Agave macroacantha can sexually reproduce by seeds and propagate vegetatively by aerial bulbils and ground-level basal shoots and rhizomes. It forms compact patches apparently generated by the multiplication of ground-level offshoots. We experimentally evaluated the establishment and survival of bulbils and seedlings of A. macroacantha in the Tehuacán Valley, Mexico, between 1991 and 1994 and studied comparatively the effectiveness of sexual reproduction against vegetative propagation.Seedlings showed low survival rates. Cohorts placed outside nurse plants died in less than 1 yr, while 1-10% of cohorts under nurse plants survived for more than 2 yr. Herbivores negatively affected seedling survival in non-nursed plots. In rainy years, survival rates increased. Bulbils showed higher survival rates than seedlings.The excavation of rosettes showed that most are derived from vegetative shoots, as indicated by remains of rhizomes in their base. Most rosettes had ground-level vegetative offspring totaling almost three shoots per rosette.In A. macroacantha, the establishment of seedlings and bulbils is a rare event that possibly only occurs under nurse plants in rainy years, while ground-level cloning is highly effective as a propagation mechanism. These results are consistent with the aggregated spatial pattern of the species.
Bulbils are small aerial rosettes that occur on the flowering stalks of semelparous Agave plants and in related families, and that are capable of acting as clones of the parent plant. We hypothesized that bulbil formation was inversely related to fruiting success in the flowering stalk, and we explored this hypothesis in A. macroacantha, a species from South-Central Mexico. Forty randomly chosen plants were divided amongst three treatments: (a) elimination of all floral buds, (b) exclusion of pollinators, and (c) control. We also studied 22 plants in which the flowering stalk had been felled by goat grazing. Between September and November 1991 we kept a record of the numbers of bulbils and capsules produced in each flowering stalk. Significant (P<0.0001) differences between treatments were found in the proportion of plants hearing capsules and bearing bulbils. The control treatment had the highest proportion of plants producing capsules, treatment a had the highest proportion of individuals bearing bulbils, while treatment b showed an intermediate response. In the goat-grazed group, 45% of the plants failed to produce any propagative structure after the stalk was cut, and half of all plants produced bulbils on the remaining stump. A significant inverse relationship between the numbers of capsules and the numbers of bulbils per plant was found for the three randomly assigned treatments. Our results suggest that once the production of the flowering stalk has been triggered and the death of the rosette is irreversible, bulbils may act as an insurance mechanism that increases the probability of successful reproduction of the genet.
This paper analyzes the correlation between leaf orientation and the environmental conditions that prevail within the biogeographic range of each of the four South American Larrea species (L. ameghinoi, L. cuneifolia, L. divaricata, and L. nitida; Zygophyllaceae). Data on the distribution of Larrea species were gathered from herbarium specimens. Measurements of leaf orientation were made throughout the Monte Desert, and in more detail in northern Patagonia, where the four species coexist. The direct solar radiation intercepted by the unshaded leaves of each species was estimated through a computer model and plotted as a function of the hourly time for the summer and winter solstices. L. ameghinoi presents horizontal leaves and prostrate growth, characters which allow its development on sites that are exposed to the Patagonian westerlies. The species, however, is an inefficient light interceptor in winter and early spring, when moisture conditions are adequate in Patagonia. Its architecture is the result of selection for cushion—type, wind resistant forms, at the expense of light interception. It is restricted to windy, open areas of the Patagonian steppe. L. cuneifolia shows erect, east—facing leaves and branches, which maximize interception in the early morning and late afternoon, keeping noon interception at a minimum. It can tolerate very hot environments by physically evading the midday sun and intercepting more early morning and late afternoon light. It colonizes the hotter and drier parts of the Monte Desert. L. divaricata has divaricate leaves with folioles uniformly distributed in all azimuthal directions, and showing an inclination of around 70°. Although it never shows maximum light interception efficiencies, it preforms relatively well in all seasons and at all hours of the day. Its distribution is wide, not only in the arid Monte, but reaching also the Chaco woodlands and the Pacific coastal deserts. L. nitida shows erect, north—facing leaves and branches. Its leaf orientation distribution allows the gradual warming of the leaf surfaces during the morning, with a maximum light interception near winter noons. The species grow in the Patagonian Monte and on the slopes of the Andes. Its general affinity with the colder west side of the Patagonian and Monte Deserts links its distribution with winter—type rains of Pacific origin. The results suggest that the contrasting leaf orientations of the four South American Larrea species reflect the prevalent selective conditions endured under long periods in isolation, and that leaf orientation is an adaptive character that influences the habitat specificity of the different species.
We did a series of observational studies and manipulative experiments on the guild of nocturnal visitors of Agave macroacantha, including (1) a description of the hourly patterns of visits by moths and bats, (2) an evaluation of the relative contribution of bats and moths to flowering success, and (3) an evaluation of the pollination efficiency of the different bat species. Scapes exposed to moths but excluded to bats yielded ∼50% fewer fruits than those exposed to both pollinator groups. Flowers exposed to the bat species Leptonycteris curasoae showed similar fruiting success to those exposed to Choeronycteris mexicana and to those exposed to the whole nocturnal visitor guild. However, the fruits originated from flowers pollinated by Leptonycteris curasoae yielded significantly more seed than those exposed to Choeronycteris mexicana or to the whole pollinator guild. It is concluded that Agave macroacantha is extremely dependent on nocturnal pollinators for its reproductive success and that bats are especially important for successful pollination. Some of these pollinators are migratory and have been reported to be steadily declining. A continuing decline in the populations of pollinators may impede the successful sexual reproduction of the plant host and may put the long-term survival of this agave species under risk.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.