The Xanthium strumarium Complexes in Australia

The Xanthium strumarium complexes have exploited ecosystems disturbed by man on all continents by virtue of a vast array of morpho-physiological differentiation. Australia, which had no indigenous populations, has been colonized by four morpho-physiological variants. The four introductions have maintained their successful adaptations through self-pollination but have produced new morpho- physiological combinations in certain floodplain habitats through hybridization of sympatric variants. The four introductions can be treated variously as members of morphological complexes: 'chinense', 'italicum', 'pennsylvanicum', and 'cavanillesii', or as members of four taxa: X. chinense Mill., X. italicum Mor., X. californicum Greene, and X. cavanillesii Schouw. The introduction of X. chinense represents a type that is photoperiodically adapted to the south- eastern United States and is similar to populations occurring on the Mississippi Delta south of New Orleans, Louisiana. This introduction in the 'chinense' complex reportedly arrived in Queensland with American cotton seed and is commonly referred to as Noogoora burr. It represents the most aggressive form of Xanthium and is widely distributed in eastern Australia from northern Victoria to Queensland and the Northern Territory. The populations of X. italicum represent a morphological form that is currently found in populations of eastern Texas, where the 'italicum' and 'chinense' complexes are sympatric, and was possibly introduced in its present intermediate morpho-physiological state. This introduction in the 'italicum' complex is photoperiodically similar to plants of central Texas and is confined to latitudes 29-33° in New South Wales. Hybrid intermediates between the populations of X. chinense and X. italicum have been detected in river floodplains of New South Wales where the two introductions are sympatric. The introduction in the 'pennsylvanicum' complex can be referred to X. californicum Greene. It represents a morpho-physiological form that occurs in northern California and may have originated in the vicinity of the San Francisco Bay. Reportedly confined in Australia to the eastern portion of South Australia and adjacent Victoria, this introduction seems to be adapted to a Mediterranean climate. Collections of Australia and California cultivated in central Texas showed poor vigour, 'stem rotting', and drying branches with no burr production. The introduction of X. cavanillesii is locally confined to the Nepean-Hawkesbury drainage north-west of Sydney and is not reported outside New South Wales. These populations of the 'cavanillesii' complex represent the morphological form that is indigenous to South America and may have originated in the vicinity of Buenos Aires, Argentina. Although the origins of Xanthium in Australia are not documented, they were probably accidentally introduced with cotton seed (X. chinense), in animal fur (X. italicum, X. cavanillesii) and with grape stock (X . Californicum). The morpho-physiological range of the four introductions represents a very small part of the total adaptive range within the eight complexes of X. strumarium and could be expanded by subsequent arrivals from other continents.