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Dapeng Zhang and Wanda Collins

To understand the prospects of applying the RAPD technique to assay genetic diversity in Ipomoea, four species (I. batatas, I. trifida, I. triloba, and I. ×leucantha) were analyzed for RAPD molecular markers. Six accessions of each species were used. Significant RAPD polymorphisms were detected within each species. Of 20 primers used, nine produced clear scorable polymorphic bands. The number of polymorphic bands produced per primer ranged from two to nine. Pair-wide genetic distance was calculated based on “band sharing”. The SAS-CLUSTER procedure was used to build a hierarchical species dendrogram. The four species were clearly separated by the clustering, which agrees with their existing taxonomic relationship. This study shows that RAPD analysis can be a powerful tool for identifying duplicates of germplasm acessions and for assessing genetic diversity. The procedures are relatively inexpensive and easy to perform and could be valuable in preliminary assessment of field genebank collections to separate species and indicate duplications in collected material.

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Dapeng Zhang and Wanda W. Collins

Randomly amplified polymorphic DNA (RAPD) analysis was performed on 18 accessions belonging to four different species of the genus Ipomoea, including sweetpotato and three related species. Twenty-two out of 30 primers tested revealed polymorphisms among these four species. Eight primers were selected on the basis of the number and repeatability of polymorphism produced. With these, a total of 98 different DNA bands were obtained and 85% of them were polymorphic. Based on the presence/absence of the bands, a genetic similarity among accessions and among species was calculated. Unweighted pair-group method with arithmetical averages (UPGMA) based on the similarity coefficients clearly discriminated these four species. Ipomoea trifida and sweetpotato share more genetic similarity. Ipomoea triloba and I. leucantha fall into another cluster. This study demonstrated that RAPD techniques can be a very useful tool for genotype/accession identification and studying the genetic relationship among genotypes/accessions of sweetpotato and among species of Ipomoea.

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Luz M. Reves and Wanda Collins

Eight populations including Ipomoea batatas and I. trifida species were assayed at six polymorphic enzyme loci. Differences in allele frequencies among populations allowed distinction of the two species and among levels of ploidy. Principal component and cluster analyses using isozyme and morphological data were performed. Results from isozyme characterization in general supported the results from morphological classification. Cluster analysis from isozyme assays indicated that the hexaploid species I. trifida is not a different species than I. batatas but may be a wild type. Significant deviations of genotypic frequencies from Hardy-Weinberg equilibrium in some of the loci were detected and were possibly due to effects of natural selection; this fact was reflected in the level of homozygosity observed among populations for the loci in disequilibrium.

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Petra Wolters and Wanda W. Collins

Ipomoea trifida (2X = 30) is purported to be the wild Ipomoea species most closely related to the commercially grown Ipomoea batatas (sweetpotato, 6X = 90). The two species can be crossed with much difficulty, but seed occur rarely. Ipomoea trifida has been shown to possess some agronomically desirable traits that are missing in sweetpotato (e.g., sweetpotato-weevil resistance). Attempts to locate morphological markers in the diploid trifida that would serve as indicators of successful crosses with sweetpotato resulted in the identification of two traits controlled by single genes: nectary color and male sterility. Both traits require flowering to identify, and flowering is often difficult to induce in Ipomoea species. An analysis of I. trifida accessions using RAPD molecular markers was undertaken. Using a segregant population resulting from crossing a green nectary, fertile plant with a yellow nectary, male, sterile plant, RAPD analysis resulted in clear markers for both the nectary color trait and the male sterility trait. These traits now can be identified in the absence of flowering plants.

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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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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 Suzanne Belding

Eight sweetpotato [Ipomoea batatas (L.) Lam.] clones were evaluated for the digestibility of their starch in animals with a simple in vitro screening method. Starch digestibility varied significantly among clones. After dry-heat treatment at 100C for 30 minutes, digestibility of the most heat-sensitive clone increased only 37.8%. Excellent repeatable results were obtained with a simple weight-loss method. This assay procedure can be used as a screening method in breeding digestible sweetpotatoes for animal feed.

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Sharon R. Funderburk and Wanda W. Collins

Crimson clover Trifolium incarnatum L.) was used as a N source for sweet potato [Ipomoea batatas (L.) Lam.]. Treatments were designed to compare estimated N delivery by clover incorporation amounts of N delivered by inorganic fertilizer. Plants were sampled every 14 days and sectioned into four parts: shoots, stem tips, fibrous and storage roots. Dry matter content was significantly influenced by time. Total plant dry matter was lowest in the highest inorganic N treatment. Nitrogen concentration (DWB) decreased over time and was highest in the highest inorganic N treatment. Similar vine weights were noted in N and clover treatments while number of storage roots per plant was unaffected by treatment as was weight per storage root, which increased linearly over time. No significant difference existed between the high and low N application treatment or late clover incorporation treatment in any grade of storage roots except culls, which were 90% lower in clover treatments than in N fertilizer treatments.

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Petra Wolters, Wanda Collins and J.W. Moyer

The establishment of a sweet potato repository in Georgia that will eventually accept and distribute true seed of sweet potato [Ipomoea batatas (L.) Lam.] raised the question of seed transmission of viruses, especially of sweet potato feathery mottle virus (SPFMV). Seedlings obtained from virus-infected parent plants were free of viral infection. Examination of virus distribution in virus-infected plants determined that SPFMV was present in vegetative tissue, but not in reproductive organs, indicating that the probability of SPFMV transmission in sweet potato through seed is very low.