Seed germination of Opuntia species from an aridity gradient in Central Mexico

In the Mexican highlands there are several aridity gradients affecting Opuntia species composition and its biological function. Since environmental variations can affect plant traits like seed morphology as well as related processes, we conducted experiments in order to determine whether seed mass, testa hardness, seed imbibition, dormancy type, polyembryony occurrence, and different habitats are related with interspecific variation of seed germination in Opuntia spp. Our results showed that there were interspecific differences of seed attributes among seven Opuntia species along an environmental gradient in Central Mexico. Seeds of most species were able to imbibe water; however, mechanical scarification treatment did not break seed dormancy, but promoted faster germination. This pattern might be a result of the testa porosity. Seed hardness was negatively correlated with seed germination in Opuntia jaliscana; thus harder seeds had lower germination percentage. In contrast, seeds of O. robusta increased their germination percentage as seed hardness increased. We found differences in seed mass among species and localities; seeds collected in moist areas were heavier. There was evidenced a correlation of seed mass with seed germination for O. jaliscana. Polyembryony was present in four of the seven species studied and it appears to be more frequent in seeds from dry sites of the gradient.