Role of Endogenous Flavonoids in Resistance Mechanism of Vigna to Aphids Vincenzo Lattanzio,* ,† Salvatore Arpaia, ‡ Angela Cardinali, § Donato Di Venere, § and Vito Linsalata § Istituto di Orticoltura e Colture Industriali, CNR, Via S. Loja, Zona Industriale, 85050 Tito Scalo (PZ), Italy; Metapontum Agrobios, S.S., Ionica 106 Km 448.2, 75010 Metaponto (MT), Italy; and Istituto sull’Orticoltura Industriale, CNR, Via Amendola 165/A, 70126 Bari, Italy Cultivated and wild species of the genus Vigna were screened for their flavonoid content. Flavonoid HPLC analyses clearly showed that cultivated lines of cowpea (Vigna unguiculata L. Walp.) are very similar from a qualitative point of view, always showing three flavonoid aglycons: quercetin, kaempferol, and isorhamnetin. In addition, a positive relationship between resistance/susceptibility characteristics against aphids and flavonoid glycoside content of cowpea lines was found. The resistant lines showed a flavonoid content higher than that of susceptible ones. In vitro bioassays proved that, among endogenous flavonoids, quercetin and isorhamnetin possess a good inhibitory aphid reproduction rate. Flavonoid HPLC analyses of wild Vigna species supported evidence for the existence of different flavonoid chemotypes in some species of section Vigna. There are kaempferol chemotypes, kaempferol being the main aglycon detected, quercetin chemotypes, containing quercetin glycosides only, and two isorhamnetin chemotypes. When the resistance characteristics to aphids in different chemotypes of the same species were tested, it became evident that quercetin or isorhamnetin chemotypes showed a higher level of resistance compared to kaempferol chemotypes in the same species, thus demonstrating a direct involvement of quercetin or isorhamnetin in the resistance mechanism. These results can provide useful information for further studies on gene expression of resistance factors. Keywords: Plant phenolics; flavonoids; bioassay; aphid resistance; Vigna species INTRODUCTION Cowpea (Vigna unguiculata L. Walp.) is an important food legume in many countries in sub-Saharan Africa and Latin America. Its yield potential is very high (1.5- 3.0 t/ha), but the actual yields in much of the developing world are far less, averaging 0.2-0.4 t/ha. The major constraints to cowpea production are insect pests, plant diseases, plant parasitic weeds, drought, and heat (Murdock, 1992; Singh et al., 1992; Thottappilly et al., 1992). There are two Aphis spp. (Homoptera: Aphididae) reported as pests of cowpeas: A. craccivora Koch, which is the main aphid infesting cowpeas throughout Africa and Asia, and A. fabae (Scopoli), which has been reported as a minor pest in Africa and whose biology appears to be similar to that of A. craccivora. Cowpea aphids primarily infest seedlings, but large populations also infest flowers and green pods of older plants (Annan et al., 1996; Singh and Jackai, 1985). The ecological relationship between plants and insects is complex one with physical as well as chemical interactions. This relationship is also affected by plant factors, insect factors, and some insect-plant factors, including hy- persensitive reaction and plant resistance to insect- borne disease. Various environmental conditions can modify the expression of these factors by acting prima- rily on the insect, the plant, or the insect-plant relationships. Each of the plant or insect mechanisms indicated may be the result of one or more genetic factors (Painter, 1941). Plant volatiles and visual and thigmotactic cues may be involved in an insect’s recognition of, and migration to, a host plant. After locomotion is arrested, probing occurs. The net result of individual feeding stimuli and deterrents determines whether the insect will remain and feed. Whether a plant is accepted or rejected as food by insects depends largely on its chemical composition in addition, of course, to physical factors such as toughness, thickness, and hairiness. In addition, chemi- cal inhibitors play an important role in the inhibition of oviposition on the host-plant and, in turn, in insect larval growth and survival of progeny (Chapman, 1974; Dethier, 1970; Stotz et al., 1999). Studies on the role of inhibitors in host-plant selection indicate that many different chemicals may be expected to have an inhibi- tory effect on feeding by different insects. Among plant constituents, phenolics are known to be involved in disease and insect resistance of crop plants (Corcuera, 1993; Elliger et al., 1980; Todd et al., 1971). Plant flavonoids affect the behavior, development, and growth of a number of insects (Hedin and Waage, 1986). Some cotton flavonoids are feeding stimulants for the boll weevil, Anthonomus grandis (Hedin et al., 1988), or oviposition stimulants of a Citrus-feeding swallowtail butterfly, Papilio xuthus L. (Nishida et al., 1987), or, * Author to whom correspondence should be addressed (telephone +0390971427249; fax +390971427222; e-mail lattanzio@ioci.pz.cnr.it). † Istituto di Orticoltura e Colture Industriali. ‡ Metapontum Agrobios. § Istituto sull’Orticoltura Industriale. 5316 J. Agric. Food Chem. 2000, 48, 5316-5320 10.1021/jf000229y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000