344 ± 49 192 ± 10 355±41 220±25 325 ± 26 225 ± 26

63 ± 29 21 ± 4 126 ± 35 65 ± 9 151 ± 20 102 ± 31

Data are means ± SE (n

= 10 styles). Adapted from Rosas and Pimienta (1986).

Mandujano et al. 1996). For Echinocactus, Ferocactus, Myr-tillocactus, Pachycereus, and Stenocereus, flower differentiation and fruit development start earlier, and fruit ripening occurs at the end of the spring (Trujillo 1982; Gibson and Nobel 1986; del Castillo 1988a; Pimienta-Barrios 1999).

Another common reproductive feature of cacti is that flower development is asynchronous. Namely, flowers in early stages of differentiation, flowers at anthesis, and young developing fruits can occur simultaneously (Fig. 5.3; Trujillo 1982; del Castillo 1988a; Pimienta-Barrios 1990; Lomelí and Pimienta 1993). The asynchronous patterns of flower and fruit production may be especially advantageous during adverse environmental conditions (Pimienta-Barrios and Nobel 1995). Also, some cacti may have a second blooming, as for Stenocereus griseus (Piña 1977).

The time between flower bud differentiation and flower opening is relatively short for cacti—30 to 35 days for Cereus peruvianus and Hylocereus spp. (Nerd and Mizrahi 1997), 40 to 50 days for Opuntia spp. (Pimienta-Barrios 1990), 45 to 60 days for Stenocereus megalanthus (Nerd and Mizrahi, 1997), and 40 days for S. queretaroen-sis (Pimienta-Barrios and Nobel 1998). This behavior is similar to that for other tropical and subtropical fruit crops, such as orange, avocado, and mango (Pimienta-Barrios 1990), but contrasts with that for temperate fruit crops (e.g., apple, peach, pear) whose flower initiation usually occurs 1 year before flowering (Faust 1989).

Flower bud initiation for Opuntia spp. is inhibited by shade and gibberellic acid (GA). Both shade and GA are effective when applied before the vegetative meristem in the areoles begins its transformation to the reproductive condition, as indicated by the flattening of its dome. Areoles in which GA inhibit flower bud differentiation show a marked increase in the number of spines (Pimienta-Barrios 1990), perhaps reflecting a reversion to the juvenile phase, as occurs for Hedera helix (Rogler and Hackett 1975).

Low temperatures in the winter influence flower bud burst for some cultivated cacti, indicating that chilling is involved (Nerd and Mizrahi 1997). Horticulturists argue that bud burst occurs at the end of the winter in cultivated cacti. In the highlands of Ayacucho, Peru, flower bud burst occurs throughout the year for Opuntia ficus-indica, suggesting that other physiological factors can be involved in the control of flower induction and bud break. Thus, flower bud initiation and control may not rely on a single factor, such as chilling.

The time of flowering in certain species of cacti is little influenced by water availability. For instance, many species of cacti in Mexico and the southwestern United States start flowering (e.g., Echinocactus platyacanthus, Trujillo 1982; Echinocereus spp., Powell et al. 1991; Neobux-baumia spp., Valiente-Banuet et al. 1997) or have their flowering peaks (e.g., Ferocactus histrix, del Castillo 1988a; Opuntia spp., Rodríguez-Zapata 1981, del Castillo and González-Espinosa 1988; Pachycereuspringlei, Fleming et al. 1994) in March or April. These months are among the driest and near the end of the longest drought period of the year. Although reproductive development occurs during the dry season in arid regions of both the Northern and Southern Hemispheres, relatively few fruits abscise and a high percentage of flowers become fruits. In contrast, the reproductive growth of many other fruit crops is highly sensitive to drought. For instance, the percentage of flowers that become fruit can be 95% for O. ficus-indica and 28% for Stenocereus queretaroenis but is often under 10% for many fruit crops, such as apple, avocado, mango, and orange (Stephenson 1981; Pimienta-Barrios 1990; Pimienta-Barrios et al. 1995). This may in part reflect the succulence of the cactus stems, which can store appreciable amounts of water that can be available to reproductive structures during drought (Gibson and Nobel 1986).

Reproductive growth for O. ficus-indica is highly re-

month (1994)

Figure 5.3. Reproductive development for Stenocereus queretaroensis under natural conditions, indicating the emerging flower buds (0), open flowers (A), and nearly ripe fruits (□). Adapted from Pimienta-Barrios and Nobel (1995).

month (1994)

Figure 5.3. Reproductive development for Stenocereus queretaroensis under natural conditions, indicating the emerging flower buds (0), open flowers (A), and nearly ripe fruits (□). Adapted from Pimienta-Barrios and Nobel (1995).

sponsive to management. Both flower number and fruit number dramatically increase in response to both mineral and organic fertilizers, water, and pruning (Pimienta-Barrios 1990). In contrast, irrigation does not affect stem extension, reproductive demography, fruit quality, or seed size for S. queretaroensis (Pimienta-Barrios and Nobel 1995).

The time from flower opening to fruit maturation is relatively short, particularly for columnar cacti. Fertilized flowers give rise to mature fruits 40 to 50 days after pollination for Pilosocereus lanuginosus, Stenocereus griseus, and Subpilocereus repandus (Petit 1995). For S. queretaroensis, fruits attain over 90% of their final length in about 40 days, and the overall development period from anthesis to maturity is about 90 days (Pimienta-Barrios and Nobel 1995). Fruits of the vinelike climbing cacti Hylocereus costaricencis, H. polyrhizus, and H. undatus have a short growth period of about 50 days, whereas fruits of Selenicereus megalanthus require 90 to 150 days (Weiss et al. 1994b, 1995).

Seed Germination

Both ovule fertilization and seed germination are critical stages in plant development. Despite the importance of seed germination in the reproduction of cacti, the pioneer studies on seed germination are relatively recent. Alcorn and Kurtz (1959) and McDonough (1964) revealed the importance of light. But Zimmer (1969) found that some species do not require light for germination and that the cactus species that require light for germination have a greater response when they are exposed to red light. Pilcher (1970) indicated the presence of dormancy for Opuntia seeds, an observation later confirmed for a large number of Opuntia species (Bregman and Bouman 1983; Pérez 1993). Although most cactus seeds germinate within a week, germination for subfamily Opuntioideae often takes a few months (Bregman and Bouman 1983; Pérez 1993; Table 5.2). Seed dormancy, characterized by the perseverance of the dormant condition even when the seeds are exposed to optimal environmental conditions for germination, has survival value for cacti growing in arid environments. Both innate and enforced dormancy are common for most cacti (Rójas-Aréchiga and Vázquez-Yanes 2000).

A low level of light (PPF < 20 pmol m-2 s-1) is required for the germination of most cactus seeds. Red light stimulates germination for soaked seeds of Stenocereus griseus, but the effect of red light is reversed by far-red light, suggesting the participation of phytochrome in the seed germination process (Martínez-Holguín 1983). The application of gibberellic acid also increases seed germination for S. griseus (Moreno et al. 1992; López-Gómez and Sánchez-Romero 1989). Thus, phytochrome may exert its control of seed germination through the synthesis of gibberellic acid. The seeds of S. griseus also maintain their viability after 12 months of storage, reaching germination percentages of 90 to 100% four days after sowing (López-Gómez and Sánchez-Romero, 1989). Maiti et al. (1994) suggest that a high germination percentage is associated with a thin testa and with the presence of starch granules. Seed germination for Stenocereus queretaroensis is relatively high even though its seeds are small (2.6 mg); the seeds have a thin testa and a relatively large amount of lipid (Pimienta et al. 1995). Seeds of many wild species that respond to light are also rich in lipids. Because of their small size and light requirements, such seeds should be near the soil surface for successful germination.

Besides light, the germination of cactus seeds requires wet conditions and responds to temperature (Rójas-Aréchiga and Vázquez-Yanes 2000). Water uptake stimulates cracks in the testa (outer seed coat) caused by the growing embryo (Bregman and Bouman 1983) and leaches possible germination inhibitors (Mondragón-Jacobo and Pimienta-Barrios 1995). The presence of soluble inhibitory substances in the testa of cactus seeds maintains the dormant state until environmental conditions are suitable for development (Rójas-Aréchiga and Vázquez-Yanes 2000). The fractional germination of seeds of some cacti increases for the first 3 years after harvest, or after the passage through the digestive tracts of rabbits. Seeds that pass through the digestive tract of cattle exhibit average germination percentages that are 50% higher than seeds re-

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