Patterns and implications of floral nectar secretion, chemical composition, removal effects and standing crop in Mandevilla pentlandiana (Apocynaceae)

Flower morphology, nectar features (chemical composition, production pattern, removal effects, and standing crop) and floral visitors are analysed in an Argentine population of Mandevilla pentlandiana. Nectar variability was examined during the lifetime of a single flower, over the course of the flowering season, and at different times of the day. Nectar is sucrose dominant. There were some variations in the proportions of sugar throughout both the flower lifespan and the flowering season. Flowers produced most nectar during bud-stage. Nectar secretion ceased near the end of the first day after flower opening. Nectar quantity varied as a function of flower age due to a combination of nectar secretion, cessation, and resorption periods. Overall sugar production was increased by nectar removal. Standing crop data showed that each open flower and inflorescence offers c. 2 and 11 mg of sugar respectively at any time of the flowering season. There was higher nectar availability at the beginning of the flowering season compared with the rest of the period. Flowers were visited by bumblebees, honeybees and hummingbirds. The greater the number of open flowers and the nectar variance, the more the mean reward quantity per flower available in the inflorescence. The sources of nectar variability in M. pentlandiana seem to be linked with both the female function (nectar resorption, nectar cessation) and the male one (early and comparatively large nectar availability, variation in nectar production as the flower ages, nectar secretion stimulation by nectar removals).     

Nectar ecology of Datura ferox (Solanaceae): an invasive weed with nocturnal flowers in agro-ecosystems from central Argentina

Plant–pollinator interactions provide highly important ecological functions, and are influenced by floral nectar characteristics. The night blooming Datura ferox is an excellent model to test general hypotheses on the relationship between nectar traits (e.g., nectar secretion patterns, nectar chemical composition), pollinators and reproductive success for invasive, weedy species in highly modified ecosystems as crop fields. We hypothesized an adjustment between nectar composition and secretion dynamics through flower anthesis and the activity and requirements of nocturnal pollinators. Nectar chemical analyses showed low quantities of amino acids and lipids, phenolics, and alkaloids were not detected. D. ferox showed sucrose-dominant nectar with comparable amount of hexoses. Sugar proportions did not vary between populations or during flowering season. Most nectar is secreted before flower opening. Nectar resorption was detected at the end of anthesis. Experimentally drained flowers of both populations increased nectar production up to 50 % in the total amount of sugar per flower compared to control flowers. Nectar standing crop was relatively constant during the flowering season, but differences were detected between populations. Nectar traits of D. ferox would be favoring cross-pollination and maintaining seed production of this weed, since recently open flowers display a higher amount of nectar and they can renew nectar after a pollinator visit or reabsorb it at the end of anthesis. This nectar source may be important for native pollinators considering that human-induced forest fragmentation is related with the impoverishment of native flora from agro-ecosystems.     

NECTAR PRODUCTION PATTERNS IN IPOMOPSIS AGGREGATA (POLEMONIACEAE)

This study describes nectar production patterns for Ipomopsis aggregata and discusses their potential adaptive and ecological significance. It also examines the influence of environmental and other factors on nectar production rate (NPR) and nectar sugar concentration. For I. aggregata there were no NPR differences with flower age. An hypothesis for the presence or absence of such differences is discussed. Ipomopsis aggregata has a relatively constant rate of nectar production during the day and production continues overnight but at a reduced rate. Newly opened flowers already have a sizeable accumulation of nectar. 24-hr nectar sugar production on overcast days was 62% of sunny day production. NPR values at the beginning of the flowering season were almost twice as great as those near the end but the sugar concentration did not change. Whether nectar was removed periodically (to simulate pollinator visits) or simply allowed to accumulate over 24 hr had no effect on total production. Nectar sugar concentration has a characteristic diurnal pattern: highest in the afternoon and lowest in the early morning, probably in response to diurnal changes in relative humidity. Sugar concentration was also lower on overcast days. These changes are not due to evaporative losses from the open end of the flower. However, evaporation did occur in flowers which had been punctured at the base of the corolla by nectar robbing bees. In general, the results of this study suggest caution in characterizing the NPR or sugar concentration of a species by making measurements at one point in time under one set of environmental conditions.     

Nectar Secretion Pattern and Removal Effects in Six Argentinean Pitcairnioideae (Bromeliaceae)

Nectar secretion pattern and effects of nectar removal were analyzed in six hummingbird-pollinated Argentinean Pitcairnioideae: Abromeitiella brevifolia. A. lorentziana, Deuterocohnia longipetala. Dyckia floribunda, D. ragonesei and Puya spathacea. Flower lifetime was determined in each case. Nectar volume, concentration and sugar production were measured from bud opening till fading of flowers at different time intervals according to the species' flower life-span. Nectar volume varies while nectar concentration and cumulative sugar production increase as a function of flower age in all the studied species. In general, total nectar volume and concentration values can be affected by periodic removal, but total sugar production is unaffected. Only in Puya spathacea did periodic nectar harvesting reduce the total amount of sugar produced.     

Reproductive biology, variability of nectar features and pollination of Combretum fruticosum (Combretaceae) in Argentina

Flower morphology, nectary structure, nectar features (chemical composition, secretion pattern, standing crop, removal effects) and flower visitors are analysed in an Argentinian population of Combretum fruticosum. The variability of these data was examined throughout the flower lifetime. Nectar is hexose dominant. Its chemical composition and concentration are constant for all flowering stageS. Nectar volume varies as a function of flower age due to a combination of nectar secretion, cessation and resorption periods. The overall sugar production is decreased by nectar removal. The plant is self-incompatible and xenogamouS. Only 16.2% of the flowers set seedS. Inflorescences with green flowers were exclusively visited by two hummingbird and three perching bird species which transfer the pollen. A clear link was observed among nectar production pattern, standing crop of nectar, and visitors' behaviour.     

Pollination ecology of Christmas Bells (Blandfordia nobilis): Patterns of standing crop of nectar

Standing crop of nectar (both volume and sugar concentrations) was measured from flowers in a population of Blandfordia nobilis. Nectar in this honeyeater-pollinated species was extremely viscous during one flowering season and averaged approximately 95% sugar, while it averaged only 25% sugar two seasons later. Nectar volume was significantly and negatively correlated with sugar concentration on nine often sampling dates. There was no significant relationship between either nectar volume or sugar concentration and a number of measures of plant size (inflorescence height, numbers of open flowers, buds, spent flowers and total flowers). Similarly, no consistent relationship was found between measures of flower size and reward level. There were significant positive correlations between the reward (both volume and sugar concentration) offered by blossoms on the same plant. The nectar volume of flowers from nearest neighbouring inflorescences were significantly and positively correlated with one another on three of six occasions while the sugar concentrations of those blossoms were significantly correlated on just two of six sampling dates. There was no significant difference in reward (both volume and sugar concentration) between open flowers and those bagged to exclude pollinators. There was significant heterogeneity in the nectar volumes offered by plants to pollinators during a single flowering season.     

Nectar dynamics and reproductive success in Saponaria officinalis (Caryophyllaceae) in southern Germany

Nectar production and flower visitors of the night-flowering Saponaria officinalis L. (Caryophyllaceae) were studied in relation to the reproductive success. Nectar production was worthwhile for nocturnal flower visitors. Nectar standing crop was about 267 μg sugar per flower, and comparison of nectar offering of covered and freely exposed flowers revealed that main nectar secretion time is mainly during the night up to the morning hours. In both covered and freely exposed flowers nectar volumes decreased over the day. In covered flowers, nectar volume, sugar concentration, and sugar amount per flower increased up to the third day; in older flowers sugar secretion ceased. In 1996 Autographa gamma (Noctuidae) was the exclusive nocturnal flower visitor, but pollen transfer experiments proved that A. gamma (Noctuidae) is a very ineffective pollinator of S. officinalis. In 1999 up to 50% of the observed visitors were Sphingidae, which resulted in a significantly higher seed set. Fruit set was constantly high independent of pollinator availability. In the nectar manipulation experiments seed set was highest in non-emasculated flowers filled with unnaturally high concentrated sucrose solutions. Differences to seed set on stalks treated with a sucrose solution mimicking naturally concentrated nectar were significant. Lowest fruit and seed set were found on inflorescences with emasculated flowers filled with a sucrose solution mimicking naturally concentrated nectar.     

Morphology of nectaries and biology of nectar production in the distylous species Fagopyrum esculentum

Background and Aims

The mechanisms of floral nectar production in buckwheat (Fagopyrum esculentum, Polygonaceae), a distylous pseudo-cereal, have received relatively little attention, prompting an investigation of the factors that regulate this process. The aim was to perform a refined study of the structures that secrete nectar and of the internal and external parameters influencing nectar volumes and sugar concentrations.

Methods

In order to control environmental parameters, plants were cultivated in growth rooms under controlled conditions. The structure of nectaries was studied based on histological sections from flowers and flower buds. Nectar was extracted using glass micropipettes and the sugar concentration was measured with a hand refractometer. Sugar concentration in the phloem sap was measured using the anthrone method. To test the influence of photosynthesis on nectar production, different light and defoliation treatments were applied.

Key Results

Unicellular trichomes were located in the epidermis at the ventral part of eight nectary glands situated on the flower receptacle alternately with stamens. Vascular bundles consisting of both phloem and xylem were identified at the boundary between a multilayered nectary parenchyma and a sub-nectary parenchyma with chloroplasts. A higher volume of nectar in thrum morphs was observed. No other difference was found in morphology or in sugar supply to inflorescences between morphs. Nectar secretion was strongly influenced by plant age and inflorescence position. Nectar volumes were higher in the upper inflorescences and during the flowering peak. Light had a dual role, (1) acting directly on reproductive structures to trigger flower opening, which conditions nectar secretion, and (2) stimulating photosynthetic activity, which regulates nectar accumulation in open flowers.

Conclusions

In buckwheat, nectar is secreted by trichomes and probably proceeds, at least in part, from phloem sap. Nectar secretion is strongly influenced by floral morph type, plant age, inflorescence position and light.Key words: Buckwheat, distyly, Fagopyrum esculentum, inflorescence position, morph comparisons, nectary histology, nectar sugar concentration, nectar volume, light intensity, organ biomass, phloem sap, plant age     

Beyond the pollination syndrome: nectar ecology and the role of diurnal and nocturnal pollinators in the reproductive success of Inga sessilis (Fabaceae)

Inga species present brush‐type flower morphology allowing them to be visited by distinct groups of pollinators. Nectar features in relation to the main pollinators have seldom been studied in this genus. To test the hypothesis of floral adaptation to both diurnal and nocturnal pollinators, we studied the pollination ecology of Inga sessilis, with emphasis on the nectar secretion patterns, effects of sequential removals on nectar production, sugar composition and the role of diurnal and nocturnal pollinators in its reproductive success. Inga sessilis is self‐incompatible and pollinated by hummingbirds, hawkmoths and bats. Fruit set under natural conditions is very low despite the fact that most stigmas receive polyads with sufficient pollen to fertilise all ovules in a flower. Nectar secretion starts in the bud stage and flowers continually secreting nectar for a period of 8 h. Flowers actively reabsorbed the nectar a few hours before senescence. Sugar production increased after nectar removal, especially when flowers were drained during the night. Nectar sugar composition changed over flower life span, from sucrose‐dominant (just after flower opening, when hummingbirds were the main visitors) to hexose‐rich (throughout the night, when bats and hawkmoths were the main visitors). Diurnal pollinators contributed less than nocturnal ones to fruit production, but the former were more constant and reliable visitors through time. Our results indicate I. sessilis has floral adaptations, beyond the morphology, that encompass both diurnal and nocturnal pollinator requirements, suggesting a complementary and mixed pollination system.     

Pollination Ecology and Effect of Nectar Removal in Macleania bullata (Ericaceae)1

The floral syndrome of Macleania bullataYeo (Ericaceae) reflects its adaptation to hummingbird pollination. Its flowers, however, are subject to high levels of nectar robbing. I examined the floral visitor assemblage of M. bullata in a tropical montane wet forest in southwestern Colombia, focusing on the behavior of the visitors. I also tested for the presence of nocturnal pollination and the effects of nectar removal on new nectar production. The principal floral visitors were the nectar robbing hummingbirds Ocreatus underwoodii (19.1% of visits) and Chlorostilbon mellisugus (18.9%). Only two species of long–billed hummingbirds visited the flowers of M. bullata as “legitimate” pollinators: Coeligena torquata (14.7% of visits) and Doryfera ludoviciae (14.3%). The remaining visits constituted nectar robbing by bees, butterflies, and other species of hummingbirds. Nocturnal pollination took place, although fruit set levels were 2.4 times higher when only diurnal pollination was allowed as opposed to exclusively nocturnal pollination. Nectar robbers removed floral nectar without pollinating the flower. Treatments of experimental nectar removal were carried out to examine if flowers synthesize more nectar after nectar removal. Nectar removal increased the total volume of nectar produced by each flower without affecting sugar concentration. Thus, nectar robbing can impose a high cost to the plants by forcing them to replace lost nectar.     

Nectar production rates of Asclepias quadrifolia: causes and consequences of individual variation

The causes and reproductive consequences of individual variation in nectar production rates within a population of Asclepias quadrifolia were investigated. Two parameters were correlated with nectar production rate per flower: the root weight of the plant and the number of flowers in the inflorescence (umbel). Nectar production increased with increasing root weight but levelled off after a root size of about 3 g was reached. Nectar production decreased with increasing umbel size, but only for umbels that were greater than average size. A total of 57% of the variance in individual nectar production could be explained by these two variables with root weight accounting for 67% of the explained variance. Root weight is a good indicator of a plant's energetic status, indicating the importance of available energy in determining quantity of nectar produced. About 30% of the energy devoted to flowering is utilized in nectar production. Nectar production was significantly correlated with the male component of reproductive fitness, pollinaria removal, but not with the female component, pollinia insertion. Since pod production is limited by resource availability rather than the number of pollinia insertions, nectar production in A. quadrifolia is most closely associated with the maximization of the male function.     

Variability of nectar production and composition in Linaria vulgaris (L.) Mill. (Scrophulariaceae)

 We studied nectar characteristics during the long flowering period (late June to end of November) in two populations of Linaria vulgaris (L.) Mill. spontaneously growing in the Botanical Gardens of Siena University (Tuscany, central Italy). The two populations were close to each other but they differed in blooming period. Plants of population 1 sprouted in May and flowered from the end of June to the end of September. Population 2 sprouted at the end of August and flowered from September to the end of November. Differences in nectar production and composition were found between and within populations. Flowers of population 1 produced a very small amount of nectar (not collectable) that remained on the nectary surface. The quantity of nectar increased in late September, when each flower produced 2–3 μl of nectar that flowed into the spur. Total sugar concentration was 175.8 mg/ml in young flowers. Flowers of population 2 produced 5–8 μl of nectar with a total sugar concentration of 200.9 mg/ml in the young stage. In bagged senescent flowers nectar volume decreased in both populations and nectar sugar concentration decreased down to 11.6 mg/ml in population 2 and increased up to 289.6 mg/ml in population 1. For both populations, the decrease in nectar volume in bagged flowers may have been due to water loss by evaporation. In population 2, the decrease in sugar concentration may have been due to nectar reabsorption that was never observed in population 1. Nectar variability is discussed in relation to insect visits and seed set. Received August 14, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Interpopulation variation in nectar production in Aconitum columbianum (Ranunculaceae)

Summary In Aconitum columbianum there are extreme interpopulation differences in rates of nectar secretion per flower. Since nectar sugar concentration varies little among populations, increased nectar secretion results in a greater mass of sugar per flower for pollinator attraction. These differences in the amount of reward offered per flower account at least in part for observed higher levels of pollinator activity in populations with high nectar production. Nectar production is correlated also with nectary depth, i.e., flowers in populations with deep nectaries have higher rates of nectar secretion than those with shallow nectaries. Nectary depth differences adapt populations to different pollinator-types. Populations with deeper nectaries are adapted to pollination by bumblebees with longer tongues and more specialized foraging behaviors. In conclusion, there are basic differences in pollination ecology among geographical races of a. columbianum, which are indicated by correlated interpopulution differences in (1) nectar production, (2) level of pollinator activity, (3) nectar depth, and (4) pollinator-type.     

Nectar Dynamics and Sugar Composition in Flowers of Silene and Saponaria Species (Caryophyllaceae)

Abstract: Nectar production in Saponaria officincilis and in five species of Silene (S. ciba, S. dioica, S. noctiflora, S. nutans, S. vulgaris ) was examined during two consecutive years (May to July 1993, and May to June 1994) in the Botanical Garden of the University of Giessen. Nectar volume and sugar concentration were studied in relation to time of day, flower sex, flower age, and flowering stage. Nectar amount in all species studied (except S. dioica ) increased in the afternoon or in the evening until midnight (or until the early morning in S. nutans ). After midnight and until midday, nectar volume in non-visited flowers (except S. dioica ) decreased. Nectar volume in non-visited S. dioica flowers increased constantly with flower age, indicating a stable nectar secretion rate, possibly favouring both day- and night-active flower visitors. Even at the time of highest nectar secretion, all species studied presented several nectarless flowers. Sucrose dominance in the nectar of the nocturnal species S. nutans and Saponaria officinalis fits well with the general syndrome of flowers pollinated by hawkmoths. The syndrome also applies to the nocturnal but regularly selfing, S. noctiflora . The more generalis-tic species S. dioica and S. vulgaris , which are regularly visited by bumblebees as well as nocturnal moths, secreted hexose-domi-nant nectar. Unexpectedly, Silene alba , the only nocturnal species that strictly excluded day-active flower visitors by closing flowers during the day, also secreted hexose-dominant nectar. In some cases, nectar volumes and nectar concentration differed significantly between hermaphroditic, male, and female flowers. Female flowers of S. alba, S. dioica , and S. nutans contained significantly less concentrated nectar than male or hermaphroditic ( S. nutans ) ones. In S. noctifiora and S. vulgaris the difference was not statistically significant but nectar concentration did show the same tendency.     

Reproductive conflicts ofPalicourea padifolia (Rubiaceae) a distylous shrub of a tropical cloud forest in Mexico

We studied a population of the distylousPalicourea padifolia (Rubiaceae) in a cloud forest remnant near Xalapa City, Veracruz, México to explore possible asymmetries between floral morphs in the attractiveness to pollinators, seed dispersers, nectar robbers, floral parasites, and herbivores. We first assessed heterostyly and reciprocal herkogamy by measuring floral attributes such as corolla length (buds and open flowers), style and anther heights, stigma and stamen lengths and the distance between the anther tip to the stigma lobe. We then estimated floral and fruit attributes such as flower size, anther height, number and size of pollen grains, fruit size, seed size, nectar production, and flower and fruit standing crops to assess differences between floral morphs in attracting and effectively using mutualistic pollinators and seed dispersers. Also, floral parasitism and nectar robbing were assessed in this study as a measure of flower attractiveness to antagonists. The system seems to conform well to classical heterostyly (e.g. reciprocal stamen/style lengths, pollen and anther dimorphism, intramorph incompatibility) yet, there were several tantalizing differences observed between pin and thrum morphs. Thrum flowers have longer corollas and larger but fewer pollen grains than pin flowers. Both morphs produced the same total number of inflorescences, developed the same number of buds, and opened the same number of flowers per inflorescence during the flowering season. Nectar production and sugar concentration were similar between floral morphs but the reward was not offered symmetrically to floral visitors throughout the day. Nectar concentration was higher in pin flowers in the afternoon. The numbers of developing, fully developed, and ripe fruits were the same between floral morphs, however, fruits and seeds were larger than those of thrums. The incidence of fly larvae was higher among thrum flowers and damage by nectar robbing was the same between floral morphs. Fruit abortion patterns of flowers manually pollinated suggest intra-morph sterility (self and intramorph incompatibility). There were no differences between morphs in fruit and seed set per flower following legitimate pollination although thrums were more leaky than the pins (intramorph compatibility).     

Reproductive biology and effect of nectar robbing on fruit production in Macleania bullata (Ericaceae)

Hummingbird-pollinated flowers are frequently subjected to nectar robbing. In this paper, I examine the impact of nectar robbing on plant reproductive success on a hummingbird-pollinated species. After studying the basic aspects of the floral morphology and reproduction of Macleania bullata (Ericaceae) in a tropical montane wet forest in southwest Colombia, I examined the percent of flowers robbed and the effect of nectar robbery on fruit set. The flowers of this species are typical for plants pollinated by long-bill hummingbirds. They are protandrous and open for four days. Fruit production requires a pollinator visit; fruit set following pollinator exclusion was zero. Fruit set following xenogamous pollen transfer (36.8%) differed significantly from that of population controls (11.9%) and of autogamous pollen transfer (6.3%). Nectar volume, sugar concentration and sugar production were measured at daily intervals from bud opening until the fading of flowers. Daily nectar production (both volume and amount of sugar) varied considerably with flower age. Sugar production peaked on the second day, coinciding with the male phase. The frequency of nectar robbing in the studied population was very high (75% of examined flowers) and was positively correlated with reduced fruit set. I discuss the probability of a relation between reduced fruit set on robbed flowers and an energetic investment. Robbing by non-pollinating visitors can suppose the plant to re-synthesize more nectar. The high incidence of nectar robbing impugns the advantage of specialization.     

Depressed Pollination of Lapageria rosea Ruiz et Pav. (Philesiaceae) in the Fragmented Temperate Rainforest of Southern South America

We studied the pollination and reproductive success in continuous and fragmented populations of Lapageria rosea, a self-compatible plant endemic to temperate forests of Chile. Pollinator abundance, visitation rates, flower abundance, nectar volume and concentration, pollen germination and fruit and seed production, were compared between continuous forest of 145 ha and four forest fragments of 6, 3, 3, and 1 ha respectively, surrounded by mature pine plantations of Pinus radiata. Flower abundance was lower in three out of four forest fragments relative to continuous forest. Nectar volume and sugar concentration did not differ between flowers in the two habitats. Pollinators of L. rosea, the hummingbird Sephanoides sephaniodes and bumblebee Bombus dahlbomii were less abundant and visited flowers of L. rosea at lower rates in fragments than in continuous forest. In addition, in vitro rates of pollen germination were lower for flowers in forest fragments. The number of seeds per fruit was also lower in forest fragments. We suggest that fragmentation affects the reproductive success of L. rosea, lowering the total numbers of seeds produced and possibly compromising long term persistence of fragmented populations.     

Nectar concentration and composition of 26 species from the temperate forest of South America

BACKGROUND AND AIMS: Floral nectar concentration and chemical composition of 26 plant species native to the temperate forest of southern South America are reported and the relationships with the flower type are evaluated. METHODS: Nectar concentration was measured with a hand refractometer and sugar composition was analysed by gas-liquid chromatography. Plant species were classified into flower type categories based not only on floral features but also on data from the literature and field observations on their pollinators. KEY RESULTS: Most data on nectar are new reports at the generic and/or specific level. Plant species in which more than one population was studied showed significant among-population variation in nectar sugar concentration and composition. Results showed a weak relationship between nectar traits and flower type. Many species had nectar containing 50 % or more sucrose (17 of 26 species), independent of the main pollinator. CONCLUSIONS: Considering that (a) nectar characteristics did not show a clear association with different flower types or with plant taxonomic membership, and (b) different populations of the same species showed large variability in sugar composition, the results suggest that other factors (e.g. historical and environmental) could be involved in determining the sugar composition of the highly endemic plant species from this region.     

Oilseed rape (Brassica napus) as a resource for farmland insect pollinators: quantifying floral traits in conventional varieties and breeding systems

Oilseed rape (OSR; Brassica napus L.) is a major crop in temperate regions and provides an important source of nutrition to many of the yield‐enhancing insect flower visitors that consume floral nectar. The manipulation of mechanisms that control various crop plant traits for the benefit of pollinators has been suggested in the bid to increase food security, but little is known about inherent floral trait expression in contemporary OSR varieties or the breeding systems used in OSR breeding programmes. We studied a range of floral traits in glasshouse‐grown, certified conventional varieties of winter OSR to test for variation among and within breeding systems. We measured 24‐h nectar secretion rate, amount, concentration and ratio of nectar sugars per flower, and sizes and number of flowers produced per plant from 24 varieties of OSR representing open‐pollinated (OP), genic male sterility (GMS) hybrid and cytoplasmic male sterility (CMS) hybrid breeding systems. Sugar concentration was consistent among and within the breeding systems; however, GMS hybrids produced more nectar and more sugar per flower than CMS hybrid or OP varieties. With the exception of ratio of fructose/glucose in OP varieties, we found that nectar traits were consistent within all the breeding systems. When scaled, GMS hybrids produced 1.73 times more nectar resource per plant than OP varieties. Nectar production and amount of nectar sugar in OSR plants were independent of number and size of flowers. Our data show that floral traits of glasshouse‐grown OSR differed among breeding systems, suggesting that manipulation and enhancement of nectar rewards for insect flower visitors, including pollinators, could be included in future OSR breeding programmes.     

EFFECTS OF DISPLAY SIZE,FLOWERING PHENOLOGY,AND NECTAR AVAILABILITY ON EFFECTIVE VISITATION FREQUENCY IN COMPARETTIA FALCATA (ORCHIDACEAE)

Comparettia falcata is an epiphytic, neotropical orchid that produces nectar as a pollinator reward. In Puerto Rico, C. falcata is allogamous and pollinated by the endemic hummingbird Chlorostilbon maugaeus. Autogamous pollinations are possible, but may result in reduced fruit set. For the 1989 and 1990 flowering seasons, the probability of pollinarium removals and natural pollinations increased with individual inflorescence display size. However, the frequency of effective pollinator visits was independent of flowering phenology in both years. A positive correlation between inflorescence size and reproductive success occurred in 1990 but not in 1989. In 1990 plants produced longer spurs, a higher standing crop of nectar, and a more concentrated nectar than in 1989. There was no relation between nectar availability (= standing crop of nectar) and sugar concentration in either year. Nectar availability and sugar concentration did not vary among the first four flowers of an inflorescence in either season. Nectar availability was not a good predictor of effective visitation. Comparettia falcata has a higher natural fruit set than tropical deceptive orchids, suggesting that pollinator visitation may be enhanced by nectar reward. The small, dilute nectar volumes secreted by C. falcata may benefit the plant by increasing interplant pollinator movement and pollen dispersal.