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

(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