Eulogio Pimienta Barrios and Rafael F del Castillo

Introduction

Areoles and Reproductive Structures

Flower Types, Pollinators, and Pollination

Outcrossing Mechanisms

Polyploidy and Reproduction

Flower Fertilization

Flower and Fruit Development

Seed Germination

Apomixis

Conclusions

Literature Cited

Introduction

Cacti are native to the Americas, where they are widely distributed. More than 70% of the species occur in arid and semiarid regions of Mexico, Peru, Argentina, and Chile (Gibson and Nobel 1986; Arias-Moreno 1997). Water is the major limiting factor for plant productivity in such environments (Fischer and Turner 1978), although other factors, such as strong drying winds, extreme temperatures, limited nutrients, and high light intensity, can exacerbate the effects of water scarcity (Berry et al. 1983). Cacti perhaps provide the best examples of adaptations to aridity, including morphological modifications (e.g., succulence, low stomatal density, thick cuticles) and Crassulacean acid metabolism (Gibson and Nobel 1986; Nobel 1995). However, with the exception of seed germination and establishment (Rójas-Aréchiga and Vázquez-Yanes 2000), the role of most of the reproductive adaptations of cacti to aridity has been poorly studied and comes mainly from ex periments with cultivated plants (Pimienta-Barrios and Nobel 1998). Nevertheless, studies on reproductive development for cacti in arid environments can help explain the evolution of reproductive mechanisms in cacti, which allow them to cope with the prevailing physical and chemical stresses. This review describes the available information on reproductive biology, relating various aspects to the adaptations of cacti to aridity and to the origin and causes of biological variation.

Areoles and Reproductive Structures

The distinctive vegetative structure of cacti is the areole, considered homologous to a lateral (axillary) bud (Buxbaum 1950). Hairs, foliar organs, reproductive organs, glochids, and roots develop from the areoles (Booke 1980). Cactus flowers are sessile and solitary, and commonly only one flower is produced per areole, which helps increase fruit set for cacti (Ramírez and Berry 1995). However, several flowers are sometimes derived from a single areole, e.g., for Pterocereus foetidus (Bravo-Hollis 1978). For Myrtillocactus, several areoles join to produce an inflorescence in which the flowers appear to be derived from a single meristem (Buxbaum 1950). For Opuntiaprolifera, a chain of fruits results from the development of flowering buds derived from pericarpic areoles. The flowers are usually campanulate with radial symmetry (actinomorphic), but in epiphytic species the flowers are lightly bilateral (zy-gomorphic), as the stamens and styles are located in the ventral part of the flower (Arreola-Nava 1997). The large, attractive flowers for which cacti are noted occur in most self-fertile polyploids (Ross 1981). For instance, the flowers of Opuntia are large, and mature flowers can vary from 6 to 10 cm in length; they commonly are borne near the apex of the cladodes (Gibson and Nobel 1986). The flowers of Cereus peruvianus, Hylocereus spp., and Selenicereus mega-lanthus are among the largest in the cactus family (Nerd and Mizrahi 1997).

Cactus flowers usually open and close during the same day. For opuntias (Fig. 5.1), flower opening typically lasts 8 to 11 hours (Rosas and Pimienta 1986; del Castillo and González-Espinosa 1988; Osborn et al. 1988; Mandujano et al. 1996), but some flowers may open a second day (Grant and Grant 1981; Rosas and Pimienta 1986). Ephemeral flowers are common in dry tropical rainforests; this flower behavior apparently decreases water loss by decreasing the time available for transpiration (Primack 1982).

The most fundamental characteristic of a cactus flower is its inferior ovary, meaning that the ovary occurs below the perianth parts and the stamens (Fig. 5.2); only a few Pereskia spp. have superior ovaries (Bravo-Hollis 1978; Gibson and Nobel 1986). Cacti are among the few plants in which the exterior of an inferior ovary is sunken into a modified stem, termed the receptacle (Booke 1980). The ovary is formed by the fusion of several carpels and consists of a single internal chamber, the locule, where a relatively high number of ovules occur in a parietal location along the ventral wall of the ovary (Fig. 5.2; Booke 1980). The number of ovules per flower is 150 to 400 for Opuntia ficus-indica (Rosas and Pimienta 1986; Weiss et al. 1993), 388 (hermaphrodites) to 406 (females) for O. robusta (del Castillo 1986a), over 1,000 for Stenocereus queretaroensis (Pimienta-Barrios and Nobel 1995), and 7,200 for Hylocereus undatus (Nerd and Mizrahi 1997).

A unique type of ovule is found in the Cactaceae. Due to unilateral growth, it first becomes anatropous and, as the curvature continues, the micropyle again points upward in the fully formed ovule (Bhojwani and Bhatnagar 1979). Embryological observations for Opuntia and Stenocereus spp. (Rosas and Pimienta 1986; Ortega, 1993;

Figure 5.1. Opuntia flower at the initial stages of opening, when only the basal part of the stigma is covered with pollen grains released before opening from anthers in contact with the basal part of the stigmatic surface.

García-Aguilar and Pimienta-Barrios 1996) reveal that most of the embryological characters are primitive, according to Grayum's (1991) criteria. The embryological characters are: (1) the inferior position of the ovary, (2) four microsporangia per anther, (3) isobilateral type of microspore tetrads, (4) bitegmic and crassinucelated ovules, (5) the functional megaspore in the chalazal position, (6) embryo of the polygon type, (7) the high number of ovules per ovary, and (8) glandular tapetal tissue (Grayum 1991; Ortega 1993).

The style can be short and thick, as for Opuntia spp., or long and thin, as for Heliocereus spp. (Arreola-Nava z997). The stigma is at the apex of the style (Fig. 5.2), where pollen is deposited, and shows a variable number of lobes—from 3, as for Mammillaria, to 24 for Hylocereus (Arreola-Nava 1997). The stigma of most cactus flowers shows characteristics suitable for insect landing, because it is over the stamens (Grant and Grant 1979b; Ross 1981; del Castillo and González-Espinosa 1988). Both the large stig-matic surface and the stickiness of the receptive surface improve the chances of catching pollen grains. The nectary usually lies at the base of the thickened style, forming a chamber at the base of the receptacle, where substantial nectar accumulates. Nectar can be rich in sucrose, as for Pachycereuspringlei (Fleming et al. 1994), and is usually secreted at the beginning of anthesis (del Castillo and González-Espinosa 1988; Mandujano et al. 1996). In the Peruvian genus Matucana, the nectar chamber is closed by a protective device that probably prevents dilution of the nectar (Bregman 1996).

The numerous stamens of a cactus flower are spirally arranged and have thigmotropic sensitivity; i.e., they move

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