Variation and Function of Cucurbitacins in Cucurbita: An Examination of Current Hypotheses

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Abstract
Though few data document the causes of qualitative and quantitative variation in cucurbitacins among Cucurbita species, the spatial and temporal patterns of cucurbitacin distribution within plants and between species are not likely to be fortuitous or satisfactorily explained by defense or non-defense hypotheses alone. Some interspecific differences in cucurbitacin concentrations correlate with xeric and mesic characteristics of cucurbit habitats and support the hypothesis that the production of secondary compounds is mediated to some extent by nutrient availability. However, defense hypotheses find support in the Cucurbita system from two fronts. Patterns of foliar response to injury indicate that Cucurbita species produce cucurbitacins in quantities large enough to discourage nonadapted herbivores but small enought to reduce production costs and detection by adapted herbivores. Furthermore, chemical responses in cucurbits against viral, fungal, and bacterial leaf pathogens do not involve cucurbitacins, suggesting that cucurbits do not employ stereotypic responses to all invading organisms. Instead, cucurbits exhibit some ability to "recognize" particular types of biotic stress and to respond through appropriate chemical defenses. We suggest that current interspecific and intraspecific patterns of cucurbitacin variation in the genus Cucurbita reflect an evolutionary compromise between several abiotic determinants of cucurbitacin metabolism and the conflicting pressures of both physiologically nonadapted and adapted cucurbit herbivores.