Watermelon [C. lanatus var. lanatus (Thunbs) Matsum & Nakai] is an important crop globally. The origin of Citrullus spp. is in central or southern Africa (Jarret et al., 1997; Mujaju et al., 2010). On that continent, a wide variation of watermelon populations exists in diverse geographical regions and the fruit is considered a vital source of water and food for the native people and animals. As a result of many years of cultivation and selection for desirable qualities, a large number of the American heirloom watermelon cultivars shares a narrow genetic base and is susceptible to diseases and pests (Levi et al., 2001a; Simmons and Levi, 2002). On the other hand, the Citrullus spp. germplasm collected mainly in central and southern Africa shows a wide phenotypic and genetic diversity (Levi et al., 2001b).
The genus Citrullus includes several species or subspecies. Among them is the bitter watermelon C. colocynthis (L.) Schrad that thrives in the deserts of North Africa, the Middle East, and Asia. It has distinct morphological and biochemical features such as thick leaves, fairly small fruits, and a bitter odor that repels insects (Simmons and Levi, 2002). Additional species, found in southern Africa, are C. ecirrhosus Cogn. and C. rehmii De Winter (Robinson and Decker-Walters, 1997). C. lanatus var. lanatus is considered the progenitor of cultivated watermelon. The C. lanatus also includes the citron watermelon, C. lanatus (Thunbs) Matsum & Nakai var. citroides (L.H. Bailey), which thrives in the deserts of southern Africa. It is known as the “Citron Watermelon,” “Cow Watermelon,” or “Tzama” (Jarret et al., 1997; Mujaju et al., 2010) and is considered a valuable germplasm source because different accessions of this subspecies contain resistance to diseases or pests (Levi et al., 2001b; Thies and Levi., 2007). The Citrullus germplasm collection maintained by the USDA-ARS Plant Genetic Resources and Conservation Unit, Griffin, GA (http://www.ars-grin.gov) includes over 1800 U.S. PIs. These PIs have been useful sources of germplasm for identifying disease or pest resistance that through intensive breeding programs could be incorporated into elite watermelon cultivars.
Whiteflies [Bemisia tabaci (Gennadius)] and aphids [Aphis gossypii Glover and Myzus persicae (Sulzer)] are major pests that feed on and transmit viruses to watermelon plants (Simmons et al., 2010; Simmons and Levi, 2002). However, several C. colocynthis PIs possess resistance to the sweetpotato whitefly, B. tabaci (Simmons and Levi, 2002), the two-spotted spider mite, Tetranychus urtichae Koch (Lopez et al., 2005), and aphids (Simmons, unpublished data). These sources of germplasm should be useful for incorporating pest resistance into watermelon cultivars.
Very little work has been reported on the flavonoids and phenolics of watermelon, especially in the leaves. Most previous investigations of flavonoid or phenolic content of watermelon have been limited to determination by colorimetric methods (Asyaz et al., 2010; Chopra et al., 1974; Ibrahim et al., 2010; Tlili et al., 2011; Venkataramaiah and Narayana, 1983). Others have used acid hydrolysis before analysis to measure individual aglycone flavonoids (Harsh and Nag, 1988; Lugasi and Hovari, 2002; Meena and Patni, 2008) or phenolic acids (Das et al., 1967; Venkataramaiah and Narayana, 1983). Delazar et al. (2006) determined a number of flavonone-C-glycosides in Citrullus colocynths fruits. To our knowledge, only the report of Maatooq et al. (1997) reports specific and novel hydroxybenzyl-flavonoids in the leaves of watermelon (Citrullus colocynthis) and only that of Chopra et al. (1974) relates phenolic content to resistance and susceptibility to disease (Alternaria cucumerina). Furthermore, there is no sufficient information on the chemical profile and compounds that may lure or repel insect pests and affect their feeding habits and reproduction on plants of watermelon cultivars vs. C. colocynthis PIs.
The objective of this study was to determine if differences exist in the chemical profiles of leaves of C. colocynthis PIs that showed whitefly resistance (Simmons and Levi, 2002) vs. those of susceptible watermelon cultivars (C. lanatus var. lanatus) and in a representative PI of the citron watermelon (C. lanatus var. citroides).
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