Photoperiodic adaptation in Xanthium strumarium populations introduced to Australia

Seedlings derived from field-collected burs from Australia were tested for photoperiodic adaptation and morphologies. Four morphophysiological types were represented from northern Victoria to southern Queensland. The widespread chinense morphological complex (Noogoora Burr) has a critical dark period of 10.5 h. The italicum morphological complex of eastern central New South Wales has a critical dark period of 10 h. The cavanillesii morphological complex near Sydney has an apparent critical night of 9.5 h and the pensylvanicum morphological complex in northwestern Victoria has an apparent critical night of 9.25–9.5 h. Two populations in the area of sympatry of the chinense and italicum complexes showed diverse photoperiodic adaptation, ripeness-to-flower (maturity) responses and morphologies, suggesting hybridization. Although Xanthium populations have been in Australia for over 100 years, their morphophysiology matches populations in America with relatively narrow distribution.