Effects of achene dimorphism on dormancy and progeny traits in the two ephemerals Hedypnois cretica and Crepis aspera (Asteraceae)

Hedypnois cretica (L.) Dum.-Cours. and Crepis aspera L. are ephemeral composites native to the Mediterranean coastal desert of Sinai, Egypt, with marked achene dimorphism. This study examined the relationship between achene morphology and both some progeny traits and germination behaviour in terms of final germination percentage and germination speed. The effects of length of achene storage and temperature of storage on germination behaviour were particularly examined. Fresh harvested seeds showed little germination. After 6 months of storage, central achenes attained higher germination percentages in the two species and germinated faster in C aspera than the peripheral achenes. The dormancy loss was faster in achenes of H. cretica than in those of C aspera. Greater germination dimorphism was associated with greater dimorphism in size and morphology and consequently in the dispersal ability of the two morphs of C. aspera than in H. cretica. Warm storage significantly enhanced the germination level and speed of the heavier peripheral achenes compared with those of the central achenes of both species, so this was dependent on storage period. Plants produced by peripheral achenes had a significantly greater size than those from central achenes. This difference was more pronounced in H. cretica and persisted until 70 days postemergence but disappeared after 40 days in C. aspera. Results are discussed in light of possible adaptive significance and selective forces, such as the differential dispersal, dormancy, and growth of the two morphs, that maintain the dimorphism in the unpredictable heterogeneous deserts.