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Jin Wook Lee, Kenneth W. Mudge, and Joseph Lardner

American ginseng (Panax quinquefolium L.) contains pharmacologically active secondary compounds known as ginsenosides, which have been shown to be affected by both genetic and environmental factors. In this greenhouse experiment, we tested the hypothesis that ginsenosides would behave as “stress metabolites” and be associated with osmoregulation in response to drought stress. Two year-old seedlings, grown in 5-inch pots, were well watered for 40 days prior to the initiation of treatments. Plants in the drought stress treatment were watered every 20 days while the controls were watered every 10 days, and the experiment was terminated after 4 and 8 dry down cycles (80 days), respectively. Predawn leaf water potential and relative water content (RWC) of drought-stressed plants during a typical dry down cycle were lower than control plants. The diameter and weight of primary storage roots were decreased in the stressed treatment. The length of the main storage root and the longest secondary (fibrous) root were significantly increased by the drought stress treatment. Leaf chlorophyll content of drought-stressed plants was lower than controls. The osmotic potential of the drought-stressed ginseng was not lower than the control, indicating that ginsenoside is not involved in osmoregulation in response to drought stress. Furthermore, ginsenosides Rb1 and Rd, and total ginsenosides were significantly lower in primary roots of drought-stressed plants compared to control plants.

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Vital Hagenimana, Ronald E. Simard, and Louis-P. Vézina

In vitro activity measurements indicate that storage sweetpotato roots contain high amounts of extractable amylolytic enzymes. These storage roots also have a very high starch content, a characteristic indicating that the in vitro measurements estimate potential amylolytic activity rather than actual physiological activity. We are interested in optimizing the use of endogenous amylases when processing sweetpotato roots and have undertaken a study to identify physiological parameters that control in vivo starch breakdown. Sweetpotato roots were allowed to germinate for 35 days in controlled conditions. Using a combination of in vitro activity measurements and immunochemical detection, the spatial distribution and changes in activity levels for the three major amylolytic enzymes in storage sweetpotato roots—α-amylase, β-amylase, and starch phosphorylase—have been followed. After 6 days, α-amylase protein increased in the outer starchy parenchymatous tissues surrounding the cambium layers, a result suggesting a de novo synthesis of the enzyme in cambium or laticifers layers. β-Amylase was abundant throughout the root at all times, and its high levels did not directly affect starch degradation rates. Starch phosphorylase protein level remained constant, while its extractable activity increased. Starch content decreased during sweetpotato seed root germination. However, the amount of starch that disappeared during germination was low compared with the calculated starch hydrolysis potential estimated by amylolytic activity measurements.

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W. R.(Bill) Jester, Charles W. Averre, and Jonathan R. Schultheis

Russet crack-like symptoms have been observed with increasing frequency on Beauregard storage roots in North Carolina and resulted in some crop failures in neighboring states. The objective of this experiment was to determine if this cracking disorder was soil-borne, seed transmissible or transmissible via grafting. Beauregard plants were obtained from cuttings from commercially available virus-indexed micropropagated plants (M), and selected symptomatic roots (culls) originating from 1992 Foundation stocks (R). In a third treatment plants from each source were alternated in a row, then M and R plants were cleft grafted. The planting was made June 30, 1993 and replicated five times (12 plants per rep). Yield was determined and roots from each hill were washed and examined for russet crack-like symptom(s), and interior color on the proximal end. M roots had 82% good color; while R roots had 19%. M plants contained 0.3% symptomatic roots; R plants 65.5%. Similarly, only 1.6% of the M plants contained a symptomatic root, while 95.0% of the R plants had symptomatic roots. One-third of the grafted M plants contained one or more roots with cracking symptoms. M outyielded R. The russet crack-like disorder was determined to not be soil-borne, but was transmissible through the seed or grafting.

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Scott Aker and William Healy

Thinning of Alstroemeria `Regina' at 0, 30, 60, or 90% did not result in induction of cyclic variation in shoot length. Thinning caused an overall decrease in stem length and final fresh weight of storage roots (SR). Number of nodes on generative shoots did not change due to thinning treatment but varied over time. Thinning by 90% reduced yield, delayed harvest and increased flower quality. In the second year, plants were rethinned and grown with supplemental HPS irradiance of either 25 or 125 μmolm-2sec-1. Weekly production diminished with increased thinning, and was amplified by increased total fluence. In a second experiment, thinning resulted in decreased shoot, rhizome and SR growth in plants sampled before and after flowering. Rhizome index increased with increased thinning, indicating a relatively smaller impact of thinning on rhizome growth compared to SR and shoot growth. The carbohydrate composition of SR tissue was unchanged by treatment. Thinning resulted in decreased SR production and decreased fresh weight per SR between thinning treatments. Change in total amount of carbohydrate reserves in the SR is therefore due to change in number & size of the SR.

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Makoto Nakatani, Masaru Tanaka, and Masaru Yoshinaga

A late-storage root-forming mutant (`KM95-A68') of sweetpotato [Ipomoea batatas (L.) Poir.] was characterized to clarify the genetic and physiological mechanisms of storage root formation. This mutant originated from a somaclonal mutation of `Kokei No. 14'. Storage roots of `KM95-A68' are rare and, when formed, develop 2 or 3 weeks later than those of `Kokei No. 14' from which it originated. Morphological characteristics of the canopy and leaf photosynthetic rates of `KM95-A68' were similar to those of `Kokei No. 14'. No apparent differences were observed in the anatomy of root cross sections of `KM95-A68' and `Kokei No. 14'. An apparent increase in the root zeatin riboside (ZR) levels were observed in `Kokei No. 14' at storage root formation. Root ZR levels differed between `Kokei No. 14' and `KM95-A68'. The onset of increase in root ZR levels was delayed by 2 or 3 weeks in `KM95-A68' in comparison to `Kokei No. 14'. Maximum root ZR levels in `Kokei No. 14' were 2.2 times higher in comparison to `KM95-A68'. This appeared to be a factor in delayed storage root formation of `KM95-A68'. Results of reciprocal grafts of `KM95-A68' and `Kokei No. 14' indicated that the late storage root-forming characteristic of `KM95-A68' is a characteristic that arises from the root itself.

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D. G. Mortley, C. K. Bonsi, W. A. Hill, P. A. Loretan, C. E. Morris, A. A. Trotman, and P. P. David

Growth chamber studies were conducted to determine growth responses of sweetpotato [Ipomoea batatas (L.) Lam) to differing photoperiods (PP) when grown by use of NFT. Four vine cuttings (15 cm length) of GA Jet and TI-155 were grown for 120 days at 12/12, 15/9, 18/6, and 21/3 light/dark PP. Irradiance averaged 427 umol m-2 s-1, with day/night temperatures of 28/22C and 70% RH. A modified half Hoagland's solution was used. Number of storage roots/plant, and storage root fresh and dry weights for GA Jet increased as PP increased from 12 to 21 h, while storage root fresh and dry weights for TI-155 increased up to 18 h PP but declined at 21 h PP. Storage root number/plant for TI-155 declined at 15 h PP but was higher at both 18 and 21 h PP. Highest foliage dry weight for GA Jet was obtained at 21 h PP while that for TI-155 was obtained at 18 h PP. Leaf area index (LAI) for GA Jet increased with increased PP, while LAI for TI-155 increased with increased PP up to 18 h then declined at 21 h PP.

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Petra J. Wolters and Wanda W. Collins

Streptomyces soil rot or pox, caused by the actinomycete Streptomyces ipomoea, is a destructive root disease of sweetpotato. Evaluation for resistance to S. ipomoea in naturally infested fields, requires much space and results may vary from year to year. In this study a greenhouse method for evaluating the response of sweetpotato clones to infection with S. ipomoea was developed. The greenhouse method used fibrous roots, developed on terminal vine cuttings. Experiments showed no time by clone interaction, indicating that this method gave consistent results when repeated. A study to determine corrrelation between field resistance of clones and resistance as found by the greenhouse method was done. Thirty-nine clones were screened for resistance using the greenhouse method and were also planted in a field naturally infested with S. ipomoea. Severity of disease on fibrous roots (greenhouse method) and on storage roots (field method) was evaluated visually using a scale of 0 to 5 (0: no symptoms. 5: severe symptoms). Although correlations between data from the greenhouse and field methods were low lo moderate (r=0.17 to 0.49). extremely susceptible or resistant clones were identified as such by both methods. These results suggest that it is possible to select clones with high resistance to S. ipomoea using the greenhouse method, which provides a better controlled environment, and requires less space than field evaluations.

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Dapeng Zhang, Wanda W. Collins, and Maria Andrade

Sweetpotato [Ipomoea batatas (L.) Lam.] is intensively used as an animal feed in many developing countries. Information about trypsin inhibitor activity (TIA), an antinutritional component in this crop, will be useful for breeding sweetpotato as animal feed. Nine sweetpotato lines were grown at two locations and fertilized or nonfertilized conditions at each location. Samples were analyzed for TIA using a substrate-specific colorimetric method. Soybean [Glycine max (L.) Merr.] seeds were used to compare the levels of TIA in sweetpotato and soybean. Activity in roots ranged from 29.5 to 55.0 units in the nine lines. The mean TIA in roots was 40.7 units averaged over lines and environments, which was ≈28% of the mean for the five soybean cultivars. Activity in sweetpotato vines was only ≈14.6% of that in the roots, and TIA in fertilized plots was 150% and 67% higher than that in nonfertilized plots in the two locations, respectively. There was a small but significant positive correlation between TIA and crude protein in roots. These results suggested that TIA in sweetpotato storage roots may be high enough to have a substantial nutritional impact on animals, whereas TIA in vines is very low and should be of less nutritional concern.

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W.J. McLaurin and S.J. Kays

Under typical field production conditions, four high-yielding sweetpotato cultivars (Centennial, Jewel, Regal and Resisto) were found to lose substantial amounts of leaves due to natural senescense rather than pathological or herbivory causes. Leaf loss by the normal harvest date ranged from 46 to 63% of the total leaves formed in 1991 and 48 to 59% in 1992. There was a strong positive correlation between leaves lost and the number of vines (r2 = 0.80) and nodes (r2 = 0.89) per plant. Positive correlations were also found between leaf loss and total dry weight of the plant (r2 = 0.67). root fresh weight (r2 = 0.65). root dry weight (r2 = 0.60), and vine dry weight (r2 = 0.68). Distinct differences were found among cultivars in dry matter allocation within the plant. Of the cultivars tested, 'Jewel' allotted a lower percentage of dry matter into vines and a greater percentage into storage roots. Estimated leaf dry matter losses due to leaf shedding ranged from 1.2 to 2.6 MT·ha-1. Amount of leaf loss appeared to be closely related to vigorous vine growth and subsequent shading of older leaves, though leaf loss did not have a negative impact on storage root yield in the cultivars tested.

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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.