4.1 Seed dormancy in Cactaceae
Different authors reported dormancy in many Cactaceae species, related to the xeric environment, with highly limited natural resources to support seed germination, seedling development, and survival. Some of these studies associate physical dormancy with mechanical resistance (Rojas-Aréchiga et al. 2011; Pérez-Molphe-Balch et al. 2015) caused by the rigid coat surrounding the embryo, which could limit the water uptake into the seeds. Seeds of different species of Cactaceae have rigid and hard coats (Archibald, 1939; Orozco-Segovia et al. 2007). However, a study on the anatomy of seeds of different Cactaceae species, including Opuntia tomentosa , demonstrated that in these seeds there are no characteristics that explain the occurrence of physical dormancy or any limitation to the water uptake (Orozco-Segovia et al. 2007). At the same time, treatments to reduce the mechanical resistance of seeds using chemical or mechanical scarification have been one of the most studied in different species of Cactaceae, but failed to achieve high rates (>90%) of seed germination (Delgado-Sánchez et al. 2010; Barrios et al. 2020; Magnani and Cardoso, 2022).
In addition, recent studies increased the evidence that only physiological dormancy exists in Cactaceae seeds (Barrios et al. 2020; Rojas-Aréchiga and Garcia-Morales, 2022).
In Melocactus , the presence of seed dormancy was widely reported, with percentages of germinated seeds between 8 and 65%, depending on the species, genotype, harvest time, germination temperature, cultivation conditions (in vitro , Petri dish or in vivo ), and pre-treatments or treatments applied to the seeds (Zamith et al. 2013; Bravo Filho et al. 2019; Chaves et al. 2021). However, the main biological evidence and reasons why 35-90% seeds did not germinate, as well as, the treatments required to further increase the germination rates of dormant seeds in Cactaceae, and more specifically inMelocactus , are not yet fully elucidated.