Photoperiodic adaptation of Xanthium strumarium in Europe, Asia Minor, and northern Africa

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Abstract
The range of photoperiodic adaptation among populations of Xanthium strumarium L. in Europe, Asia Minor, and northern Africa is nearly as broad as among populations in North America. Photoperiodic diversity within the indigenous morphological complex, strumarium, is relatively restricted, including night requirements primarily less than 8 h. American introductions include plants with reproductive requirements ranging from 7.5-h nights in northern populations to 10.5–11-h nights in southern populations. Populations in northern Europe in indigenous strumarium and introduced complexes, echinatum and italicum, show a photoperiodic response thinly separated from day neutrality. Southern European populations show reproductive diversity among introduced plants and include sympatric occurrences of three morphological complexes, cavanillesii of South America, chinense of North America, and orientale possibly of North America. The chinense introductions to northern Africa (Morocco, Egypt) are similar to those of Spain with a night requirement of 10.25–10.5 h. Those of Asia Minor include the italicum complex in Turkey and Israel, 9.0–9.25 h, and the chinense complex in southern U.S.S.R., 9.75 h, and in Israel and Iraq, 10.5–11.0 h. Although some of the photoperiodic diversity of the cockleburs of the Old World probably resulted from hybridization between indigenous and introduced plants, the broad adaptive potential was imported from America.

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