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Zana C. Somda and Stanley J. Kays

Changes in leaf distribution of the sweet potato [Ipomoea batatas (L.) Lam.] cultivar Jewel were assessed bi-weekly for 18 weeks at three plant densities (15, 30, and 45 cm × 96-cm spacing). The distribution of leaves on the branches and the timing at which leaf number stabilized were affected by the plant density. Plant density resulted in significant differences in the number of leaves and percentage of missing leaves during the growing season. Leaf number and total leaf area varied substantially in response to plant density, but individual lamina and petiole lengths and leaf area did not vary. Average petiole and leaf lengths and leaf size increased during the season, with the maximum length and area dependent on the type of branch on which the leaf was formed. Average petiole length per branch and the susceptibility to leaf loss increased with descending branch hierarchy (secondary branch < primary branch < main stem). Leaf losses after the 4th week tended to parallel a progressive increase in petiole length of new leaves, suggesting shading as a primary cause of leaf shedding and the loss of the oldest leaves first.

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Stanley J. Kays, Yan Wang and Wayne J. McLaurin

Sweetness, which is known to vary significantly among clones, is the dominant sensory attribute characterizing the flavor of sweetpotatoes [Ipomoea batatas (L.) Lam.]. The relative sweetness of baked roots, expressed as sucrose equivalents, was determined for 272 clones from the U.S. Department of Agriculture National Plant Germplasm System collection. The clones were from 34 countries that collectively produced 93% of the world's sweetpotato production in 2002. Individual clones were separated into five categories based upon the concentration and relative sweetness of individual sugars, expressed numerically as sucrose equivalents per 100 g dry mass: very high ≥38; high 29-37; moderate 21-28; low 12-20; and nonsweet ≤12. Based upon the mean sucrose equivalents of the clones for each country, only 9% of the countries, which accounted for only 2.1% of the total annual production of the countries surveyed, had sweetpotatoes that were classified as very high. While the majority (62%) of the countries surveyed had clones that were categorized as high, they represented only 4.4% of the total production of sweetpotatoes. None of the countries had mean sucrose equivalent values that were categorized as low or nonsweet, although a few individual clones were ranked as low and one as nonsweet. Countries that account for the majority (87%) of the sweetpotatoes grown worldwide had a mean sucrose equivalent ranking of moderate. Sweetness is derived from the composite of endogenous sugars (sucrose, glucose, fructose) and maltose formed via starch hydrolysis during baking. Maltose accounted for only 42% of the average contribution to the total sucrose equivalents. The range in the concentration of individual sugars among clones was substantial as was their contribution to sucrose equivalents. Sucrose equivalents due to maltose in individual clones ranged from 0.6 to 21.9 while endogenous sugars ranged from 6.4 to 46.9. The results indicate that essentially all of the sweetpotato clones tested from around the world were classified as equal to or greater than moderate in sucrose equivalents, and that there is substantial genetic diversity within the genepool such that the potential exists for tailoring the flavor of new cultivars, via significantly increasing or decreasing sugar content, to meet specific consumer preferences and/or product uses.

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Ki-Cheol Son, Ray F. Severson and Stanley J. Kays

Methodology was developed for the rapid quantitative and qualitative screening of sweetpotato [Ipomoea batatas (L.) Lam.] germplasm for the concentration of the major sweetpotato weevil oviposition stimulant, boehmeryl acetate, and its alcohol, boehmerol. The major surface components were rapidly quantified, using a minimum of plant material. Boehmeryl acetate, present in methylene chloride root extracts, did not degrade when held under normal laboratory conditions for 45 days. Boehmeryl acetate and boehmerol were found only in the outer 1 to 1.2 mm of periderm and the distribution of the compounds appeared to be relatively uniform over the surface of the root.

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Ki-Cheol Son, Ray F. Severson and Stanley J. Kays

Levels of major root surface components for two sweetpotato [Ipomoea batatas (L.) Lam.] cultivars that differ in susceptibility to the sweetpotato weevil [Cylas formicarius elegantulus (Summers)] were determined. Analyses were made 30 days before harvest, at harvest, after curing, and after 2 months of storage during two seasons. Significant variation in the amounts of individual components, especially boehmeryl acetate, which is known to be an ovipositional stimulant for the weevil, was found before and after harvest, with season, and between cultivars. These results suggest that variation in field susceptibility of cultivars displaying moderate levels of resistance may be due in part to seasonal variation in the level of ovipositional stimulants.

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Jyh-Bin Sun, Ray F. Severson and Stanley J. Kays

We describe a relatively simple collection procedure for quantifying volatiles in baked sweetpotato [Ipomoea batatas (L.) Lam.]. Volatiles formed during baking `Jewel' and `Centennial' sweetpotatoes at 204C were purged from a baking vessel with He or a HeO2 mixture, collected in cold methylene chloride, and reduced in volume using a Kuderna-Danish concentrator. Volatile components were quantified by capillary gas chromatography and characterized using gas chromatographic-mass spectrometer analysis. Quantitatively, the major components were identified as 2-furaldehyde; 2-furanmethanol; benzaldehyde; 5-methyl-2-furfural; phenylacetaldehyde; 3-hydroxy-2-methyl-4 H -pyran-4-one; 2,3-dihydro-3,5-dihydroxy-6-methyl-4 H- pyran-4-one; and 5-hydroxy-methyl-2-furancarboxaldehyde. Some quantitatively minor compounds were also identified. The volatile collection system is reproducible for quantitative comparisons among breeding lines.

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Stanley J. Kays, Jyh-Bin Sun and Ray F. Severson

Changes in the concentration of individual sugars in sweetpotato storage roots with cooking and their relationship to the formation of volatile compounds were studied. During cooking maltose concentration increased from 0.03% fwt at 25.C to a maximum of 4.33% at WC. Microwave pretreatment (2-4 minutes) resulted in a significant decrease in amounts of maltose and volatiles formed. At 80°C, approximately 80% of maltose synthesis was inhibited when pretreated with microwaves. Adding maltose into microwave pretreated samples and then cooking in a convection oven restored most of the volatile profile with the exception of phenylacetaldehyde. Upon heating (200°C), sweetpotato root material that was insoluble in both methanol and methylene chloride produced similar volatile profiles to those from sweetpotatoes baked conventionally. Volatiles derived via thermal degradation of the non-polar methylenc chloride fraction and the polar methanol fraction did not display chromatographic profiles similar to those from conventionally baked sweetpotatoes. Initial reactions in the formation of critical volatiles appear to occur in the methanol and methylene chloride insoluble components. Maltol (3-hydroxy-2-methyl-4-pyrone) was found to be one of the critical components making up the characteristic aroma of baked sweetpotatoes. It was concluded that maltose represents a primary precursor for many of the volatile compounds emanating from baked `Jewel' sweetpotatoes and the formation of these volatiles appears to involve both enzymatic and thermal reactions.

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Creighton K. Thomas, Kwang Jin Kim and Stanley J. Kays

Volatile organic compounds (VOCs) in homes and offices represent a potentially serious health problem for exposed individuals. While certain indoor plants have been shown to remove VOCs in small test chambers, the results have not consistently translated to larger, more complex structures. We report the results of a mathematical model that assesses the effect of plants on the removal of benzene or other VOCs in buildings, incorporating the primary variables modulating indoor air VOC concentration. Building air volume, amount of plant material, VOC concentration and air exchange, VOC emanation, and plant phytoremediation rates can be altered over ranges reported in the literature, clarifying the relationship among these parameters and thereby identifying the most appropriate interior air remediation options. The results indicate existing published phytoremediation rates determined using small test chambers are far higher than can be achieved with static potted plants in buildings, and facilitated air movement through the plant media will most likely be essential for phytoremediation to be a viable means of improving indoor air quality.

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Kwang Jin Kim, Eun Ha Yoo and Stanley J. Kays

Begonia maculata, Ardisia crenata, and Ardisia japonica plants exposed to 3.5 ppm toluene in air for 12 h displayed a pronounced stimulation (358%, 318%, and 252%, respectively) in subsequent toluene removal potential. The duration of the stimulation effect, monitored over 3 weeks, was short-lived decaying to prestimulation levels within 1 to 7 days depending on species. Elevated phytoremediation rate was dependent on the continued presence of toluene. The rapid rate of increase in phytoremediation and subsequent decay points toward a response mediated by changes in gene expression by the plant, microorganisms within the media, or both rather than an alteration in microbe population. A better understanding of the stimulation response may facilitate the use of plants for indoor air remediation in homes and offices.

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Stanley J. Kays, Jason Hatch and Dong Sik Yang

Selection emphasis on cyme size and flower color of Heliotropium arborescens L. has led to cultivars with diminished floral fragrance. As a preliminary inquiry into the fragrance chemistry of the species, we identified 41 volatile compounds emanating from the flowers of 'Marine' via isolation (Tenax trapping) and gas chromatography–mass spectrometry. The majority of the volatile compounds emanating from the flowers were terpenes (camphene, p-cymene, δ-3-carene, α-humulene, δ-1-limonene, linalool, (E)-β-ocimene, α-pinene, and β-thujone), benzenoids of which benzaldehyde was the most abundant, aldehydes (decanal, heptanal, nonanal and octanal), and hydrocarbons (decane, heneicosane, heptadecane, hexadecane, nonadecane, nonane, octadecane, tetradecane, tridecane and undecane) along with a cross-section of other compounds. Subsequent identification and quantification of critical ordorants will facilitate selecting new cultivars with quantitative and qualitative improvements in fragrance.

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Teresa A. Morrison, Russell Pressey and Stanley J. Kays

Staple-type lines of sweetpotato [Ipomoea batatus (L.) Lam.] do not sweeten significantly upon cooking as compared to the traditional-type lines. Four lines exhibiting distinct differences in sweetness after cooking were evaluated for changes in α- and ß-amylase activity and reducing sugars (by HPLC) at harvest, after curing, and at intervals during 180 days of storage. The traditional cultivar `Jewel' and staple-type line `Sumor' displayed high a- and ß-amylase activities, which rose from low levels at harvest to peak levels ≈ 90 days into the storage period. Staple-type lines `99' and `86' displayed significantly lower a- and ß-amylase activities. By using polyclonal sweetpotato ß-amylase antibody and western blot following native- and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it was confirmed that a lower level of ß-amylase synthesis existed in `99' and `86'. Quantitatively, `Jewel', `Sumor', and an additional staple-type line, `HiDry', had 361,374, and 365 μg ß-amylase protein per gram of fresh storage root tissue, respectively, while `99' and `86' possessed <60 and 12 μg·g-1, respectively. In raw roots, individual (glucose, fructose, and sucrose) and total sugar concentrations were significantly higher in `Jewel' than in `Sumor', `99', or `86'. Only trace amounts of maltose were found in raw roots of any line. Sucrose, glucose, and fructose concentrations decreased with baking in all lines except `86', in which they increased. There was substantial maltose produced by baking `Jewel' and `Sumor', but only trace amounts found in baked `99' and `86'. Sweetpotato germplasm can be separated into four general classes based on initial sugar concentration and changes during cooking: 1) low sugars/low starch hydrolysis, 2) low sugars/high starch hydrolysis, 3) high sugars/low starch hydrolysis, and 4) high sugars/high starch hydrolysis. At least two mechanisms may confer the lack of starch hydrolysis and subsequent sweetening in staple-type sweetpotato: 1) inhibition of ß-amylase synthesis, and 2) a nonenzyme mediated mechanism.