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  • Author or Editor: M. Snook x
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These studies were initiated to investigate severe growth inhibition observed when some vegetable crops were infested with corn spurry (Spergula arvensis L.). Interference by a natural population of the weed reduced the shoot weights of English pea (Pisum sativum L.) and collard (Brassica oleracea L.) by 93% and 72%, respectively. In a greenhouse experiment where light competition by corn spurry was prevented, broccoli (Brassica oleracea L.) shoot weights were reduced by corn spurry, but pea weights were not different from the controls. Homogenized corn spurry shoot tissue incorporated into a greenhouse potting medium inhibited the growth of both species, and a concentration effect was observed. Sequential hexane, dichloromethane, methanol, and 50% aqueous methanol extracts of corn spurry root and shoot tissue were tested for inhibitory activity using millet seed germination and broccoli seedling growth bioassays. Dichloromethane, methanol, and aqueous methanol shoot extracts were inhibitory to broccoli; whereas all shoot extracts inhibited millet germination. Shoot extracts were more inhibitory than root extracts. Further fractionation of the inhibitors using a combination of reversed-phase sephadex LH-20 and silicic acid column chromatographic procedures showed that a major portion of the millet germination inhibition was due to sucrose esters (SE). Preliminary characterization of the esters showed that there were four different SE groups. The major groups contained either octanoic or dodecanoic acid along with butanoic and petanoic acids. All groups inhibited seed germination at concentrations as low as 20 ppm. This is the first report of the SE class of defense chemicals in plant species outside of the solanaceae family.

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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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