Search

The carbohydrate content of floral nectars of 12 Marantaceae species was analyzed by high-performance thin-layer chromatography (HPTLC). The main interest was not only to determine the sugar content of the nectar produced by the flowers but also to find potential differences within the genera and species. A total of 131 samples of this plant family, which grows in the tropical understory rainforest, were collected in the Golfo Dulce Region, Costa Rica, and analyzed. The nectar sugars found in the neotropical samples comprised melezitose, fructose, sucrose, glucose, and xylose (listed ascending in hRF values), whereof the main sugars were sucrose, glucose, and fructose. The nectars were successfully analyzed with regard to their individual sugar composition and classified according to their ratios. The overall mean sugar content of the studied species was 36%. The highest sugar contents of ≥40% were found in the nectar of the species Calathea lutea, Calathea donnell-smithii, Calathea marantifolia, Hylaeanthe hoffmannii, and Pleiostachya pruinosa. All species studied showed a ratio of sucrose to glucose and fructose between 1.3 for Calathealutea and 11.2 for Calathea donnell-smithii and, hence, were classified as sucrose dominated. HPTLC was found to be a good analytical method for nectar analysis, which took less than 1.5 h for 21 samples analyzed in parallel (ca. 4 min per sample).

The Namaqua rock mouse Aethomys namaquensis, a rodent pollinator of certain geoflorous Protea species, consumes nectar containing xylose. Xylose is not known to be efficiently utilized by mammals. However, it is fermented by certain bacteria, yeasts, and fungi, particularly gastrointestinal bacteria. The end products of microbial fermentation are utilized by the host in oxidative metabolism. Here we investigate the degree to which intestinal bacteria of A. namaquensis contribute to xylose metabolism. Mice were caught during Protea humiflora flowering and nonflowering seasons and given an oral dose of 14C-labeled xylose. Exhaled CO2 and excreted urine and feces were continuously collected for 30 h thereafter, and label recovery was determined. Each mouse was then treated with antibiotics to reduce gut microflora, and the experiment was repeated. With their natural gut flora population intact, mice caught during the flowering season exhaled significantly more 14CO2 than did mice caught during the nonflowering season. Also, during both seasons, mice exhaled significantly more 14CO2 before antibiotic treatment than after. Antibiotic treatment caused a significant increase in the proportion of 14C-labeled xylose that was excreted in the urine. The mouse diet likely influences the composition of the gastrointestinal community. Aethomys namaquensis relies on its gut microflora to ferment xylose, thereby converting it into end products that are used by the mice for metabolism.

Studies of nectar sugar composition in the Proteaceae, an ancient southern hemisphere plant family, have demonstrated that xylose comprises up to 39% of nectar sugar in two genera, Protea and Faurea, and may therefore represent a substantial fraction of the energy available to pollinators of these plants. Although insect and bird pollinators of Protea species are averse to xylose, mice (Aethomys namaquensis) will drink pure xylose, which is metabolized either by gut bacteria or by the mouse tissues. In the form of xylan polymers, the pentose sugar D-xylose is a structural component of plant cell walls, and there is considerable biotechnological interest in xylose fermentation. Bacteria and yeasts convert D-xylose to D-xylulose and thence via the pentose phosphate pathway to fructose-6-phosphate, which is either oxidized or fermented to ethanol. Gut symbionts of rodent pollinators may be analogous to ruminal xylose-metabolizing bacteria. The presence of xylose in Protea and Faurea nectar remains puzzling in view of pollinator aversions: even for rodent pollinators, it is the least preferred nectar sugar. In the generalized pollination systems of the Proteaceae, a coevolutionary explanation for nectar xylose as an attractant for mammalian pollinators is probably less likely than one involving plant physiology, with xylose in phloem sap being secreted passively into the nectar.

1. Subject and sourcePuya raimo ndii Harm s is the largest bromel iad in the world, reachin g up to 12 m in height (F oste r,1950; Hornung-Leoni and Sosa, 2005 ). It gro ws at high altitudes in the Puna of scatte red loca tions in the Ande s of Peru and Boli viaand is kno wn as the OOQueen of the PunaO O (Raimondi , 1874 ; Smith and Do wns , 1974 ). Only a few plan ts of everypopulation of this monocar pic species produc e insoresc ences in a given year or even in a period of 2 or 3 year sand plan ts take from 80 to 150 year s to s ower (F oster ,1984; Benzing, 2000; Hor nung-Le oni and So sa, 2004 ). Nectarof P. raimondi iwas collec ted in two areas of Peru to determin e quantity ,sugar concentr ation and com position. Theywere Huascar a«n P ark, in the OOCordil lera BlancaO O (around 6000 m); and Canchay llo, in the OOCordil lera Ne graO O(3600 e 4100 m), 147 km from Lima. In Huascar a«n three indi vidua ls from four populations were blooming, inCanchay llo only one plant was bloomi ng. V oucher speci mens hav e been deposit ed in Peru at the USM herba rium(Hornun g 1118; Hor nung 1120 e 1126).2. Pre vious wor kPre vious studies of nect ar in bromelia ds sho wed difference s in sugar conce ntration (8.2 e 45.2% ) and volume(0.06 e 43.2 ml) in species such as Dyckia ragones ei, Puya alpes tris and Puy a soccosa (Galet to and Bernard ello,1992; Scogi n and Freeman ,1984; Ba ker and Ba ker ,1990 ). In Puya ,differences in nectar com position hav e repor tedthat in species of subgen us Puya ,the nectar is dominated by hexoses (glucose and fructose) and in speci es of subgen usPuyopsi s the nect ar is dominated by sucr ose (Scog in and Freeman , 1984 ; Baker and Baker , 1990 ). In P. raimondi i,which belongs to subgen us Puy a,a sugar ratio (Sucrose/ Glucose ! Fruct ose) of 0.05 was reporte d, while P. soccosafrom subgen us Puyopsi s a sugar ratio of 1.13 was accou nted (Baker and Ba ker , 1990 ). Ho we ver , analyses with thinlayer chrom atograph y or gas chromatogr aphy hav e never been conduc ted in the nect ar of P. raim ondii to determ ineother sugars in the nectar in addi tion to sucr ose, glucose or fruc tose.

Nectarivore sugar preferences and nectar composition in the Cape Floristic Kingdom (southern Africa) differ from trends reported for analogous systems in America and Europe in that sugarbirds and sunbirds show no aversion to sucrose, which is the dominant nectar sugar in many of their food plants. To elucidate the physiological bases (if any) of nectarivore sugar preferences, we determined apparent sugar absorption efficiencies in a passerine endemic to this region, the Cape sugarbird Promerops cafer. Apparent absorption efficiencies for the three major nectar sugars, sucrose, glucose, and fructose, were extremely high (99%), as in other specialized avian nectarivores. Xylose, a pentose sugar recently reported in the nectar of some Proteaceae, was absorbed and/or metabolized inefficiently, with a mean of 47.1% of ingested sugar recovered in cloacal fluid. We did not measure the proportions of xylose that were absorbed and/or metabolized. We also compared three methods of estimating absorption efficiency: (1) measurements of total sugar in cloacal fluid with refractometry, without correction for differences between volumes of ingesta and excreta; (2) the same measurements combined with correction for volume differences; and (3) HPLC analyses quantifying individual sugars in cloacal fluid, with correction for volume differences. Refractometry has been frequently used in previous studies. For all sugars except xylose, method 1 yielded results similar to those obtained with method 2, but the convergence was artifactual, and we do not recommend use of this method. Apparent absorption efficiencies calculated with method 2 underestimated true absorption efficiency, because refractometry measures nonsugar solutes, but this error is biologically significant only when efficiencies are low.

1. In contrast to hummingbirds, passerine frugivores prefer hexose sugars to sucrose and absorb sucrose poorly. It has been predicted that the sugar preferences and digestive physiology of specialized passerine nectarivores will be similar to those of frugivores. 2. We examined the preferences of 11 Lesser Double-collared Sunbirds, Nectarinia chalybea (Nectariniidae), for 20% (w/w) solutions of sucrose, glucose, fructose, a 1: 1 glucose+fructose mixture, and xylose. The order of preference was found to be sucrose = glucose + fructose = fructose > glucose > xylose. The birds rejected xylose, which has recently been found in the nectar of Protea and Faurea (Proteaceae). 3. Preferences for different sugar concentrations were also tested. In the case of sucrose and fructose, the sunbirds preferred 20% to 10% solutions, but were indifferent to 20% vs 30%. They preferred 20% to 30% glucose, but were indifferent to 10% vs 20%. 4. The concentration and sugar composition of the cloacal fluid were measured after feeding on known diets. The sunbirds absorbed sucrose, glucose and fructose from the ingested food at close to 100% efficiency, but xylose was excreted. 5. The results of this study thus contradict the belief that passerine nectarivores are inefficient at digesting sucrose and reject sucrose in favour of hexose sugars.

The sugars occurring in the extrafloral nectar of Andropogon gayanus var. bisquamulatus were examined by paper and gas chromatography. They were found to be principally sucrose, glucose and fructose with small amounts of maltose, raffinose, arabinose, xylose and three unidentified sugars.

Various structurally diverse compounds appear in the nectar of excised flowers of snapdragon (Antirrhinum majus) when added to the culture solution. Quantitative analyses of individual nectar constituents were made to obtain information on the interrelations of movement of sucrose, which is the major solute in snapdragon nectar, other solutes that normally are present only in very small amounts or absent, and water. Nectar yields were altered experimentally either by changing the concentration of sucrose in the culture solution or by adding indoleacetic acid (IAA) to it. A representative of each of three different groups of compounds was tested in combination with sucrose. There was a common pattern of variation in yield, differing from that of sucrose, for sodium chloride, urea, and xylose. Sucrose promoted nectar flow and its yield in nectar was depressed by IAA, whereas the other solutes did not promote nectar flow and their yields were insensitive to IAA. Nectar concentrations of the other solutes were not affected by the concentration of sucrose in the culture solution, but their yields were increased proportionately to the sucrose-induced increase in nectar volume. The results are interpreted as indicating a selective transport of sucrose, an indirect influence of sucrose on yield of other solutes through its effect on water movement, and independence of water and solute movement at some stage in the production of nectar in excised snapdragon flowers.

Sugar preferences and absorption efficiencies were investigated in the Cape White-eye Zosterops pallidus, a generalist frugivore. Unlike the mainly American frugivorous passerines previously studied, Cape White-eyes preferred 20% (weight: weight) solutions of sucrose to glucose or fructose, and apparently possess high sucrase activity because absorption efficiencies for all three sugars were close to 100%. Cape White-eyes rejected 20% solutions containing a fourth nectar sugar, xylose, recently found in the nectar of Protea and Faurea (Proteaceae). Absorption efficiencies for xylose averaged 61%, but we do not know whether this sugar is utilized by the birds. Cape White-eyes thus resemble specialized southern African nectarivores (Nectariniidae, Promeropidae) in their responses to nectar sugars.