Seed development and germination of Strombocactus species (Cactaceae): A comparative morphological and anatomical study

Seed development, germination and seedling establishment were compared between Strombocactus species, S. corregidorae, S. disciformis ssp. disciformis and S. disciformis ssp. esperanzae, emphasizing the origin and role of the seed appendage, which is present only in both S. disciformis subspecies. The development and morpho-anatomy of both seeds and seedlings were evaluated using standard light microscopy and scanning electron microscopy. Embryo and seed development proceeded similarly in all three taxa. However, we unequivocally demonstrate the funicular origin and aerenchymatic structure of the seed appendage, the aril in S. disciformis ssp. disciformis and S. disciformis ssp. esperanzae, which differentiates soon after fertilization. Other distinguishing features include the delayed growth of the testa in the micropyle in S. corregidorae, and testa micromorphology in mature seeds. Seed potential and production were similar between species, although seed efficiency was higher in both subspecies of S. disciformis, which may reflect pollination problems in S. corregidorae natural populations. Seed flotation occurs in the three taxa, due to the presence of air spaces in the lumen of the seed coat cells and the aril aerenchymatic, when present. Neither physiological nor structural dormancy were detected in Strombocactus, and a high percentage of seeds germinated in the different taxa, as no scarification or disinfection treatments were necessary. The absence of the aril in S. corregidorae shows that further genetic, phylogenetic and comparative ontogenic studies that include the three taxa of Strombocactus and the nearest genera, such as Ariocarpus, Turbinicarpus and Epithelantha, are required in order to determine if this structure was gained or lost during evolution of Strombocactus. This research provides a greater understanding of the reproductive biology of this endangered genus endemic to Mexico, necessary for the establishment of future restoration and conservation programs.