Revue internationale d'écologie méditerranéenne International ...

186
◆ A. ABBAD, A. EL-HADRAMI & A. BENCHAABANE
have a greater flexibility for the phenotypic feature than
for the reproductive activity what probably ensures to
species its preservation and its existence under harsh conditions.
The important reduction of the number of female
flowers compared to the male ones observed for the nonirrigated
treatment may be due to the protandry of the
species. While male flower formation occurs in June and
July, female flowers formation essentially occurs during
the month of August just prior to the onset of the rainy
season. Given limited water availability during female
flowers production, it is significant that the proportion of
female flowers was lower in the non-irrigated treatment.
The response of the three studied populations to this
water availability is variable. The population from the
mesic site (Sidi Bouzid) responded most to irrigation
by producing more female flowers while the population
from the Saharan site responded the least. Thus, larger
variation of the floral sex ratio in Sidi Bouzid population
comparatively to the Idelssen and Marrakech populations.
In the light of these data, it appears that the variation of
the floral sex ratio within the studied populations is largely
dependant of the population and consequently is of a
genetic origin. Nevertheless, it remains influenced by the
environmental conditions. This hypothesis is in agree
with what is reported for other plant species (Freeman
et al., 1981; McArthur & Freeman, 1982; McArthur et
al., 1992).
Irrigation also affects the weight of the seeds. In fact,
non-irrigated individuals (of all three populations) produced
seeds of lesser weights than they did in the irrigated
treatment. Ferchichi (2002) observed similar findings
for A. halimus in Tunisia, and the phenomenon is known
for other Atriplex (Ungar, 1987; Mandak, 1999). In our
survey, the variation of the seed weight that is expressed
by a variation of the size appears to be epigenetic and
linked to environmental conditions. This is in agreement
with observations made by other authors for other plant
species (Pitelka et al., 1983; Hendrix, 1984; Mandak,
1999). In contrast, the rate of insect infestation and the
production of immature seeds depend more on the population
with the lowest rate of immature seeds recorded
for the Saharan Idelssen population. That appears like a
selective adaptation of the Saharan ecotype to the harsh
conditions. Indeed, the ecotype face to resources limited
in its natural environment produces less female flowers.
This situation would be compensated by a relatively low
rate of infestation what enables him to ensure a greater
production of mature seeds. The susceptibility of plants to
predation is well known to have a strong genetic component
(Cobb et al., 2002) and thus we are not surprised to
find that the rate of insect infestation had a large genetic
component in this study.
On the phenotypic traits, the irrigation treatment stimulates
the vegetative development with a tendency to
develop the width of the crown, marked by low values of
H max/CW max. Inversely, in non-irrigated treatment,
the growth in height was more important than in width.
The same observation has been made by Pendleton et al.
(1992) for the hermaphroditic individuals of the species
Atriplex canescens in the United States. In our case, this
phenotypic parameter seems to depend solely on the
mode of treatment (irrigation) and represent therefore a
reaction of the species to an edaphical aridity. In agreement
with what was shown for the species Calendula
micrantha (Gardocki et al., 2000), the irrigation increase
significantly the number of inflorescence. However,
only the population of the arid regions (Marrakech and
Idelssen) respond significantly to this irrigation.
This work support the idea of the existence of a very
important polymorphism within the natural population of
Atriplex halimus in Morocco. This polymorphism would
be generated in part by the genetic diversity resulting
from an ecotypic differentiation of the species. In addition,
there is a great evidence that environmental factors
cause changes in phenological and phenotypic characteristics
in this species. The widening of the survey on
several natural populations of A. halimus, in situ as well
as in a common garden, for more years, can bring other
arguments that can sustain some hypotheses advanced in
these preliminary study.
ACKNOWLEDGEMENTS
This work was financially supported by the European
Community within the framework of INCO project
(Program ERB IC18 CT98 0390).
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ecologia mediterranea, tome 29, fascicule 2, 2003