Biomechanics of the columnar cactus Pachycereus pringlei

We report the longitudinal variations in stiffness and bulk density of tissue samples drawn from along the length of two Pachycereus pringlei plants measuring 3.69 and 5.9 m in height to determine how different tissues contribute to the mechanical stability of these massive vertical organs. Each of the two stems was cut into segments of uniform length and subsequently dissected to obtain and mechanically test portions of xylem strands, stem ribs, and a limited number of pith and cortex samples. In each case, morphometric measurements were taken to determine the geometric contribution each tissue likely made to the ability of whole stems to resist bending forces. The stiffness of each xylem strand increased basipetally toward the base of each plant where stiffness sharply decreased, reaching a magnitude comparable to that of strands 1 m beneath the stem apex. The xylem was anisotropic in behavior, i.e., its stiffness measured in the radial and in the tangential directions differed significantly. Despite the abrupt decrease in xylem strand stiffness at the stem base, the contribution made by this tissue to resist bending forces increased exponentially from the tip to the base of each plant due to the accumulation of wood. A basipetal increase in the stiffness of the pith (and, to limited extent, that of the cortex) was also observed. In contrast, the stiffness of stem rib tissues varied little as a function of stem length. These tissues were stiffer than the xylem in the corresponding portions of the stem along the upper two-fifths of the length of either plant. Tissue stiffness and bulk density were not significantly correlated within or across tissue types. However, a weak inverse relationship was observed for these properties in the case of the xylem and stem rib tissues. We present a simple formula that predicts when stem ribs rather than the xylem strands serve as the principal stiffening agents in stems. This formula successfully predicted the observed aspect ratio of the stem ribs (the average quotient of the radial and tangential dimensions of rib transections), and thus provided circumstantial evidence that the ribs are important for mechanical stability for the distal and younger regions of the stems examined.     

Tetrahydroisoquinoline alkaloids of the Mexican columnar cactus Pachycereus Weberi

Eight tetrahydroisoquinoline alkaloids have been crystallized and identified from the nonphenolic and phenolic extracts of the giant Mexican cereoid cactus, Pachycereus weberi (Coult.) Br. and R. The identities were established as 5,6,7-trimethoxy-1,2,3,4- tetrahydroisoquinoline (nortehuanine) 1; 7,8-dimethoxy-1,2,3,4-tetrahydroisoquinoline (lemaireocercine) 2; 7-methoxy-1, 2,3,4-tetrahydroisoquinoline (weberidine) 3; 5,6,7,8-tetramethoxy-1,2,3,4-tetrahydroisoquinoline (weberine) 4; 6,7- dimethoxy-1,2,3,4-tetrahydroisoquinoline (heliamine) 5; 2-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (N- methylheliamine or oxymethyl-corypalline) 6; 2-methyl-5,6,7-trimethoxy-1,2,3,4-tetrahydroisoquinoline (tehuanine) 7; and 1,2-dimethyl-6,7-dimethoxy-8-hydroxy-1,2,3,4-tetrahydroisoquinoline (pellotine) 8. Compounds 1- 4 have not been identified previously as natural compounds, while compounds 5-8 are previously known cactus alkaloids.     

Generalized and complementary pollination system in the Andean cactus Echinopsis schickendantzii

The considerable floral diversity present in the cactus family has often been associated with the specificity of its pollinators. However, many cactus pollination systems are generalized as their flowers are pollinated by a wide spectrum of animals. For example, cactus species with white flowers, nocturnal anthesis and extended floral cycles would present generalized pollination systems in which both nocturnal and diurnal visitors could be effective pollinators. In this article, we tested this hypothesis by studying the pollination biology of Echinopsis schickendantzii, an Andean cactus with sphingophilous flowers. In addition, we evaluated whether the cactus’s pollination system is complementary or redundant regarding the relative contributions of nocturnal and diurnal pollinators. Specifically, we studied the floral cycle, the reproductive system and the pollination effectiveness of floral visitors. The flowers of E. schickendantzii are self-incompatible; they opened at crepuscule and have an extended floral cycle. Moths were frequent visitors at night, whereas bees were frequent visitors during the day; both were effective pollinators of the cactus. Our results indicated that the flowers of this species present phenotypic, functional and ecological generalization, and their fruit set is determined by the contributions of both pollinator functional groups, i.e., they have complementary pollination systems. These results support the hypothesis that cacti in the extra-tropical deserts of South America have generalized pollination systems.     

Influence of vesicular-arbuscular mycorrhizae on biomass production by the cactus Pachycereus pecten-aboriginum

This study reports the effect of vesicular-arbuscular mycorrhizal (VAM) fungi on dry matter production by Pachycereus pecten-aboriginum (Engelm.) Britt & Rose, an arborescent cactus of arid and tropical dry forest in Mexico. Seedlings in the presence or absence of VAM fungi were grown in soil between two plates of glass (20 × 30 cm) for 8 months inside growth chambers (30/25° C, 13/11 h day/night and a light intensity of 400 mol m^-2 s^-1). VAM seedlings had significantly (P<0.01) higher dry matter production (0.418 versus 0.169 g), root/shoot ratios (0.26 versus 0.14) and specific root length (0.65 versus 1.41 mm mg^-1) than non-VAM seedlings, suggesting a more efficient exploitation of soil resources by the VAM cacti. The data point to a role for VAM fungi in the establishment, growth, water relations and nutrition of cacti in the arid tropics.     

Effect of habitat disturbance on pollination biology of the columnar cactus Stenocereus quevedonis at landscape‐level in central Mexico

Stenocereus quevedonis (‘pitire’) is a columnar cactus endemic to central Mexico, grown for its edible fruit. Phenology, pollination biology and behaviour of flower visitors of this species were compared in six conserved and disturbed sites, hypothesising that: (i) pitire pollination is self‐incompatible, requiring animal vectors; (ii) higher incidence of radiation on plants in cleared forest may lead to a higher number of flowers per pitire plant and longer blooming season, and disturbing and differential spatial availability of flower resources may determine differential attraction of pollinators to conserved and disturbed areas; (iii) if pitire pollination system is specialised, reproductive success would decrease with pollinator scarcity, or other species may substitute for main pollinators. In all sites, pitire reproduction started in January, flowering peak occurring in April, anthesis duration was 15 h and predominantly nocturnal (9 h), pollen was released at 23:00 h, nectar was produced throughout anthesis, and breeding system was self‐incompatible. Flower production per plant was similar in disturbed and conserved sites, but flower availability was higher (because of higher tree density) and longer in disturbed sites. Pollination is nocturnal, the most frequent legitimate pollinator being the bat Leptonycteris yerbabuenae; diurnal pollination is rare but possible, carried out by bee species. Fruit and seed set in control and nocturnal pollination treatments at disturbed sites were higher than in conserved sites. Frequency of L. yerbabuenae visits was similar among site types, but more visits of complementary nocturnal and diurnal pollinators were recorded in disturbed sites, which could explain differences in reproductive success.     

Pollination system of the Pilosocereus leucocephalus columnar cactus (tribe Cereeae) in eastern Mexico

It has been suggested that there is a geographic dichotomy in the pollination systems of chiropterophilous columnar cacti: in intra‐tropical areas they are pollinated almost exclusively by bats, whereas in extratropical areas they are pollinated by bats, birds and bees. However, currently the studies are clumped both taxonomically (mainly Pachycereeae species) and geographically (mainly in the Tehuacan Valley and the Sonoran Desert). This clumping limits the possibility of generalising the pattern to other regions or cactus tribes. Only four of the 36 chiropterophilous cacti in Pilosocereus have been studied. Despite the tropical distribution of two Pilosocereus species, bees account for 40–100% of their fruit set. We examined how specialised is the pollination system of P. leucocephalus in eastern Mexico. As we studied tropical populations, we expected a bat‐specialised pollination system. However, previous studies of Pilosocereus suggest that a generalised pollination system is also possible. We found that this cactus is mainly bat‐pollinated (bats account for 33–65% of fruit set); although to a lesser degree, diurnal visitors also caused some fruit set (7–15%). Diurnal visitors were more effective in populations containing honeybee hives. P. leucocephalus is partially self‐compatible (14–18% of fructification) but unable to set fruit without visitors. Despite the variation in pollination system, P. leucocephalus shows more affinity with other columnar cacti from tropical regions than with those from extratropical regions. Although we report here that a new species of tropical Pilosocereus is relatively bat‐specialised, this Cereeae genus is more flexible in its pollination system than the Pachycereeae genera.     

The pollination spectrum in the southwestern American cactus flora

The cacti of the American Southwest, defined as the region from southern California to Texas, are surveyed for types of pollination systems and their frequencies. Four types of pollination systems are known to occur in the southwestern cactus flora: bee, hummingbird, hawkmoth, and bat pollination. Two other modes are suspected but not documented: miscellaneous smallinsect pollination and autogamy.—Bee flowers comprise a wide and nearly continuous series of size classes from very small to very large. The large bee flowers, with perianths 5.5 to 12.5 cm in diameter, form a prominent but arbitrarily delimited subcategory in the bee pollination system. Promiscuous flowers and autogamous flowers, if they occur, are included with the medium-sized and small bee flowers in our present classification, due to lack of information, and will have to be separated out when our knowledge is more complete. The overwhelming majority of species in the southwestern cactus flora are bee flowers. Between 39 and 44% of the species are large bee flowers. Another 50 to 56% of the species are classified as medium-sized and small bee flowers; while some of these may turn out to be promiscuous flowers and autogamous flowers, most of them are undoubtedly bee flowers.—Hummingbird, hawkmoth, and bat pollination are conspicuous but statistically minor components of the pollination spectrum, occurring in only one or a few species (see Table 2). Hummingbird pollination has arisen from bee pollination in the Southwest. The species or species groups with hawkmoth and bat pollination, on the other hand, are basically tropical groups which developed their advanced pollination systems in the tropics.Pollination of North American Cacti, V. See alsoGrant & Grant (1979a),Grant & al. (1979),Grant & Grant (1979b),Grant &Hurd (1979).     

Geographic variation in the breeding system and the evolutionary stability of trioecy in Pachycereus pringlei (Cactaceae)

The Sonoran Desert columnar cactus Pachycereus pringlei has a geographically variable, non-hermaphroditic breeding system. It is trioecious (separate males, females and hermaphrodites) in the northern two-thirds of its range in Sonora, Mexico, and in the southern three-quarters of its range in Baja California, Mexico, and is gynodioecious (separate females and hermaphrodites) elsewhere. Trioecy occurs near known maternity roosts of its major pollinator, the nectar-feeding bat Leptonycteris curasoae; gynodioecy occurs>50km from known bat roosts. The observed geographic patterns cannot be explained by limited gene flow or by the geographic distributions of diurnal avian pollinators. Our field observations plus a theoretical analysis suggest that the abundance of chiropteran pollinators plays an important role in the maintenance of trioecy in this plant. Under pollinator limitation, trioecy can be a stable breeding system in this species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Seedling performance in a trioecious cactus,Pachycereus pringlei: Effects of maternity and paternity

We studied seed germination and the growth and survivorship of seedlings of females and hermaphrodites ofPachycereus pringlei (cardon), a Mexican columnar cactus whose geographically variable breeding system includes trioecy and gynodioecy. Results of a two-year field experiment conducted near Bahia Kino, Sonora, Mexico and a ten-month laboratory experiment were similar and did not support the hypothesis that seedlings of females outperform those of hermaphrodites. In the field, percent seed germination and 2-yr seedling survivorship averaged 66% and 95%, respectively and did not differ among six treatment classes. Seedlings of hermaphrodites generally were larger than those of females at the end of both experiments. Selfed seedlings of hermaphrodites did not grow more slowly than outcrossed seedlings of hermaphrodites or females. Hermaphrodite seedlings performed best when pollinated with hermaphrodite pollen; female seedlings performed best with male pollen. We conclude that superior seedling performance cannot explain why females are able to coexist with hermaphrodites in populations of this cactus. Instead, we postulate that greater annual seed production, which averaged 1.6 times higher in females than in hermaphrodites in two years, may be sufficient to allow females to co-occur with hermaphrodites in this large, longlived plant, especially if sex determination involves cytoplasmic-nuclear inheritance.     

Adaptation of five columnar cactus species to various conditions in the negev desert of Israel

Five species of columnar cacti were examined with the aim of introducing new orchard crops to the Negev desert of Israel. The species comprisedStenocereus gummosus, S. griseus, S. thurberi andPachycereus pringlei, all found in semi-arid regions of Mexico, andCereus peruvianus, which is grown in sub-tropical regions. Young seedlings were planted at four sites in the Negev. The sites differ in climatic conditions and water quality. Growth data, expressed as total stem length and stem biomass, was obtained after five to six years in the orchards. They showed thatC. peruvianus grew best in the site with moderate temperatures and good-quality water, whereas the other species also did well under more extreme conditions of high temperatures and brackish water. Growth of all species was significantly retarded at the site having water with the highest NaCl content. In the sixth year a severe drop in temperature at one of the sites caused extensive injury toC. peruvianus andS. griseus plants, raising doubts as to the suitability of that site for their cultivation. At this stageC. peruvianus is the most promising candidate for domestication. Its growth rate was much higher than that of the other species, yield was precocious and fruit of good quality. Stem tissue ion concentrations (dry weight basis) showed that accumulation of K⁺was similar at all four sites and that Na⁺and Ct were significantly higher (probably in toxic concentrations) at the site with the highest concentrations ofNa⁺and Cl~ ions in the water. At all sites Cl~ concentration inC. peruvianus was much lower than that in the other species indicating that the exclusion of Cl^- from the stems was not correlated to salt tolerance.     

Reproductive biology and the process of domestication of the columnar cactus Stenocereus Stellatus in Central Mexico

Pollination biology, breeding system, and floral phenology of the columnar cactus Stenocereus stellatus were studied in wild, wild managed in situ and cultivated populations of central Mexico, in order to examine whether these aspects have been modified under domestication and whether they determine reproductive barriers between wild and manipulated individuals. Individuals of both wild and manipulated populations are self-incompatible, indicating that artificial selection has not modified the breeding system. Their pollination biology is also similar. Anthesis is mainly nocturnal, with a peak of nectar production between 0200 and 0400 when the stigma presents maximum turgidity. Nocturnal visitors are the effective pollinators. Nearly 75% of flowers exposed for nocturnal pollination set fruit, while none of the flowers exposed for diurnal pollination produced fruits. The bats Leptonycteris curasoae, L. nivalis, and Choeronycteris mexicana (Glossophaginae) are the most likely pollinators, and their time of foraging is synchronized with the time of nectar production and stigma receptivity in S. stellatus. Bats potentially move pollen over a considerable distance, so there is apparently no spatial isolation to prevent pollen exchange between wild and cultivated populations. Phenological studies showed that there are also no apparent temporal barriers. However, manual cross pollination failed between some domesticated and wild phenotypes, suggesting that gene flow between wild and cultivated populations might be limited by pollen incompatibility.     

The evolution of obligate pollination mutualisms: senita cactus and senita moth

We report a new obligate pollination mutualism involving the senita cactus, Lophocereus schottii (Cactaceae, Pachyceereae), and the senita moth, Upiga virescens (Pyralidae, Glaphyriinae) in the Sonoran Desert and discuss the evolution of specialized pollination mutualisms. L. schottii is a night-blooming, self-incompatible columnar cactus. Beginning at sunset, its flowers are visited by U. virescens females, which collect pollen on specialized abdominal scales, actively deposit pollen on flower stigmas, and oviposit a single egg on a flower petal. Larvae spend 6 days eating ovules before exiting the fruit and pupating in a cactus branch. Hand-pollination and pollinator exclusion experiments at our study site near Bahia Kino, Sonora, Mexico, revealed that fruit set in L. schottii is likely to be resource limited. About 50% of hand-outcrossed and open-pollinated senita flowers abort by day 6 after flower opening. Results of exclusion experiments indicated that senita moths accounted for 75% of open-pollinated fruit set in 1995 with two species of halictid bees accounting for the remaining fruit set. In 1996, flowers usually closed before sunrise, and senita moths accounted for at least 90% of open-pollinated fruit set. The net outcome of the senita/senita moth interaction is mutualistic, with senita larvae destroying about 30% of the seeds resulting from pollination by senita moths. Comparison of the senita system with the yucca/yucca moth mutualism reveals many similarities, including reduced nectar production, active pollination, and limited seed destruction. The independent evolution of many of the same features in the two systems suggests that a common pathway exists for the evolution of these highly specialized pollination mutualisms. Nocturnal flower opening, self-incompatible breeding systems, and resource-limited fruit production appear to be important during this evolution. Received: 19 August 1997 / Accepted: 24 November 1997     

Demography of the columnar cactus Neobuxbaumia macrocephala: a comparative approach using population projection matrices

Neobuxbaumia macrocephala is a long-lived columnar cactus endemic to the Tehuacán-Cuicatlán Valley in south-central Mexico. This plant has a very restricted distribution and few recruitment events have been detected in its populations. In this study, we analyze the N. macrocephala demographic pattern using a projection matrix in order to determine the main limiting factors of this species. To accomplish this goal, we compare our results with those obtained for another species of the same genus, N. tetetzo. Considering that both species inhabit the same valley, we believe that this comparative study will offer insights into the main demographic limitations of N. macrocephala. Results showed that these species of columnar cacti have similar demographic patterns in which survival is the process with the highest relative contribution to λ, followed by growth and reproduction. Of all the life cycle stages, seeds and seedlings have the lowest survival probabilities due to a high mortality caused by seed predation and effects of direct solar radiation on germinated seeds. The estimated growth rates indicate that populations of these species of Neobuxbaumia are in a numerical equilibrium. With respect to reproduction, N. macrocephala produce a lower number of seeds per plant than N. tetetzo. This low level of sexual reproduction may decrease the probability of establishment of new individuals in N. macrocephala populations. It is suggested that pollen limitation and pre-dispersal seed predation could be some factors that limit the distribution and abundance of this columnar cactus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Morphological consequences of the trade-off between growth and reproduction in a columnar cactus (Lophocereus schottii)

Since cacti have very small surface/volume ratios, the amount of light they intercept is limited, and so is photosynthesis. However, many cacti shade themselves by means of dense hairy mats, called cephalia, which protect the floral buds. There is a trade-off between allocating a surface to photosynthesis or to the protection of reproductive structures. We found that proportion of the stem covered by cephalium affected fitness in the columnar cactus Lophocereus schottii. Too little cephalium holds few floral buds while too much reduces fecundity – probably through diminished photosynthesis – with an intermediate amount maximising reproduction. The optimal proportion of cephalium becomes smaller as the plant grows, probably because self-shading by new branches causes a further reduction in light interception. Smaller plants had significantly less cephalium than the optimal amount to maximise current reproduction. Since the largest individuals produced more reproductive structures, small cacti may find it more profitable to grow fast to achieve a large fecundity, than to assign resources to immediate reproduction via cephalium development.     

Species-specificity of nurse plants for the establishment, survivorship, and growth of a columnar cactus

? Premise of the study: Seedling establishment and early survivorship are crucial steps for the regeneration of plant populations because both have long-lasting effects on plant population dynamics. For species recruiting through facilitation, species-specific facilitative effects might affect early fitness, an overlooked aspect in studies of facilitation considering groups of nurse species. ? Methods: We experimentally evaluated the roles of 10 nurse species and open space on the early performance of the columnar cactus Neobuxbaumia mezcalaensis. We measured establishment, survivorship, and growth of individuals over 3 years. Moreover, to study an extended period of the ontogeny of the interaction between this cactus and its nurse plants, we also monitored survivorship and growth rates of individuals between 3 to 12 cm tall during a 3-year period. ? Key results: Neobuxbaumia mezcalaensis performance varied significantly among nurse species, and only six yielded positive effects on early fitness. Densely canopied plants were the best nurses for this cactus. However, even among densely canopied species, some produced negative effects on the early fitness of N. mezcalaensis, indicating that similar nurse plants may elicit either facilitative or interference effects on beneficiary species. ? Conclusions: Our results emphasize the importance of species-specific facilitative interactions in the crucial early stages in the life cycle of N. mezcalaensis and how different nurse species modify the effect of seed-rain and contribute significantly to the population dynamics of the species.     

Pollination biology of Oreocereus celsianus (Cactaceae), a columnar cactus inhabiting the high subtropical Andes

The geographical dichotomy hypothesis suggests that columnar cacti in the tropics depend primarily on bats for pollination. This dependence may to be less in the outer tropics where many columnar cactus species (or their populations) show a relatively generalized pollination system with both nocturnal (moths and bats) and diurnal pollinators (bees and hummingbirds) (geographical dichotomy hypothesis). This hypothesis has been mostly tested in the northern tropics; nonetheless, our knowledge of columnar cactus species inhabiting the southern tropics is still scarce. The aim of this project was to evaluate the pollination biology of Oreocereus celsianus, a columnar cactus with restricted distribution in the subtropical Andes, to determine if the pollination system of this cactus tends to be more generalized than specialized because of the geographical position where it occurs. Observations of frequency of visit showed that Patagona gigas (Giant Hummingbird) is the main pollinator of the flowers, visiting them when they are opening (afternoon of the first day). Bees, wasps and moths were occasional visitors of the flowers. None of them seem to act as pollinator. Autogamy, geitonogamy and xenogamy treatments produced high fruit set, showing that O. celsianus has an unusual mixed mating system. The results suggest that this Andean columnar cactus is partially specialized on hummingbirds, with most pollination service performed by a single species, and it has the capacity of selfing (??fail-safe?? pollination system). This mixed mating system (both outcrossing and selfing) may be a response to the unpredictable environment of the Prepuna in the subtropical Andes.     

Geographic variation of floral scent in a highly specialized pollination mutualism

Floral scents are important signals for communication between plants and pollinators. Several studies have focused on interspecific variation of these signals, but little is known about intraspecific variation in flower scent, particularly for species with wide geographic distributions. In the highly specific mutualism between Ficus species and their pollinating wasps, chemical mediation is crucial for partner encounter. Several studies show that scents, i.e. blends of volatiles, are species-specific, but no studies address interpopulation variation of scents in fig pollination mutualisms, which often have broad geographic distributions. In this study, using absorption/desorption headspace techniques, we analyzed variation in floral scent composition among three populations of each of two widely distributed Asian Ficus species. We identified more than 100 different volatile organic compounds, predominantly terpenes. In both species, significant differences were found between scent bouquets of East Asian and Indian populations. These differences are discussed in relation to geographical barriers that could disrupt gene exchange between these two areas, thereby isolating Indian populations from those of Eastern Asia.     

The genetic structure of a columnar cactus with a disjunct distribution: Stenocereus gummosus in the Sonoran desert

Stenocereus gummosus is a columnar cactus endemic to the Sonoran desert that exhibits a disjunct distribution: it is widely distributed in Baja California and restricted to a small coastal area in mainland Sonora. In this paper, we examine the genetic structure and the mating system of this species in order to explore the origin of the disjunction and describe aspects of the pollination biology. Flowers are nocturnal, pollinated mainly by sphingids and self-incompatible. Polymorphism for allozymes (11 loci) was relatively high (P=75%) but moderate levels of heterozygosity were detected (H(o)=0.103 and H(e)=0.261). Sonoran populations exhibited higher levels of genetic variation than peninsular populations. H(e) declined with latitude when just peninsular and two island populations are included. Substantial levels of inbreeding within populations (f=0.60), moderate differentiation among populations (theta=0.10), and no evidence of isolation by distance were detected. The neighbor-joining phenogram showed Sonoran and island populations nested within peninsular populations. Mainland populations showed greater genetic similarity to island populations, supporting a dispersal hypothesis for the origin of the disjunction. Future studies using DNA markers are suggested in order to better understand the forces that have shaped the genetic structure of this species.