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18 E. Imbert nearly equally to changes in salinity (Khan et al. 1998; Khan & Gul 1998). The absence of chlorophyll in yellow seeds of Salsola volkensii also affects dormancy (Negbi & Tamari 1963). Differences in pericarp morphology do not always influence germination. Arctotis fastuosa, Arthrocnemum macrostachyum, Centaurea soltistialis, Charieis heterophylla, Crepis sancta, Dimorphotheca pluvialis, Galinsoga parviflora and Hypochoeris glabra are all heteromorphic but the different morphs do not have different germination requirements (references in Appendix). Furthermore, heterochrony of germination can vary among populations (Venable et al. 1987; Kigel 1992). For instance, in Bidens bipinnata heterochrony between peripheral and central achenes is important in Asian populations (Dakshini & Aggarwal 1974), but reduced in South African populations (Brown & Mitchell 1984). The last point related to dormancy is the conservation of seed viability, i.e. the ability to remain viable when embryos cannot germinate. This character is poorly documented in species with heteromorphic seed, but is observed in heterocarpic Asteraceae (Imbert 1999). Yet, the delayed germination could be successful only if ungerminated seeds remain viable in the seed bank (Cohen 1966). Interspecific comparisons showed that large seeds, which contain more storage material and a thicker seed coat, could remain viable longer than small seeds (Priestley 1986). However, Thompson et al. (1993) suggested that there is a negative correlation between seed size and longevity. This pattern has been observed in Bidens pilosa (Rocha 1996), whereas in Crepis sancta the peripheral achenes are heavier and remain viable longer than central ones (Imbert 1999). Actually, it seems that seed viability is mainly determined by the hardness of the seed coat, which acts as a physical defence against humidity and fungal infection (Mohamed-Yasseen et al. 1994). For instance, in Atriplex semibaccata and Blackiella inflata hard and dark coloured seeds remain viable longer than soft and light ones (Beadle 1952). Seedling emergence, seedling survival and growth Differences among morphs are often associated with differences in embryo size; therefore, a difference in seedling success is expected. A positive relationship between seed size and seedling survival has been documented for a few species with heteromorphic seed (Koller & Roth 1964; Budd et al. 1979; Venable & Levin 1985a; Rai & Tripathi 1987; Venable et al. 1987). Initial seedling size also influences reproductive output for some heteromorphic species (Weiss 1980; Cheplick & Quinn 1982; Venable & Levin 1985b; Schnee & Waller 1986; Ellison 1987; Beneke et al. 1993b). Plants germinating from the largest seeds often have a competitive advantage (Weiss 1980; Perspectives in Plant Ecology, Evolution and Systematics (2002) 5, 13–36 Cheplick & Quinn 1982; Venable & Levin 1985b; Rai & Tripathi 1987; Imbert et al. 1997). For some species, plants from one seed morph appear to be more resistant to water stress (Koller & Roth 1964; Bendall 1973; Cheplick & Quinn 1982; Venable 1985b) or to nutrient deficiency (Galinsoga parviflora; Rai & Tripathi 1987). Such a difference suggests that the root/shoot ratio differs between plants from different seed morphs. Indeed, interspecific comparisons tend to show that larger embryos have a greater root/shoot ratio (Gleeson & Tilman 1994; but see Marañon & Grubb 1993). This difference has been also observed in intraspecific comparisons (Wulff 1986b), but in species with heteromorphic seed comparisons between morphs failed to show such difference (Baker & O’Dowd 1982; Beneke et al. 1993b). For instance, in the Asteraceae Crepis sancta peripheral achenes are three times heavier than central ones (0.27 vs. 0.10 mg; Imbert et al. 1996). Consistently, seedlings from peripheral achenes are larger than those from central ones (Table 2), and have greater above– and belowground parts, but the root/shoot ratio does not differ between the morphs (Table 2). Further experiments have shown that both morphs are equally affected by nutrient depletion (Imbert et al. 1997). Zhang (1995) reports similar results for Cakile maritima. Seed heteromorphism as a bet-hedging strategy It therefore appears that ecological differences between morphs can be important. This confirms the assertion of Harper (1977) who associated seed heteromorphism with a strategy combining “seeds for different ends or function ...”. The mixed strategy for germination, which has been established for at least thirty species, is presented as the major ecological conse- Table 2. Comparisons of seedling size between achene morphs in Crepis sancta (Asteraceae). For each achene morph one thousand achenes were germinated in Petri dishes, each dish containing one disk of Whatmann paper which was regularly supplied with distilled water. Measurements were made on the first hundred seedlings of each achene morph. Maximal diameter of the two cotyledons was measured immediately after emergence. Length of the radicle was measured when the seedling was totally separated from its seed coat. Once measured, seedlings were removed from the Petri dish. Seedlings were visited every 12 hours (means ± SE). Achene morph Cotyledon Radicle Root/shoot length (mm) length (mm) ratio Peripheral 5.40 ± 0.19 6.49 ± 0.22 1.41 ± 0.05 Central 3.96 ± 0.05 4.67 ± 0.06 1.37 ± 0.04 F 1,198 74.54 13.91 0.28 P-value
quence of seed heteromorphism (Harper 1977; Lloyd 1984; Silvertown 1984; Venable 1985a). Spreading offspring in time, resulting from variation in germination time, could be efficient to reduce sib competition (Cheplick 1996a). The efficiency of spreading individuals in time also represents a bet-hedging strategy, reducing temporal variance in fitness, which is advantageous in highly variable and unpredictable habitats (Slatkin 1974; Gillespie 1977; Kaplan & Cooper 1984). This major consequence of seed heteromorphism has been clearly formalised with the High Risk/Low Risk strategy by Venable (1985a). Identically, intra-individual variation in seed morphology could optimise the respective seed shadows by maximising the spread of seeds in space (Augspurger & Franson 1993). For instance, in wind-dispersed seeds the dispersal distance depends on the terminal velocity that in turns depends on seed morphology (Sheldon & Burrows 1973). Therefore, any morphological variation contributes to variation in dispersal distance (Greene & Johnson 1989). Such variation is important to reduce density-dependent effects, and thus sib-competition, and to increase colonisation ability of new habitats. As presented above, in several species with heteromorphic seed, one morph has greater dispersal ability than the other one. This difference in dispersal is of interest, because the proportion of seeds that are potentially dispersed produced by a single individual can be related to the dispersal rate of its progeny. A number of theoretical models exist for evolution of dispersal rate, although these are typically used for animals (e.g. Johnson & Gaines 1990; Ronce et al. 2001). Thus, species with heteromorphic seed are appropriate to empirically test theoretical predictions about the dispersal rate in plant species (Olivieri & Gouyon 1985; Imbert 2001; Imbert & Ronce 2001). Furthermore, there is an obvious trend that associates low dispersal ability with high seed dormancy, while seed morphs with high dispersal show reduced dormancy (see also Olivieri & Berger 1985). This pattern fits with theoretical expectations about trade-offs between dispersal in space and in time (Venable & Lawlor 1980; Olivieri 2001). All these ecological differences represent ultimate factors assuring the evolutionary success of mixed strategy in seed morphology. Indeed, seed heteromorphism is adaptive when seed morphs differ in their ecological behaviour (Lloyd 1984; Venable 1985a). However, the maintenance of seed heteromorphism can also be explained by the positive effect of seed size on competitive ability, and the trade-off between seed size and the number of seeds produced by a single individual. Geritz (1995) showed that, considering these two selective forces, intra-individual variation in seed size represents an evolutionarily stable strategy (ESS) provided Consequences and ontogeny of seed heteromorphism 19 there is spatial variation in seedling density. Indeed, in safe sites where seed density is high, the heaviest seeds are advantageous, while in low-density conditions, a small seed size is not disadvantaged. As small seeds are more numerous, they have a greater probability to join sites where the density is low. This has to be related to the relation between seed size and dispersal ability (Venable & Brown 1988). It can also be shown that intra-individual variation for seed size represents an ESS in situations where seed predation is positively related to seed size (Geritz 1998). Recently, Fenner et al. (2002) demonstrated a positive relationship between capitulum size and pre-dispersal seed predation in some Asteraceae species. Therefore, considering the theoretical predictions obtained by Geritz (1998) and these observations, it can be suggested that seed size variation should be positively correlated to capitulum size. As a corollary, seed heteromorphism is supposed to occur more often in species with large capitula than in species with small capitula. More data concerning the predation rate in species with heteromorphic seed, and in particular data concerning differences in predation rate among seed morphs, are thus needed. Seed heteromorphism in the genus Crepis Because seed heteromorphism represents a mixed strategy reducing temporal variance in fitness, species with heteromorphic seed should mainly occur in unpredictable habitats such as the desert (Zohary 1962) and disturbed environments (Harper 1965). For instance, using the Flora of Israel, Ellner & Shmida (1981) observed that heteromorphism was more frequent in desert habitats (13.2% of 604 species) than in Mediterranean habitats (0.7 % of 1560 species). Telenius & Torstensson (1991) showed that Spergularia species with heteromorphic seeds mainly occurred in saltmarshes which can be considered as a frequently disturbed. It is also often stated that seed heteromorphism mainly occurs in monocarpic species, in particular because polycarpy allows spreading the risk of reproduction over several years, and thus it is also a bet-hedging strategy (Venable & Brown 1988). Consistently, Plitmann (1986) reported a significant association between annual life cycle and heterocarpy in Turkish Asteraceae. In contrast, this relation was not observed in the genus Spergularia (Telenius & Torstensson 1991; see also Ellner & Shmida 1981). It is not possible to test directly both statements with the species surveyed in the present review, because ecological and phenological information about non-heteromorphic sister species is missing in most cases. To deal with this topic, I focused on the genus Crepis which is well represented within the Appendix. Perspectives in Plant Ecology Evolution and Systematics (2002) 5, 13–36
Page 1 and 2: Vol. 5/1, pp. 13-36 © Urban & Fisc
Page 3 and 4: 250,000 angiosperm species, the pre
Page 5: melina benghalensis (Commelinaceae;
Page 9 and 10: and presence/absence of seed hetero
Page 11 and 12: Table 3. Number of heterocarpic and
Page 13 and 14: ner & Shmida 1984) and Bidens pilos
Page 15 and 16: Bachmann K & Price HJ (1979) Variab
Page 17 and 18: Gutterman Y (1994) Long-term seed p
Page 19 and 20: Ruiz de Clavijo E (1994) Heterocarp
Page 21 and 22: Appendix Consequences and ontogeny
Page 23 and 24: Family Species References Consequen

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