Showing papers on "Sucrose published in 2005"

Fructose, insulin resistance, and metabolic dyslipidemia

Abstract: Obesity and type 2 diabetes are occurring at epidemic rates in the United States and many parts of the world. The "obesity epidemic" appears to have emerged largely from changes in our diet and reduced physical activity. An important but not well-appreciated dietary change has been the substantial increase in the amount of dietary fructose consumption from high intake of sucrose and high fructose corn syrup, a common sweetener used in the food industry. A high flux of fructose to the liver, the main organ capable of metabolizing this simple carbohydrate, perturbs glucose metabolism and glucose uptake pathways, and leads to a significantly enhanced rate of de novo lipogenesis and triglyceride (TG) synthesis, driven by the high flux of glycerol and acyl portions of TG molecules from fructose catabolism. These metabolic disturbances appear to underlie the induction of insulin resistance commonly observed with high fructose feeding in both humans and animal models. Fructose-induced insulin resistant states are commonly characterized by a profound metabolic dyslipidemia, which appears to result from hepatic and intestinal overproduction of atherogenic lipoprotein particles. Thus, emerging evidence from recent epidemiological and biochemical studies clearly suggests that the high dietary intake of fructose has rapidly become an important causative factor in the development of the metabolic syndrome. There is an urgent need for increased public awareness of the risks associated with high fructose consumption and greater efforts should be made to curb the supplementation of packaged foods with high fructose additives. The present review will discuss the trends in fructose consumption, the metabolic consequences of increased fructose intake, and the molecular mechanisms leading to fructose-induced lipogenesis, insulin resistance and metabolic dyslipidemia.

Sugar signalling and gene expression in relation to carbohydrate metabolism under abiotic stresses in plants

Abstract: Sucrose is required for plant growth and development. The sugar status of plant cells is sensed by sensor proteins. The signal generated by signal transduction cascades, which could involve mitogen-activated protein kinases, protein phosphatases, Ca 2+ and calmodulins, results in appropriate gene expression. A variety of genes are either induced or repressed depending upon the status of soluble sugars. Abiotic stresses to plants result in major alterations in sugar status and hence affect the expression of various genes by down- and up-regulating their expression. Hexokinase-dependent and hexokinase-independent pathways are involved in sugar sensing. Sucrose also acts as a signal molecule as it affects the activity of a proton-sucrose symporter. The sucrose trans-porter acts as a sucrose sensor and is involved in phloem loading. Fructokinase may represent an additional sensor that bypasses hexokinase phosphorylation especially when sucrose synthase is dominant. Mutants isolated on the basis of response of germination and seedling growth to sugars and reporter-based screening protocols are being used to study the response of altered sugar status on gene expression. Common cis-acting elements in sugar signalling pathways have been identified. Transgenic plants with elevated levels of sugars/sugar alcohols like fructans, raffinose series oligosaccharides, trehalose and mannitol are tolerant to different stresses but have usually impaired growth. Efforts need to be made to have transgenic plants in which abiotic stress responsive genes are expressed only at the time of adverse environmental conditions instead of being constitutively synthesized.

Consuming fructose-sweetened beverages increases body adiposity in mice

Abstract: Author(s): Jurgens, Hella; Haass, Wiltrud; Castaneda, Tamara R; Schurmann, Annette; Koebnick, Corinna; Dombrowski, Frank; Otto, Barbel; Nawrocki, Andrea R; Scherer, Philipp E; Spranger, Jochen; Ristow, Michael; Joost, Hans-Georg; Havel, Peter J; Tschop, Matthias H | Abstract: ObjectiveThe marked increase in the prevalence of obesity in the United States has recently been attributed to the increased fructose consumption. To determine if and how fructose might promote obesity in an animal model, we measured body composition, energy intake, energy expenditure, substrate oxidation, and several endocrine parameters related to energy homeostasis in mice consuming fructose.Research methods and proceduresWe compared the effects of ad libitum access to fructose (15% solution in water), sucrose (10%, popular soft drink), and artificial sweetener (0% calories, popular diet soft drink) on adipogenesis and energy metabolism in mice.ResultsExposure to fructose water increased adiposity, whereas increased fat mass after consumption of soft drinks or diet soft drinks did not reach statistical significance (n = 9 each group). Total intake of energy was unaltered, because mice proportionally reduced their caloric intake from chow. There was a trend toward reduced energy expenditure and increased respiratory quotient, albeit not significant, in the fructose group. Furthermore, fructose produced a hepatic lipid accumulation with a characteristic pericentral pattern.DiscussionThese data are compatible with the conclusion that a high intake of fructose selectively enhances adipogenesis, possibly through a shift of substrate use to lipogenesis.

Physiological characterization of Mg deficiency in Arabidopsis thaliana

Abstract: Although the symptoms of magnesium deficiency are well documented in plants, the primary physiological effects of low Mg availability remain largely unknown. This paper describes the physiological responses of Mg starvation in Arabidopsis thaliana. Growth characteristics, Mg and sugar concentration, and photochemical performance were measured at regular intervals during the induction of Mg deficiency. These data show that Mg deficiency increased the sugar concentration and altered sucrose export from young source leaves before any noticeable effect on photosynthetic activity was seen. The decline in photosynthetic activity might be elicited by increased leaf sugar concentrations. Transcript levels of Cab2 (encoding a chlorophyll a/b protein) were lower in Mg-deficient plants before any obvious decrease in the chlorophyll concentration. These transcriptional data suggest that the reduction of chlorophyll is a response to sugar levels, rather than a lack of Mg atoms for chelating chlorophyll.

Adverse effects of dietary fructose.

Abstract: The consumption of fructose, primarily from high-fructose corn syrup (HFCS), has increased considerably in the United States during the past several decades. Intake of HFCS may now exceed that of the other major caloric sweetener, sucrose. Some nutritionists believe fructose is a safer form of sugar than sucrose, particularly for people with diabetes mellitus, because it does not adversely affect blood-glucose regulation, at least in the short-term. However, fructose has potentially harmful effects on other aspects of metabolism. In particular, fructose is a potent reducing sugar that promotes the formation of toxic advanced glycation end-products, which appear to play a role in the aging process; in the pathogenesis of the vascular, renal, and ocular complications of diabetes; and in the development of atherosclerosis. Fructose has also been implicated as the main cause of symptoms in some patients with chronic diarrhea or other functional bowel disturbances. In addition, excessive fructose consumption may be responsible in part for the increasing prevalence of obesity, diabetes mellitus, and non-alcoholic fatty liver disease. Although the long-term effects of fructose consumption have not been adequately studied in humans, the available evidence suggests it may be more harmful than is generally recognized. The extent to which a person might be adversely affected by dietary fructose depends both on the amount consumed and on individual tolerance. With a few exceptions, the relatively small amounts of fructose that occur naturally in fruits and vegetables are unlikely to have deleterious effects, and this review is not meant to discourage the consumption of these healthful foods.

Assessment of antioxidant activity of cane brown sugars by ABTS and DPPH radical scavenging assays: determination of their polyphenolic and volatile constituents.

Abstract: Seven cane brown sugars (four from La Reunion, two from Mauritius, and one from France) were investigated for their polyphenol content and volatile composition in relation to their free radical scavenging capacity determined by ABTS and DPPH assays. The thin layer coated on the sugar crystal was extracted by Soxhlet extractor with dichloromethane. The volatile compounds of brown sugars were studied by GC-MS, and 43 compounds were identified. The total phenolic content of brown sugars was determined according to the Folin-Ciocalteu method. Phenolic compounds were quantified in the brown sugar extracts by LC-UV-ESI-MS. Brown sugar aqueous solutions exhibited weak free radical scavenging activity in the DPPH assay and higher antioxidant activity in the ABTS assay at relatively high concentration. The brown sugar extracts showed interesting free radical scavenging properties despite the low concentration of phenolic and volatile compounds. Sugar is a common foodstuff traditionally used for its sweetening properties, which might be accompanied by antioxidant properties arising from molecules (polyphenols, Maillard products) other than sucrose of the cane brown sugars.

Magnesium deficiency in sugar beets alters sugar partitioning and phloem loading in young mature leaves.

Abstract: Magnesium deficiency has been reported to affect plant growth and biomass partitioning between root and shoot. The present work aims to identify how Mg deficiency alters carbon partitioning in sugar beet (Beta vulgaris L.) plants. Fresh biomass, Mg and sugar contents were followed in diverse organs over 20 days under Mg-sufficient and Mg-deficient conditions. At the end of the treatment, the aerial biomass, but not the root biomass, of Mg-deficient plants was lower compared to control plants. A clear inverse relationship between Mg and sugar contents in leaves was found. Mg deficiency promoted a marked increase in sucrose and starch accumulation in the uppermost expanded leaves, which also had the lowest content of Mg among all the leaves of the rosette. The oldest leaves maintained a higher Mg content. [14C]Sucrose labelling showed that sucrose export from the uppermost expanded leaves was inhibited. In contrast, sucrose export from the oldest leaves, which are close to, and export mainly to, the roots, was not restricted. In response to Mg deficiency, the BvSUT1 gene encoding a companion cell sucrose/H+ symporter was induced in the uppermost expanded leaves, but without further enhancement of sucrose loading into the phloem. The observed increase in BvSUT1 gene expression supports the idea that sucrose loading into the phloem is defective, resulting in its accumulation in the leaf.

Hepatic adaptations to sucrose and fructose

Abstract: The liver is an important site of postprandial glucose disposal, accounting for the removal of up to 30% of an oral glucose load The liver is also centrally involved in dietary lipid and amino acid uptake, and the presence of either or both of these nutrients can influence hepatic glucose uptake The composition of ingested carbohydrate also influences hepatic glucose metabolism For example, fructose can increase hepatic glucose uptake In addition, fructose extraction by the liver is exceedingly high, approaching 50% to 70% of fructose delivery The selective hepatic metabolism of fructose, and the ability of fructose to increase hepatic glucose uptake can, under appropriate conditions (eg, diets enriched in sucrose or fructose, high fructose concentrations), provoke major adaptations in hepatic metabolism Potential adaptations that can arise in response to these conditions and putative mechanisms driving these adaptations are the subject of this review

Urinary Sucrose and Fructose as Biomarkers for Sugar Consumption

Abstract: The use of 24-hour urinary sucrose and fructose as potential biomarkers for sugars consumption was investigated in two studies of 21 healthy participants living in a volunteer suite where dietary intake was known and all specimens collected. The dose-response was assessed in 12 males using a randomized crossover design of three diets containing constant levels of 63, 143, and 264 g of sugars for 10 days each. Both sugars and sucrose intake were significantly correlated with the sum of sucrose and fructose concentration in urine (0.888; P < 0.001). To assess effects with volunteers consuming their habitual varying diets, seven males and six females were fed their usual diet (assessed beforehand from four consecutive self-completed 7-day food diaries) for 30 days under controlled conditions in the volunteer suite. The mean (+/-SD) calculated total sugars intake was 202 +/- 69 g/d, 41% from sucrose. Mean (+/-SD) urinary sucrose and fructose were 36.6 +/- 16.6 and 61.8 +/- 61.3 mg/d, respectively. The sum of sucrose and fructose in urine was significantly correlated with sugars (0.841; P < 0.001) and sucrose intake (0.773; P = 0.002). In the regression, 200 g of sugars intake predicted approximately 100 mg of sucrose and fructose in urine. The correlation between individual means of randomized 16 days of sugars intake and 8 days of sugars excretion data (as used in validation studies) remained as high as that obtained with the means of 30-day measurements and the regression estimates were very similar. Twenty-four-hour urinary sucrose and fructose could be grouped into a new category of biomarkers, predictive biomarkers, that can be used in studies determining the structure of dietary measurement error in free living individuals and to relate sugars intake to disease risk.

Fruit carbohydrate metabolism in an introgression line of tomato with increased fruit soluble solids.

Abstract: A tomato line (IL9-2-5) of the cultivated species, Lycopersicon esculentum, carrying a 9 cM introgression from the wild species, Lycopersicon pennelli, produces fruit with high soluble solids content (Brix), an important determinant of fruit quality for processing. Two quantitative trait loci (QTLs) relating to fruit soluble solids content have been identified within the introgressed segment. One of these QTLs (PW-9-2-5) is silent under the growth conditions used in this study, while a second (Brix-9-2-5) has been shown to encode a fruit apoplastic invertase (Lin5) with altered kinetic properties. In this study, we have undertaken a detailed biochemical analysis of the introgression line to attempt to gain an understanding of the metabolic changes associated with increased fruit soluble solids. Increased Brix in ripe fruit was shown to be the result of increased sucrose and glucose, with a more minor contribution from aspartate and alanine. The introgression leads to a pronounced increase in apoplastic invertase activity in the columella tissue that extends throughout fruit development. Furthermore, columella tissue from IL9-2-5 fruit has a greater capacity to take up exogenously supplied sucrose, an observation that is consistent with the kinetic properties of the introgressed Lin5 allele. Apart from the increase in mature fruit sugar and increases in some amino acids, metabolite profiling revealed few other metabolic perturbations in fruit from IL9-2-5. The only other major change was a dramatic increase in starch accumulation at earlier stages of fruit metabolism. This occurred without any increase in the activity of the enzymes of sucrose metabolism or starch synthesis and may therefore be driven by increased availability of sucrose. We conclude that the major factor that leads to increased fruit sugar in IL9-2-5 is an increase in the capacity to take up sucrose unloaded from the phloem.

Heterotrophic high cell‐density fed‐batch cultures of the phycocyanin‐producing red alga Galdieria sulphuraria

Abstract: Growth and phycocyanin production in batch and fed-batch cultures of the microalga Galdieria sulphuraria 074G, which was grown heterotrophically in darkness on glucose, fructose, sucrose, and sugar beet molasses, was investigated. In batch cultures, specific growth rates and yields of biomass dry weight on the pure sugars were 1.08-1.15 day-1 and 0.48-0.50 g g-1, respectively. They were slightly higher when molasses was the carbon source. Cellular phycocyanin contents during the exponential growth phase were 3-4 mg g-1 in dry weight. G. sulphuraria was able to tolerate concentrations of glucose and fructose of up to 166 g L-1 (0.9 M) and an ammonium sulfate concentration of 22 g L-1 (0.17 M) without negative effects on the specific growth rate. When the total concentration of dissolved substances in the growth medium exceeded 1-2 M, growth was completely inhibited. In carbon-limited fed-batch cultures, biomass dry weight concentrations of 80-120 g L-1 were obtained while phycocyanin accumulated to concentrations between 250 and 400 mg L-1. These results demonstrate that G. sulphuraria is well suited for growth in heterotrophic cultures at very high cell densities, and that such cultures produce significant amounts of phycocyanin. Furthermore, the productivity of phycocyanin in the heterotrophic fed-batch cultures of G. sulphuraria was higher than is attained in outdoor cultures of Spirulina platensis, where phycocyanin is presently obtained.

Formation of furan from carbohydrates and ascorbic acid following exposure to ionizing radiation and thermal processing.

Abstract: This study was conducted to investigate the formation of furan from sugars, ascorbic acid, and organic acids as affected by ionizing radiation and thermal treatments. Results showed that both thermal treatments and irradiation induced formation of furan from ascorbic acid, fructose, sucrose, or glucose. Little furan was produced from malic acid or citric acid. The pH and concentration of sugars and ascorbic acid solutions had profound influences on furan formation due to either irradiation or thermal treatment. The rate of irradiation-induced furan formation increased with decreasing pH from 8 to 3. Approximately 1600 times less furan was formed at pH 8 as apposed to pH 3. At the same pHs, the amounts of furan formed from irradiation of ascorbic acid, fructose, and sucrose were always higher than from glucose. As pH decreased from 7 to 3, an increase in thermally induced furan was observed for sucrose and ascorbic acid solutions, but for glucose solution, less furan was formed at pH 3 than at pH 7. The levels of sugars commonly found in fruits and fruit juices, upon irradiation, would be high enough to potentially produce low parts per billion (ppb) levels of furan. The concentration of ascorbic acid at which a maximum of furan was produced upon irradiation was about 0.5 mg/mL, a level commonly found in some foods. Five furan derivatives were tentatively identified in thermally treated ascorbic acid solution, while one furan derivative was tentatively found in both irradiated and thermally treated samples.

Sugars in Mediterranean Floral Nectars: An Ecological and Evolutionary Approach

Abstract: High-pressure liquid chromatography analyses of 73 plant species showed that the nectars of phrygana (East Mediterranean garrigue) mainly contain sucrose, glucose, and fructose, and traces of 10 minor sugars. Although the sucrose/hexose ratio was not related to plant life habit, ecological constraints had a detectable effect in shaping sugar composition. This was detected by distinguishing the phryganic plant species into “spring–summer” and “winter” flowering, with the distinction made on the basis of the water deficit in the study area. Plants flowering in spring–summer had a higher rate of “high sucrose” (i.e., sucrose/hexose ratio ≥0.5; 60.8% of the plant species) vs. “low hexose” nectars (i.e., ratio <0.5; 39.2%). The ratio was reversed in winter flowering species (36.4% vs. 63.6% with “high sucrose” and “high hexose,” respectively). Sucrose/hexose ratios were associated with plant family. The highest values were those of Lamiaceae, which differed significantly from the “low sucrose” Liliaceae and Apiaceae. Based on recorded plant–pollinator interactions in the community, the present data provide evidence of a partitioning of nectar resources by the existing pollinator guilds within the community, based on the sugar profiles of nectar (all sucrose/hexose ratios for all interactions). Among all major groups, bees and wasps (aculeates) preferred “high sucrose” nectars, which differed significantly from syrphids, anthomyid a.o. flies, and beetles that visited “low sucrose” nectars. Similarly, butterflies visited “lower sucrose” nectars compared to bees. Within families, only Megachilidae could be clearly characterized as “high sucrose” consumers, differing in this respect from all the remaining insect groups including most other bee families. This confirms previous findings that Megachilidae have a key position in Mediterranean communities where they probably constitute a selective factor for “high sucrose” nectars.

Physicochemical characterization and biological effects of inulin enzymatically synthesized from sucrose.

Abstract: We first developed the method to produce inulin from sucrose using an enzyme from Bacillus sp. 217C-1. Synthesized inulin consists of a linear polymer having beta(2-1) linkages of d-fructose with one terminal glucose. The synthesized inulin has similar properties (pH and thermal stability, Maillard reaction, and in vitro fermentation) to plant-derived inulin. The marked difference is the polydispersity of the inulin chain length. Synthesized inulin with a narrow degree range of fructose polymerization shows better solubility in water than plant-derived inulin. Synthesized inulin (5%, w/w) treatment for 12 weeks reduced the elevation in body weight and serum and liver lipids in rats fed high fat- and high sucrose-supplemented diets, and blood glucose in rats fed a standard diet. Synthesized inulin (15%, w/w) significantly suppressed the elevation in blood glucose of human healthy subjects after dextrin loading. These results suggest that daily intake of synthesized inulin modulates carbohydrate and lipid metabolism.

Analyses of enzyme II gene mutants for sugar transport and heterologous expression of fructokinase gene in Corynebacterium glutamicum ATCC 13032

Abstract: Corynebacterium glutamicum ATCC 13032 has four enzyme II (EII) genes of the phosphotransferase system in its genome encoding transporters for sucrose, glucose, fructose, and an unidentified EII. To analyze the function of these EII genes, they were inactivated via homologous recombination and the resulting mutants characterized for sugar utilization. Whereas the sucrose EII was the only transport system for sucrose in C. glutamicum, fructose and glucose were each transported by a second transporter in addition to their corresponding EII. In addition, the ptsF ptsG double mutant carrying deletions in the EII genes for fructose and glucose accumulated fructose in the culture broth when growing on sucrose. As no fructokinase gene exists in the C. glutamicum genome, the fructokinase gene from Clostridium acetobutylicum was expressed in C. glutamicum and resulted in the direct phosphorylation of fructose without any fructose efflux. Accordingly, since fructokinase could direct fructose flux to the pentose phosphate pathway for the supply of NADPH, fructokinase expression may be a potential strategy for enhancing amino acid production.

Transcriptional Regulation and Characterization of a Novel β-Fructofuranosidase-Encoding Gene from Bifidobacterium breve UCC2003

Abstract: An operon involved in fructooligosaccharide breakdown was identified in the genome of Bifidobacterium breve UCC2003. This 2.6-kb transcriptional unit was comprised of three genes that encoded a putative permease, a conserved hypothetical protein, and a β-fructofuranosidase. Active transcription of the operon was observed when B. breve UCC2003 was grown on sucrose or Actilight, while transcription appeared to be repressed when the organism was grown on glucose, fructose, a combination of glucose and sucrose, or a combination of fructose and sucrose. The β-fructofuranosidase encoded by this operon was purified and biochemically characterized. The optimum pH and temperature for catalytic activity were determined to be pH 6.0 and 37°C, respectively, and there was a dependence on bivalent cations, particularly manganese. The Km and Vmax values for sucrose hydrolysis were determined to be 25 ± 2 mM and 24 ± 3 μmol min−1 mg−1, respectively. Interestingly, the enzyme was shown to specifically catalyze cleavage of the β(2-1) glycosidic bond between glucose and its neighboring fructose moiety in sucrose and other fructooligosaccharides with a relatively low degree of polymerization, and there was no detectable activity towards the β(2-1) glycosidic bond between two fructose moieties within the same substrate. To our knowledge, such an enzymatic activity has not previously been described in bifidobacteria or other gram-positive bacteria.

Kinetics of acrylamide formation and elimination during heating of an asparagine-sugar model system.

Abstract: The kinetics of acrylamide (AA) was analyzed by heating a simple model system consisting of asparagine and glucose, fructose, or sucrose (0.01 M, pH 6) at temperatures between 140 and 200 °C. The A...

VvHT1 encodes a monosaccharide transporter expressed in the conducting complex of the grape berry phloem

Abstract: The accumulation of sugars in grape berries requires the co-ordinate expression of sucrose transporters, invertases, and monosaccharide transporters. A monosaccharide transporter homologue (VvHT1, Vitis vinifera hexose transporter 1) has previously been isolated from grape berries at the ve´raison stage, and its expression was shown to be regulated by sugars and abscisic acid. The present work investigates the function and localization of VvHT1. Heterologous expression in yeast indicates that VvHT1 encodes a monosaccharide transporter with maximal activity at acidic pH (pH 4.5) and high affinity for glucose (Km570 lM). Fructose, mannose, sorbitol, and mannitol are not transported by VvHT1. In situ hybridization shows that VvHT1 transcripts are primarily found in the phloem region of the conducting bundles. Immuno- fluorescence and immunogold labelling experiments localized VvHT1 in the plasma membrane of the sieve element/companion cell interface and of the flesh cells. The expression and functional properties of VvHT1 suggests that it retrieves the monosaccharides needed to provide the energy necessary for cell division and cell growth at an early stage of berry development.

Pink-pigmented facultative methylotrophic bacteria accelerate germination, growth and yield of sugarcane clone Co86032 (Saccharum officinarum L.)

Abstract: The existence of Methylobacterium as a symbiont with sugarcane and its influence on crop growth at various stages was examined. Pink-pigmented facultative methylotrophic bacteria (PPFMs) strains isolated from different parts of the sugarcane clone Co86032 showed growth on methanol, and were further confirmed based on the mxaF gene encoding the α-subunit of the methanol dehydrogenase by polymerase chain reaction amplification using specific primers. True seeds inoculated with PPFMs had a higher germination percent and rate of germination than the control. A combined treatment of seed imbibition, soil application and phyllosphere spray increased specific leaf area, plant height, number of internodes, and cane yield. Immunological determination of cytokinin in young and mature leaves significantly increased when the epiphytic population on the leaf surface increased. Trends in sugar qualities in the form of Pol (sucrose) % in cane, Brix % in cane, and commercial cane sugar were similar to that of cane yield. These effects might be mediated by the production or synthesis of plant hormones.

Fermentation of 10% (w/v) sugar to D: (-)-lactate by engineered Escherichia coli B.

Abstract: Derivatives of ethanologenic Escherichia coli K011 were constructed for d(−)-lactate production by deleting genes encoding competing pathways followed by metabolic evolution, a growth-based selection for mutants with improved performance Resulting strains, SZ132 and SZ186, contain native genes for sucrose utilization No foreign genes are present in SZ186 Strain SZ132 also contains a chromosomally integrated endoglucanase gene (Erwinia chrysanthemi celY) Strain SZ132 produced over 1 mol lactate per liter of complex medium containing 10% (w/v) sugar (fermentation times of 48 h for glucose, 120 h for sucrose) Both strains produced 667–700 mmol lactate per liter of mineral salts medium Yields for metabolized sugar ranged from 88% to 95% in both media

Leaf:fruit ratio and irrigation supply affect seasonal changes in minerals, organic acids and sugars of mango fruit

Abstract: To determine the effects of assimilate and water supply on the determination of mango fruit quality, the seasonal variations of minerals, acids and sugar concentrations were investigated over two successive years. To manipulate the assimilate supply, selected branches were girdled to provide ratios of 10, 25, 50 and 100 leaves per fruit. Irrigation was managed to provide two types of water supply treatments. Fruit growth rate was greater when increasing the leaf:fruit ratio. Structural dry matter content and total dry matter content of flesh were higher in fruit with higher leaf:fruit ratios. Treatments had no effect on the structural to total dry matter ratio of flesh. Potassium and magnesium to structural dry weight ratios were not affected by treatments, whereas the calcium to structural dry weight ratio was higher in the flesh of fruit grown under low leaf:fruit ratios. Low assimilate supply increased the ratios of malle and citric acid to structural dry weight. This treatment had little effect on acid concentrations. Glucose and fructose to structural dry weight ratios were higher when assimilate supply was lower. Low leaf:fruit ratios increased fructose concentration but not glucose concentration. Irrigation treatment strongly affected fructose concentration. Sucrose concentration, based either on structural dry matter or on fresh matter, was significantly increased by higher leaf-to-fruit ratios. When the fruit was close to maturity, levels of sucrose storage and starch breakdown were positively correlated with assimilate supply. Levels of starch breakdown were correlated with irrigation supply. The effects of these treatments on sugar concentrations may change fruit taste. (Resume d'auteur)

Antisense inhibition of sorbitol synthesis leads to up-regulation of starch synthesis without altering CO2 assimilation in apple leaves

Abstract: Sorbitol is a primary end-product of photosynthesis in apple (Malus domestica Borkh.) and many other tree fruit species of the Rosaceae family. Sorbitol synthesis shares a common hexose phosphate pool with sucrose synthesis in the cytosol. In this study, ‘Greensleeves’ apple was transformed with a cDNA encoding aldose 6-phosphate reductase (A6PR, EC 1.1.1.200) in the antisense orientation. Antisense expression of A6PR decreased A6PR activity in mature leaves to approximately 15–30% of the untransformed control. The antisense plants had lower concentrations of sorbitol but higher concentrations of sucrose and starch in mature leaves at both dusk and predawn. 14CO2 pulse-chase labeling at ambient CO2 demonstrated that partitioning of the newly fixed carbon to starch was significantly increased, whereas that to sucrose remained unchanged in the antisense lines with decreased sorbitol synthesis. Total activities of ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), sucrose-phosphate synthase (EC 2.4.1.14), and ADP-glucose pyrophosphorylase (EC 2.7.7.27) were not significantly altered in the antisense lines, whereas both stromal and cytosolic fructose-1,6-bisphosphatase (EC 3.1.3.11) activities were higher in the antisense lines with 15% of the control A6PR activity. Concentrations of glucose 6-phosphate and fructose 6-phosphate (F6P) were higher in the antisense plants than in the control, but the 3-phosphoglycerate concentration was lower in the antisense plants with 15% of the control A6PR activity. Fructose 2, 6-bisphosphate concentration increased in the antisense plants, but not to the extent expected from the increase in F6P, comparing sucrose-synthesizing species. There was no significant difference in CO2 assimilation in response to photon flux density or intercellular CO2 concentration. We concluded that cytosolic FBPase activity in vivo was down-regulated and starch synthesis was up-regulated in response to decreased sorbitol synthesis. As a result, CO2 assimilation in source leaves was sustained at both ambient CO2 and saturating CO2.

Sugar Preferences in Nectar‐ and Fruit‐Eating Birds: Behavioral Patterns and Physiological Causes1

Abstract: Sucrose, glucose, and fructose are the three sugars that commonly occur in floral nectar and fruit pulp. The relative proportions of these three sugars in nectar and fruit in relation to the sugar preferences of pollinators and seed dispersers have received considerable attention. Based on the research of Herbert and Irene Baker and their collaborators, a dichotomy between sucrose-dominant hummingbird-pollinated flowers and hexose-dominant passerine flowers and fruits was proposed. Data on sugar preferences of several hummingbird species (which prefer sucrose) vs. a smaller sample of passerines (which prefer hexoses) neatly fitted this apparent dichotomy. This hummingbird‐passerine dichotomy was strongly emphasized until the discovery of South African plants with sucrose-dominant nectars, which are pollinated by passerines that are able to digest, and prefer sucrose. Now we know that, with the exception of two clades, most passerines are able to assimilate sucrose. Most sugar preference studies have been conducted using a single, relatively high, sugar concentration in the nectar (ca 20%). Thus, we lack information about the role that sugar concentration might play in sugar selection. Because many digestive traits are strongly affected not only by sugar composition, but also by sugar concentration, we suggest that preferences for different sugar compositions are concentration-dependent. Indeed, recent studies on several unrelated nectar-feeding birds have found a distinct switch from hexose preference at low concentrations to sucrose preference at higher concentrations. Finally, we present some hypotheses about the role that birds could have played in molding the sugar composition of plant rewards.