B2

Adapted from Mondragón-Jacobo and Pimienta-Barrios (1995).

moved from ripe fruits (Potter et al. 1984). For cacti, the optimal temperature for seed germination ranges from 17 to 34°C, with a mean of 25°C (Nobel 1988). Alternating temperatures give higher germination yields than constant temperatures (Rójas-Aréchiga and Vázquez-Yanes, 2000). For Opuntia species, optimum constant temperatures vary from 25 to 35°C (Mondragón-Jacobo and Pimienta-Barrios 1995). For Astrophytum myriostigma, germination rates are highest (80-98%) at temperatures from 20 to 25°C, and decrease at higher temperatures under conditions of diffuse light (Moreno et al. 1995).

For species of Opuntia and Stenocereus, among others, seed dormancy is apparently related to the time of fruit ripening. For instance, fruit ripening for S. queretaroensis and Myrtillocactus geometrizans occurs at the end of the spring, in contrast with Opuntia, for which fruit ripening for most species occurs from the middle of summer to the beginning of the autumn. The occurrence of fruit ripening at the end of the spring for S. queretaroensis and M. geometrizans may be optimal, because seed maturation occurs just before the start of the summer, which favors seed germination and establishment because of favorable soil moisture and shading by surrounding natural vegetation in their habitats (Fig. 5.4; Nobel 1988; Pimienta-Barrios and Nobel 1998). In contrast, seed development for Opuntia spp. occurs near the middle or the end of the rainy season, so its seed maturation and dispersal coincide with the beginning of the dry season and the reduction of air temperatures in Mexico.

To avoid drought stress and frost damage, seeds of

Opuntia spp. have dormancy periods of 7 to 8 months; seed germination is initiated at the start of the next rainy season (Pérez 1993). However, seed germination for other cacti indicates that dormancy is not related to the time of fruit ripening. Seeds of some barrel cacti (e.g. Ferocactus flavovirens, F. histrix, F. latispinus, and F. robustus) and certain columnar cacti (e.g., Cephalocereus chrysacanthus, Neobuxbaumia, and Pachycereus hollianus) do not show dormancy, as high percentages of their seeds germinate without scarification or washing, once the fruit is mature (Bregman and Bouman 1983; del Castillo 1986b; Rojas-Aréchiga et al. 1998). However, the seeds of Echinocereus polyacanthus do not germinate in the winter, at the time of fruit ripening, suggesting that low temperatures or short days can induce dormancy for this species (Trujillo 1982). Given the short and unpredictable rain periods occurring in most arid and semiarid habitats, the absence of dormancy for cactus seeds may be considered a selective advantage, as a quick onset of germination would enable seedling establishment despite an erratic environment.

Apomixis

In addition to sexual reproduction, two types of asexual reproduction are present in cacti: vegetative apomixis and agamospermy. In vegetative apomixis, a new plant is derived after the rooting of plant fragments or ramets detached from a parent plant, as occurs for Myrtillocactus geometrizans. Among the Cactaceae, Opuntia spp. are probably the best examples of reproductive versatility, using a wide array of sexual and asexual methods. Indeed,

Drought

Rainy

Drought

Rainy

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