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Title: Partial leaf chemical profiles of a desert watermelon species (Citrullus colocynthis) and heirloom watermelon cultivars (Citrullus lanatus var. lanatus)

item OGBUJI, KELECHI - Claflin University
item MCCUTCHEON, GLORIA - Claflin University
item Simmons, Alvin
item Snook, Maurice
item Harrison Jr, Howard
item Levi, Amnon

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2012
Publication Date: 5/18/2012
Citation: Ogbuji, K., McCutcheon, G.S., Simmons, A.M., Snook, M.E., Harrison, H.F., Levi, A. 2012. Partial leaf chemical profiles of a desert watermelon species (Citrullus colocynthis) and heirloom watermelon cultivars (Citrullus lanatus var. lanatus). HortScience. 47:580-584.

Interpretive Summary: Whiteflies and aphids are major insect pests that transmit viruses and feed on watermelon plants. Finding natural resistance to these insect pests in wild watermelons and moving the resistance into watermelon cultivars is an effective approach that could reduce pesticide applications in watermelon fields. Natural resistance to whiteflies and aphids has been identified in the desert watermelon Citrullus colocynthis that thrives in deserts of North Africa, the Middle East, and Asia. This natural resistance may be the result of a compound produced in the leaves of the desert watermelon that repels the insect pests. In this study, scientists at the U.S. Vegetable Laboratory, in cooperation with entomologists with Clafflin University investigated the chemical profile of the desert watermelon leaves using a technology called “high pressure liquid chromatography” (HPLC). Results showed that several compounds called “flavonoids”, that are known to have inhibitory activity against organisms or pests, are abundant in the leaves of the desert watermelon plants, but not in watermelon cultivars. These compounds will be examined in further studies to determine if they are associated with natural resistance to insect pests and if they can be incorporated into elite watermelon cultivars to enhance their resistance to insect pests. Insect resistant watermelon cultivars should require less pesticides which will benefit growers, consumers and the environment.

Technical Abstract: 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. plant introductions (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 UV and 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 about 4 to 18 times higher in C. colocynthis accessions, and 7 to 9 times higher in C. lanatus. var. citroides, as compared to 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), read from a glucometer, were statistically the same among the various germplasm entries. These results will help in the development of pest resistant watermelon.