Dormancy of Urena lobata L. seeds. I. Development of sulphuric ...

Dormancy of Urena lobata L. seeds. I. Development
of sulphuric acid scarification techniques
P. J. C. HARRIS
Department of Biological Sciences, Njala University College, Sierra Leone (Present address: Department
of Biological Sciences, Coventry (Lanchester) Polytechnic, Priory Street, Coventry, UK.)
SUMMARY
An investigation was undertaken to determine the optimum
conditions fOJovercoming Urena lobata L. seed dormancy
by sulphuric acid scarification. Soaking seeds in 18 M
sulphuric acid for 30-90 min resulted in more than 85 per
cent of viable seeds germinating under laboratory conditions
compared with a maximum of 4.2 per cent of untreated
seeds. A 30-min acid treatment was most effective between
30 and 50 "C, and 60-min at 30 or 40 "c. Concentrations
lower than 18 M were less effective even with extended
exposure. Scarification required a minimum volume of
0.67 dm'/kg (seed). Recycling residual acid was
unsatisfactory owing to a decline in acid efficiency.
Scarification was effective at all times tested during the 4
months following harvest. No significant loss of viability
occurred in scarified seeds stored over the same period.
Original scientific paper. Received 9 Oct 81; revised 25 Apr
86.
Introduction
The development of Urena lobata as a fibre
crop in Sierra Leone has involved primarily the
introduced cultivar, Ex-Mokwa (Das Gupta, 1971,
1973a, 1973b). More recently, attention has been
given to desirable characters of indigenous U
lobata and selections from wild populations have
been introduced into cultivation. While seeds of
the former cultivar exhibit little dormancy under
field conditions, those from indigenous plants
possess a form of dormancy imposed by an
impermeable testa. This has been shown to be the
principal cause ~f1ow percentage field emergence
I Ghana Jnl agrie. SeL 14-19,79-84 (1981-86)
RESUME
HARRIS, P. J. C.: Prefloraison des graines dUrena lobata L.
I. Development des techniques de scarification a I'aide de
I'acide suifitrique. Une etude a ete fait pout determiner des
conditions optimales pour combattre Ie prefloraison par la
scarification a I'aide de I'acide sulfuriqile. Trempant les
graines dans I'acide sulfurique de 18 M de ·concentration
pour 30 a 90 min a donne plus de 85% de cas de germination
au laboratoire, alors que ce pourcentage ne fut que 4.2 % au
maximum pour des graines non-traitees. Un traitement de
30 min avec I'acide etait plus efficace entre 30 et 50 "C,
alors que pour 60 min, iIetait 30 ou 40 "C.Des concentrations
de I'acide moins de 18M etaient moins efficace, meme pour
des temps longues. Un volume minimum de 0.67 dm J de
I'acide/kg du graine etait necessaire pour la scarification. Le
reutilization de I'acide restant n' a pas ete satisfaisant car
cet acide n'est plus tout a fait actif. La scarification etait
efficace pour tous les temps dans les quatre mois apres la
recolte. Aucune porte significante de viabilite n'est apparu
dans des graines scarifiee~ et stockees pendant la meme
periode.
from such seeds (Harris, 1981). Slow or uneven
germination, suggestive of dormancy, has been
reported by Kirkby (1963) and by Crane & Acuna
(1945). Dormancy due to seed coat effects have
been confirmed in U lobata by Horn & Natal Colon
(1942), Juillet(1952) and Gerrard (1955).
Dempsey (1975) has suggested that the process
of removing carpels by rubbing the fruit in sand
acts additionally to improve germination by
mechanical scarificatic;m. Greater success in
overcoming dormancy appears to have been
achieved with the well established technique of
sulphuric acid scarification. Horn & Natal Colon
Accra: National Science & Technology Press I

(1942), Juillet(l952) andDempsey & Baumann
(1970) all report favourably on acid scarification of
U. /obata seeds. However, with the exception of
Juillet (1952) and to a lesser extent Horn & Natal
Colon (1942), little comparative information has
been published with regard to optimum acid
concentration, duration and temperature of
treatment. This study reports a series of experiments
designed to reinvestigate these factors and to
obtain new information on the. quantitative
requirements for acid. The study also investigates
the effect of scarification on seed quality during
storage, with a view to making specific
recommendations applicable or adaptable to local
requirements and conditions.
Materials and methods
Seeds of Urena /obata L. were obtained in
January 1980 from a wild population located at
Taiama Junction in the Southern Province of Sierra
Leone. Fruit, harvested by hand, was sun-dried
and the seeds, with a moisture content of 4-5 per
cent on a dry weight basis, extracted from the
carpels in a large wooden mortar and pestle. Most
experiments were performed soon after harvest on
seeds stored in paper bags until required. Seeds to
be treated at intervals between harvest and normal
field sowing time in May were stored either in
sealed glass containers or in cloth bags. In all cases
the seeds were kept in darkness at room temperature.
All scarification treatments employed sulphuric
acid, obtained as concentrated ('raw') battery acid
and diluted to the required strength with distilled
water. Scarification was carried out in glass beakers
and followed by thorough washing with tap water
and air-drying. Unless specified, an excess volume
of sui phuric acid equivalentto 6 dm 3 /kg (seed) was
used. In one series of experiments the temperature
of the sulphuric acid was controlled (0.5°C) with a
heated water bath and an ice bath. All other
experiments were carried out at a mean laboratory
air temperature of 27.3°C. Under the latter
conrlitio.ns, the temperature of the sulphuric acid
increased by I-2°C above ambient during
scarification.
Eight replicates of30 seeds were obtained for
each treatment and tested for germination in II-cm
petri dishes containing one layer of moist blotting
paper. The numbers of normal seedlings were
counted after 5 days incubation at room temperature.
Germination was expressed as a percentage of
those seeds viable before treatment. Estimates of
initial seed viability varied from 88 to 93 per cent
during the investigation. Seed viability tests were
performed at the same time as germination tests,
with the same number of seeds and replicates.
Viability was tested by soaking seeds in a 10 mg
cm· 3 solution of2, 3, 5-triphenyl tetrazolium chloride
for 24 h in darkness, after removing part ofthe testa
with a razor blade. Seeds were counted as viable if
the embryo stained red or pink. Analysis of variance
was performed to test the significance of differences
between means, and least significant differences
(LSD) were calculated from the pooled standard
deviations.
Results
The response of U. /obata seeds to sulphuric
acid treatments of varying concentrations and
duration are shown in Table I. The percentage of
Effects of Su/phuric Acid Concentration and Duration
of Treatment on the Percentage Germination of Urena
lobata Seeds
Concen- Soaking time (min) LSD
tration 30 60 90 180 P=0.05 P=O.OI
(M)
0 1.4 1.9 1.4 4.0 NS
3 3.3 2.4 2.4 1.5 NS
6 1.9 3.3 2.4 3.5 NS
9 1.9 1.9 1.4 2.0 NS
12 4.3 13.7 24.2 24.8 6.9 9.5
15 41.1 53.7 ,59.6 43.9 13.4 18.3
18 85.1 98.0 96.5 24.9 10.6 14.3
LSD
P=0.05 6.7 6.7 5.0 17.9
P=O.OI 8.9 8.5 6.7 23.8

viable seeds germinating after treatment with
distilled water alone was very low; such seeds
failed to imbibe water. Similarly, concentrations of
sulphuric acid of 9 M or less were ineffective in
breaking dormancy over the range of treatment
times tested. The highest germination percentages
were obtained by treatment with 18 M su'lphuric
acid for30-90min which allowed a rapid imbibition
of water when the seeds were sown. Extendingthe
duration of soaking in 18 M sulphuric to 180 min
caused a subsequent reduction in the percentage
germination. Sulphuric acid at a concentration of
12 or IS M increased germ ination compared with
that obtained with the distilled water treatment, but
failed to induce the high germination percentages
obtained with 18 M acid. In the case oflS Mas well
as 18M sulphuric acid, a 180 min exposure exceeded
the optimum soaking time.
Since the data in Table I were obtained without
temperature control, further experiments were
conducted to determine the influence of
temperature on the effectiveness of sulphuric acid
in overcoming dormancy. Table 2 shows the
percentage germ ination of U. lobata seeds treated
with 18 Msulphuric acid at controlled temperatures
for 30 or 60 min. With both treatment times
germ ination increased with increasing tem"perature,
reaching an optimum at SO°Cwith a30 nlinexposure
and at30°C with a60 min exposure. At It'mreratures
Percentage Germination q( Urena lobata Seeds Treated
with 18 M Sulphuric Acid at Various Temperatures
LSD
P = 0.05
P = 0.01
Soaking time (min)
30 60
33.2
76.7
89.2
94.6
97.8
47.5
9.3
12.4
29.6
79.4
94.2
92.8
51.1
9.9
8. I
10.8
higherthan these optima, gennination was markedly
decreased.
The minimum volume of sulphuric acid
necessary to successfully scarify donnant U. lobata
seeds was 0.67 dm 3 /kg(seed) (Table 3). No
significant improvement could be demonstrated
Percentage Germination of Urena lobata Seeds Treated
with Various Volumes of 18 M Sulphuric Acid at Room
Temperature
LSD
P = 0.05
P = 0.01
Volume of acid
(dmJlkg seed)
0.00
0.) 7
0.33
0.67
1.00
1.33
1.67
1.9
32.5
66.2
&7.0
·90.8
86.6
91.3
8.2
10.8
with higher volume:weight ratios. The possibility
of reducing the volume of sui phuric acid required
by recycling it was examined. Successive batches
of seeds were treated for 30 min in sulphuric acid
initially at a concentrat'ion of 18 M and followed
repeatedly with the recycled acid. The consumption
of the acid and its effectiveness in breaking
dormancy after each treatment were monitored.
The first three batches of seeds were successfulIy
scarified resulting in high gennination percentages.
However, subsequent batches of seeds showed
progressively lower germination percentages,
indicating a decline in the effectiveness of the
sulphuric acid in overcoming dennancy (Table 4).
The results of experiments carried out on seeds
stored before treatment in cloth bags or in sealed
glass containers are shown in Table·S. Scarification
treatments were highly effective in breaking
donnancy at alI times tested. No significant
differences were found between samples scarified
at different times, with respect to their percentage

Percentage Germination of Successive Batches of Urena
lobata Seeds Treated for 30 min with Repeatedly
Recycled Sulphuric Acid
LSD
Initial acid
volume
(dmJ/kg seed)
Volume of
acid consumed
(dmJ/kg seed)
Germination
(%)
I 10.00 1.67 92.4
2 8.33 1.26 93.3
3 7.07 0.87 93.7
4 6.20 0.87 83.4
5 5.33 1.33 64.6
6 4.00 0.87 46.6
7 3.13 0.80 21.5
8 2.33 0.93 16.6
9 1.40 0.60 11.7
10 0.80 0.50 2.7
P = 0.05 7.5
P = 0.01 10.0
Treatment time
(months before
sowing date)
Stored in cloth bags
o I
2
3
4
Untreated
Stored in sealed glass containers
o I
2
3
4
Untreated
Germination ("A,)
Immediately At sowing
after treatment date
97.7
98.2
85.6
91.4
82.5
93.5
100.0
91.4
germmation or viability. Furthermore, no evidence
was obtaIned for loss of viability or ability to
germinate during storage of seeds scarified with
sulphuric acid and returned to either storage
condition.
Discussion
The percentage germination of unscarified seeds
was consistently low, the highest recorded
percentage in this investigation being 4.2 per cent.
The data presented here are thus consistent with
previous statements concerning the need for an
effective dormancy breaking technique for seeds
of this type (Harris, 1981). The results confirm that
scarification with sulphuric acid can be a highly
successful method of overcoming U. lobata seed
dormancy. The most effective treatment tested
was to immerse seeds in 18 M sulphuric acid for
between 30 and 90 min. This conclusion is similar
to those reached by previous workers. Horn &
Natal Colon (1942) increased germination from 22
to 87 per cent by a I-h treatment with 96.5 pe cent
sulphuric acid. This was considerd the optimum
91.9
100.0
94.5
91.1
86.0
4.2
94.5
94.9
95.0
86.5
96.2
0.8
Viability (%)
Immediately At sowing
after treatment date
91.7
90.4
89.6
91.7
95.4
89.6
85.0
91.7
92.9
86.7
91.3
88.8
92.1
90.8
90.4
89.6
91.7
93.8
88.3
91.7
soaking time for hulled seeds and germination was
less with longer or shorter soaking times. Juillet
(1952) suggested a sulphuric acid treatment for

Dormancy of Urena lobata L. seeds
hulledseedsforO.75hat30"Corl hat 12-15°C. Such
treatments increased germination from 30 to 79.5
per cent in the laboratory and from 40 to a maximum
of 65 per cent when used in conjunction with
fungicides on seed sown in compost in a
greenhouse.
Extendingthesoakingtimein 15or I8Msulphuric
acid to 180 min reduced germination. This time was
probably sufficient for acid not only to increa~e
testa permeability but also to reach and damage the
embryo. It appears unlikely that a reduction in the
concentration of acid used could adequately be
compensated for by an extension of the soaking
time. With lower concentrations inherent variation
in testa permeability may result in dormancy being
broken rapidly in some seeds but only slowly in
others. A proportion of seeds would thus be
damaged owing to acid uptake while others
remained unscarified, causing a less than maximum
percentage germination.
Although temperature affects the response of
seeds to sulphuric acid, successful scarification
was achieved over a wide range of temperatures
with a 30 min treatment. At temperatures likely to
be encountered in practice, both 30 and 60 min
treatments are appropriate. Although the
temperature of sulphuric acid was shown to increase
during scarification, control of this environmental
parameter should be unnecessary under normal
field or laboratory conditions.
A minimum volume of sulphuric acid is required
to successfully scarify U. lobata seeds. If the
volume of acid used is too little, even though it may
be sufficient to moisten the entire seed surface,
less than complete scarification is obtained.
Working on the assumptions ofa suggested plant
density for fibre production of 400,000 plants/ha, a
dry seed weight of 12 mg/seed, 100 per cent seed
viability, and a minimum acid requirement of 0.67
dm 3/kg (seed), arequirementof3.22 dmlfhamay be
computed. After appropriate adjustments for the
viability of particular seed batches, required plant
densities, and allowances for wastage, this value
could be used in considering the economics of
scarification with regard to local availability and
cost of sulphuric acid. Experiments in which the
residual sulphuric acid was removed and used to
scarify further batches of seeds proved generally
unsuccessful. The efficiency of the acid declined
rapidly even while the volume present remained
well inexcess of the previously determined minimum.
Furthermore, the average loss of acid during removal
and washing of the seeds exceeded the minimum
volume required for scarification when the acid was
utilized once only.
Loss of viability of U. lobata seeds due to
delayed sowing has been reported by Crane
&Acuna (1945). The present author found no
evidence for viability loss in seeds of U. lobata
indigenous to Sierra Leone with a storage time of
up to 4 months after harvest (Harris, 1981). The data
presented here have further indicate that loss of
viability does not occur when seeds are stored from
one growing season to the next, even if they are
scarified immediately after harvest. There is thus
considerable flexibility in the time at which seeds
may be scarified. This fact could have important
implications if seeds were to be produced and
scarified centrally before distribution to individual
growers.
The results of this study indicate that sulphuric
acid scarification of U. lobata seeds is a highly
effective method of breaking their dormancy. The
technique for scarifying the seeds is relatively
simple and may easily be carried out under normal
working conditions without strict control of
environmental conditions or of the date and duration
of treatment. An exposure to 18 M sulphuric acid
for 30-60 min followed by thorough washing and
air-drying is recommended. A minimum volume of
0.67 dm 3 /kg (seed) should be used and the residual
acid discarded.· Against the advantages of the
technique must be set disadvantages such as the
dangers faced by inexperienced operators handling
concentrated sulphuric acid and the recurrent cost
involved in seed scarification. Thus, while efforts
should be directed towards developing mechanical
and low cost treatments and to selecting nondormant
seeds, suIphuric acid scarification has a

84 P. J. C. Harris (1986) Ghana Jnl agric. Sci. 14-19, 79-84
~iM;hcant role to play :n U.lobata cultivatIon. DasGupta, D. K. (I 973b) Potential for commercial fibre
Acknowledgement
The author wishes to express his gratitude to the
University of Sierra Leone Research Fund for
financing the study, and to Mr S. A. Harris for his
technical assistance.
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