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ADEL DHIEF, MUSTAPHA GORAI, SAMIRA ASCHI-SMITI, MOHAMED NEFFATI 24 Table 1 – Month and year of seed collection, seed size and seed dry mass of three Calligonum species. Species for 30 min (47, 44 and 43% for C. comosum, C. azel and C. arich, respectively). Seeds of C. arich had significantly lower seed mortality than those of C. comosum and C. azel relative to the control and other treatments (Figure 5). C. arich had larger seeds (9.30 mm ± 1.28) than C. comosum and C. azel (6.59 mm ± 0.70 and 7.52 mm ± 0.74) respectively, but C. comosum had a greater seed dry mass (47.50 mg ± 9.42) than C. arich and C. azel (39.50 ± 12.03 mg and 44.50 ± 12.44 mg) respectively (Table 1). Discussion Date of seed collection Seeds of many desert and semi-desert plants fails to germinate because of the impermeable seed coat. Under natural conditions, numerous factors act alone or in combination to produce the crack of the tegumentary barrier: temperature oscillation, dry-wet period alternation, soil microorganism action and chemical scarification (Baskin & Baskin 1998). Following the release of Calligonum seeds over summer, seeds are in contact with a high temperature and a low water potential soil (Dhief et al. 2009). Length of seed (mm) Diameter of seed (mm) Seed mass (mg) C. comosum May 2007 6.59 ± 0.70 4.13 ± 0.53 47.50 ± 9.42 C. azel May 2007 7.52 ± 0.74 3.86 ± 0.49 44.50 ± 12.44 C. arich June 2007 9.30 ± 1.28 3.45 ± 0.30 39.50 ± 12.03 Seed size and seed dry mass are mean ± SE of 20 randomly selected seeds from each species. Table 2 – Two-way ANOVA of the effects of species (S), treatment (T) and their interaction on germination characteristics of three Calligonum species. Parameter Main factors Species (S) Treatment (T) Interaction (S × T) Degrees of freedom 2 6 12 Germination 7.15 ** 72.85 *** 0.80 ns Relative germination 14.33 *** 38.75 *** 0.87 ns Germination index 12.23 *** 49.88 *** 1.43 ns Dormancy 21.46 *** 7.30 *** 1.10 ns Relative dormancy 10.01 *** 20.50 *** 0.87 ns Mortality 11.39 *** 12.02 *** 1.19 ns Numbers are F-values significant at ** p
Many treatments have been proposed to overcome the hard testa imposed dormancy and to improve the germination rate in other species of the same genus. Yu & Wang (1998) showed that the seed dormancy rates of the three Calligonum species are more than 95%, and the dormant degrees are rather deep. Ren & Tao (2004) studied the effects of abrasion, sulphuric acid, boiling water, cold stratification and seed exudates (water-soluble inhibitors from the testa) treatments on the germination of 10 Calligonum species. They concluded that the rate and the percentage of the germination of these species are greatly increased by mechanical or chemical scarification; however, the lowest values are obtained with the exudate treatment. According to Zhang (1992), the most appropriate pretreatments for Calligonum species are sulfuric acid and cold stratification. In the present study, the seed germination percentage of the three species was enhanced by an immersion time in sulphuric acid and increased with time until 30 min. Zou & Chen (2008) showed that germination potential and rate of Sophora flavescens were modified by different treatments: H2SO4 (98%), grinding with sand and scalding with boiling water. H2SO4 (98%) shows the best result and improves the germination potential to 82.0% and the germination rate to 86.7%. The seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate (Finch- Savage & Leubner-Metzger 2006). Analyzing the seed phylogenetic tree based on the internal morphology of the embryo and endosperm in mature angiosperm seeds, these authors noticed that seeds of the Polygonaceae family present a non dormancy (ND) and a physiological dormancy (PD). In the present study, scarifying seeds of the three Calligonum species with sulphuric acid for 30 min could break the dormancy of viable seeds and increase their germination compared to the control and other treatments. The seed viability rates of the three Calligonum species were low (22-57%). Therefore, it was recommended to use a large number of seeds in order to increase the germination rate. Our results were in accordance with those from the study by Yu & Wang (1998) showing that seed viability is variable for 10 Calligonum species originated in China, but is generally between 30 and 50%. At the end of the experiments, the ungerminated seeds of the studied species showed a large fraction of mortality. ecologia mediterranea – Vol. 38 (1) – 2012 Effects of some seed-coat dormancy breaking treatments on germination of three Calligonum species occurring in Southern desert of Tunisia Yu & Wang (1998) showed that the major causes of seed dormancy are the inhibitors existing in peels and seeds. The mortality of seeds may be caused by water-soluble exudate inhibitors in the seed coat that significantly reduce germination and increase dormancy (Ren & Tao 2003). The dormancy should not just be associated with the absence of germination, It is a characteristic of the seed that determines the conditions required for germination (Vleeshouwers et al. 1995; Thompson 2000; Fenner & Thompson 2005). Gardarin (2010) showed that the seed mortality of 13 herbaceous species decreases according to the increasing seed coat thickness. The same authors showed that the thickness of the seed coat, which is varying between 17 and 231μm over the studied range, can protect the seed from external attacks and slow down the seed decay. Yan et al. (2003) showed, that the process of the seed water absorbing consists of a rapid water absorbing phase, slows the water absorbing phase and the saturation phase of 13 species of Calligonum L. The balance time of water absorbing was in 28 hours, and the phase of rapid water absorbing was between four to eight hours. The seed characteristics of water absorbing had a great similarity within Calligonum genus but varied among species. The capacity of water absorbing and it speed had a close relationship with temperature. Being a negatively photoblastic species, it is shown that light inhibits the germination of C. comosum seeds and they need to be covered by sand in order to germinate (Koller 1956). This author demonstrated that light, high temperature (>30 o C) and close contact with water are factors that inhibit germination of this shrub. Studying the germination of 10 Calligonum species. Ren et al. (2002) found that seeds germinate at soil depths ranging from 2 to 14 cm, but as burial depth increases both seed germination and seedling emergence decrease. The same author showed that the seed dormancy does not exist because all the seeds (or fruits) can absorb water and expand after having been soaked in water for 24 hours under room temperature, and their morphology and structure are favorable to water absorbing and gas (O 2 ) exchanging. Bond et al. (1999) showed that the seed germination is directly related to the seed size and the seed depth. Müller et al. (2006) showed, that the endosperm is a barrier for radicle protrusion of many angiosperm seeds. Rupture of the testa (seed coat) and rupture of the endosperm are two sequential 25
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TAHAR TLIG, MUSTAPHA GORAI, MOHAMED
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