Seed response to temperature of Mexican cacti species from two life forms: an ecophysiological interpretation

The effect of seven different constant temperatures and five ranges of alternating temperatures on seed germination of seven species of cacti from Puebla, México was analyzed. Six cacti species germinated in a wide range of temperature. Columnar cacti were more tolerant to low temperatures and germinated in a wider range of temperature than barrel cacti. One of the barrel cacti studied (Ferocactus recurvus) only reached full germination at 25 °C . Temperature fluctuations did not produce significant effects on germination compared to the results obtained at constant temperatures. This may reveal differing ecophysiological adaptations with respect to temperature requirements during the establishment conditions for each life form. Columnar cacti may become established mainly under the shade of desert shrubs, whereas barrel cacti maybe can also become established in open areas, beneath the shade of small rocks or soil irregularities. In both cases, temperature fluctuations are attenuated by the shade, but mean temperatures may be higher in the second condition than beneath the shade of plants.     

Analysis of form and function in North American columnar cacti (tribe Pachycereeae)

Simple and multivariate linear models were used to demonstrate the influence of mechanical design and climate on stem morphology and branching architecture in 25 species of North American columnar cacti. The effect of phyletic inertia was tested by the method of independent contrasts. Stem girth was found to increase significantly slower with increased height within taxa (cross-sectional stem area ;ps [plant height] 0.603), than across taxon (cross-sectional stem area ;ps [plant height] 1.451). Juveniles are shown to be mechanically overbuilt and subsequently grow into more slender adult forms determined in part by structural limitations and the optimization of other stem functions. We make a structural analogy of relatively rigid columnar cacti to concrete columns and compare plants and models with similar growth forms lacking woody skeletons (barrel cacti). Taxa with woody support achieved a surface-to-volume ratio six times greater than taxa without woody support. Across taxon, cooler winter temperatures were associated with larger stem girths, and greater annual precipitation was associated with less frequent branching. The relationship between total plant surface and volume approaches isometry within taxa, but across taxon average individuals are scaled replicates. We hypothesize that architecture and average plant height are adjusted, in an evolutionary sense, to maintain geometric similitude between surface and volume along a climatic gradient.     

Population Structure and Growth Patterns of Opuntia echios var. gigantea along an Elevational Gradient in the Galípagos Islands1

Cacti growing in forests potentially experience growth limitation due to reduced light availability. To test this hypothesis, we studied the population structure of Opuntia echios var. gigantea at 15 sites on the south side of Isla Santa Cruz, Galípagos Islands, Ecuador. Populations were located in communities ranging from arid scrub at low elevations to closed‐canopy tropical dry forest at higher elevations. Ordination confirmed the existence of a strong elevation‐vegetation gradient. Opuntia abundance peaked at lower elevations (ca 30 m), with lower densities in closed‐canopy sites. For populations in scrub vegetation, density declined fairly regularly with plant height. Populations in forested sites had few plants of intermediate height, suggesting periodic recruitment. Scrub populations had random dispersion, while those in forests were aggregated. The change in spatial pattern may be related to a change in primary reproductive mode from asexual propagation via fallen fruits to propagation via fallen cladodes. Height was significantly correlated with stem diameter. Intercepts of these relationships increased toward higher elevations, probably in response to the increasing height of the surrounding canopy.     

Resource availability, population dynamics and diet of the nectar-feeding bat Leptonycteris curasoae in Guaymas, Sonora, Mexico

Nectar-feeding bats play important roles in the pollination of columnar cacti. Migratory bats such as Leptonycteris curasoae are vulnerable to extinction due to their migratory behavior and cave-roosting habit. The conservation of migratory bats requires the identification and protection of key roosting sites and their floral resources. In this paper we describe the availability of resources, the population dynamics and the diet of L. curasoae in the Guaymas region, and its diet at three additional sites in coastal Sonora, Mexico. Our results indicate that L. curasoae is a seasonal resident in the Guaymas area, with local occupation of around 4 months. Seasonal occupation seems to be determined by the availability of flowers and fruits of columnar cacti as peak abundance and peak availability coincide in time. The analyses of fecal and fur samples revealed that columnar cacti constitute the major food source during their seasonal occupation of coastal Sonora. The frequency of pollen from Agave and Bombacaceae in fecal samples was generally low but increased in importance in lower latitudes. Our results suggest that the availability of resources provided by columnar cacti determine seasonal occupation of L. curasoae in Sonora. Given that we detected newborns in the studied colony, we suggest active cave protection for the conservation of the maternity roosting site in Guaymas, Sonora, Mexico.     

Reproductive biology of Ferocactus recurvus (Mill.) Borg subsp. recurvus (Cactaceae) in the Tehuacán‐Cuicatlán Valley,Mexico

• Mexico has one of the highest diversities of barrel cacti species worldwide; however, all are threatened and require conservation policies. Information on their reproductive biology is crucial, but few studies are available. Ferocactus recurvus subsp. recurvus is a barrel cactus endemic to the Tehuacán‐Cuicatlán Valley. Our research aimed to characterise its floral and pollination biology. We hypothesised bee pollination, as suggested by its floral morphology and behaviour, and self‐incompatibility, like most barrel cacti studied.
• Three study sites were selected in the semiarid Zapotitlán Valley, Mexico. We examined 190 flowers from 180 plants to determine: morphometry and behaviour of flowers, flower visitors and probable pollinators, and breeding system.
• Flowers showed diurnal anthesis, lasting 2–5 days, the stigma being receptive on day 2 or 3 after the start of anthesis. Flowers produced scarce/no nectar and main visitors were bees (Apidae), followed by flies (Muscidae), ants (Formicidae), thrips (Thripidae) and hummingbirds (Throchilidae); however, only native bees and occasionally wasps contacted the stigma and anthers. Pollination experiments revealed that this species is self‐incompatible and xenogamous. In natural conditions, fruit set was 60% and cross‐pollination fruit set was 100%. Percentage seed germination resulting from cross‐pollination was higher than in the control treatment.
• Our results provide ecological information for conservation programmes to ensure a high probability of breeding and seed production in natural populations of F. recurvus.

     

Distribution of plant life forms along an altitudinal gradient in the semi‐arid valley of Zapotitlán,Mexico

Abstract. Plant life‐form abundance along a 600 m altitudinal gradient (1600–2200 m a.s.l.) in the semi‐arid valley of Zapotitlán, México was correlated with soil characteristics and climatic variables. One mixed soil sample was taken and analysed for each of six elevations, temperature was estimated using a terrestrial thermal gradient, and precipitation using a linear regression between total annual precipitation and the elevation of the weather stations in the valley. Rosettes, microphanerophytes, therophytes and nanophanerophytes were well represented throughout the gradient. Columnar cacti were restricted to the 1600–1800 m range, and geophytes to the 1700–1800 m range. In general, abundance of life forms was inversely associated with altitude. Multiple regression analysis did not show parameters to significantly explain the abundance of rosettes, nanophanerophytes, epiphytes, geophytes and hemiparasites; altitude and nitrogen proved significant for columnar cacti, succulents and chamaephytes; altitude, pH, electrical conductivity and nitrogen were significant for globose cacti; pH was significant for therophytes; and altitude was significant for microphanerophytes.     

Estudio Fenológico de Cactíceas en el Enclave Seco de la Tatacoa,Colombia1

A one‐year phenological study of three columnar cacti, Stenocereus griseus (Haw.) Britton & Rose, Pilosocereus sp., Cereus hexagonus (L.) Mill., and a decumbent cactus Monvillea cf. smithiana (Britton & Rose) Backeberg., was carried out in the Andean arid region of La Tatacoa, Colombia. Pollinators and/or dispersers of the cacti species also were studied monthly, and fecal samples were collected for the identification of pollen and seeds. The flowering of all species was prolonged and showed bimodal, multimodal, or irregular patterns. Fruiting in all species also was prolonged and followed flowering with a lag of less than two months. Although there were no simple correlations between rainfall and flowering or fruiting, flower production during the dry season was higher for 5. griseus, while Pilosocereus sp. and C. hexagonus showed higher flower production during the wet season. Fruit production was also seasonal, with higher production during the wet season for 5. griseus and C. hexagonus. The patterns of flowering and fruiting in M. cf. smithiana showed no relationships with dry and wet seasons. The bats Glossophaga longirostris, Carollia perspicillata, Sturnira lilium, the birds Melanerpes rubricapillus (Picidae) and Mimus gilvus (Mimidae), and moths of the family Sphingidae, were identified as pollinators and/or fruit consumers of these cacti species.     

Diurnal and nocturnal pollination of Marginatocereus marginatus (Pachycereeae: Cactaceae) in Central Mexico

BACKGROUND AND AIMS: Chiropterophillous and ornithophillous characteristics can form part of a single reproductive strategy in plants that have flowers with diurnal and nocturnal anthesis. This broader pollination strategy can ensure seed set when pollinators are scarce or unpredictable. This appears to be true of hummingbirds, which presumably pollinate Marginatocereus marginatus, a columnar cactus with red nocturnal and diurnal flowers growing as part of dense bat-pollinated columnar cacti forests in arid regions of central Mexico. The aim of this study was to study the floral biology of M. marginatus, and evaluate the effectiveness of nocturnal vs. diurnal pollinators and the contribution of each pollinator group to overall plant fitness. METHODS: Individual flower buds were marked and followed to evaluate flower phenology and anthesis time. Flowers and nectar production were measured. An exclusion experiment was conducted to measure the relative contribution of nocturnal and diurnal pollinators to seed set. KEY RESULTS: Marginatocereus marginatus has red hermaphroditic flowers with nocturnal and diurnal anthesis. The plant cannot produce seeds by selfing and was pollinated during the day by hummingbirds and during the night by bats, demonstrating that both pollinator groups were important for plant reproduction. Strong pollen limitation was found in the absence of one of the pollinator guilds. CONCLUSIONS: Marginatocereus marginatus has an open pollination system in which both diurnal and nocturnal pollinators are needed to set seeds. This represents a fail-safe pollination system that can ensure both pollination, in a situation of low abundance of one of the pollinator groups (hummingbirds), and high competition for nocturnal pollinators with other columnar cacti that bloom synchronously with M. marginatus in the Tehuacan Valley, Mexico.     

Equilibrium mechanics of monolayered epithelium

In order to fully understand the epithelial mechanics it is essential to integrate different levels of epithelial organization. In this work, we propose a theoretical approach for connecting the macroscopic mechanical properties of a monolayered epithelium to the mechanical properties at the cellular level. The analysis is based on the established mechanical models—at the macroscopic scale the epithelium is described within the mechanics of thin layers, while the cellular level is modeled in terms of the cellular surface (cortical) tension and the intercellular adhesion. The macroscopic elastic energy of the epithelium is linked to the energy of an average epithelial cell. The epithelial equilibrium state is determined by energy minimization and the macroscopic elastic moduli are calculated from deformations around the equilibrium. The results indicate that the epithelial equilibrium state is defined by the ratio between the adhesion strength and the cellular surface tension. The lower and the upper bounds for this ratio are estimated. If the ratio is small, the epithelium is cuboidal, if it is large, the epithelium becomes columnar. Importantly, it is found that the cellular cortical tension and the intercellular adhesion alone cannot produce the flattened squamous epithelium. Any difference in the surface tension between the apical and basal cellular sides bends the epithelium towards the side with the larger surface tension. Interestingly, the analysis shows that the epithelial area expansivity modulus and the shear modulus depend only on the cellular surface tension and not on the intercellular adhesion. The results are presented in a general analytical form, and are thus applicable to a variety of monolayered epithelia, without relying on the specifics of numerical finite-element methods. In addition, by using the standard theoretical tools for multi-laminar systems, the results can be applied to epithelia consisting of layers with different mechanical properties.     

Evolution under domestication: ongoing artificial selection and divergence of wild and managed Stenocereus pruinosus (Cactaceae) populations in the Tehuac��n Valley, Mexico

Background and Aims

The Tehuacán Valley in Mexico is a principal area of plant domestication in Mesoamerica. There, artificial selection is currently practised on nearly 120 native plant species with coexisting wild, silvicultural and cultivated populations, providing an excellent setting for studying ongoing mechanisms of evolution under domestication. One of these species is the columnar cactus Stenocereus pruinosus, in which we studied how artificial selection is operating through traditional management and whether it has determined morphological and genetic divergence between wild and managed populations.

Methods

Semi-structured interviews were conducted with 83 households of three villages to investigate motives and mechanisms of artificial selection. Management effects were studied by comparing variation patterns of 14 morphological characters and population genetics (four microsatellite loci) of 264 plants from nine wild, silvicultural and cultivated populations.

Key Results

Variation in fruit characters was recognized by most people, and was the principal target of artificial selection directed to favour larger and sweeter fruits with thinner or thicker peel, fewer spines and pulp colours others than red. Artificial selection operates in agroforestry systems favouring abundance (through not felling plants and planting branches) of the preferred phenotypes, and acts more intensely in household gardens. Significant morphological divergence between wild and managed populations was observed in fruit characters and plant vigour. On average, genetic diversity in silvicultural populations (H_E = 0·743) was higher than in wild (H_E = 0·726) and cultivated (H_E = 0·700) populations. Most of the genetic variation (90·58 %) occurred within populations. High gene flow (Nm_FST > 2) was identified among almost all populations studied, but was slightly limited by mountains among wild populations, and by artificial selection among wild and managed populations.

Conclusions

Traditional management of S. pruinosus involves artificial selection, which, despite the high levels of gene flow, has promoted morphological divergence and moderate genetic structure between wild and managed populations, while conserving genetic diversity.