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Takaya Moriguchi, Tetsuro Sanada, and Shohei Yamaki

Sugar levels and composition were determined in developing `Hakuto' peach (Prunus persica Batsch var. vulgaris Maxim.) fruit. Glucose and fructose in nearly equal amounts were the predominant sugars detected during the early stage of development. Sucrose subsequently began to accumulate and was the predominant sugar in mature fruit. Sorbitol remained at a low level throughout development. The large increase in the amount of sucrose was accompanied by a rapid increase in sucrose synthase (EC 2.4.1.13) activity. Sucrose phosphate synthase (EC 2.4.1.14) was also detected in flesh extracts, but the activities were low throughout development. Acid invertase (EC 3.2.1.26) activity was highest in young fruit and declined with development. Activity, however, increased again at a later stage of development. Peach fruit contained appreciable sorbitol oxidase activity, while other sorbitol-related enzymes were barely detectable, suggesting that transported sorbitol was predominantly converted to glucose. These results suggest that the supply of glucose and fructose depends on acid invertase and sorbitol oxidase, and that accumulation of sucrose depends on-sucrose synthase.

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Riccardo Lo Bianco, Mark Rieger, and She-Jean S. Sung

Terminal portions of `Flordaguard' peach roots [Prunus persica (L.) Batsch] were divided into six segments and the activities of NAD+-dependent sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX), sucrose synthase (SS), soluble acid invertase (AI), and soluble neutral invertase (NI) were measured in each segment 10, 15, and 20 days after seed germination. The same type of experiment was conducted with terminal portions of `Flordaguard' and `Nemaguard' peach shoots except that one of the six segments consisted of the leaflets surrounding the apex. Independent of the age of individual roots, activities of SDH and AI were consistently highest in the meristematic portion and decreased with tissue maturation. In shoots, AI was the most active enzyme in the elongating portion subtending the apex, whereas SDH was primarily associated with meristematic tissues. A positive correlation between SDH and AI activities was found in various developmental zones of roots (r = 0.96) and shoots (r = 0.90). Sorbitol and sucrose contents were low in roots regardless of distance from tip, while sucrose showed a decreasing trend with distance and sorbitol, fructose, and glucose increased with distance from the meristem in shoots. Activity of SDH in internodes, but not apices, correlated with shoot elongation rate of both cultivars, whereas activities of other enzymes did not correlate with shoot elongation rate. We conclude that AI and SDH are the predominant enzymes of carbohydrate catabolism and the best indicators of sink growth and development in vegetative sinks of peach.

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Dongfeng Liu, Junbei Ni, Ruiyuan Wu, and Yuanwen Teng

trees, sorbitol is the main product of photosynthesis and the form of carbohydrate transported in the phloem, and is also an important accumulated sugar alcohol in fruit ( Loescher, 1987 ; Loescher et al., 1982 ). In pear, sorbitol content in mature

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Douglas D. Archbold

Abbreviations: AGR, absolute growth rate; DW, dry weight; RGR, relative growth rate; SAR, sorbitol accumulation. The investigation reported in this paper (no. 91-10-86) is in connection with a project of the Kentucky Agricultural Experiment Station

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Sylvia Letícia Oliveira Silva, Renato de Mello Prado, Gilmara Pereira da Silva, Gabriel Barbosa da Silva Júnior, Monica Lanzoni Rossi, and Leónides Castellanos González

increase foliar uptake efficiency is to prepare a spray syrup (polyols such as sorbitol) with the appropriate amount of the nutrient (e.g., B) that would ensure optimal leaf nutrient levels. This is because the polyols have a wetting action, thus delaying

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Margarita Pérez-Jiménez, Alfonso Guevara-Gázquez, Antonio Carrillo-Navarro, and José Cos-Terrer

given a central role. Sucrose, fructose, glucose, and sorbitol are the most commonly used sugars used as carbon source and osmotic stabilizer ( Scozzoli and Pasini, 1991 ), although their concentration is more important for the osmolarity of the medium

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Riccardo Lo Bianco, Mark Rieger, and She-Jean S. Sung

Sorbitol is the major photosynthetic product in peach. In sink tissues, sorbitol is converted to fructose via the NAD-dependent enzyme sorbitol dehydrogenase (SDH). A new assay is described that allows rapid, simple quantitation of SDH activity in growing shoot tips, root tips, and fruits. The activity was measured on the crude extract desalted with a Saphadex G-25 column to eliminate small molecules such as sugars and nucleotides. Optimum buffer type and pH for the enzyme as well as degradation by proteolytic enzymes and stability over time were determined in the present study. Inhibition by dithiothreitol (DTT) was detected at an inhibitor concentration as low as 2 mM, proving the similarity with mammalian SDH. Storage of samples at 4°C overnight resulted in significant loss of enzyme activity. Using this assay, we also correlated SDH activity with sink strength in peach.

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Wayne Loescher, Tad Johnson, Randolph Beaudry, and Sastry Jayanty

Sorbitol is the major carbohydrate translocated into apple fruit where it is normally metabolized to fructose. In watercored apple fruit tissues, however, the intercellular spaces become flooded and sorbitol content is consistently higher than in nonwatercored apples, suggesting a defect in sugar alcohol metabolism or transport. Our previous results have identified and characterized two sorbitol transporters, MsSOT1 and MsSOT2, in apple fruit tissues. Sorbitol transporter gene expression has been implicated in development of watercore with MsSOT expression diminished or absent in certain watercored fruit tissues. To explore this further, we have investigated the relationships between watercore, fruit maturation, fruit composition, and MsSOT expression in a number of apple cultivars that differ in watercore susceptibility. We also compared transporter expression between affected (watercored) and healthy parts of the same fruit and between watercored and nonwatercored fruits throughout the maturation and ripening processes. The MsSOT expression was often dramatically reduced in fruit tissues exhibiting watercore. Thus, in susceptible cultivars, maturing (ripening) fruit parenchyma cells lose the ability to transport sorbitol, and this in turn leads to sorbitol accumulation in the apoplastic free space and subsequent flooding of these spaces. These results are consistent with a relationship between watercore and sorbitol transport and also with a genetic susceptibility to the disorder.

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Ben-Hong Wu, Shao-Hua Li, Marta Nosarzewski, and Douglas D. Archbold

Sorbitol is the primary photosynthetic product and the major translocated sugar during the growing season in some species of the Rosaceae family, including apple ( Bieleski, 1969 ; Loescher, 1987 ; Webb and Burley, 1962 ). Sorbitol is also an

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Zhongchun Wang and Gary W. Stutte

Previous results showed that active sorbitol accumulation occurs under water stress. We tested the hypotheses that sorbitol accumulation is due to reduced sorbitol export from leaves or from increased synthesis of glucose to sorbitol. To test the hypotheses, 230 μl 14C-sucrose was introduced through the stems to detached `Jonathan' apple shoots which had either water stress or no stress. Following uptake of 14C-sucrose, 0% or 10% PEG was applied to shoots for 24 hours. The results showed that 73% of 14C-sucrose in non-stressed leaves was broken down within 1 hour and 44% was recovered in sorbitol. PEG initially stimulated the breakdown of 14C-sucrose to glucose and fructose, but further conversion to sorbitol was reduced. However, the percentage of 14C-sorbitol in mature leaves increased gradually in 10% PEG until it exceeded that of control at 24 hours. In contrast to mature leaves, young leaves and stems showed significantly less sorbitol under 10% PEG 24 hours after treatment. These results supported the hypothesis that sorbitol accumulation under water stress was due to the reduced sorbitol transport.