Revue internationale d'écologie méditerranéenne International ...

ecologia mediterranea, tome 29, fascicule 2, 2003, p. 153-164
INTERPRETING GERMINATION RESULTS BASED ON SEED SIZE, MASS AND ECOLOGICAL… ◆
firmed by Zahran (1975) in his studies on Juncus rigidus
seeds that succeeded to germinate at salinity level of 3%
NaCl solution. Accordingly, he reported that the success
or failure of seed germination in saline soil might be an
indicator for the success or failure of seed propagation of
salt tolerant plant in saline soil.
Annuals in desert ecosystems exploit top centimeters
of the soil, and are abundant only when surface moisture
is relatively high. Moisture conditions of the surface
soil vary less spatially than they do temporally (Olsvig-
Whittaker et al., 1983). Hence, spatial heterogeneity of
annuals as a response to local environmental variation
may be relatively low, although countered by biologically
caused patterning. Baker (1972) compared seed weight
of the California flora with moisture availability of each
habitat. He reported that among herbaceous species there
was a significant negative correlation between moisture
availability and seed weight. Westoby et al. (1992) argued
that Baker’s data should be regarded cautiously. However,
in the present study, there was a highly significant correlation
between seed weight, seed size of annuals and the
moisture availability of the natural habitats they collected
from as well as the water supply allowed in the laboratory
experiments. This trend is disagreeing with Baker
(1972). On the other hand, Mazer (1989) working on
Indian Dunes flora reported no significant association
between seed mass and moisture availability. This trend
is also noted in this study where there is no significant
correlation between moisture level and seed mass of the
species collected from xeric and dried habitats, such as
the inland plateau, saline depressions, sand dunes and
rocky cliffs habitats (see values of correlation in table 1).
Similarly, Telenius and Torstensson (1991) did not find
any association between seed weight and moisture availability
in 48 species from the genus Spergularia. However,
the comparative evidence for an association between large
seed mass and dry habitats is quite limited, despite its
general acceptance in the literature. On the other hand,
Guerrero-Campo and Fitter (2001) reported that plant
species with shallow or thin main root had smaller seeds
than plants with fibrous or other adventitious roots. They
concluded that seed size was related to plant height, but
this association was weaker than that between seed size
and root depth. Their results suggested that traditional
ecological explanation do not explain the relation between
seed and adult plant size, and they proposed that deeprooted
plants have large seeds because of allometric and
developmental constraints, which means that only large
seeds can produce the thick roots that can grow rapidly
to depth. The abundant literature that demonstrates the
adaptation of plants to the conditions prevailing in their
habitats showed that natural selection operates, at a very
fine level, genotypes that are most efficient at gathering
the resources of the local habitats (Heslop-Harrison,
1964). The changes in moisture and salinity in different
habitats are reflected in the changes of the produced
progeny (seeds). It seems that with the greater austerity
in available moisture resources most of the plant species
directed to produce small seeds. As stated above larger
seed size is not necessarily associated with larger mass.
It is unlikely that the advantage of large seeds was due to
the ability to emerge from greater depth in the soil, where
soil moisture is more favorable. Experimentation on the
behavior of the studied species should be the next step
in clarifying the mode of that behavior and its relevant
to the habitats.
ACKNOWLEDGMENTS
The author sincerely wishes to thank Dr. Nabil El-
Hadidi, Professor of plant taxonomy and flora & the keeper
of Cairo University Herbarium for the identification
of the plant specimens. The author also acknowledges
the valuable comments of Dr. M. Abdel-Razik & Dr. S.
El-Darier, Professors of plant ecology and Dr. S.Y.
Barakat, emeritus Professor of plant physiology, Faculty
of Science Alexandria University. I wish to thank my
colleague Dr. H. Deif for his kind help in preparing the
photographs.
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