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Jeffrey Adelberg, Kazuhiro Fujiwara, Chalermpol Kirdmanee, and Toyoki Kozai

Two triploid clones of melon from the same tetraploid parent were grown in vitro with and without sugar, rooted without sugar in media both in a laboratory controlled environment chamber (in vitro) and a greenhouse acclimatization unit (ex vitro), and compared for subsequent nursery growth in the greenhouse unit. The clone `(L-14 c B) × L-14' produced more shoots in both photomixotrophic (with sucrose) or photoautotrophic (sugar-free) conditions. Both genotypes were equally likely to root in sugar-free media, and `(L-14 × B) × L-14' rooted as well from either photoautotrophic and photomixotrophic shoots but `(L-14 × B) × Mainstream' rooted less frequently from photoautotrophic shoots. Seventy-six percent (76%) of the shoots were able to root photoautotrophically in vitro, whereas 47% of the ex vitro shoots were rooted. About 85% of plantlets from all treatments survived after transfer to the nursery. After growth in the greenhouse nursery, the sizes of plants (fresh and dry weight, leaf area) were the same for either clone, from either photoautotrophic or photomixotrophic shoots. Also, after growth in the nursery, plantlets that had been rooted in vitro were larger than those rooted ex vitro. Photoautotrophic rooting demonstrates a concept for integrating micropropagation and plug-type vegetable transplant production.

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T.M.M. Malundo, E.A. Baldwin, R.L. Shewfelt, H. Sisson, and G.O. Ware

Fruit flavor is a function of sensory perception of taste, aromatic and chemical feeling factor components in the mouth. The specific role of sugars and acids in potentiating flavor perception of volatile compounds and chemical feeling factors is not well known for many fruits. This study was conducted to determine the effects of selected levels of sugars and acids on perception of 3 taste (sweet, sour, bitter), 6 aromatic (banana, grassy, orange peel, peach, pine/turpentine, sweet potato), and 2 chemical feeling factor (astringent, biting) flavor notes in diluted, fresh mango homogenate using a trained descriptive panel. Perception of all flavor descriptors except sour were enhanced by increasing the sugar concentration. An increase in acid concentration enhanced perception of sweet, sour and biting notes while lowering perception of the astringent, peach and pine/turpentine notes. Brix-to-acid ratio (BAR) was found to be an effective chemical indicator for perception of sourness but was not effective for perception of sweetness. These results provide insight into optimum balances of sugars and acids as they influence mango flavor perception specifically in preparation of juice blends, selection of cultivars for specific fresh markets, or determination of optimum ripeness in the marketplace.

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Graham H. Barry, William S. Castle, and Frederick S. Davies

Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of Citrus L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of Citrus species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [C. sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose. Sucrose concentration was not affected, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.

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Dominique-André Demers, André Gosselin, and H. Chris Wien

Sweet pepper (Capsicum annuum L.) plants were grown under natural or supplemental lighting that extended thephotoperiods to 16, 20, or 24 hours. Increasing the photoperiod to 16 and 20 hours increased pepper plant yields, but continuous light (24 hours) decreased yields compared to the 20-hour photoperiod. In a second experiment, plants were exposed to a photoperiod of 14 or 24 hoursand either pruned to one fruit every four nodes or not pruned. During the first weeks of treatments, plants grown under continuous light had higher shoot mass (fresh and dry) and yields. After 7 to 8 weeks of treatments, plants under continuous light grew more slowly than plants exposed to a 14-hour photoperiod. At the end of the experiment, shoot mass and yields of plants grown under a 14-hour photoperiod were equal to or higher than plants under continuous light. So, it seems possible to provide continuous lighting for a few weeksto improve growth and yields. Limiting the number of fruit per plant increased shoot mass and decreased yields, but had no effect on the general response of pepper plants to photoperiod treatment. Leaf mineral composition was not affected by photoperiod treatment, indicating that reduced growth and yields under continuous light were not due to unbalanced mineral nutrition. Leaf starch and sugar contents were increased under continuous light. However, fruit pruning treatments did not modify the pattern of starch and sugar accumulation under the different photoperiod treatments. Reduced growth and yields measured under a 24-hour photoperiod are probably explained by starch and sugar accumulation in leaves as a result of leaf limitations rather than a sink limitation.

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F. Kultur, H.C. Harrison, and J.E. Staub

Muskmelon (Cucumis melo L.) genotypes, Birdsnest 1 [`Qalya' (BN1)], Birdsnest 2 (BN2), and `Mission' (V) were used to determine the effects of plant architecture and spacing on fruit sugar concentration and yield. The BN1 and BN2 genotypes possessed a highly branched growth habit specific to birdsnest melon types but not characteristic of standard indeterminate vining types (e.g., `Mission'). Experiments were conducted at the Hancock (sandy soil, <1% organic matter) and Arlington (heavy, praire loam soil, >4% OM) Experimental Farms in Wisconsin. Plant response to two within-row spacings [35 cm (72,600 plants/ha) and 70 cm (36,300 plants/ha)] in rows on 210-cm row centers was examined. Genotypes were grown in a randomized complete-block design with four replications at each location and evaluated for primary lateral branch number, fruit number per plant, fruit number per hectare, average fruit weight, yield (g) per plant, yield (MT) per hectare, and fruit sugar concentration. All genotypes produced higher yield, fruit number and sugar concentration on the mineral soil at Arlington compared to the sands at Hancock. The main effect of genotypes was significant for all traits examined. BN1 and V genotypes had greater yield (gram per plant, yield per hectare, and average fruit weight) as well as higher fruit quality (fruit sugar concentration) than the BN2 genotype. Spacing affected all traits examined except primary branch number and fruit sugar concentration. As withinrow spacing increased from 35 to 70 cm, fruit number per plant, yield per plant and average fruit weight increased. However, yield (MT) per hectare and fruit number per hectare decreased. Fertility was adjusted according to soil tests for the two different soil types at the two farm locations.

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Xuan Liu, James Sievert, Mary Lu Arpaia, and Monica A. Madore

Avocado (Persea americana Mill.) tissues contain high levels of the seven-carbon (C7) ketosugar mannoheptulose and its polyol form, perseitol. Radiolabeling of intact leaves of `Hass' avocado on `Duke 7' rootstock indicated that both perseitol and mannoheptulose are not only primary products of photosynthetic CO2 fixation but are also exported in the phloem. In cell-free extracts from mature source leaves, formation of the C7 backbone occurred by condensation of a three-carbon metabolite (dihydroxyacetone-P) with a four-carbon metabolite (erythrose-4-P) to form sedoheptulose-1,7-bis-P, followed by isomerization to a phosphorylated d-mannoheptulose derivative. A transketolase reaction was also observed which converted five-carbon metabolites (ribose-5-P and xylulose-5-P) to form the C7 metabolite, sedoheptulose-7-P, but this compound was not metabolized further to mannoheptulose. This suggests that C7 sugars are formed from the Calvin Cycle, not oxidative pentose phosphate pathway, reactions in avocado leaves. In avocado fruit, C7 sugars were present in substantial quantities and the normal ripening processes (fruit softening, ethylene production, and climacteric respiration rise), which occurs several days after the fruit is picked, did not occur until levels of C7 sugars dropped below an apparent threshold concentration of ≈20 mg·g-1 fresh weight. The effect of picking could be mimicked by girdling the fruit stalks, which resulted in ripening on the tree. Again, ripening followed a decline in C7 sugars to below an apparent threshold level. Taken together, these data indicate that the C7 sugars play important roles in carbon allocation processes in the avocado tree, including a possible novel role as phloem-mobile ripening inhibitors.

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J.C. Beaulieu, J.M. Lea, G. Eggleston, and Z. Peralta-Inga

Markedly higher average sucrose (58.1%) was recovered from mesocarp tissue of six orange-flesh cantaloupe (Cucumis melo L.) genotypes over three seasons compared to glucose (17.5%) and fructose (25.6%). A significant decrease in sucrose concentration was observed in the fall for all six genotypes, and the glucose (21.2%) and fructose (33.5%) ratios were also higher in the fall; markedly different than the spring fruit averages. The female inbreds had significantly (P = 0.05) lower glucose, fructose, sucrose, and total sugars than the commercial hybrids. Compared to the male and female inbreds, commercial hybrids had significantly (P = 0.05) higher concentrations of fructose, sucrose and total sugars, but not glucose. Two refractometric digital measures of °Brix (°Brix-At and °Brix-II) in homogenized slurries were positively correlated (r = 0.914; P ≤ 0.001), and were also correlated with total sugars (r ≥ 0.839) and sucrose (r ≥ 0.752). °Brix of cubes (°Brix-cube) was significantly correlated with sucrose and total sugars (r ≥ 0.627). Total sugar was positively correlated with sucrose (r = 0.843; P ≤ 0.001). Eastern-type U.S. melons had significantly (P = 0.05) higher °Brix-cube and °Brix-At compared to U.S. western shipper-types. Female inbreds were significantly (P = 0.05) lower in mean °Brix (all three measures) compared to the hybrids and male inbreds, and female inbreds had higher pH than the male inbreds. Western shippers had significantly (P = 0.05) higher pH compared to eastern genotypes. The predominant organic acid in all six genotypes was succinic acid, generally followed by oxalic, citric/isocitric, then malic acid. Succinic acid recovery was significantly higher in all six genotypes harvested in the fall, compared to spring. Eastern genotypes had significantly (P = 0.05) lower organic acids compared to western genotypes. Results indicate that maternal inheritance appears to confer lower sugar accumulating capacity and higher pH, which, is associated with vacuolar acid invertase (AI) and hexose balance. Breeding programs should focus on hybrid vigor derived through accentuating homozygous female inbreds with lower pH and higher capacity for sucrose accumulation, as well as morphological and agronomic traits often carried in the female line.

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John R. Stommel

Sugar accumulation throughout fruit development in the cultivated tomato (Lycopersicon esculentum) and a wild green-fruited species (L. peruvianum) are being examined. Results obtained using HPLC demonstrate that the fruit of L. peruvianum accessions accumulate the disaccharide, sucrose, in addition to the monosaccharides, glucose and fructose, common to L. esculentum. When detectable, sucrose in the L. esculentum cultivar FM6203 was present at very low levels throughout development. Analysis of mature fruit of L. esculentum var. cerasiforme, L. pimpinellifolium, and L. cheesmanii accessions indicate glucose and fructose as the primary storage sugars. Similar to L. peruvianum, mature fruit of the green-fruited species, L. hirsutum f. typicum and L. hirsutum f. glabratum, accumulate sucrose in addition to glucose and fructose.

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Manfredo J. Seufferheld and Cecil Stushnoff

Strawberry plantlets, regenerated from leaf disks, were used as a model system to study the effect of high concentrations of sugars and dehydration on survival during cryopreservation. After cold acclimation, plantlets imbibed for 3 days (one day each) in 0.5, 0.7 and 1.2 M sucrose and (1.0M sucrose + 0.2M raffinose) and desiccated to 25 % moisture (fwb) in alginate capsules consistently survived cryopreservation. Differential scanning calorimetry revealed only a very small exotherm between -20C and -28C during freezing; a glass transition at -50C and a small melting event at -10C during warming. Conversely, samples with the lowest survival rate, had a large nucleation exotherm at -30C and a devitrification exotherm between -70 and -40C. We conclude that imbibition with sugars, coupled with desiccation treatments, may be used to manipulate freeze tender tissues of strawberry to permit successful cryopreservation.

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Kenji Katayama, Katsumi Komaki, and Seiji Tamiya

Near infrared analysis was used to predict the starch, moisture, and sugar content in sliced fresh sweetpotato [Ipomoea batatas (L.) Lam.] storage roots. Samples were collected in each of three growing years. The best calibration equation for starch from combined samples (1989 to 1991) showed a multiple correlation coefficient (R) of 0.949, a standard error of calibration (sec) of 2.01, and a standard error of prediction (sep) of 1.91. The R, sec, and sep for moisture and sugar were 0.930, 1.85, and 2.00, and 0.837, 1.30, and 1.21, respectively. Calibrations based on samples from a given year adequately predicted the variables but could not account for variances introduced by samples from other years. Multiyear calibrations based on several years of data adequately predicted starch and moisture content in root slices. Thus, multiyear calibrations with annual bias adjustments can be applied to screening sweetpotato breeding germplasm for these two variables.