Sugar preferences and digestive efficiency in an opportunistic avian nectarivore, the Dark-capped Bulbul Pycnonotus tricolor

It has recently been recognized that flowers pollinated by generalist opportunistic nectarivores tend to have different nectar properties to those pollinated by specialist nectarivores (including both hummingbirds and specialist passerines). While renewed interest in specialist avian nectarivore sugar preferences and digestive physiology has helped explain the concentrated sucrose-dominated nectar of plants they feed on, there has been little progress in understanding why generalist or occasional nectar-feeding birds tend to be associated with flowers that have dilute hexose-dominated nectar. We examined sugar preferences and assimilation efficiencies over a range of concentrations, and concentration preferences, in Dark-capped Bulbuls Pycnonotus tricolor, one of the more common occasional avian nectarivores in southern Africa. Dark-capped Bulbuls showed significant preference for hexose sugar solutions, irrespective of concentration, when given a choice between hexose and sucrose solutions in equicaloric pair-wise choice tests conducted at five different concentrations (5–25%). This contrasts with results from specialist nectarivore groups which generally show a significant concentration-dependant switch in preference from hexose at low concentrations to sucrose at high concentrations for equicaloric solutions. In addition, Dark-capped Bulbuls showed an unusual lack of preference for solutions of higher sugar concentration when simultaneously offered four solutions varying in concentration from 10 to 25%. Dark-capped bulbuls also showed a unique effect of concentration on sugar assimilation efficiency, assimilating relatively more energy on 5% diets than on 25% diets. Although able to assimilate sucrose effectively, assimilation rates of hexose sugars were marginally higher. These results shed new light on pollination systems involving occasional nectarivores and, in particular, help to explain the prevalence of low concentration hexose-dominated nectars in flowers pollinated by these birds.     

Effects of feeding frequency and sugar concentrations on lifetime reproductive success of Meteorus pulchricornis (Hymenoptera: Braconidae)

Many species of parasitoids can increase their reproduction through feeding on supplemental sugar sources. We evaluated the longevity both with and without hosts and progeny production of a uniparental strain of Meteorus pulchricornis by varying concentration of the sugar solution and feeding frequency. A 1:1:1 sucrose–glucose–fructose-mixture was tested in a range of concentrations (0–70% (w/v)). The parasitoid lifespan did not increase with the concentration of sugar solutions, but parasitoids feeding on a 30% sugar solution attained greater longevity than parasitoids feeding on other sugar concentrations in the absence of hosts. The same effect of sugar concentration was also observed on progeny production, where when the parasitoid was fed 30% (w/v) sugar solution, its lifetime progeny production doubled in comparison to that fed water, and increased by 50% compared to those fed the other sugar solutions. In the last experiment, we studied the effect of feeding intervals on the longevity and progeny production of M. pulchricornis. Parasitoids fed with a 30% (w/v) 1:1:1 sucrose–glucose–fructose-mixture continuously lived six times longer than unfed parasitoids, three times longer than parasitoids fed only once and two times longer than parasitoids fed once every two days in the absence of hosts. Parasitoids with continuous access to the sugar solution produced considerably more progeny than those with fewer feeding frequencies. This increasing progeny production was accomplished more through an increase in daily offspring production rather than through extending reproductive lifespan.     

Do foraging bumblebees scent-mark food sources and does it matter?

Summary The foraging of worker bees of Bombus terrestris visiting artificial feeders in a climatic test chamber was investigated. The behaviour of worker bees visiting rewarding and unrewarding feeders is completely different. Of all flower visits to rewarding feeders 94% are probing-visits, i.e. the bees land on the flower and probe for nectar. In contrast, only 0.3% of all visits to unrewarding feeders are probing-visits, whereas 47% are approach-visits, i.e., the bees approach the feeders without landing. Exchanging feeder discs proves that the signal used for discrimination must be associated with the plastic disc used as landing platform. Most probably it involves scent-marking of the rewarding feeders with components of high and low volatility. The mean foraging efficiency of bees in a scent-marked foraging arena is 5.7 mg sugar/min and drops to 2.8 mg sugar/min after the scent marked discs are replaced by clean ones. Three components generate this drop in foraging efficiency: (1) the between-flower flight time increases, i.e. the bees search for a longer time before landing on flowers, (2) the bees no longer discriminate between rewarding and unrewarding feeders, and (3) the bees probe empty feeders longer than necessary; obviously they expect to find nectar.     

虎掌菌菌丝富硒培养的研究

为生产合适的硒源提供一种思路,以菌丝体生物量、含硒量、还原糖、氨态氮和蛋白质为指标,采用四因素三水平正交设计法优化虎掌菌的富硒发酵条件,探讨不同浓度的硒对虎掌菌固体培养菌丝生长和液体培养产生的生物组分的影响。结果表明,高浓度的硒抑制虎掌菌菌丝体生长;正交试验选择不同的衡量指标,由极差分析得出各因素的影响程度大小,结合证实试验得到以还原糖为指标的最优组合为葡萄糖6%(质量分数)、酵母浸膏3%(质量分数)、KH_2PO_4 0.1%(质量分数)、Na_2SeO_3 0.6 mmol/L,其菌丝体生物量和氨态氮含量较高。与对照相比,加硒后虎掌菌发酵液中氨态氮、还原糖和总糖含量显著增加(P0.05);当硒浓度为0.5 mmol/L时,氨态氮、还原糖和总糖含量均达到最高值。菌丝体生物量和可溶性蛋白质分别在硒浓度为0.2 mmol/L和0.4 mmol/L时达到最大值。虎掌菌富硒培养后,发酵液的营养成分含量会发生变化。     

Valve closure responses of the Asiatic clam Corbicula fluminea exposed to cadmium and zinc

The valve movement patterns of immobilized Asiatic clams (Corbicula fluminea) were monitored during exposure to constant concentrations of cadmium (0.0, 0.1, 0.2, 0.3, and 0.4 mg 1^–1) or zinc (0.0, 0.1, 0.3, 0.5, and 0.9 mg 1^–1) for 24 h following a 24-h acclimation period. Data indicate that the duration of response was concentration dependent and toxicity related. Durations of periods with valves parted declined as the concentration of heavy metal increased. Behavior was consistent for both mean time to first closure following the initial exposure and mean time per valve parting episode over a 24-h exposure period. Mean time per valve parting episode during the 24-h exposure period ranged from 600 minutes for control trials to 36 and 69 minutes for the highest concentrations of cadmium and zinc tested, respectively. There was no association between durations of periods with valves sealed and solutions of cadmium; however, lengthening durations of periods with valves sealed coincided with exposure of clams to progressively more concentrated zinc solutions. In addition, Asiatic clams demonstrated a greater rate of response (decline in the duration of periods with valves parted) to progressively more concentrated solutions of cadmium than to comparable increases in the concentrations of zinc solutions.     

Thorax vibrations of a stingless bee (<Emphasis Type="Italic">Melipona seminigra</Emphasis>). II. Dependence on sugar concentration

Using a laser vibrometer we studied the influence of the foods sugar concentration on different parameters of the thorax vibrations produced by foragers of Melipona seminigra during trophallaxis in the nest. The concentrations tested (20–70% sugar w/w) were within the biologically relevant range. They substantially influenced different parameters of the thorax vibrations. An increase of energy gains at the food source due to an increased sugar concentration was followed by an increase of both the pulse duration and the duty cycle and by a decrease of the pause duration between two subsequent pulses. These findings further support the hypothesis that the temporal pattern of the thorax vibrations reflects the energy budget of a foraging trip rather than food source distance. Likewise, the steep increase of pulse duration variability with sugar concentration is hard to reconcile with the assumption that pulse duration conveys reliable information about food source distance when bees collect at high-quality food sources.     

Exogenous sugars overcome incompatibility in hazelnut (Corylus avellana L.)

Summary Petreatment of hazelnut (Corylus avellana L.) stigmas with L M or 2 M sugar solutions before pollination with incompatible pollen, or pollination with 11 mixtures of incompatible pollen and finely ground sugars, prevented rejection of incompatible pollen by the stigma surface. Lower sugar concentrations or lower ratios of sugar to pollen were less effective. No specificity for overcoming incompatibility was observed among 18 simple sugars and related sugar compounds.Oregon Agricultural Experiment Station Technical Paper No. 7613     

Application of immobilized invertase to continuous hydrolysis of concentrated sucrose solutions

Invertase immobilized onto corn grits was utilized in the hydrolysis of highly concentrated sucrose solutions producting liquid sugar solutions containing glucose and fructose. Comparisons of conversion efficiencies of this immobilized invertase in a continuous stirredtank reactor and a plug-flow reactor indicated that the plug-flow reactor has an higher efficiency. Continuous sucrose hydrolysis was then performed in 0.1- and 1-L tubular reactors. This tenforld scaling-up was achieved without any noticeable loss in efficiency. This process thus was scaled-up to a 17.6-L pilot reactor set in a cane sugar refinery. This reactor was fed with highly concentrated sucrose solutions [71% (w/w)] to produce invert sugar syrup with the desired inversion degree. It allows a productivity equal to 9.1 kg sucrose hydrolyzed/h in the case of a 69% (w/w) sucrose initial concentration with a 72% conversion rate.     

Sugarcane molasses and yeast powder used in the Fructooligosaccharides production by Aspergillus japonicus-FCL 119T and Aspergillus niger ATCC 20611

Different concentrations of sucrose (3–25% w/v) and peptone (2–5% w/v) were studied in the formulation of media during the cultivation of Aspergillus japonicus-FCL 119T and Aspergillus niger ATCC 20611. Moreover, cane molasses (3.5–17.5% w/v total sugar) and yeast powder (1.5–5% w/v) were used as alternative nutrients for both strains’ cultivation. These media were formulated for analysis of cellular growth, β-Fructosyltransferase and Fructooligosaccharides (FOS) production. Transfructosylating activity (U _t ) and FOS production were analyzed by HPLC. The highest enzyme production by both the strains was 3% (w/v) sucrose and 3% (w/v) peptone, or 3.5% (w/v) total sugars present in cane molasses and 1.5% (w/v) yeast powder. Cane molasses and yeast powder were as good as sucrose and peptone in the enzyme and FOS (around 60% w/w) production by studied strains.     

Saccharification and protein enrichment of sugar beet pulp byPleurotus florida

Summary Pleurotus florida was grown in submerged culture with different concentrations of sugar beet pulp with a view to its nutritional upgrading. Micelial growth, protein enrichment and dry weight loss of the solid residues of cultures, as well as the evolution of reducing sugars in the liquid medium, were followed for 14 days. A product with 45% (w/w) protein and a saccharification extent as high as 35% (w/w) were obtained after 7 days with 1% (w/v) sugar beet pulp.     

Influence of pectin and glucose on growth and polygalacturonase production by Aspergillus niger in solid-state cultivation

The solid-state production of endo- and exo-polygalacturonases (PG) by Aspergillus niger was studied in a media containing wheat bran, salts, and different citric pectin and/or glucose concentrations. Kinetic analysis of the process indicated that the formation of PG and the growth of A. niger are associated processes. By increasing citric pectin from 0 to 16% (w/w), the maximum A. niger concentration (X _m) was raised from 94 to 121 mg/g dry medium suggesting that pectin can be used by A. niger as a growth substrate besides its role as an inducer. With 16% (w/w) pectin, 281 U exo-PG/gdm and 152 U endo-PG/gdm were obtained. Otherwise, pectin concentrations from 20 to 30% (w/w) hindered both production and growth. A. niger concentrations of 108–113 mg/gdm were achieved in runs with glucose from 5 to 12% (w/w), whereas at 16 and 20% (w/w) glucose, lower X _m values (ca. 100 mg/gdm) were measured. The addition of glucose to the wheat bran medium, up to 10% (w/w) led to maximum endo-PG titers slightly lower than those found in the absence of glucose. Nevertheless, exo-PG formation in these media was strongly increased and activities over 370 U/gdm were achieved. The results suggest that in experiments with pectin concentrations until 16% (w/w), exo-PG production was repressed by pectin-degradation products although these same substances had favored biomass growth. When glucose concentrations over 10% (w/w) were added to the media, the maximum activities of both enzymes decreased drastically, suggesting that glucose at high concentrations also exerts a repressive effect on PG production.     

Pollination by passerine birds: why are the nectars so dilute?

Bird-pollinated flowers are known to secrete relatively dilute nectars (with concentrations averaging 20–25% w/w). Many southern African plants that are pollinated by passerine birds produce nectars with little or no sucrose. Moreover, these hexose nectars are extremely dilute (10–15%). This suggests a link between sugar composition and nectar concentration. Nectar originates from sucrose-rich phloem sap, and the proportion of monosaccharides depends on the presence and activity of invertase in the nectary. Hydrolysis of sucrose increases nectar osmolality and the resulting water influx can potentially convert a 30% sucrose nectar into a 20% hexose nectar, with a 1.56 times increase in volume. Hydrolysis may also increase the gradient for sucrose transport and thus the rate of sugar secretion. When sucrose content and refractometer data were compared, some significant correlations were seen, but the occurrence of sucrose-rich or hexose-rich nectars can also be explained on phylogenetic grounds (e.g. Erythrina and Protea). Hexose nectars may be abundant enough to drip from open flowers, but evaporation leads to much variability in nectar concentration and increases the choices available to pollinators.     

Sugar assimilation and digestive efficiency in Wahlberg's epauletted fruit bat (Epomophorus wahlbergi)

Fruit- and nectar-feeding bats have high energy demands because of the cost of flight, and sugar is a good fuel because it is easily digested and absorbed. This study investigated the digestive efficiency of different sugars at different concentrations in Wahlberg's epauletted fruit bat (Epomophorus wahlbergi). We predicted that the sugar type and concentration would affect the total amount of solution consumed, while the total energy gained and the apparent assimilation efficiency would be high, irrespective of sugar type or concentration. Equicaloric solutions of two sugar types, glucose and sucrose, at low (10%), medium (15%) and high (25%) concentrations were offered in separate trials to bats. Total amount of solution consumed, total energy gained from each solution, and apparent assimilation efficiency, were measured. Bats had higher total volumetric intake of glucose and sucrose at the low concentrations than at the higher concentrations. However, bats maintained similar total energy intake on the respective glucose and sucrose concentrations. Bats were found to have high assimilation efficiencies on both glucose and sucrose irrespective of concentration. As bats used both sugars efficiently to maximize and maintain energy gain, it is expected that they feed opportunistically on fruit in the wild depending on temporal and spatial availability to obtain their energy requirements. Furthermore, fruit with high sucrose or glucose content will be consumed.     

Comparison of the hydrolytic activity of commercial cellulase preparations

Summary Two different quality types of sugar-cane molasses containing a total sugar content of 48%–50% (w/v) and 35%–42% (w/v) were investigated for Zymomonas biothanol production. Molasses concentrations of up to 250 g/l (1:3 dilution) were successfully fermented within 24 h despite a higher salt concentration in the lower grade molasses. Higher molasses concentrations (300 g/l) led to fructose accumulation. The addition of sucrose to a final sugar concentration of 15% (w/v) led to 10% (v/v) ethanol with conversion efficiencies up to 96%. Sorbitol levels were negligible, but increased up to tenfold upon addition of invertase. Offprint requests to: H. W. Doelle     

Enzymatic hydrolysis of cellulose: Evaluation of cellulase culture filtrates under use conditions

Culture filtrates from three mutant strains of Trichoderma reesei grown on lactose and on cellulose were compared under use conditions on four cellulose substrates. Cellulose culture filtrates contained five to six times as much cellulase as lactose culture filtrates. Unconcentrated cellulose culture filtrates produced up to 10% sugar solutions from 15% cellulose in 24 h. Specific activity in enzyme assays and efficiency in saccharification tests were low for enzymes from all the mutants. Over a wide range the percent saccharification of a substrate in a given times was directly proportional to the logarithm of the ratio of initial concentrations of enzyme and substrate. As a result of this, dilute enzyme is more efficient than concentrated enzyme, but if high sugar concentrations are desired, very large quantities of enzyme are required. Since the slopes of these plots varied, the relative activity of cellulase on different substrates may be affected by enzyme concentration.     

Biological detoxification of different hemicellulosic hydrolysates using Issatchenkia occidentalis CCTCC M 206097 yeast

This work had as its main objective to contribute to the development of a biological detoxification of hemicellulose hydrolysates obtained from different biomass plants using Issatchenkia occidentalis CCTCC M 206097 yeast. Tests with hemicellulosic hydrolysate of sugarcane bagasse in different concentrations were carried out to evaluate the influence of the hydrolysate concentration on the inhibitory compounds removal from the sugarcane bagasse hydrolysate, without reduction of sugar concentration. The highest reduction values of inhibitors concentration and less sugar losses were observed when the fivefold concentrated hydrolysate was treated by the evaluated yeast. In these experiments it was found that the high sugar concentrations favored lower sugar consumption by the yeast. The highest concentration reduction of syringaldehyde (66.67%), ferulic acid (73.33%), furfural (62%), and 5-HMF (85%) was observed when the concentrated hydrolysate was detoxified by using this yeast strain after 24 h of experimentation. The results obtained in this work showed the potential of the yeast Issatchenkia occidentalis CCTCC M 206097 as detoxification agent of hemicellulosic hydrolysate of different biomass plants.     

Effects of growth period,plant age and changes in solution aluminium concentrations on aluminium toxicity in wheat

The effects of growth period (time between transplanting and harvesting), plant age at which aluminium (Al) was added to solution, changes in Al concentration, and solution culture techniques (monitoring and adjusting solution Al concentrations thrice weekly or weekly replacement of the solutions) were investigated using a low ionic strength (2.7×10^–3 M) solution culture technique. The wheat (Triticum aestivum L.) cultivars Waalt (Al-tolerant) and Warigal (Al-sensitive), or the near isogenic lines bred from these cultivars (RR for the Al-tolerant line and SS for the Al-sensitive line) were grown. In all experiments and treatments, Al additions were required to maintain the nominal concentration. The decline in solution Al concentrations was partially attributed to formation of an Al-hydroxy-phosphate precipitate with an Al:P molar ratio of 2.8 to 4.0. Increasing the growth period from 14 to 28 days increased Al sensitivity in Warigal but not in Waalt. When plants were exposed to Al for the same time, increasing the age of the plants that Al was added to solution decreased sensitivity to Al. Differential Al tolerance between the two lines was evident when solutions were monitored thrice weekly or replaced weekly. However, the Al concentration required to reduce relative yield by a given amount when the solutions were replaced weekly was about twice that when the solutions were monitored. With a constant growth period of 28 days, increasing solution Al concentrations for 3 or more days resulted in decreased yields at harvest. The exact effect depended on the cultivar, plant part (tops or roots), when solution Al concentrations were increased and the duration of the increase. For example, increasing Al concentrations from 5 M to 20 M for 10 days reduced yield in the RR line by approximately 50% in the tops and 30% in the roots beyond the effect of 5 M but had no effect in the SS line due to yields already being low at 5 M. Adding 10 M Al to solution for 6 days at the beginning of the experiment reduced yield by 25% in the RR line and 50% in the SS line. In contrast, adding 10 M Al for 6 days in the middle of the growth cycle had no effect on the RR line but reduced yield by approximately 25% in the SS line. These results show that growth period, the age of the plants at which Al is added and the technique used (monitored or weekly replacement) all need to be considered when comparing results from different experiments. These results also show that the Al concentrations in solution need to be regularly monitored in long term experiments.     

Effects of nectar volume and concentration on sugar intake rates of Australian honeyeaters (Meliphagidae)

Summary Sugar intake rates of captive Australian honeyeaters (Meliphagidae) feeding at artificial flowers varied across species, and as a function of nectar volume and concentration within each species. Red Wattlebirds (Anthochaera carunculata, 110 g), achieved higher intake rates than New Holland Honeyeaters (Phylidonyris novaehollandiae, 20 g), and both achieved higher rates than Eastern Spinebills (Acanthorhynchus tenuirostris, 10 g). These results reflect differences in bill and tongue dimensions as well as in body mass. Sugar intake rates for all three species increased with volume (5–50 l) at any given concentration (10–60% mass/mass sucrose). For a given volume, sugar intake rates peaked at intermediate concentrations: 40–50% for the two larger species, and 30–40% for the smallest species. Published studies for other nectarivores foraging at unlimited volume feeders also show optimal nectar concentrations of 30–50%. However, biophysical theory predicts optima at 20–26% for small volumes, and plants presumed to be adapted for bird-pollination often have dilute nectar (20–30%). To explore this discrepancy further, we presented New Holland Honeyeaters with a range of sucrose concentrations (10–50%) using two presentation schemes. In the first we varied concentration but kept volume constant, thus varying gross sugar reward available in each concentration. This gave maximum sugar intake rates at 50%. In the second we varied both volume and concentration so that gross sugar rewards were equal for all solutions, decoupling high concentrations and large sugar rewards. This gave optima at 20%. We argue that variation among plants in nature more closely resembles the latter, equal sugar presentation scheme, and therefore, that dilute nectars may indeed represent adaptations for bird pollination.     

Sugarcane-pressmud as a novel substrate for production of citric acid by solid-state fermentation

Sugarcane-pressmud, a by-product of cane-sugar manufacture, was used as a substrate for production of citric acid by Aspergillus niger CFTRI 30, in a solid-state fermentation system. Of the 170 g of sugar supplied, 131 g were consumed, with a 79% yield of citric acid over 120 h. Potassium ferrocyanide improved the conversion to about 88% and lowered the fermentation time by 24 h. Enrichment with sugar and NH₄NO₃ was essential to improve productivity. About 174 g citric acid/kg dry sugarcane-pressmud were produced after 120 h in ferrocyanid-treated medium which initially contained 12.5% (w/w) effective sugar and 0.1% (w/w) NH₄NO₃. About 3% (w/w) of the original sugar present in the sugarcane-pressmud was non-utilizable. This is the first report on the potential of sugarcane-pressmud for citric acid production.V.S. Shankaranand and B.K. Lonsane are with the Fermentation Technology and Bioengineering Discipline, Central Food Technological Research Institute, Mysore-570 013, India     

Effects of freezing on isolated plant mitochondria

Mitochondria isolated from spinach leaves (Spinacia oleracea L.) and potato tubers (Solanum tuberosum L.) were partly injured when subjected to freezing for 2 to 4 h at-25°C in salt solutions in the absence of cryoprotectants. Damage was manifested by the inactivation of respiratory properties and increase in the permeability of the mitochondrial membranes. Decrease in respiratory control indicated that the control mechanism of the electron transport chain was influenced by freezing. Oxidative phosphorylation was only slightly more affected than electron transport. The inactivation of the membrane systems was caused by an increase in the concentration of membrane-toxic solutes. This was confirmed by treatment of the organelles at 0°C in solutions of high salt concentrations. When sugar was present in the course of freezing, mitochondria were partly or completely protected. On a molar basis, sucrose was more effective in membrane protection than glucose. Under certain conditions amino acids, e.g., proline and hydroxyproline, also stabilized isolated mitochondria during freezing.Abbreviations BSA bovine albumin - HEPES N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid - MOPS 2-N-morpholinopropane sulfonic acid - PVP polyvinyl pyrrolidone - RC respiratory control - Tris tris (hydroxymethyl) aminomethane