Seed germination, salt stress tolerance and seedling growth of Opuntia ficus-indica (Cactaceae), invasive species in the Mediterranean Basin

The present study, focuses on seed germination ecology and seedling growth of Opuntia ficus-indica (Cactaceae), a native species of arid and semi-arid regions of Mexico and invasive in the Mediterranean Basin. Shortly after collecting seeds in December 2014, scarification as well as the intra-specific variability in the responses of germination to light, temperature, salt stress and germination recovery were evaluated among three naturalized populations of this species in the Mediterranean. Moreover, seedling biomass production and the growth rate were measured. The most suitable method of scarification resulted the nail-clippers chipping. For all the populations, the germination trend was similar under light and darkness, but higher values of germination occurred under light. The highest germination occurred, for all the tested populations, at temperatures of 20 and 25 °C. Opuntia ficus-indica seeds were able to germinate up to 500 mM NaCl, but inter-population differences were observed. Nevertheless, seeds from all provenances showed a low ability to recover their germination after NaCl exposure. Biomass production of seedlings was similar for Sardinian and Sicilian populations, but not compared with the Spanish population. The growth rate of the Spanish population was, like the seedling biomass, different from those of Sardinia and Sicily. This study confirmed the presence of physical dormancy in O. ficus-indica seeds, due to lignified teguments in the seed testa and a secondary dormancy induced by the exposition to salinity. Even if many naturalized populations occur in the Mediterranean ecosystems, it seems this species has not acquired the Mediterranean germination syndrome. The ability of the seeds to germinate at high NaCl concentrations reveals a potential growth of O. ficus-indica in close coastline areas and in soils with high salt concentrations. These results could be important to predict where this species will spread in the Mediterranean Basin, threatening the biodiversity of coastal ecosystems, especially in islands.