DR 0.254 (0.046) 2.34 (0.39) 0.12 (0.01) 19.3 (1.32)
DR 0.050 (0.020) 2.20 (0.17) 0.10 (0.01) 21.8 (0.61)
TVu-9698 Ki and r constrained within
40C/35% RH (4d)/ DR
and DR and r constrained
within 40C/35% RH (2d) and
(6d)/ DR
40C/35% RH (2d)/ DR 0.211 (0.034) 2.76 (0.30) 0.11 (0.01) 25.9 (1.10)
40C/35% RH (4d)/ DR 0.101 (0.049) 1.52 (0.24) 0.08 (0.01) 19.5 (1.42)
40C/35% RH (6d)/ DR 0.136 (0.044) 1.89 (0.25) 0.11 (0.01) 17.7 (1.12)
40C/35% RH (8d)/ DR — 1.04 (0.11) 0.07 (0.00) 15.1 (0.85)
DR 0.101 (0.049) 1.52 (0.24) 0.08 (0.01) 19.5 (1.42)
TVu-11980 Ki and r constrained within
40C/35% RH (2d) and (6d)
/ DR and DR
40C/35% RH (2d)/ DR 0.065 (0.021) 2.28 (0.19) 0.10 (0.01) 22.6 (0.61)
40C/35% RH (4d)/ DR — 1.65 (0.13) 0.09 (0.01) 17.7 (0.72)
40C/35% RH (6d)/ DR 0.065 (0.021) 2.28 (0.19) 0.10 (0.01) 22.6 (0.61)
40C/35% RH (8d)/ DR 0.095 (0.068) 1.72 (0.42) 0.11 (0.01) 16.1 (2.10)
DR 0.065 (0.021) 2.28 (0.19) 0.10 (0.01) 22.6 (0.61)
TVu-13193 Ki and r constrained within all
treatments40C/35% RH (2d)/ DR 0.258 (0.017) 2.42 (0.18) 0.10 (0.01) 23.7 (0.59)
40C/35% RH (4d)/ DR
40C/35% RH (6d)/ DR
40C/35% RH (8d)/ DR
DR
TVu-13209
40C/35% RH (2d)/ DR Ki and r constrained within
40C/35% RH (2d), (4d), and
(6d)/ DR
0.040 (0.013) 2.54 (0.17) 0.14 (0.01) 18.7 (0.38)
40C/35% RH (4d)/ DR 0.040 (0.013) 2.54 (0.17) 0.14 (0.01) 18.7 (0.38)
40C/35% RH (6d)/ DR 0.040 (0.013) 2.54 (0.17) 0.14 (0.01) 18.7 (0.38)
40C/35% RH (8d)/ DR 0.175 (0.057) 1.99 (0.39) 0.10 (0.01) 20.6 (1.70)
DR — 1.30 (0.10) 0.08 (0.00) 17.2 (0.76)
Harvest B
TVu-8851 Ki and r constrained within
40C/35% RH (4d) and (6d)
/ DR and DR
40C/35% RH (2d)/ DR — 0.97 (0.10) 0.06 (0.00) 15.1 (0.94)
40C/35% RH (4d)/ DR 0.156 (0.028) 2.19 (0.26) 0.12 (0.01) 18.6 (0.74)
40C/35% RH (6d)/ DR 0.156 (0.028) 2.19 (0.26) 0.12 (0.01) 18.6 (0.74)
40C/35% RH (8d)/ DR — 1.21 (0.13) 0.08 (0.01) 14.4 (0.83)
DR 2.19 (0.26) 0.12 (0.01) 18.6 (0.74)
TVu-9698 Ki and r constrained within all
treatments40C/35% RH (2d)/ DR 0.124 (0.043) 1.75 (0.22) 0.09 (0.01) 20.1 (1.15)
40C/35% RH (8d)/ DR
DR
TVu-11980 Ki and r constrained within
40C/35% RH (2d), (4d), and
(6d)/ DR
40C/35% RH (2d)/ DR — 1.64 (0.07) 0.08 (0.00) 21.2 (0.45)
40C/35% RH (4d)/ DR — 1.64 (0.07) 0.08 (0.00) 21.2 (0.45)
40C/35% RH (6d)/ DR — 1.64 (0.07) 0.08 (0.00) 21.2 (0.45)
40C/35% RH (8d)/ DR — 1.96 (0.14) 0.11 (0.01) 18.2 (0.68)
DR 0.149 (0.030) 2.57 (0.36) 0.10 (0.01) 25.7 (1.06)
TVu-13193 Ki and r constrained within
40C/35% RH (2d) and (4d)
/ DR and DR and within
40C/35% RH (6d) and (8d)
40C/35% RH (2d)/ DR 0.176 (0.035) 1.91 (0.22) 0.09 (0.01) 21.6 (1.08)
40C/35% RH (4d)/ DR 0.176 (0.035) 1.91 (0.22) 0.09 (0.01) 21.6 (1.08)
40C/35% RH (6d)/ DR 0.290 (0.031) 2.26 (0.32) 0.10 (0.01) 21.9 (1.25)
40C/35% RH (8d)/ DR 0.290 (0.031) 2.26 (0.32) 0.10 (0.01) 21.9 (1.25)
DR 0.176 (0.035) 1.91 (0.22) 0.09 (0.01) 21.6 (1.08)
TVu-13209 Ki and r constrained within 40C
(4d) and (8d)/ DR and DR40C (2d)/ DR — 1.76 (0.11) 0.11 (0.01) 15.8 (0.62)
40C (4d)/ DR 0.105 (0.025) 2.32 (0.20) 0.11 (0.01) 21.8 (0.69)
40C (6d)/ DR 0.114 (0.028) 2.67 (0.44) 0.15 (0.02) 17.4 (0.86)
40C (8d)/ DR 0.105 (0.025) 2.32 (0.20) 0.11 (0.01) 21.8 (0.69)
DR 0.105 (0.025) 2.32 (0.20) 0.11 (0.01) 21.8 (0.69)
Source: Ellis and Roberts, 1980.24
The parameters shown are for the simplest model that could be fitted without a significant ( p < 0.05) increase in the residual deviance
compared with the best fit model. Due to the low yield of accession TVu-9698, there were insufficient seeds from harvest B to carry out the
full set of drying treatments. Therefore, storage experiments were only carried out for two of the four high-temperature treatments (2 and
8 days), plus the DR control. For those samples that showed a reduced initial viability, the ‘‘control mortality’’ was applied to determine the
proportion of responding seeds within the population.13
s.e., standard error.
332
to 8 days, resulted in lower values of Ki (initial viability)
compared with the DR control, seeds lost viability at a
slower rate. In both harvests, accession TGm-1014 showed
the greatest longevity following drying at 40C/35% RH
after 8 (harvest A) and 6 days (harvest B), with values of p50
estimated at 76 and 66.6 days for harvests A and B, re-
spectively. Accession TGm-22, from both harvests, was the
only accession where drying for an initial period at 40C/
35% RH did not improve subsequent storage longevity
compared with the DR control. In harvest A, there was no
significant difference ( p < 0.05) in the longevity between
seeds dried at 40C/35% RH for 4 and 6 days and the DR
control, with seeds dried for 8 days showing the lowest
value of Ki and estimate of p50 (Table 3). On the other hand,
in harvest B, all drying treatments could be constrained to a
common line, that is, there was no significant difference in
longevity between any of the drying treatments.
Discussion
Cowpea and soya bean are valuable and economically
important agricultural commodities, helping to sustain millions
FIG. 2. Ability to germinate
when tested during experi-
mental storage at 45C and
60% RH for seeds of four ac-
cessions of soya bean har-
vested on January 31, 2017
(Harvest A), and February 6,
2017 (Harvest B), which were
immediately dried after har-
vest in the DR (17C/15%
RH) or initially at 40C/35%
RH for 2, 4, 6, or 8 days before
equilibrium drying in the dry-
room. The viability model21
was fitted to the data with or
without parameter constraints;
the results shown are for the
model with the fewest param-
eters that could be fitted with-
out a significant increase in the
residual deviance compared
with the best-fit model (Ta-
ble 3). The dashed lines cor-
respond to treatments that
could be constrained to a sin-
gle curve ( p> 0.05). For those
seed lots that showed a reduced
initial viability (<100%), an
additional parameter was ap-
plied to determine the propor-
tion of responding seeds within
the population.26
GENEBANK STANDARDS FOR DRYING ORTHODOX SEEDS 333
Table 3. Results of Fitting the Viability Model for Samples of Soya Bean Harvested on January 31, 2017
(Harvest A), and February 6, 2017 (Harvest B),Which Were Immediately Dried After Harvest in the Dryroom
(17C/15% Relative Humidity) or Initially at 40C/35% Relative Humidity for 2, 4, 6, or 8 Days
before Equilibrium Drying in the Dryroom
Treatment Model
Proportion of
nonresponders
(s.e.)
Ki (s.e.)
(NED)
s-1 (s.e.)
(days-1)
p50 (s.e.)
(days)
Harvest A
TGm-22 Ki and r
-1 constrained within
40C/35% RH (4d) and
(6d)/ DR and DR and
r-1 constrained within
40C/35% RH (2d) and
(8d)/ DR
40C/35% RH (2d)/ DR — 5.59 (0.39) 0.19 (0.01) 29.0 (0.55)
40C/35% RH (4d)/ DR — 5.36 (0.29) 0.14 (0.01) 39.4 (0.39)
40C/35% RH (6d)/ DR — 5.36 (0.29) 0.14 (0.01) 39.4 (0.39)
40C/35% RH (8d)/ DR — 4.65 (0.33) 0.19 (0.01) 24.1 (0.54)
DR — 5.36 (0.29) 0.14 (0.01) 39.4 (0.39)
TGm-891 Ki and r
-1 constrained within
40C/35% RH (2d) and
(6d)/ DR and within
40C/35% RH (4d) and
(8d)/ DR
40C/35% RH (2d)/ DR — 2.64 (0.11) 0.05 (0.00) 50.5 (0.89)
40C/35% RH (4d)/ DR — 2.17 (0.09) 0.04 (0.00) 60.9 (1.40)
40C/35% RH (6d)/ DR — 2.64 (0.11) 0.05 (0.00) 50.5 (0.89)
40C/35% RH (8d)/ DR — 2.17 (0.09) 0.04 (0.00) 60.9 (1.40)
DR — 5.18 (0.45) 0.14 (0.01) 37.0 (0.66)
TGm-1013 Ki and r
-1 constrained within
40C/35% RH (6d)/ DR
and DR and r-1 constrained
within 40C/35% RH (2d),
(4d), and (8d)/ DR
40C/35% RH (2d)/ DR 0.153 (0.022) 3.87 (0.66) 0.08 (0.01) 50.8 (2.71)
40C/35% RH (4d)/ DR 0.153 (0.023) 4.68 (0.82) 0.08 (0.01) 61.4 (2.47)
40C/35% RH (6d)/ DR 0.058 (0.009) 6.11 (0.57) 0.15 (0.01) 40.1 (0.73)
40C/35% RH (8d)/ DR 0.135 (0.017) 5.45 (0.92) 0.08 (0.01) 71.5 (3.11)
DR 0.058 (0.009) 6.11 (0.57) 0.15 (0.01) 40.1 (0.73)
TGm-1014 Ki and r
-1 constrained within
40C/35% RH (2d)/ DR
and DR and r-1 constrained
within 40C/35% RH (4d)
and (8d)/ DR
40C/35% RH (2d)/ DR 0.028 (0.007) 7.30 (0.85) 0.16 (0.02) 44.9 (0.62)
40C/35% RH (4d)/ DR 0.090 (0.014) 3.86 (0.43) 0.07 (0.01) 59.1 (1.36)
40C/35% RH (6d)/ DR 0.085 (0.011) 10.22 (1.36) 0.17 (0.02) 58.6 (0.73)
40C/35% RH (8d)/ DR 0.078 (0.01) 4.96 (0.49) 0.07 (0.01) 76.0 (2.22)
DR 0.028 (0.007) 7.30 (0.85) 0.16 (0.02) 44.9 (0.62)
Harvest B
TGm-22 Ki and r
-1 constrained within
all treatments40C/35% RH (2d)/ DR — 3.79 (0.14) 0.14 (0.01) 26.7 (0.29)
40C/35% RH (4d)/ DR —
40C/35% RH (6d)/ DR —
40C/35% RH (8d)/ DR —
DR —
TGm-891 Ki and r
-1 constrained within
40C/35% RH (2d), (4d),
and (8d)/ DR and r-1
constrained within 40C/
35% RH (6d)/ DR
and DR
40C/35% RH (2d)/ DR — 2.22 (0.07) 0.04 (0.00) 56.0 (0.92)
40C/35% RH (4d)/ DR — 2.22 (0.07) 0.04 (0.00) 56.0 (0.92)
40C/35% RH (6d)/ DR 3.49 (0.01) 0.12 (0.01) 28.7 (0.10)
40C/35% RH (8d)/ DR — 2.22 (0.07) 0.04 (0.00) 56.0 (0.92)
DR 4.31 (0.28) 0.12 (0.01) 35.6 (0.70)
TGm-1013 Ki and r
-1 constrained within
40C/35% RH (2d), (4d),
and (8d)/ DR
40C/35% RH (2d)/ DR — 3.12 (0.13) 0.06 (0.00) 55.8 (0.74)
40C/35% RH (4d)/ DR — 3.12 (0.13) 0.06 (0.00) 55.8 (0.74)
40C/35% RH (6d)/ DR 0.015 (0.006) 6.61 (0.69) 0.15 (0.02) 44.1 (0.82)
40C/35% RH (8d)/ DR — 3.12 (0.13) 0.06 (0.00) 55.8 (0.74)
DR 0.010 (0.009) 3.34 (0.36) 0.09 (0.01) 37.2 (1.03)
TGm-1014 r-1 constrained within 40C/
35% RH (2d) and (4d)/
DR
40C/35% RH (2d)/ DR 0.122 (0.015) 8.79 (1.03) 0.15 (0.02) 56.9 (0.88)
40C/35% RH (4d)/ DR 0.012 (0.006) 6.28 (0.66) 0.15 (0.02) 40.6 (1.20)
40C/35% RH (6d)/ DR — 2.88 (0.21) 0.04 (0.00) 66.6 (2.13)
40C/35% RH (8d)/ DR 0.042 (0.013) 3.41 (0.38) 0.06 (0.01) 54.1 (1.52)
DR — 3.72 (0.30) 0.13 (0.01) 29.7 (0.69)
Ellis and Roberts, 1980.24
The parameters shown are for the simplest model that could be fitted without a significant ( p < 0.05) increase in the residual deviance
compared with the best fit model. For those samples that showed a reduced initial viability, the ‘‘control mortality’’ was applied to
determine the proportion of responding seeds within the population.25
334
of people within Africa and Asia. The IITA Genetic Re-
sources Center currently holds the largest and most diverse
collection of cowpea (>15,000 accessions), and nearly 5000
accessions of soya bean, which are made available for food
and agricultural research across the globe. Seed longevity is
an important agronomic factor, which, if not maximized, can
lead to loss in seed vigor and viability during storage, thus
negatively impacting seedling establishment and the overall
yield. It is important for genebanks to predict the longevity
of their accessions as an unexpected loss in viability will
incur more frequent regeneration, which is costly economi-
cally and risky genetically.4,26,27 This is particularly prob-
lematic for species that are inherently short-lived as they
already have short regeneration intervals. In comparison with
cowpea (oil content of 1%),28 which has a predicted lon-
gevity (r) of 301 years under medium-term storage (6.1%
MC and 5C) at IITA (estimated using the SID),23 soya bean
(oil content of 21%29) seeds are relatively short-lived with an
estimated longevity of only 92 years based on the viability
constants given in Ellis et al.30 and Dickie et al.31.
As in cultivated rice, longevity in legumes increases
during seed development, particularly during seed matura-
tion.32,33 Previous research on rice has provided evidence
that seed quality15 and subsequent seed longevity12–14,16 can be
improved ex planta by drying seeds immediately after harvest,
which are still within the maturation drying phase of seed de-
velopment (MCs close to 16.5%),12–14 for an initial period at a
high temperature (45C–60C) before final drying at a lower
temperature and humidity, as recommended in the Genebank
Standards.2 The results of this study have major implications for
genebanks globally as it has shown that such benefits are not
limited to rice, and potentially, a two-stage drying procedure
could also improve storage longevity of other orthodox species.
In general, cowpea accessions did not show the same,
consistent, beneficial response to high-temperature drying as
observed in soya bean, with only three seed lots (from ac-
cessions TVu-9698, TVu-13209, and TVu-13193) showing
an increase in longevity, all after different durations of
drying, compared with the DR control (Fig. 2 and Table 3).
Furthermore, despite this positive response, the improve-
ment in longevity was relatively low, with an improvement
in longevity by 33%, 20%, and 1% for accessions TVu-
9698, TVu-13209, and TVu-13193, respectively. In soya
bean, however, high-temperature drying has the potential to
improve the longevity of accessions by more than 100%, as
the value of p50 in seeds of accession TGm-1014 more than
doubled following 6 days of drying at 40C/35% RH
(66.6 days) compared with the DR control (29.7 days)
(Table 3). In fact, TGm-22 was the only accession that did
not show any improvement in longevity in response to high-
temperature drying, nor a negative effect, with all other
accessions showing the greatest response following 6 or
8 days of drying at 40C/35% RH. This is consistent with
the previous research carried out by Whitehouse et al.12–14
who also reported, in rice, that while most of the drying
occurred after only 1–2 days at the higher temperature,
longevity continued to improve with drying duration.
Although in most cases in this study, drying at the higher
temperature for 8 days tended to reduce the initial viability of
the seed lot (lower Ki value) compared with the DR control,
the seeds remained viable for longer during storage (lower
value of r-1). This implies that high-temperature exposure
may have been detrimental to short-lived seeds within the
population, which were already on the cusp of losing via-
bility, but was beneficial to longer-lived seeds, enabling them
to continue to accrue longevity. It has recently been reported
in soya bean that an increase in longevity during the matu-
ration drying phase is associated with the increase in tran-
scription factors and gene expression involved in encoding
protective proteins (e.g., heat-shock proteins and chaperones)
and other protective mechanisms such as sugar metabolism
(raffinose family oligosaccharides/sucrose ratio).33 It is likely
that the high temperature is enhancing this stress response,
contributing to, and potentially increasing, the stabilization
of tissues during desiccation and survival in air-dry storage.13
The long-term conservation of germplasm comes at a cost
and, as genebanks largely rely on public funding, managers
are under pressure to reduce their expenditure by optimizing
and increasing the efficiency of their management procedures.
The frequency of both viability monitoring and regeneration,
which incurs the greatest costs, can be reduced by ensuring
seeds are at their maximum possible longevity when placed
into storage.34 Therefore, improving the storage longevity of
seeds has the potential to reduce the number of accessions
regenerated each year, but only if the quantity of seeds
stored is sufficient to provide enough for use before via-
bility drops below the 85% standard, that is, a higher
quality seed lot calls for a larger sample to be stored.35,36
To conclude, the benefits, originally shown in rice, of an
initial period of high-temperature drying on subsequent seed
longevity have now been observed in an independent study on
soya bean and perhaps also for cowpea, although the data are
somewhat variable. Although seed storage experiments (as used
in this study), where seeds are stored at a high temperature and
MC to accelerate the natural aging process, are commonly used
in research to compare the longevity between different seed lots
(within or between species), there is still a debate as to how
comparable these estimates are to those observed under con-
ventional air-dry storage. However, the storage experiments
carried out in this study were conducted at an MC and tem-
perature where the effects of the changes in these variables are
well defined by the seed viability equations.24 Further to this,
the seeds were also hermetically sealed inside aluminum foil
packets to not only limit any fluctuation in MC but also to
restrict the availability of oxygen (which if freely available
can lead to an overestimation in longevity [r] compared
with that predicted by the viability equations37). Therefore,
under such conditions, we expect the improvement in
longevity in response to initial high-temperature drying to
also be apparent in genebank storage.
These results, and those obtained from similar studies, have
major implications for ex situ conservation as it is possible that
other orthodox species may benefit from alternative drying
conditions, especially in regard to temperature. Future re-
search should be carried out to determine whether a two-stage
drying procedure is beneficial to other economically important
crops, especially those with poor storage longevity. In light of
further evidence, it may be necessary, at least for those species
where the benefits of a two-stage drying regime have been
confirmed, that the FAO standards are adapted accordingly.
Acknowledgments
The authors are grateful toResearch forDevelopment, IITA;
Genetic Resources Centre, IITA, Ibadan, Nigeria; Global Crop
Diversity Trust, Bonn, Germany; and International Rice
GENEBANK STANDARDS FOR DRYING ORTHODOX SEEDS 335
Research Institute, Philippines (IRRI). TheGenebankPlatform
provides support for the maintenance of the collection at IITA.
Author Disclosure Statement
No conflicting financial interests exist.
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Address correspondence to:
Katherine J. Whitehouse, PhD
Australian Grains Genebank
Agriculture Victoria Research Division
Department of Economic
Development, Jobs, Transport and Resources
Private Bag 260
Horsham, Victoria 3401
Australia
E-mail: katherine.whitehouse@ecodev.vic.gov.au
336 WHITEHOUSE ET AL.