Phytotoxic Effects and a Phytotoxin from the Invasive Plant Xanthium italicum Moretti

Xanthium italicum Moretti (family Asteraceae, Compositae), a noxious annual invasive weed in China, is mainly distributed in six provinces. In 2007, it was listed on the “People’s Republic of China imported plant quarantine pests list”. Although X. italicum is currently distributed in only a few places in China, ecological modeling has predicted that it has the potential to spread further to most regions in China, except for several areas with extreme environmental conditions such as Tibet.

Biological invasions cause great damage worldwide, especially to fragile ecosystems. Arid ecosystems are relatively simple in structure and function compared with other terrestrial ecosystems, which makes them more susceptible to invasions. The recent invasion of X. italicum has raised concerns regarding the possible ecological consequences it could bring to the arid land. However, the mechanism underlying the successful invasion of this plant remains unclear. There have been many hypotheses attempting to elucidate the mechanism of exotic plant invasions. Considering the fact that allelopathy is a notoriously difficult mechanism to demonstrate, researchers decide to start with the isolation and identification of phytotoxins in X. italicum plant which might work as potential allelochemicals after being released into the environment via leaching, root exudation or litter decomposition. Therefore, allelopathic effects of different parts of the plant X. italicum were evaluated by conducting bioassays against two dicot plants, amaranth (Amaranthus mangostanus L.) and lettuce (Lectuca sativa L.), and two monocot plants, wheat (Triticum aestivum Linn) and ryegrass (Lolium multiforum) in Xinjiang.

Leaf and fruit extract possessed the strongest biological activity, killing all seeds of four test species at 0.05 g/mL concentration. Fruits were chosen for further investigation because of their high biomass. This led to the isolation and identification of a phytotoxin—Xanthinosin—a known sesquiterpene lactone. Xanthinosin significantly affected seedling growth of all test species at 160 µM concentration. Cultivating seeds in 800 μM xanthinosin solution resulted in a great decrease in seedling growth of all test species, especially for the two dicot plants, amaranth and lettuce, whose root length was inhibited by 78% and 89%, respectively. By comparison, the numbers were 69% lower for wheat, and 66% for ryegrass. When treated with 4 mM xanthinosin solution, seed germination of all test plants was almost completely inhibited. The possibility of utilizing xanthinosin as an eco-friendly herbicide was also discussed in this study.

The result has been published on Molecules, 2012, 17: 4037-4046. The paper is also archived at http://www.mdpi.com/1420-3049/17/4/4037.