Net Sugar Research Articles

Model-assisted comparison of sugar accumulation patterns in ten fleshy fruits highlights differences between herbaceous and woody species.

Sugar concentration is a key determinant of fruit quality. Soluble sugars and starch concentrations in fruits vary greatly from one species to another. The aim of this study was to investigate similarities and differences in sugar accumulation strategies across ten contrasting fruit species using a modelling approach. We developed a coarse-grained model of primary metabolism based on the description of the main metabolic and hydraulic processes (synthesis of compounds other than sugar and starch, synthesis and hydrolysis of starch, and water dilution) involved in the accumulation of soluble sugars during fruit development. Statistical analyses based on metabolic rates separated the species into six groups according to the rate of synthesis of compounds other than sugar and starch. Herbaceous species (cucumber, tomato, eggplant, pepper and strawberry) were characterized by a higher synthesis rate than woody species (apple, nectarine, clementine, grape and kiwifruit). Inspection of the dynamics of the processes involved in sugar accumulation revealed that net sugar importation, metabolism and dilution processes were remarkably synchronous in most herbaceous plants, whereas in kiwifruit, apple and nectarine, processes related to starch metabolism were temporally separated from other processes. Strawberry, clementine and grape showed a distinct dynamic compared with all other species. Overall, these results provide fresh insights into species-specific regulatory strategies and into the role of starch metabolism in the accumulation of soluble sugars in fleshy fruits. In particular, inter-specific differences in development period shape the co-ordination of metabolic processes and affect priorities for carbon allocation across species. The six metabolic groups identified by our analysis do not show a clear separation into climacteric and non-climacteric species, possibly suggesting that the metabolic processes related to sugar concentration are not greatly affected by ethylene-associated events.

МИРОВОЙ РЫНОК САХАРА КАК КОНКУРЕНТНАЯ СРЕДА ДЛЯ РОССИЙСКОГО ЭКСПОРТА

This article analyzes the problematic aspects of the development and emerging trends of the world sugar market as an external economic environment for realizing the export potential of the sugar beet subcomplex of the Russian agro-industrial complex. The global sugar market is characterized by problems, including an increased level of government intervention, imperfect competition, excess production and associated price volatility. For the economy of the Russian sugar industry, it seems to be an adequate strategy of gradually increasing competitiveness to a level that makes it possible to fight for new sales markets by modernizing agro-industrial and infrastructural capacities and providing conditions for guaranteed sales of sugar in foreign markets. Russia has all the necessary prerequisites to acquire the status of a net sugar exporter in the medium term and to take its rightful place among the leaders of the world sugar market.

Overview of Sugar Industry in Kenya and Prospects for Production at the Coast

This paper utilized national data obtained mainly from published sources as well as information obtained from field visits, key informant interviews, reports and records in the coastal region of Kenya. It is a sector review of the sugar industry in Kenya, with special focus on opportunities for expanding production in the coastal region. Sugar, or sugarcane (Saccharum Hybrids spp.) is among the top six commercial crops grown in Kenya. The others are tea, cut flowers, vegetables, coffee and maize. Industrial sugar was introduced in Kenya in 1902 and the first processing factory opened in 1922. In the 1960s, new government policy encouraged expansion of commercial sugar production with new factories opening in western Kenya. By the mid-1970s, Kenya was a sugar exporter. But from the 1980s, the sugar sector started to decline both in production and profitability, with the country becoming a net sugar importer by the 1980s. Although the area under sugarcane has expanded over the years to cover 220,000 ha, productivity is low achieving only 55 tonnes/hectare. Meanwhile, production costs have been increasing, averaging US $1007/tonnes in 2018. Over 80 percent of the sugar is grown rainfed by smallholder farmers mostly in western Kenya using low inputs, with consequent poor productivity. The sugar sector has numerous challenges including: agronomic, technological, economic, management and policy limitations. This paper traces the main issues impacting the sugar industry in Kenya. It finds a sector in turmoil that requires significant reforms. It explores prospects of the next sugar revolution emerging from the coast. The coastal region has land available for rainfed and irrigated sugarcane, a warmer climate and possibilities to grow short-maturing cane varieties of higher sucrose concentrations. But other constraints are also inherent at the coast. These must be overcome for sugar to become a major cash crop in the region.

Ultrasound enhanced enzymatic hydrolysis of Parthenium hysterophorus: A mechanistic investigation

This study has attempted to establish the mechanism of the ultrasound-induced enhancement of enzymatic hydrolysis of pretreated and delignified biomass of Parthenium hysterophorus. A dual approach of statistical optimization of hydrolysis followed by application of sonication at optimum conditions has been adopted. The kinetics of hydrolysis shows a marked 6× increase with sonication, while net sugar yield shows marginal rise of ∼20%. The statistical experimental design reveals the hydrolysis process to be enzyme limited. Profile of sugar yield in ultrasound-assisted enzymatic hydrolysis has been analyzed using HCH-1 model coupled with Genetic Algorithm optimization. The trends in the kinetic and physiological parameters of HCH-1 model reveal that sonication enhances enzyme/substrate affinity and reaction velocity of hydrolysis. The product inhibition of enzyme in all forms (free, adsorbed, complexed) also reduces with ultrasound. These effects are attributed to intense micro-convection induced by ultrasound and cavitation in the liquid medium.

Evaluation of Selected White-Rot Fungal Isolates for Improving the Sugar Yield from Wheat Straw

Biological pretreatment of lignocellulosic biomass by fungi can represent a low-cost and eco-friendly alternative to physicochemical methods to facilitate enzymatic hydrolysis. However, fungal metabolism can cause cellulose loss and it is therefore necessary to use the appropriate fungal strain-biomass type combination. In this work, the effects of biological pretreatments carried out by five different fungi on enzymatic hydrolysis of wheat straw were investigated. The best results were obtained with a Ceriporiopsis subvermispora strain, which minimized weight and cellulose losses and gave the highest net sugar yield (calculated with respect to the holocellulose content of the untreated straw), up to 44 % after a 10-week pretreatment, more than doubling the yields obtained with the other isolates. Moreover, prolonging the pretreatment from 4 up to 10 weeks produced a 2-fold increase, up to 60 %, in digestibility (sugar yield, calculated considering the holocellulose content of the pretreated material). The hemicellulose content of the pretreated material resulted inversely correlated with digestibility, and it could thus be utilized as an index of the pretreatment efficacy. Finally, a correlation was also found between digestibility and the difference between the absorbance values at 290 and 320 nm of pretreated wheat straw extracts.

Assessing the comparative productivity advantage of bioenergy feedstocks at different latitudes

We evaluate the comparative productivity of maize and sugarcane biofuel feedstocks as a function of latitude. Solar radiation for photosynthesis varies by latitude and contributes to differential productivity of tropical and temperate zones. We calculate comparative productivity in two ways—the amount of net sugar energy produced per unit area, and the amount produced per unit of net primary productivity (NPP). NPP measures the accumulation of energy in an ecosystem and can be used as a proxy for the capacity of an ecosystem to support biodiversity and a broader array of ecosystem services. On average sugarcane produces three times more energy per unit area than does maize. The comparative productivity advantage of sugarcane decreases with increases in latitude. Latitudes closer to the equator have higher NPP, so there is a greater trade-off between biofuel production and ecosystem productivity in the equatorial zones. The comparative productivity of sugarcane relative to maize is reduced when comparing biofuel energy per unit of NPP. Sugarcane is still twice as productive on average compared to maize in the amount of biofuel energy produced per unit of NPP. Regions near the equator have lower biofuel energy per unit NPP, making them less attractive for biofuels production.

Hexose Transporter GLUT1 Harbors Several Distinct Regulatory Binding Sites for Flavones and Tyrphostins

The facilitative hexose transporter GLUT1 activity is blocked by tyrosine kinase inhibitors that include natural products such as flavones and isoflavones and synthetic compounds such as tyrphostins, molecules that are structurally unrelated to the transported substrates [Vera, et al. (2001) Biochemistry, 40, 777-790]. Here we analyzed the interaction of GLUT1 with quercetin (a flavone), genistein (an isoflavone), and tyrphostin A47 and B46 to evaluate if they share one common or have several binding sites on the protein. Kinetic assays showed that genistein, quercetin, and tyrphostin B46 behave as competitive inhibitors of equilibrium exchange and zero-trans uptake transport and noncompetitive inhibitors of net sugar exit out of human red cells, suggesting that they interact with the external surface of the GLUT1 molecule. In contrast, tyrphostin A47 was a competitive inhibitor of equilibrium exchange and zero-trans exit transport and a noncompetitive inhibitor of net sugar entry into red cells, suggesting that it interacts with the cytoplasmic surface of the transporter. Genistein protected GLUT1 against iodide-elicited fluorescence quenching and also decreased the affinity of d-glucose for its external binding site, while quercetin and tyrphostins B46 and A47 promoted fluorescence quenching and did not affect the external d-glucose binding site. These findings are explained by a carrier that presents at least three binding sites for tyrosine kinase inhibitors, in which (i) genistein interacts with the transporter in a conformation that binds glucose on the external surface (outward-facing conformation), in a site which overlaps with the external binding site for d-glucose, (ii) quercetin and tyrphostin B46 interact with the GLUT1 conformation which binds glucose by the internal side of the membrane (inward-facing conformation), but to a site accessible from the external surface of the protein, and (iii) the binding site for tyrphostin A47 is accessible from the inner surface of GLUT1 by binding to the inward-facing conformation of the transporter. These data provide groundwork for a molecular understanding of how the tyrosine kinase inhibitors directly affect glucose transport in animal cells.

Evaluation of the Effects of Water Stress and Different Levels of Nitrogen on Sugar Beet (Beta Vulgaris)

In order to investigate the effects of water stress and different levels of N fertilizer on yield and water useefficiency in sugar beet (Var. BP) a split plots based on randomized complete block design was carried out with4 replications in Khorasan Agricultural and Natural Resources Research Center (KANRC) during croppingseason in 2006. Water treatments comprising three levels including control (without water stress), initial waterstress and continuous water stress as main plots and different amounts of N fertilizer in 4 levels vis: control (0),50, 100 and 150 Kg net N. ha-1 as sub plots were assessed. Analysis of variation showed that effects of watertreatments on root yield and gross sugar content was significant (at 1% level). Different levels of N also havesignificant effect on root yield, net and gross sugar percent at 1% level while didn't influence net sugar yieldsignificantly. Interaction of water and N wasn’t significant for all the traits. Water use efficiency for root(WUEr) and for sugar (WUEs) were significant in irrigation amounts (P< 0.05) However, results showed thatincreased root yield under no water stress conditions with increased N amount was much more than it in waterstress conditions. In terms of root yield, no water stress treatment using 150 Kg Net N.ha-1 had the highest yield.But water stress treatment particularly constant water stress, caused the maximum water use efficiency.

The role of Latin America in the world sugar trade

This paper reviews the major changes and trends in the raw and white sugar trade flows involving Latin American exporters and their partners. The paper assesses the recent absolute and relative growth in the volume of sugar exports from Brazil (the region’s and the world’s dominant exporter) and other major regional exporters such as Guatemala, Cuba, Colombia, Mexico and Argentina. Latin America has emerged as the world’s largest net sugar exporting region. Significant volumes of raw sugar of Latin American origin are now used by a large number of new destination refineries which have been set up in the Middle East and Asia. Indeed, the share of Latin America in global raw sugar exports has increased from 62.8% on average between 2002 and 2004 to 67.3% on average between 2006 and 2008. This paper also evaluates the impact of preferential trade agreements, including the CAFTA and the EPA, for Central American and Caribbean sugar exporters, as well as the implications of NAFTA for Mexico’s sugar. Finally, the paper discusses the potential gains and benefits that diversification into ethanol and cogeneration have provided to the major Latin American sugarcane industries.

Evaluation of the influence of irrigation methods and water quality on sugar beet yield and water use efficiency

Abstract Rapid urbanization and industrialization have increased the pressure on limited existing fresh water to meet the growing needs for food production. Two immediate responses to this challenge are the efficient use of irrigation technology and the use of alternative sources of water. Drip irrigation methods may play an important role in efficient use of water but there is still limited information on their use on sugar beet crops in arid countries such as Iran. An experiment was conducted to evaluate the effects of irrigation method and water quality on sugar beet yield, percentage of sugar content and irrigation water use efficiency (IWUE). The irrigation methods investigated were subsurface drip, surface drip and furrow irrigation. The two waters used were treated municipal effluent (EC = 1.52 dS m−1) and fresh water (EC = 0.509 dS m−1). The experiments used a split plot design and were undertaken over two consecutive growing seasons in Southern Iran. Statistical testing indicated that the irrigation method and water quality had a significant effect (at the 1% level) on sugar beet root yield, sugar yield, and IWUE. The highest root yield (79.7 Mg ha−1) was obtained using surface drip irrigation and effluent and the lowest root yield (41.4 Mg ha−1) was obtained using furrow irrigation and fresh water. The highest IWUE in root yield production (9 kg m−3) was obtained using surface drip irrigation with effluent and the lowest value (3.8 kg m−3) was obtained using furrow irrigation with fresh water. The highest IWUE of 1.26 kg m−3 for sugar was obtained using surface drip irrigation. The corresponding efficiency using effluent was 1.14 kg m−3. Irrigation with effluent led to an increase in the net sugar yield due to an increase in the sugar beet root yield. However, there was a slight reduction in the percentage sugar content in the plants. This study also showed that soil water and root depth monitoring can be used in irrigation scheduling to avoid water stress. Such monitoring techniques can also save considerable volumes of irrigation water and can increase yield.

Effects of Elevated Atmospheric CO2 and O3 on Hill Activity and Ca2+/Mg2+-ATPase Activity of Pinus tabulaeformis Carr.

The main photo-physiological characteristics of Pinus tabulaeformis Carr. were analyzed in open-top chambers under elevated carbon dioxide and ozone concentrations. The results indicated that the leaves net photosynthetic rates (p < 0.05), Hill activity, Ca2+/Mg2+-ATPase activity, soluble sugar and starch contents all increased under elevated carbon dioxide concentration in whole growing season. While under elevated ozone concentration, the leaves net photosynthetic rates, Hill activity, Ca2+/Mg2+-ATPase activity, soluble sugar and starch contents all decreased. Under elevated carbon dioxide and ozone concentration, the leaves net photosynthetic rates, Hill activity, soluble sugar and starch contents all increased, but Ca2+-ATPase activity increased during the earlier growing season, decreased in later growing season, while Mg2+-ATPase activity responded contrarily.

Response of Pepper Plants to Different Rates of Mineral Fertilizers After Soil Biofumigation and Solarization

ABSTRACT A greenhouse experiment was conducted to examine the effect of using three different rates (zero, low-input, and high-input) of a mineral fertilizer (nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)) on growth and yield of pepper cultivated in a soil after biofumigation (with horse manure at the rate of 4 kg· m− 2) and solarization. Several physiological traits related with pepper plant development (leaf mineral concentration, net photosynthesis rate, transpiration, leaf sugar, and chlorophyll concentration) and fruit yield were determined. In T-1, chlorophyll was significantly lower and sugar concentration was significantly higher than in those where mineral fertilizers had been added (T-2 and T-3). There were no significant differences in photosynthetic rate among treatments. Increasing mineral fertilizer rates increased vegetative growth at the expense of fruit yield. Leaf nutrient concentrations most affected by the treatments were the N-fractions, and changes in the other parameters measured are discussed on the basis of these differences.

Evaluation of pretreatment with Pleurotus ostreatus for enzymatic hydrolysis of rice straw

The effects of biological pretreatment of rice straw using four white-rot fungi (Phanerochaete chrysosporium, Trametes versicolor, Ceriporiopsis subvermispora, and Pleurotus ostreatus) were evaluated on the basis of quantitative and structural changes in the components of the pretreated rice straw as well as susceptibility to enzymatic hydrolysis. Of these white-rot fungi, P. ostreatus selectively degraded the lignin fraction of rice straw rather than the holocellulose component. When rice straw (water content of 60%) was pretreated with P. ostreatus for 60 d, the total weight loss and the degree of Klason lignin degraded were 25% and 41%, respectively. After the pretreatment, the residual amounts of cellulose and hemicellulose were 83% and 52% of those in untreated rice straw, respectively. By enzymatic hydrolysis with a commercial cellulase preparation for 48 h, 52% holocellulose and 44% cellulose in the pretreated rice straw were solubilized. The net sugar yields based on the amounts of holocellulose and cellulose of untreated rice straw were 33% for total soluble sugar from holocellulose and 32% for glucose from cellulose. The SEM observations showed that the increase in susceptibility of rice straw to enzymatic hydrolysis by pretreatment with P. ostreatus is caused by partial degradation of the lignin seal. When the content of Klason lignin was less than 15% of the total weight of the pretreated straw, enhanced degrees of enzymatic solubilization of holocellulose and cellulose fractions were observed as the content of Klason lignin decreased.

Cooperative nucleotide binding to the human erythrocyte sugar transporter.

The human erythrocyte glucose transport protein (GluT1) is an adenine nucleotide binding protein. When complexed with cytosolic ATP, GluT1 exhibits increased affinity for the sugar export site ligand cytochalasin B, prolonged substrate occlusion, reduced net sugar import capacity, and diminished reactivity with carboxyl terminal peptide-directed antibodies. The present study examines the kinetics of nucleotide interaction with GluT1. When incorporated into resealed human red blood cell ghosts, (2,3)-trinitrophenyl-adenosine-triphosphate (TNP-ATP) mimics the ability of cytosolic ATP to promote high-affinity 3-O-methylglucose uptake. TNP-ATP fluorescence increases upon interaction with purified human red cell GluT1. TNP-ATP binding to GluT1 is rapid (t(1/2) approximately 0.5 s at 50 microM TNP-ATP), cooperative, and pH-sensitive and is stimulated by ATP and by the exit site ligand cytochalasin B. Dithiothreitol inhibits TNP-ATP binding to GluT1. GluT1 preirradiation with saturating, unlabeled azidoATP enhances subsequent GluT1 photoincorporation of [gamma-32P]azidoATP. Reduced pH enhances azidoATP photoincorporation into isolated red cell GluT1 but inhibits ATP modulation of sugar transport in resealed red cell ghosts and in GluT1 proteoliposomes. We propose that cooperative nucleotide binding to reductant-sensitive, oligomeric GluT1 is modulated by a proton-sensitive saltbridge. The effects of ATP on GluT1-mediated sugar transport may be determined by the number of ATP molecules complexed with the transporter.

Molecular determinants of sugar transport regulation by ATP.

Intracellular ATP inhibits human erythrocyte net sugar transport by binding cooperatively to the glucose transport protein (GluT1). ATP binding produces altered transporter affinity for substrate and promotes substrate occlusion within a post-translocation vestibule formed by GluT1 cytosolic domains. The accompanying paper (Cloherty, E. K., Levine, K. B., Graybill, C., and Carruthers, A. (2002) Biochemistry 41, 12639-12651) demonstrates that reduced intracellular pH promotes high-affinity ATP binding to GluT1 but inhibits ATP-modulation of GluT1-mediated sugar transport. The present study explores the role of GluT1 residues 326-343 (a proposed GluT1 ATP-binding site subdomain) in GluT1 ATP binding by using alanine scanning mutagenesis. Cos-7 and HEK cells were transfected with a cDNA encoding full-length human GluT1 terminating in a carboxyl-terminal hemagglutinin (HA)-His6 epitope. The transporter (GluT1.HA.H6) is expressed at the surface of both cell-types and is catalytically active. In HEK cells, both parental GluT1- and GluT1.HA.H6-mediated sugar transport are acutely sensitive to cellular metabolic inhibition. Isolated, detergent-solubilized GluT1.HA.H6 is photolabeled by [gamma-32P]-azidoATP in an ATP-protectable manner. Alanine substitution of E329 or G332/R333/R334 enhances GluT1.HA.H6 [gamma-32P]azidoATP photoincorporation but blocks acute modulation of net sugar transport by cellular metabolic inhibition. These actions resemble those of reduced pH on ATP binding to and modulation of red cell GluT1. It is proposed that cooperative nucleotide binding to GluT1 and nucleotide modulation of GluT1-mediated sugar transport are regulated by a proton-sensitive saltbridge (Glu329-Arg333/334).

Seasonal Changes of Dry Matter and Sugars in the Fleshy Roots of Asparagus Seedlings

ABSTRACT Fleshy roots of asparagus (Asparagus officinalis L.) seedlings were analyzed from August 1999 to May 1991 (6 to 15 months after seeding) to determine dry matter, total, and reducing sugars. Significant seasonal changes were observed in all variables. Sugar concentration decreased as the reserves were used to support formation of new buds, shoots and roots in September as well as initiation of new bud and shoot growth in March. Sugar concentration increased from April to August and from October to November, when the fern expanded and became well developed. A sharp decrease of sugars was observed in December, probably due to high temperatures during that period permitting continued root metabolism and to the absence of net sugar storage resulting from fern senescence by the end of November. The unexpected rise in sugar levels observed in late winter might suggest a physiological role for sugars in the sprouting of the dormant buds. The root dry matter decreased from August to September (period of d...

ATP-dependent substrate occlusion by the human erythrocyte sugar transporter.

Human erythrocyte sugar transport presents a functional complexity that is not explained by existing models for carrier-mediated transport. It has been suggested that net sugar uptake is the sum of three serial processes: sugar translocation, sugar interaction with an intracellular binding complex, and the release from this complex into bulk cytosol. The present study was carried out to identify the erythrocyte sugar binding complex, to determine whether sugar binding occurs inside or outside the cell, and to determine whether this binding complex is affected by cytosolic ATP or transporter quaternary structure. Sugar binding assays using cells and membrane protein fractions indicate that sugar binding to erythrocytes is quantitatively accounted for by sugar binding to the hexose transport protein, GluT1. Kinetic analysis of net sugar fluxes indicates that GluT1 sugar binding sites are cytoplasmic. Intracellular ATP increases GluT1 sugar binding capacity from 1 to 2 mol of 3-O-methylglucose/mol GluT1 and inhibits the release of bound sugar into cytosol. Reductant-mediated, tetrameric GluT1 dissociation into dimeric GluT1 is associated with the loss of ATP and 3-O-methylglucose binding. We propose that sugar uptake involves GluT1-mediated, extracellular sugar translocation into an ATP-dependent cage formed by GluT1 cytoplasmic domains. Caged or occluded sugar has three possible fates: (1) transport out of the cell (substrate cycling); (2) interaction with sugar binding sites within the cage, or (3) release into bulk cytosol. We show how this hypothesis can account for the complexity of erythrocyte sugar transport and its regulation by cytoplasmic ATP.

Membrane-bound glyceraldehyde-3-phosphate dehydrogenase and multiphasic erythrocyte sugar transport.

Net sugar import by human erythrocytes consists of ATP-modulated rapid and slow phases while sugar export consists of a single slow phase. We have proposed that this behaviour results from obligate substrate tunnelling from transporter to bulk cytosol through a complex containing high-affinity, low-capacity sugar binding sites (Cloherty, Sultzman, Zottola & Carruthers, 1995). Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is known to compartmentalize ATP delivery to erythrocyte membrane ATPases and interact directly with the erythrocyte glucose transporter in vitro. The present study examines the possibility that GAPDH is an obligate component of the hypothesized sugar-binding complex. GAPDH remains associated with the erythrocyte membrane following cell lysis and remains associated with the cytoskeleton under conditions where more than 99% of the membrane glucose transport protein (GLUT1) is released by detergent (Triton X-100). GAPDH is released from erythrocyte membranes upon exposure to Mg.ATP or to NADH. ATP displacement of membrane-bound GAPDH is half-maximal at 200 microM ATP and appears to involve ATP interaction with multiple, co-operative sites. GAPDH interaction with purified tetrameric GLUT1 is saturable, co-operative and also inhibited by ATP. ATP inhibition of GAPDH binding to purified tetrameric GLUT1 is less effective than ATP inhibition of GAPDH binding to intact erythrocyte membranes. Removal of cellular GAPDH by exposing erythrocyte membranes to NADH prior to membrane resealing neither affects ATP modulation of sugar transport nor reduces biphasic net sugar uptake to a single phase. We conclude that ATP-sensitive GAPDH interaction with the cytoplasmic surface of erythrocyte membranes and GLUT1 is responsible neither for ATP modulation of sugar transport nor for multiphasic net sugar import by human red cells.

Effects of manipulated grape berry transpiration on post-veraison sugar accumulation

A series of experiments were conducted on container-grown Pinot Noir and Sangiovese grapevines (Vitis vinifera L.) to investigate whether changes in berry water loss at veraison influence the pattern of sugar accumulation. Berry transpiration was induced to vary either by changing the vapour pressure deficit (VPD) around bunches through temperature or relative humidity (RH) manipulations, or by applying drying-accelerating emulsion or a hydrophobic coating (vaseline) over the berry skin. In all the experiments, every berry of every bunch was rated prior to treatment as to softness and colour intensity. Transpiration rates were derived either from measurements of attached bunches using a custom-built open gas-exchange system or from weight loss calculated for single excised berries. Berry development and ripening were monitored throughout each experiment as deformability, fresh weight, and sugar concentration and content. Berries either did not respond to VPD-enhancement or showed reduced water loss when bunches were subjected to high temperature. Low berry transpiration in the latter treatment led to lower sugar content per berry up to harvest, and berry transpiration and net sugar intake were linearly correlated up to 0.20–0.25 mmol/m2.s. A similar correlation was found also in the coating experiment within the first five days after treatments. When berry transpiration was restricted by applying vaseline, sugar accumulation was retarded. Low values of VPD, induced by raising the RH around the bunches, lowered sugar concentration but not sugar content per berry. The present study provides preliminary evidence that sugar accumulation in grapevine berries responds to changes in the evaporative demand around the bunch at veraison.

Net sugar transport is a multistep process. Evidence for cytosolic sugar binding sites in erythrocytes.

Human erythrocyte net sugar transport is hypothesized to be rate-limited by reduced cytosolic diffusion of sugars and/or by reversible sugar association with intracellular macromolecules [Naftalin, R.J., Smith, P.M., & Roselaar, S.E. (1985) Biochim. Biophys. Acta 820, 235-249]. The present study examines these hypotheses. Protein-mediated 3-O-methylglucose uptake at 4 degrees C by human erythrocytes and by resealed, hypotonically lysed erythrocytes (ghosts) is inhibited by increasing solvent viscosity. Protein-mediated transport and transbilayer diffusion of the slowly transported substrate 6-NBD glucosamine are unaffected by increasing solvent viscosity. These findings suggest that protein-mediated 3-O-methylglucose transport is diffusion-limited in erythrocytes. More detailed analyses of red cell 3-O-methylglucose uptake (at 4 degrees C and at limiting extracellular sugar levels) reveal that net influx is a biexponential process characterized by rapid filling of a small compartment (C1 = 29 +/- 6% total cell volume; k1 = 7.4 +/- 1.7 min-1) and slow filling of a larger compartment (C2 = 71 +/- 6% total cell volume k2 = 0.56 +/- 0.11 min-1). Erythrocyte D-glucose net uptake at 4 degrees C is also a biphasic process. Transmembrane sugar leakage is a monoexponential process indicating that multicomponent, protein-mediated uptake does not result from sugar uptake by two cell populations of differing cellular volume. Sugar exit at limiting 3-O-methylglucose concentrations is described by single exponential kinetics. This demonstrates that multicomponent sugar uptake does not result from influx into two populations of cells with widely different sugar transporter content. We conclude that biexponential sugar uptake results from slow (relative to transport) exchange of sugars between serial, intracellular sugar compartments. Biexponential sugar uptake is observed under equilibrium exchange conditions (intracellular sugar concentration = extracellular sugar concentration) but only at 3-O-methylglucose concentrations of less than 1 mM. Above this sugar concentration, exchange uptake is a monoexponential process. Because diffusion rates are independent of diffusant concentration, this suggests that multicomponent uptake results from high-affinity sugar binding within the cell. The concentration of cytosolic binding sites (30 microM, Kd(app) = 400 microM) was estimated from the equilibrium cellular 3-O-methylglucose space. Biexponential net 3-O-methylglucose uptake is also observed in human erythrocyte ghosts, in control human K562 cells, and in K562 cells induced to synthesize hemoglobin by prolonged exposure to hemin. This demonstrates that neither membrane-bound nor free cytosolic hemoglobin forms the sugar-binding complex.(ABSTRACT TRUNCATED AT 400 WORDS)