Revue internationale d'écologie méditerranéenne International ...

156
◆ L. M. M. BIDAK
conditions should be a function of seed reserve and size
(Solbrig et al., 1977).
The present study is initiated to achieve two main
objectives:
(i) to test the hypothesis that large seed size provides
an advantage for seedlings establishing in conditions of
low soil moisture content. A range of species of different
seed sizes and growth forms collected from different
habitats and geographical regions are used to test the
hypothesis;
(ii) to evaluate the germination efficiency of the studied
species, in order to have a better knowledge on seed
ecology of these species.
MATERIALS AND METHODS
The studied species
The seeds of sixty-six species (36 annuals and 30
perennials) belonging to 27 families representing different
habitats and life forms (table 1) were collected for
the present study. The study species were identified following
Täckholm (1974) and the nomenclature updated
according to Boulos (1995).
Soil analysis
Three soil samples were collected from each habitat.
These samples were pooled, air dried and passed through
a 2 mm sieve to eliminate the gravel and debris and used
for the determination of texture, water holding capacity,
electric conductivity and pH. Soil texture was determined
by the Hydrometer method and soil water extracts of 1:5
were prepared and used in the determination of electric
conductivity (EC) and pH by an electrical conductivity
meter (LF 65) and a glass electrode pH meter. All these
procedures were followed Allen et al. (1974). Soil description
of the studied habitats is provided in table 2.
Seed measurements
The dry seed mass was taken as the reserve mass of
seed (i.e. the embryo, endosperm, seed coat, excluding
seed dispersal structure like pappus). This provides the
best measure of the amount of available resources to
the developing seedlings (Westoby & Leishman, 1994).
Seed mass (mg) was the average of 20 air-dried seeds.
Seeds smaller than 0.1mg were weighed in 10 groups of
twenty seeds each. In most species seed length (L) and
seed width (W) were measured by the micrometer and
the multiplication of “L x W” was considered to represent
the size of the seed in the present study.
Germination experiments
At first, some seeds have been treated to promote
germination and/ or to break dormancy by incubation
at 30-35 o C, cold incubation, dipping in dilute H 2SO 4 or
scarification prior to planting. The different treatments
applied to break dormancy or promote germinations of
the seeds of some of the studied species are mentioned
in table 3. Emergence of radicle was taken as the criterion
of successful germination. As species differ in their
speed of germination, seeds were first left to germinate
in Petri dishes in the laboratory. At varying times prior
to planting to asses whether they need pretreatment.
Germination was carried out with moist filter paper
and 12-hours days at 27 o C. however, seeds used in
the germination experiments were not pretreated except
those seeds of Juncus acutus, Bidens bipinnata, Carduus
getulus, Calligonum comosum, Kickxia aegyptiaca, Peganum
harmala, Polygonum equisetiforme, Roemeria hybrida and
Forsskalea tenacissima.
In preliminary experiments the soil collected from the
sand dunes was mixed with potting compost (John Innes
no. 2 compost) by a ratio of 1:3 weight/weight in plastic
pots (each of 25 cm diameter and 10 cm height) resulting
a mixture which had adequate drainage to improve
soil and seed germination. Later on all the experiments
were done with natural sandy soil collected from sand
dunes to investigate the effect of different water regimes
on germination and survival. All seeds were sowed at the
same depth (so that no seed was at more favorable soil
moisture level than the other) in the laboratory under normal
condition of light and temperature. Emergence and
survival of seedlings were monitored daily until 18 days
after planting, where mortality has ceased (see table 4).
Preliminary experiments have been done to determine
the water field capacity of the soil. All pots were kept
moist on the first two days, then two water regimes were
applied by the rate of 100 ml/pot/day; representing full
water treatment (favorable moisture), and 50 ml/pot/day:
representing half water treatment (low moisture availability).
Each treatment was represented by 4 replicates.
RESULTS
Of the main physiographic factors taken into consideration
by the present study was the presence of gravel near
ecologia mediterranea, tome 29, fascicule 2, 2003