Factors affecting growth and reproduction of Noogoora burr ( Xanthium occidentale Bertol.)

Plant density and availability of light and water significantly affected growth and reproduction of Noogoora burr plants grown in a glasshouse. Burr yield was reduced most by low levels of available water. Low light intensity, low water availability and high plant density reduced net assimilation rates and, in combination, caused the greatest reduction in burr production. Under field conditions the ratio of burr dry weight to total shoot dry weight decreased with increasing plant density. In terms of production per unit area, dry weight of main stems and main stem leaves increased with increasing plant density at the expense of burr dry weight. We conclude that the success of Noogoora burr as a fleece contaminant could be attributed, in part, to: the insensitivity of burr production to variation in soil fertility and length of growing season; the ability to regulate plant density under conditions of limiting light or water; and the ability to regulate plant components so that the efficiency of burr production increases with a decrease in plant density.