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associated with organic acids, bitterness is often the result of phenolic compounds, saltiness is attributable to sodium or potassium, and sweetness is the result of sugars, including fructose, glucose, and sucrose ( Sims and Golaszewski, 2003 ). Sugars not

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In all eukaryotic cells, fructose 2,6-bisphosphate (fru 2,6P 2 ) is an important regulator of carbohydrate metabolism ( Okar and Lange, 1999 ). In plants, this molecule coordinates the rate of CO 2 assimilation and carbon partitioning between

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Cytosolic fructose-1,6-bisphosphatase (cytoFBPase) (EC 3.1.3.11) occupies a strategic site in sucrose synthesis and has been demonstrated to play a key role in carbon partitioning between sucrose and starch in non-sorbitol forming plants. In addition to sucrose and starch, Sorbitol is the primary photosynthetic end product in the leaves of many tree fruit species in the Rosaceae family. To understand the biochemical regulation of photosynthetic carbon partitioning between sorbitol, sucrose and starch in sorbitol synthesizing species, we purified cytoFB-Pase to apparent homogeneity from apple leaves. The enzyme was a homotetramer with a subunit mass of 37 kDa. It was highly specific for fructose-1,6-bisphosphate with a Km of 3.1 μm and a Vmax of 48 units/mg protein. Either Mg2+ or Mn2+ was required for its activity with a Km of 0.59 mm and 62 μM, respectively. Li+, Ca2+, Zn2+, Cu2+ and Hg2+ inhibited whereas Mn2+ enhanced the Mg2+-activated enzyme activity. Fructose-6-phosphate was found to be a mixed type inhibitor with a Ki of 0.47 mm. Fructose 2,6-bisphosphate (F2,6BP) competitively inhibited the enzyme activity and changed the substrate saturation curve from hyperbolic to sigmoidal. Adenosine monophosphate (AMP) was a non-competitive inhibitor for the enzyme. F2,6BP interacted with AMP to inhibit the enzyme in a synergistic way. Dihydroxyacetone phosphate did not have inhibitory effect on apple leaf cytosolic FBPase activity. Sorbitol increased the susceptibility of the enzyme to the inhibition by F1,6BP. The presence of sorbitol in the reaction mixture led to a reduction in the enzyme activity.

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Abstract

The concentration of photosynthate in midribs of crisphead lettuce at various times of the day may be involved in the susceptibility of this crop to various postharvest disorders (3). Measuring the soluble solids content (SSC) of the juice expressed from midribs is simple when a hand-refractometer is used, and would be an ideal method for field use. However, lettuce is low in sugar content (5); thus, SSC may be no more of a valid measure of sugar content in lettuce than in cucumbers, which also have a low sugar content (4). Consequently, we adapted (1, 7) procedures for the determination of glucose, fructose, and sucrose, the principal sugars in head lettuce (type not specified, 5) to midribs of crisphead lettuce utilizing HPLC.

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This study was undertaken to determine the effect of low-O2 atmosphere on the concentration of fructose 2,6-bisphosphate (Fru-2,6-P2), which can activate the enzyme pyrophosphate-dependent:phosphofructokinase (PPi-PFK) to catalyze the reaction from fructose 6-phosphate to fructose 1,6-bisphosphate (Fru-1,6-P2). Fru-2,6-P2 remained unchanged in carrot (Daucus carota L.) root shreds stored under air, but it increased 3.0- and 5.3-fold at 2% and 0.5% O2 atmosphere, respectively, at 5C, and the increases were almost twice as great at 15C. The concentration of PPi ranged from 17 to 33 nmol·g-1 fresh weight, which is more than sufficient for the PPi-PFK to proceed. Thus, low-O2 atmosphere appeared to hasten glycolysis of carrot shreds by increasing Fru-2,6-P2, which activated PPi-PFK toward glycolysis.

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Abstract

Filbert (Corylus avellana L.) pollen germination on sucrose-agar media with B and/or Ca, Mg, and K, was affected adversely by increasing pH. Cultivar differences in germination of pollen were apparent: ‘Tombul’ and ‘Ennis’ germinated well; ‘Hall’s Giant’ intermediate; whereas ‘Barcelona’ and ‘Butler’ showed a poor germination percentage. Germination decreased as temperature increased from 15° to 30°C. Among concentration ranges of the sugars tested at 10% to 25%, sucrose was most effective, glucose permitted good germination at lower concentrations, and fructose inhibited germination.

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showed that 80% of the soluble carbohydrate that accumulated in cotyledons was sucrose, with less than 5% each of glucose and fructose ( Patrick and McDonald, 1980 ). Significant accumulation of seed dry weight begins ≈8–12 d after fertilization, stops

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·L −1 sucrose, 20 g·L −1 fructose, or 20 g·L −1 glucose, alone or plus 200 mg·L −1 8-HQS; and distilled water as a control ( Table 1 ). These concentrations were chosen according to previous literature ( Ichimura and Korenaga, 1998 ; Islam et al

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The mechanism of C2H4 action on plant respiration is not well understood. In the present work we treated peeled sweet potato roots (Ipomea batatas cv. MD715) with 10 ppm C2H4 in air and 3% O2 Analytical data showed a close relationship between respiration and activity of phosphofructokinase while the activity of pyrophosphate fructose-6-phosphate phosphotransferase remained constant under all experimental treatments. At the respiratory peak there was an increase in both pyruvate and fructose-2,6-diphosphate. The change in the levels of pyruvate, followed closely that of the respiration drift, while those of fructose-2,6-diphosphate did not correlated so closely. The data indicate that the stimulation of respiration by C2H4 in sweet potato roots is closely associated with an enhancement of glycolysis. The levels of ATP also increased with the rise in respiration and reflected the magnitude of the respiratory increment.

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a polymer of fructose with a terminal glucose moiety ( Chatterton et al., 1989 ). Increased catabolism and decreases in levels of fructans have been observed in grasses in response to defoliation ( Morvan-Betrand et al., 2001 ; Prud'homme et al

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