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Don La Bonte and Michael Courtney

Sweetpotato[Ipommabatatas (L.) Lam.] is a major subsistence crop in southern Africa, where iron and zinc deficiency in humans is an important health problem. A cultivar of sweetpotato that is suited for subsistence farming in this region and that is high in iron and zinc could be an important means of combatting these deficiencies. As part of a program of the International Potato Center (CIP) to develop such a cultivar, we are working to identify the high and low range of iron and zinc in sweetpotato cultivars grown throughout the world by testing a number of cultivars for these nutrients. Subsidiary objectives include determining the heritability of iron and zinc levels and surveying the variability in the levels of these nutrients from root to root on the same plant, from plant to plant of the same cultivar, from the proximal to the distal end of a given root, and from cambium to cortex.

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Don R. La Bonte and John A. Juvik

A single-kernel, sugar analysis technique was used to study the genetic relationship between morphological and metabolic traits previously associated with expression of the sugary enhancer (se) endosperm mutation in a su-1 sweet corn (Zea mays L.) background. Analysis of sucrose and total carotene content in su-1 kernel populations segregating for se showed that light-yellow kernel color was a reliable phenotypic indicator for kernels homozygous for the se gene. High levels of kernel maltose was not always indicative of su-1 se kernels in mature (55 days after pollination) kernel populations. Characteristic high levels of percent moisture in su-1 se kernels at 28 and 35 days post-pollination were identified as an expression of high sugar content. Kernels homozygous for su-1 se were also found to weigh less at maturity than su-1 Se kernels, and se was found to be partially expressed in a heterozygous condition.

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Frank A. Buffone and Don R. La Bonte

Chlorotic Leaf Distortion (CLD) is a common disease of sweetpotato caused by Fusarium lateritium. This fungus is unique among Fusarium species in that it grows on the epidermis of leaves and shoot tips of sweetpotato. Fusarium lateritium appears as a white epiphytic material and under bright sunlight causes leaf chlorosis. When cloudy weather persists for several days, all symptoms disappear.

Researchers who use RAPD to examine banding patterns of sweetpotato DNA assume that foreign DNA present in the cTAB extract is quantitatively low and will not appreciably amplify and appear as bands. In this study we found the modified cTAB procedure used to amplify sweetpotato DNA also amplifies DNA of Fusarium lateritium cultures. DNA banding patterns of infected leaves was compared with those free of the disease. No differences in banding patterns were observed in this preliminary study.

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Arthur O. Villordon and Don R. La Bonte

Our research compared the extent of genomic variability between plants originating from adventitious sprouts and nodal cultures. Plant materials, derived from a single sprout and originating from a storage root each of `Jewel,' `Sumor,' and L87-95, were clonally propagated for seven generations nodally and through adventitious sprouts. PCR-based analysis using 15 random primers identified 58 scorable molecular markers, 37 (63.79%) of which were shared by all three genotypes represented by 60 samples (10 nodal and 10 adventitiously derived plants/genotype). Of 29 putatively polymorphic markers, 24 (82.75%) were putative polymorphisms across the entire data set. The remaining four (13.79%) represented putatively fixed genotypic differences that were monomorphic within genotypes. A multidimensional scaling analysis differentiated seven (23.33%) adventitiously derived phenotypic marker variants, compared to four (13.33%) among nodal materials. Our results support previous findings that, relative to nonmeristematic tissues, meristematic regions strictly control cell division and DNA synthesis that exclude DNA duplication and other irregularities.

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Michael W. Courtney and Don La Bonte

Sweetpotato [Ipomœa batatas (L.) Lam.] is a major subsistence crop in southern Africa, where iron and zinc deficiency in humans is an important health problem. A cultivar of sweetpotato that is suited for subsistence farming in this region and that is high in iron and zinc could be an important means of combatting these deficiencies. As part of a program of the HarvestPlus program, under the auspices of the International Potato Center (CIP) to develop such a cultivar, we are working to identify the high and low range of iron and zinc in sweetpotato cultivars grown throughout the world by testing a number of cultivars for these nutrients. Subsidiary objects include determining the heritability of iron and zinc levels and surveying the variability in the levels of these nutrients from root to root on the same plant, from plant to plant of the same cultivar, from the proximal to the distal end of a given root, and from cambium to cortex. For the roughly 80 cultivars in the genotypic variability study, results showed a three-fold difference between the high- and low-yielding cultivars on a fresh weight basis and a two-fold difference on a dry weight basis, for both iron and zinc.

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Arthur Villordon, Don La Bonte, and Robert Jarret

The presence of copia-like retrotransposon sequences in sweetpotato [Ipomoea batatas (L.) Lam.] was investigated. PCR-based amplification using primers to highly conserved copia-like reverse transcriptase sequences produced several products corresponding to the expected target size (≈300 bp) that were subsequently isolated and cloned. A random sample of the clones were sequenced and all six reading frames were translated into their corresponding amino acid sequences. Sequence analysis revealed the presence of 22 copia-like reverse transcriptase sequences corresponding to various subfamilies. The presence of several sequence families in the genome is indicative of past or recent transposition activity. Southern blot analysis suggested that these copia-like sequences were present in several hundred copies in the sweetpotato genome. Data also showed retrotransposon insertion polymorphisms between a limited sample of virus-tested and virus-infected sweetpotato clones, indicating putative activity and mobility. This investigation documented the presence of copia-like retrotransposon sequences in the sweetpotato genome. This is an important step in clarifying the possible association between mobile genetic elements and the unusually high incidence of somatic mutations that may result in clonal decline in sweetpotato and other asexually propagated crops. Data presented provides information on the possible use of retrotransposons as genetic markers for sweetpotato crop improvement.

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Frank A. Buffone, Don R. La Bonte, and Christopher A. Clark

DNA isolated from Fusarium lateritium Nees: Fr.-infected `Jewel' sweetpotato [Ipomoea batatas (L.) Lam.] plants was compared to F. lateritium-free `Jewel' plants for differences in random amplified polymorphic DNA (RAPD) marker products. Differences in RAPD marker products were detected. Amplified DNA isolations from F. lateritium-infected `Jewel' plants generated additional, unique DNA fragments not found in amplified DNA isolations of F. lateritium-free `Jewel' plants. These unique amplified DNA fragments were consistent with those obtained from amplified DNA isolations of the F. lateritium isolate, 91-27-2, used for inoculation. We found that F. lateritium DNA successfully competes with sweetpotato DNA in the polymerase chain reaction for priming sites in a 3: 1 ratio of sweetpotato DNA to F. lateritium DNA. Our results indicate the importance of avoiding plant material infested with pathogens to avoid spurious marker bands.

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Diego Fajardo, Don R. La Bonte, and Robert L. Jarret

The USDA gene bank currently maintains 668 accessions of cultivated sweetpotato and 219 accessions of related Ipomoea species. Information on the genetic diversity of the collection does not exist due to funding constraints. The development of a core collection would provide a subset of accessions that represent the genetic diversity of the main collection with a minimum of repetitiveness. The small size of the core collection would facilitate the evaluation of the accessions for economically important traits. The objective of this research is to develop a core collection of Papua New Guinea sweetpotato germplasm using the Amplified Fragment Length Polymorphisms (AFLPs) marker system. This approach to quantifying genetic diversity would later serve as a model for the development of a USDA sweetpotato germplasm core collection. The germplasm choosen for this study was collected from this crop's secondary center of genetic diversity based on its potential as a source of new traits. All genotypes were fingerprinted using four primer combinations that generated 224 markers. The molecular data was then analyzed using NTSYSpc 2.0 program to determine the relatedness of the genotypes. The molecular analysis showed a homogeneous genetic constitution. The extent of diversity among accessions was correlated with the geographic origin of the plant material.

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Don R. La Bonte, Howard F. Harrison, and Carl E. Motsenbocker

Field experiments were conducted to assess how sweetpotato [Ipomoea batatas (L.) Lam.] clones interfere with weeds and how clones tolerate weed interference. Eleven clones with architecturally different canopies were evaluated for yield, canopy surface area and dry mass, weed dry mass, and light interception at ground level. A 2-fold difference in ground area covered by canopy surface area was observed among the eleven clones 42 days after planting, and a 3-fold difference in canopy dry mass at harvest. Yields were reduced from 14% to 68% by weed interference. The yields of high-yielding clones, `Beauregard', `Excel', L87-125, `Regal', `Centennial', and W-274, were reduced to a significantly greater extent by weeds than were yields of the other five clones. No differences were observed between clones for weed suppression as measured by weed dry mass at harvest and ground light interception. Short-internode and long-internode clones had similar competitive abilities. Yield of high-yielding clones was impacted more by weed interference than was that of low-yielding clones.

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Don R. La Bonte, David H. Picha, and Hester A. Johnson

The quantity and pattern of carbohydrate-related changes during storage root development differed among six sweetpotato cultivars [Ipomoea batatas (L.) Poir. `Beauregard', `Heart-o-Gold', `Jewel', `Rojo Blanco', `Travis', and `White Star']. Measurements were taken for individual sugars, total sugars, alcohol-insoluble solids (AIS, crude starch), and dry weight (DW) at 2-week intervals from 7 to 19 weeks after transplanting (WAT) in two separate years. Sucrose was the major sugar during all stages of development, representing at least 68% of total sugars across all cultivars and dates. Pairwise comparisons showed `Heart-o-Gold' had the highest sucrose content among the cultivars. Sucrose content increased by 56% for `Heart-o-Gold' over the 12 weeks of assay, ranking first among the cultivars at 17 and 19 WAT and possessing 27% more sucrose than the next highest ranking cultivar, `Jewel', at 19 WAT. Fructose content profiles varied among and within cultivars. `Beauregard' showed a consistent increase in fructose throughout development while `Whitestar' showed a consistent decrease. The other cultivars were inconsistent in their fructose content profiles. Glucose content profiles were similar to those for fructose changes during development. The relationship between monosaccharides was fructose = 0.7207 × glucose + 0.0241. Cultivars with the highest fructose and glucose content could be selected by breeders after 13 WAT. Early clonal selection for high sucrose and total sugars is less promising because substantive changes in clonal rank occurred for sucrose and total sugars after 15 WAT. Cultivars ranking the highest in total sugars had either more monosaccharides to compensate for a lower sucrose content or more sucrose to compensate for a lower monosaccharide content. The relationship between DW and AIS was similar (AIS = 0.00089 × DW), and DW and AIS increased with time for most cultivars. Cultivars with high DW and AIS can be selected early during storage root development.