Photoperiodic responses ofXanthium strumarium L. (Compositae) introduced and indigenous in eastern asia

Photoperiodic responses among populations ofXanthium strumarium L. in eastern Asia from Vladivostok, USSR, to Hong Kong and Taiwan include a wide range of adaptation. Among indigenous populations of thestrumarium morphological complex, responses range from near day neutrality (Vladivostok) to approximately a night length requirement of 9.5 hr (Hong Kong). Introduced populations in Japan include a night length requirement of 9.75 hr in theitalicum morphological complex and of 10.25–10.5 hr in thechinense morphological complex. The range of photoperiodic adaptation is nearly as wide as among populations of North America.