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Howard F. Harrison, Joseph K. Peterson and Maurice E. Snook

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Joseph K. Peterson, Howard F. Harrison and Maurice E. Snook

After removal of the periderm, cortex tissue of the sweetpotato cultivar Regal was collected. Polar extracts of this tissue strongly inhibited germination of proso-millet seed. C18 preparative, step-gradient chromatography (H2O → 100% methanol) gave some 50+ fractions, all of which were assayed for inhibitory properties. Analytical HPLC, using diode array detection and signal processing, showed the presence of chlorogenic, p-coumaric and caffeic acid, scopolin and some unknown phenolic acids. Most fractions were inhibitory to some degree; however, the least polar ones (in 90% and 100% methanol), containing unknown compounds, were most inhibitory. Semi-prep HPLC of these fractions produced eight major peaks (λmax at 210–213 nm, λ2 at 281–284 nm). In our bioassays, the compounds produced 50% inhibition of proso-millet seed germination at ≈60 ppm. It is likely that these compounds contribute significantly to the allelopathic properties of sweetpotato.

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Howard F. Harrison, Joseph K. Peterson and Maurice E. Snook

Bioasssay-guided investigation of constituents possibly contributing to the allelopathic potential of sweetpotato led to the isolation of a nonpolar seed germination inhibitor in sweetpotato (Ipomoea batatas L.) roots. Mass spectral data supported by HPLC s pectroscopic analyses and data obtained from hydrolysis products revealed the presence of three monogalactosyl-diglycerides (MGDGs) (galactosyl-di-linoleneoyl glyceride, galactosyl-linoleneoyl-linoleoyl glyceride, and galactosyl-di-linoleoyl glyceride) in storage roots. The compounds inhibited proso millet germination, and at 100 ppm inhibition was about 90%. MGDG with fully saturated fatty acids (galactosyl-distearoyl glyceride) was not inhibitory in the bioassay. An efficient method for quantitation of individual MGDGs was developed, and the contents of each compound in the storage root tissues of 12 genetically diverse cultivars and breeding lines were determined. On a dry weight basis, total MGDG contents ranged between 107 and 452 μg/g in the periderm, 298 and 807 μg/g in the cortex, and 296 and 755 μg/g in the stele. Also, large differences in the ratios of the three compounds between clones and between tissues within a clone were noted. The differences between clones indicate that manipulating total content and ratios of MGDGs through plant breeding is feasible.

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

Methanol extracts of external (outer 3 mm) and interior root tissue of four sweetpotato [Ipomoea batatas (L.) Lam.] cultivars (`Centennial', `Jewel', `Regal', and `Resisto') having different levels of susceptibility to the sweetpotato weevil [Cylas formicarius elegantulus Summer] were analyzed for simple carbohydrates (fructose, glucose, sucrose, inositol) and organic acids (malic, citric, quinic) by gas chromatography and for phenolics (caffeic acid, caffeoylquinic acids, rutin) by high-performance liquid chromatography. There were significant differences among cultivars in the concentrations of total sugars and phenolics in the external tissue (P < 0.05). In addition, the distribution of carbohydrates, organic acids, and chlorogenic acid [3-O-caffeoylquinic acid] differed between external and interior tissues. Sucrose was the major water-soluble carbohydrate in all cultivars. With the exception of malic acid, the concentration of carbohydrates, organic acids, and phenolics did not correlate with cultivar susceptibility to the sweetpotato weevil.

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Kelechi Ogbuji, Gloria S. McCutcheon, Alvin M. Simmons, Maurice E. Snook, Howard F. Harrison and Amnon Levi

Whiteflies [Bemisia tabaci (Gennadius)] and aphids [Aphis gossypii Glover and Myzus persicae (Sulzer)] are serious threats to watermelon by direct feeding and by transmitting viruses of important virus diseases. The desert watermelon Citrullus colocynthis (L.) has been shown to exhibit resistance to these insect pests and could be a useful source for breeding resistance into watermelon [Citrullus lanatus var. lanatus (Thunbs) Matsum & Nakai]. Using high-performance liquid chromatography (HPLC), we found differences among the chemical profiles of two U.S. PIs of C. colocynthis, one PI of C. lanatus var. citroides, and two heirloom watermelon (C. lanatus var. lanatus) cultivars (‘Charleston Gray’ and ‘Mickey Lee’). Flavonoid and caffeic acid derivatives were identified in the leaf extracts by a combination of ultraviolet (UV) and mass spectrometry (MS) spectral analyses. Four phenolic derivatives of caffeic and/or ferulic acid were found to be essentially unique to C. colocynthis. Total flavonoid content was found to be approximately four to 18 times higher in C. colocynthis accessions and seven to nine times higher in C. lanatus var. citroides as compared with watermelon cultivars. Caffeoyl-glucose was also identified in the leaves of watermelon cultivars for the first time. Leaf sugar concentrations (198 to 211 mg·dL−1), read from a glucometer, were statistically the same among the various germplasm entries. These results will help in the development of pest-resistant watermelon.

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Howard F. Harrison Jr., Joseph K. Peterson, Christopher A. Clark and Maurice E. Snook

Sweetpotato [Ipomoea batatas (L.) Lam.] periderm components were tested for their effect on four fungi that infect sweetpotato roots: Fusarium oxysporum Schlecht. f. sp. batatas (Wollenw.) Snyd. & Hans. and F. solani (Sacc.) Mart., both of which cause stem and root disease; and Lasiodiplodea theobromae (Pat.) Griffon & Maubl. and Rhizopus stolonifer (Ehr. ex Fr.) Lind., both of which cause storage root disease. Sequential extracts of `Regal' sweetpotato periderm with hexane, methanol, and 50% methanol were inhibitory to the four fungi when incorporated into potato dextrose agar medium in petri dish bioassays. The methanol and 50% methanol extracts were much more active than the hexane extract and were combined for further study. Sephadex LH-20 column chromatography of the combined extracts, followed by bioassay with F. oxysporum indicated that the most inhibitory fraction contained the least polar components of the extract. Resin glycosides isolated from `Regal' periderm inhibited F. oxysporum, but the glycosides exhibited little concentration effect and were not as active on a tissue weight basis as other components. Periderm extracts from 10 sweetpotato clones exhibited large differences in inhibitory activity in bioassays with the four fungi. The sensitivity of the fungi to inhibition by the periderm extracts suggests that periderm components may provide protection against soil pathogens, but a relationship between such components and disease resistance was not established.

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Joseph K. Peterson, Howard F. Harrison, D. Michael Jackson and Maurice E. Snook

Periderm and cortex tissues of 14 genetically diverse sweetpotato [Ipomoea batatas (L.) Lam.] clones were grown under low stress conditions and analyzed for their content of scopoletin ((7-hydroxy-6-methoxycoumarin) and scopolin (7-glucosylscopoletin). A wide range of concentrations of both compounds was found in both tissues. The two compounds were tested in vitro for their biological activity (concentration-activity relationships) using several bio assays: germination of proso-millet (Panicum milliaceum L.) seed; mycelial growth of the sweetpotato fungal pathogens Fusarium oxysporum Schlecht. f. sp. batatas (Wollenw.) Snyd. & Hans, F. solani (Sacc.) Mart., Lasiodiplodia theobromae (Pat.) Griffon & Maubl., and Rhizopus stolonifer (Ehr. ex Fr.) Lind; and growth and mortality of diamondback moth[Plutella xylostella (L.)] larvae on artificial diet. The glycoside scopolin showed little activity, except moderate inhibition of F. oxysporum. The aglycone scopoletin inhibited seed germination and larval growth; however, at much higher concentrations than were measured in the tissues. Mycelial growth of the four pathogenic fungi, however, was inhibited at concentrations occurring in some sweetpotato clones.

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Howard F. Harrison Jr, Trevor R. Mitchell, Joseph K. Peterson, W. Patrick Wechter, George F. Majetich and Maurice E. Snook

Caffeoylquinic acid compounds are widespread in plants. They protect plants against predation and infection and may have several beneficial functions in the human diet. The contents of chlorogenic acid and the 3,4-, 3,5-, and 4,5- isomers of dicaffeoylquinic acid (DCQA) in the storage root tissues of 16 sweetpotato [Ipomoea batatas (L.) Lam.] genotypes were determined. Averaged over genotypes, the contents of the four compounds were highest in the cortex, intermediate in the stele, and lowest in the periderm. Among the genotypes, chlorogenic acid contents ranged from 16 to 212 μg·g−1 in periderm, from 826 to 7274 μg·g−1 in cortex, and from 171 to 4326 μg·g−1 in stele. The 3,5-DCQA isomer comprised over 80% of total DCQA. In most genotypes, 3,5-DCQA and chlorogenic acid contents were similar in cortex and stele tissues, but chlorogenic acid was lower than 3,5-DCQA in periderm tissue. Among the 16 genotypes, total DCQA contents ranged from 0 to 1775 μg·g−1 dry weight in periderm, from 883 to 8764 μg·g−1 in cortex, and from 187 to 4768 μg·g−1 in stele. The large differences found in a small germplasm collection suggest that selecting or breeding sweetpotato genotypes with high caffeoylquinic acid content is possible. The four caffeoylquinic acid compounds comprised over 3% of the dry weight of storage roots of the sweetpotato relative, bigroot morningglory [Ipomoea pandurata (L.) G.F.W. Meyer], indicating that it may be a good source for the compounds. The effect of DCQAs isolated from sweetpotato and I. pandurata tissue and caffeic and chlorogenic acid standards were tested in proso millet (Panicum milliaceum L.), Fusarium solani (Sacc.) Mart., and bacterial growth bioassays. Caffeic acid, chlorogenic acid, and 3,5-DCQA were most inhibitory in millet and F. solani bioassays, but 3,5-DCQA was the least inhibitory compound in bacterial growth bioassays. Their activity in the bioassays suggests that the caffeoyl quinic acid compounds contribute to the allelopathic potential and resistance to root diseases of some sweetpotato clones.