Revue internationale d'écologie méditerranéenne International ...

A. DJABEUR, M. KAID-HARCHE, D. CÔME, F. CORBINEAU
96
(Gutterman 1996), Phacelia tanacetifolia
(Rollin 1972) and Echinops spinosissimus
(Hammouda and Bakr 1969). In all cases, the
far-red and blue spectral regions are the most
effective in inhibition of germination by white
light (Bewley and Black 1982).
Dormancy of L. spartum dispersal units was
broken during dry storage at room temperature
(Figure 2). This phenomenon, usually
termed “after-ripening”, is well known in
seeds of cereals (Côme et al. 1984; Simpson
1990) and many species from hot arid and
semi-arid regions (Baskin & Baskin 1998).
Germination of after-ripened L. spartum seeds
was less sensitive to environmental factors
(temperature, light and water potential of the
medium) than that of freshly harvested ones
(Figures 2, 3 and 4). Water potential of the
PEG solution that reduced by 50% the germination
of non-dormant dispersal units was
close to – 1 MPa and – 0.4 MPa for seeds collected
in arid highlands and semi-arid littoral
coast, respectively (Figure 5). Similar water
potentials of NaCl solutions have been
reported for other herbaceous perennials from
hot semideserts and deserts (Baskin & Baskin
1998). In natural conditions, L. spartum seeds
cannot germinate during summer when the
rainfall is too low (Table 1), and although germination
tests were not performed after 3-6
months of storage in ambient conditions (20-
25 o C), seeds probably come out of dormancy
at the end of October, but their germination is
delayed until temperature and soil moisture
become non limiting, i.e. in spring (March-
April) when temperature is higher than 10 o C
(minimal temperature for germination, Figure
3). In more extreme desert habitats, there
are plant species, like Hordeum spontaneum
(Gutterman & Nevo 1994; Gutterman et al.
1996) and Schismus arabicus (Gutterman
1996), the seed germination of which did not
occur under a range of optimal temperatures,
unless they were previously exposed for at
least 2 months to high temperatures when
they were dry. This strong dormancy prevents
germination after a late rain in the growing
season, before the summer for 4 to 5 months
without rain (Gutterman 2002).
Although the main characteristics of germination
were similar for seeds collected in arid
highlands and semi-arid littoral coast, seeds
of the polyploid cytotype were more dormant,
i.e. more sensitive to the environmental factors
(Figures 2, 4 and 5). After the same duration
of after-ripening (12 months of dry stor-
age at room temperature), they germinated in
a wider range of temperatures than those collected
in arid highlands (Figure 2), but they
remained more sensitive to light (Figure 4)
and water potential of the medium (Figure 5).
Variability in dormancy intensity resulted
probably from the climatic conditions during
seed development on the mother plants (Bewley
& Black 1982; Baskin & Baskin 1998).
Since the differences in temperature registered
at both collection regions were small
(Table 1), it seems that ambient temperature
was not the cardinal factor affecting seed germinability,
but the rainfall regime recorded in
the same areas during seed formation might
result in differences in the onset of dormancy,
dormancy intensity and length of the dormancy
period. However our results do not
allow to exclude any genetic effects, because
the seed lots used in this study were from two
cytotypes which differ in spikelet morphology
and reproductive capacity (Djabeur et al.
2008).
Numerous studies performed with cereals
(wheat, barley and oat) (Côme et al. 1984)
and grasses (Simpson, 1990) have shown that
dormancy of whole seeds is mainly due to an
inhibitory action of the grain covering structures
(pericarp + seed coat) and lemma when
they remain around the caryopsis. This
inhibitory effect generally results from a
reduced oxygen supply to the embryo through
a fixation of oxygen via a polyphenoloxidase
mediated oxidation of phenolic compounds
present in high amounts in the tissues (Lenoir
et al. 1983; Côme et al. 1984), this barrier to
oxygen diffusion rising with increasing temperature
(Côme et al. 1984; Corbineau &
Côme 1996). In both cytotypes of L. spartum,
lemma is strongly involved in the dormancy
of dispersal units (Figure 2 and Table 3).
However, the embryo itself and/or the grain
covering structures also play a role in this
phenomenon since the isolated caryopses do
not perfectly germinate (Tables 2 and 3). The
inhibitory action of the lemma was almost nil
after 12 months of dry storage, since isolated
caryopses and dispersal units expressed the
same sensitivity to temperature (Figure 3 and
Table 3). Although dispersal units were more
sensitive to light and water potential of the
medium than isolated caryopses, their responsiveness
to these two environmental factors
depended mainly from the caryopsis itself,
and the inhibitory effect of the lemma did not
markedly change during dry storage (Figures
4 and 5, Tables 4 and 5).
ecologia mediterranea – Vol. 36 (1) – 2010