Variation In Fruit Set Research Articles

Honey bee (Apis mellifera) colony strength and its effects on pollination and yield in highbush blueberries (Vaccinium corymbosum).

Many pollination studies with honey bees have examined the effect of colony density on crop yield and yet overlook the effect of variation in the population size of these colonies. High colony density in northern highbush blueberry has been met with concerns from beekeepers who feel higher densities will intensify outbreaks of European foulbrood (EFB, Melissococcus plutonius, Truper and dé Clari), a honey bee brood disease. The purpose of this study was to confirm the prevalence of EFB in colonies pollinating blueberries and to determine whether field-level variation in the population of adult workers in colonies explained variation in blueberry fruit set and/or yield. We addressed these objectives over the course of two production seasons at 13 commercial blueberry fields in Oregon, USA, stocked with identical densities of 10 colonies/ha. We confirmed that all colonies had negligible symptoms of EFB at the start of blueberry pollination, but 53% of colonies in 2019 and 41% in 2020 had symptoms immediately following the pollination season. We also validated a method for rapidly assessing adult honey bee colony populations, namely by counting the rate of foragers returning to colonies, and it was found to be strongly correlated to true internal adult bee population independent of year and ambient temperature at the time of evaluation. Using returning forager counts, we determined there was considerable variation in the average population of colonies at each field, ranging from an estimated 10,300 to 30,700 adult worker bees per colony. While average colony strength did not predict variation in fruit set, it was related to variation in yield, independent of year. Our linear model of flight count (as a proxy for colony strength) predicts estimated yield increases of up to 25,000 kg/ha of blueberries could be achieved by colonies stronger than the recommended six frame minimum, suggesting that higher pollination benefits could be achieved without increasing hive density if stronger colonies are promoted.

Seed production potentiality in yield and quality of eggplant (Solanum melongena L.) Grown under summer and winter seasons

Two separate field experiments were conducted at the Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur during April to October 2012 (summer season) and October 2012 to March 2013 (winter season) to evaluate and compare the seasonal effect on fruit set, seed yield and seed quality of eggplant. Six eggplant varieties (BARI Begun-1, BARI Begun-5, BARI Begun- 6, BARI Begun-8, BARI Begun-9 and Khotkhotia) were grown separately with proper isolation in both the seasons. Significant variation in fruit set per plant, seed yield per fruit and seed quality of eggplant were observed due to execution of growing seasons. The highest seed yield per fruit was obtained from BARI Begun-6 in both the seasons. Number of fruits per plant, seeds per fruit and 1000-seed weight showed the highest in winter season (October to March) as a result the highest seed yield was obtained from the same season. Winter season also showed the best seed quality attributes like germination (%), co-efficient of germination and vigour index irrespective of variety used. Based on seed yield and seed quality, winter season (October to March) found to be more favourable for quality seed production of eggplant in Bangladesh condition.Bangladesh J. Agril. Res. 42(3): 437-446, September 2017

Seasonal variation in the mating system of a selfing annual with large floral displays.

Flowering plants display considerable variation in mating system, specifically the relative frequency of cross- and self-fertilization. The majority of estimates of outcrossing rate do not account for temporal variation, particularly during the flowering season. Here, we investigated seasonal variation in mating and fertility in Incarvillea sinensis (Bignoniaceae), an annual with showy, insect-pollinated, 'one-day' flowers capable of delayed selfing. We examined the influence of several biotic and abiotic environmental factors on day-to-day variation in fruit set, seed set and patterns of mating. We recorded daily flower number and pollinator abundance in nine 3 × 3-m patches in a population at Mu Us Sand land, Inner Mongolia, China. From marked flowers we collected data on daily fruit and seed set and estimated outcrossing rate and biparental inbreeding using six microsatellite loci and 172 open-pollinated families throughout the flowering period. Flower density increased significantly over most of the 50-d flowering season, but was associated with a decline in levels of pollinator service by bees, particularly on windy days. Fruit and seed set declined over time, especially during the latter third of the flowering period. Multilocus estimates of outcrossing rate were obtained using two methods (the programs MLTR and BORICE) and both indicated high selfing rates of ∼80 %. There was evidence for a significant increase in levels of selfing as the flowering season progressed and pollinator visitation declined. Biparental inbreeding also declined significantly as the flowering season progressed. Temporal variation in outcrossing rates may be a common feature of the mating biology of annual, insect-pollinated plants of harsh environments but our study is the first to examine seasonal mating-system dynamics in this context. Despite having large flowers and showy floral displays, I. sinensis attracted relatively few pollinators. Delayed selfing by corolla dragging largely explains the occurrence of mixed mating in I. sinensis, and this mode of self-fertilization probably functions to promote reproductive assurance when pollinator service is limited by windy environmental conditions and competition from co-occurring flowering plants.

Spatial and temporal variation on fruit set in <i>Epipactis</i> thunbergii (Orchidaceae) from southern Korea

ABSTRACT: Spatio-temporal variation in fruit set in orchids would affect long-term population viability and willinfluence genetic diversity over many generations. The aim of this study was to examine the breeding system ofthe nectariferous terrestrial orchid Epipactis thunbergii, to specifically determine levels of fruit set in terms oftime and space under natural conditions. We examined pollination under natural conditions and conducted handpollination experiments during a 2-year survey in four populations located along 1.5 km of coastal line in Jingui-ri (rual village) [Jeollanam-do (province), southern Korea]. We found that, over a 2-year period, levels of per-centage of fruit set were similar within patches of the four populations. By contrast, we detected significant dif-ferences in the percentage of fruit set among patches. We also found that plants with larger inflorescence sizeproduced significantly more fruits than plants with fewer flowers. Over a 2-year period, the percentage of fruitset for E. thunbergii was similar but low (14.1%) compared to that averaged for eighty-four rewarding species(37.1%). However, an increase in fruit set was achieved by hand-pollinations: artificial self-pollination (90.5−95.2%), artificial geitonogamy (94.7−95.0%), and cross-pollination (artificial xenogamy, 91.3−91.4%). Noemasculated flowers produced fruits and no automatic pollination was found in E. thunbergii. Our findings sug-gest that E. thunbergii is a self-compatible terrestrial orchid that depends on pollinators (insects) to achieve fruitset in natural habitats, and that local environmental conditions were similar over a period of 2 years in the studyarea. Our results also highlight the cryptic variation of fruit production in time, but more pronounced variabilityin space.

Relative density and dispersion pattern of two southern African Asteraceae affect fecundity through heterospecific interference and mate availability, not pollinator visitation rate

Summary Since co‐flowering plants often share pollinators, their fecundity is likely affected by pollen transfer within and among plant species. Changes in pollinator visitation rates (e.g. through facilitation or competition for pollinators) are usually thought to exert the predominant influence on plant fecundity. However, the transfer of conspecific relative to heterospecific pollen between individuals may also play an important role. Indeed, the relative importance of these determinants of fecundity is expected to depend on the density, relative abundance and spatial arrangement of con‐ and heterospecifics. We investigated the effects of con‐ and heterospecific density and spatial distribution pattern on pollinator visitation and plant fecundity within and between two annual, self‐incompatible co‐flowering species [Dimorphotheca pinnata and Ursinia cakilefolia (Asteraceae)] by manipulating their relative abundance, overall patch density and dispersion patterns in experimental arrays in Namaqualand, South Africa. We quantified pollinator visitation rates and fruit set in arrays of varying density and aggregation. This enabled us to determine which mechanism(s) were driving variation of fecundity, particularly through their influence on visitation rates, mate availability and heterospecific pollen interference. To test whether autogamy offers reproductive assurance when individuals are scattered among a dense population of heterospecifics, we included an autogamous species (U. anthemoides) in a separate experiment. We found that increased fecundity with increasing conspecific density was not the result of higher visitation rates, but rather increased mate availability. Furthermore, increased spatial aggregation of conspecifics at low density significantly increased fecundity through reduced heterospecific interference. In contrast to results for self‐incompatible species, fruit set in U. anthemoides was consistently high and unaffected by scattered distribution patterns. This suggests that autogamy may offer reproductive assurance when mates are limited and the potential for interspecific pollen transfer (IPT) is high. Synthesis. In this study of annual daisies, variation in fruit set is primarily driven by factors that affect the transfer of conspecific relative to heterospecific pollen, independent of pollinator visitation rate. Our findings demonstrate that mate limitation and IPT negatively affect fruit set and that these effects can be mitigated by intraspecific aggregation and the ability to autonomously self‐pollinate.

Variation in pollen limitation and floral parasitism across a mating system transition in a Pacific coastal dune plant: evolutionary causes or ecological consequences?

Evolutionary transitions from outcrossing to self-fertilization are thought to occur because selfing provides reproductive assurance when pollinators or mates are scarce, but they could also occur via selection to reduce floral vulnerability to herbivores. This study investigated geographic covariation between floral morphology, fruit set, pollen limitation and florivory across the geographic range of Camissoniopsis cheiranthifolia, a Pacific coastal dune endemic that varies strikingly in flower size and mating system. Fruit set was quantified in 75 populations, and in 41 of these floral herbivory by larvae of a specialized moth (Mompha sp.) that consumes anthers in developing buds was also quantified. Experimental pollen supplementation was performed to quantify pollen limitation in three large-flowered, outcrossing and two small-flowered, selfing populations. These parameters were also compared between large- and small-flowered phenotypes within three mixed populations. Fruit set was much lower in large-flowered populations, and also much lower among large- than small-flowered plants within populations. Pollen supplementation increased per flower seed production in large-flowered but not small-flowered populations, but fruit set was not pollen limited. Hence inadequate pollination cannot account for the low fruit set of large-flowered plants. Floral herbivory was much more frequent in large-flowered populations and correlated negatively with fruit set. However, florivores did not preferentially attack large-flowered plants in three large-flowered populations or in two of three mixed populations. Selfing alleviated pollen limitation of seeds per fruit, but florivory better explains the marked variation in fruit set. Although florivory was more frequent in large-flowered populations, large-flowered individuals were not generally more vulnerable within populations. Rather than a causative selective factor, reduced florivory in small-flowered, selfing populations is probably an ecological consequence of mating system differentiation, with potentially significant effects on population demography and biotic interactions.

Demographic effects on fruit set in the dioecious shrub Canada buffaloberry (Shepherdia canadensis).

The effects of pollen limitation on reproductive success in plants have been well-documented using pollen supplementation experiments. However, the role of local demographics in determining pollen limitation, particularly in terms of the additive and interactive effects of pollen availability and competition are not well known. We measured fruit set in the dioecious shrub Canada buffaloberry (Shepherdia canadensis) in Central Alberta, Canada to evaluate whether local demographics measured at three spatial scales (25, 50, and 100 m2) affect fruit set in buffaloberry. We test whether density-dependence (population density), pollen donor (measured as male density, distance to nearest male plant and size of nearest male plant), female competitor (measured as female density and distance to nearest female plant), or the combined pollen donor and competitor hypotheses best explain natural variations in fruit set for a population of Canada buffaloberry. Support was highest for the combined pollen donor and competitor hypothesis at an intermediate spatial scale of 50 m2. Proportion fruit set increased with male shrub density (pollen donors) and decreased with female shrub density (pollen competitors), but was more affected by the presence of males than females. This illustrates that access to male shrubs within a 3.99 m radius affects pollen availability, while nearby females compete intra-specifically for pollen.

Is pollen removal or seed set favoured by flower longevity in a hummingbird-pollinated Salvia species?

The period between the beginning of anthesis and flower senescence modulates the transport of pollen by pollinators among conspecific flowers, and its length may therefore influence reproductive success. This study evaluated whether floral longevity favours pollen removal from the anthers over fecundity (seed set) in an ornithophilous species that does not undergo pollen limitation. Field investigations were conducted on floral longevity, nectar production, pollinator behaviour, and variations in fruit set (FS), mean number of seeds per fruit (MSF) and pollen removal by hummingbirds (PR) during the anthesis of Salvia sellowiana in south-east Brazil. Anthesis of flowers exposed to pollinators lasted 4 d, as well as on flowers with pollen removed from the anthers or deposited on the stigma. The longevity of bagged flowers was significantly higher (approx. 9 d). FS and PR reached 87.2 and 90 %, respectively, in natural conditions. PR increased gradually over the period of anthesis; however, FS and MSF reached their maxima in the first hours of anthesis. Nectar production was continuous, but the secretion rate was reduced after pollination. The removal of nectar from non-pollinated flowers stimulated its production. The longevity of anthesis in S. sellowiana seems to be related to the mechanism of gradual dispensing of pollen, resulting in greater male reproductive success. This is in agreement with the pollen-donation hypothesis. The small number of ovules (four) of S. sellowiana and the high frequency and the foraging mode of its pollinators may favour the selection for floral longevity driven by male fitness in this system.

Despite their apparent integration, spur length but not perianth size affects reproductive success in the moth‐pollinated orchidPlatanthera bifolia

Summary1. The evolution of many floral traits is thought to have been shaped by pollinator‐mediated selection for increased attractiveness or an improved mechanical fit of flowers to pollinators. Yet, few studies have examined experimentally the independent and interactive effects of different aspects of flower morphology on plant reproductive success.2. In the orchidPlatanthera bifolia,perianth size and spur length are positively correlated within and among populations. To test the hypothesis that pollination success and seed output increases with increasing perianth size and spur length, we manipulated the two traits in a factorial design in a long‐spurred population ofP. bifoliapollinated by long‐tongued hawkmoths. Additionally, to determine whether differences in selfing rate can explain variation in fruit set and fruit size, we performed controlled self‐ and cross‐pollination.3. Plants with long spurs had more flowers pollinated, more pollen removed and produced more and larger fruits compared to plants with short spurs. In contrast, perianth size did not affect the pollination success or fruit production ofP. bifolia.4. Fruit production and fruit size did not differ among flowers pollinated with self‐ and cross pollen, respectively. This indicates that reduced pollen deposition rather than pollinator‐mediated self‐pollination caused the reduction in fruit set and fruit volume observed after shortening of the spur.5. The results demonstrate that spur length, but not perianth size, is critical for reproductive success inP. bifolia, and suggest that among‐population differentiation in perianth size may reflect a correlated response to selection on spur length. The results are consistent with the hypothesis that visual display is less important than other cues for the reproductive success ofP. bifolia, and underscore the necessity to experimentally examine the functional significance of putatively adaptive traits.

Temporal and spatial variation in flower and fruit production in a food-deceptive orchid: a five-year study

Deceptive orchids are generally characterized by low levels of fruit set; however, there may be substantial variations in fruit set between sites and years. Within a single population, individual plants may also differ greatly in their reproductive output as a result of differences in inflorescence size or local density. In this study, we determined flower and fruit production over 5 years in two populations of the food-deceptive orchid, Orchis purpurea. All plants were monitored annually for survival and flowering at each site to determine whether flowering and fruiting induced costs. The number of flowers per inflorescence varied considerably from year to year (min: 36.6, max: 49.5). Average fruit set was low (7%) and varied considerably among years and populations. A considerable proportion of plants also failed to set any fruit. However, the probability of producing at least one fruit was not affected by inflorescence size or local density. The number of fruits was significantly related to inflorescence size, but proportional fruit set was not. Local density also did not affect the number of fruits, nor proportional fruit set. There was also no evidence that plants with large inflorescence size or high fruiting success had a larger probability of remaining vegetative the year after flowering than plants with small inflorescence size or low fruiting success. Our results suggest that pollinator-mediated selective forces on inflorescence size through female reproductive success alone are weak, most likely because of the low overall level of visitation and the resulting uncertainty of pollination at the individual level. Our results further demonstrate that investigation of patterns of fruit set over several years is needed to better understand the variability in female reproductive success that is typical of most plant-pollinator interactions.

Evolution of autonomous selfing accompanies increased specialization in the pollination system of Schizanthus (Solanaceae)

The co-occurrence of elaborate flowers visited by specific groups of pollinators and capacity for autonomous selfing in the same plant species has puzzled evolutionary biologists since the time of Charles Darwin. To examine whether autonomous selfing and floral specialization evolved in association, we quantified the autofertility level (AFI) in nine Schizanthus species characterized by a wide range of pollination specialization, revealing AFI values of 0.02 to complete selfing. An independent contrasts analysis conducted on AFIs and number of functional pollinator groups showed that autonomous selfing evolved from an ancestral outcrossing system as plants became increasingly specialized (r = -0.82). To assess whether autonomous selfing together with specialization acts as a reproductive assurance mechanism, we estimated spatial and interannual variation in fruit set due to pollinator failure in two closely related high Andean Schizanthus species differing in their specialization levels. Variation in pollinator failure rate was more pronounced and autonomous selfing increased fruit production over biotically assisted pollination in the more specialized species. Our study suggests that specialized pollination deems species more vulnerable to pollinator fluctuation thus promoting the evolution of delayed autonomous selfing.

Large population sizes mitigate negative effects of variable weather conditions on fruit set in two spring woodland orchids

Global circulation models predict increased climatic variability, which could increase variability in demographic rates and affect long-term population viability. In animal-pollinated species, pollination services, and thus fruit and seed set, may be highly variable among years and sites, and depend on both local environmental conditions and climatic variables. Orchid species may be particularly vulnerable to disruption of their pollination services, as most species depend on pollinators for successful fruit set and because seed germination and seedling recruitment are to some extent dependent on the amount of fruits and seeds produced. Better insights into the factors determining fruit and seed set are therefore indispensable for a better understanding of population dynamics and viability of orchid populations under changing climatic conditions. However, very few studies have investigated spatio-temporal variation in fruit set in orchids. Here, we quantified fruit production in eight populations of the orchid Orchis purpurea that does not reward pollinators and 13 populations of the rewarding Neottia (Listera) ovata during five consecutive years (2002-2006). Fruit production in large populations showed much higher stability than that in small populations and was less affected by extreme weather conditions. Our results highlight the potential vulnerability of small orchid populations to an increasingly variable climate through highly unpredictable fruit-set patterns.

SURVIVAL ANALYSIS OF FLOWER AND FRUIT ABORTION IN SWEET PEPPER

In order to obtain a crop growth model that can simulate inter- and intra-plant variation in fruit set, fruit abortion times in sweet pepper were analysed by means of survival analysis. Survival analysis is a statistical technique dealing with the timing of events. The Cox proportional hazards model estimates the instantaneous baseline probability of abortion per time (hazard rate), given that the fruit has not aborted yet. It also estimates the effect of explanatory factors (covariates) on the abortion rate. Two important factors known to influence abortion in sweet pepper are the supply and demand for assimilates (source and sink strength, respectively). A plant density experiment was analysed, as density influences plant source and indirectly also sink strength. Flowering as well as abortion or harvest dates were recorded for all individual flowers and fruits. LAI was measured to calculate the source strength with a photosynthesis-based simulation model. Empirical curves showed that survival of flowers and fruits (per plant) was higher at 1.6 plants/m2 than at 3.1 or 4.6 plants/m2. Plant density, source strength and source/sink ratio all correlated with the treatment plant density, but source/sink ratio explained most of the variation in the data in a Cox proportional hazards model. Averaging the source/sink ratio over seven days gave the best fit. Adding the position within the node (main or side branch) improved the fit; flowers from the side branch had a higher probability of abortion per unit time than flowers from the main branch. The susceptibility for fruit abortion differed among plants. The baseline hazard rate indicated that between 0 and 13 days after anthesis, flowers were most susceptible for abortion.

Male sterility and reproductive output in distylous Erythroxylum suberosum (Erythroxylaceae)

Distribution of male and female organs on individual plants is diverse. Thus, accurate descriptions of plant sexuality are necessary in order to characterize changes in gender. In this study, we evaluated the level of male sterility and its implication for reproductive output in the distylous species Erythroxylum suberosum A. St.-Hil. We recorded that both thrum and pin plants are partially male-sterile, featuring a range of abnormalities in the androecium. Male sterility of donors acts as an important factor responsible for variation in fruit set. This correlation was found for E. suberosum only between thrum pollen donors and pin recipient plants, indicating that there are differences in individual performance of pollen donors between the two floral morphs. However, male sterility occurs at a low frequency in both morphs, and there is no reproductive compensation, in terms of resource reallocation, for the loss of the male function. Under these circumstances, distyly is expected to be evolutionarily stable in E. suberosum © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 88, 465–474.

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Fruit set of highland coffee increases with the diversity of pollinating bees.

The worldwide decline of pollinators may negatively affect the fruit set of wild and cultivated plants. Here, we show that fruit set of the self-fertilizing highland coffee (Coffea arabica) is highly variable and related to bee pollination. In a comparison of 24 agroforestry systems in Indonesia, the fruit set of coffee could be predicted by the number of flower-visiting bee species, and it ranged from ca. 60% (three species) to 90% (20 species). Diversity, not abundance, explained variation in fruit set, so the collective role of a species-rich bee community was important for pollination success. Additional experiments showed that single flower visits from rare solitary species led to higher fruit set than with abundant social species. Pollinator diversity was affected by two habitat parameters indicating guild-specific nesting requirements: the diversity of social bees decreased with forest distance, whereas the diversity of solitary bees increased with light intensity of the agroforestry systems. These results give empirical evidence for a positive relationship between ecosystem functions such as pollination and biodiversity. Conservation of rainforest adjacent to adequately managed agroforestry systems could improve the yields of farmers.

Effects of flower and seed predators and pollinators on fruit production in two sequentially flowering congeners

Entomophilous species that bloom in early spring are more frequently confronted with severe pollen limitation and low fruit set than species that bloom later. On the other hand, early flowering species may benefit by escaping the peak of flower and/or seed predator activity. Previous studies have implicated unpredictable weather conditions and the inactivity of pollinator and predator insects early in spring as causes of severe pollen limitation and light predation in early flowering species. However, differences not only in flowering schedule, but also in floral morphological traits and breeding systems, could induce variations in fruit set and intensity of predation among species. Comparing two species while simultaneously observing individual flowers within species could help determine the cause of these interspecific differences. For four years, I followed the fate of individual flowers, from bud to fruit, in two Vaccinium species that bloom successively in spring and share pollinators and pre-dispersal seed predators. My predictions were as follows: (1) the early flowering species, V. hirtum, would not easily gain pollinator services because of low ambient temperature, and its fruit set would be low in comparison with the late flowering species, V. smallii; (2) the early flowering species would avoid attacks by flower and pre-dispersal seed predators via phenological escape from the predators’ activity peak. Hand-pollination experiments showed both species had incomplete self-compatibility, with bumblebees required for pollination. As predicted, only the early flowering species seemed to suffer from pollen limitation. However, Studies of individual flowers suggested that pollen limitation resulted not only from pollinator limitation early in spring, but also from geitonogamy and reproductive interference between plant species. Moth and fly larvae consumed flowers and fruit, while the pre-dispersal seed predator was a weevil (Mecysmoderes sp.). Far from escaping flower and seed predation, the early flowering species suffered more severe insect attacks than the late flowering species in two years out of four. Studies of individual flowers clearly showed that flower predators’ activity was not limited, even in the early weeks of the flowering season.

Floral morphology and reproductive success in the orchid Epipactis helleborine : regional and local across-habitat variation

The terrestrial orchid Epipactis helleborine is a morphologically variable species with a wide distribution in Europe. It is pollinated by social wasps, and most populations show the morphological characteristics of outcrossing species. However, local predominantly selfing subspecies and varieties have been documented from drier habitats. To document geographic variation in floral morphology, ability to produce seeds through autogamy, and reproductive success in E. helleborine, we sampled 13 populations from three geographic regions along a latitudinal gradient of c. 1000 km from northern to southern Sweden. In the southernmost region, populations in dry and mesic habitats were compared. Supplemental hand-pollination was conducted to determine whether among-population variation in fruit set could be explained by differences in the natural level of pollination, and whether any relationship between floral morphology and fruit production could be explained by interactions with pollinators. Bagging experiments showed no evidence of autogamy in any of the study populations. Number of flowers, pollinia removal and fruit set varied significantly among populations but did not differ among regions. Pollinia removal was positively correlated with population size and both pollinia removal and fruit set were lower in dry than in mesic habitats. At the level of the individual plant, the number of pollinia removed increased more rapidly with flower number than did number of fruits produced. The hand-pollination experiment indicated that the positive relationship between number of flowers and fruit production was due to a higher degree of pollen limitation in plants with few flowers than in plants with many flowers. The experiment also showed that variation in the level of pollen limitation could only partly explain variation in fruit set among populations.

Variation in fruit‐ and seed set among and within inflorescences of Melampyrum roseum var. japonicum (Scrophulariaceae)

Abstract Variations in fruit set and seed set among and within inflorescences of the annual herb, Melampyrum roseum var. japonicum, were studied. Under natural conditions, although the mean fruit set was slightly different among inflorescences, the mean seedset was not significantly different among inflorescences within the plants. In constrast, within the inflorescences, the flowers located at a lower position of the inflorescence and which opened earlier showed higher fruit set than those at a higher position and which opened later. However, the seed set of matured capsules were not significantly different from each other, regardless of the position of flowers within the inflorescences. Patterns of the fruit‐ and seed set under open pollination indicated that variation in seed reproduction of M. roseum is due to variation in fruit production. The results of clipping experiments of flowers revealed that there was no functional limitation in seed production among flowers located at various positions within the inflorescence. It seemed that the variation in the fruit set within the inflorescences of M. roseum was not attributable to ‘architectural effects’. Reduction of the number of flowers within the inflorescences resulted in an increase of fruit set and seed weight, indicating that the flowers in an inflorescence compete for resources. This phenomenon supports the ‘resource competition hypothesis’, and variation in fruit set within the inflorescence is attributable to competition among flowers within the inflorescence for limited resources. Consequently, it was concluded that, under natural conditions, the early blooming flowers located at lower positions of the inflorescences obtain more resources and produce more fruits than the late blooming flowers located at higher positions in M. roseum.

Sonoran Desert Columnar Cacti and the Evolution of Generalized Pollination Systems

We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant–pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species. Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied three-fold over six years. In senita, peak flowering varied by as much as 5–8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years. Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64–87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species. Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The “missing” pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was complementary. The four species of cactus that we studied occur at the northern geographic limits of Mexican columnar cacti where many vertebrate pollinators are seasonal migrants. In the Sonoran Desert, variation in rainfall and spring temperatures affects timing of flowering and the extent of competition between cacti for pollinator visits and causes the relative importance of particular pollinators, especially Leptonycteris bats, for fruit set to vary annually. Under such conditions, selection has favored generalized pollination systems (as seen in organ pipe) or shifts from reliance primarily on nocturnal pollinators (as seen in cardon) to reliance primarily on diurnal pollinators (as seen in saguaro). Nonetheless, as exemplified by the senita–senita moth system, highly specialized pollination mutualisms can also evolve in this habitat in plants that rely on sedentary insects rather than migratory bats and birds for pollination.