The effect of field storage and laboratory conditions on germination of five Xanthium species

Abstract

Noogoora burr (Xanthium occidentale) and Bathurst burr (X. spinosum) are widely distributed in wool-growing areas of eastern Australia. Hunter burr (X. italicum), Californian burr (X. orientale) and South American burr (X. cavanillesii) have not shown the same ability to spread. After the burrs of these five Xanthium species had been recovered from field storage in three microenvironments at five sites in spring and autumn 1980, their germination responses were assessed in growth cabinets set at 25/15 and 35125¦C (8/16 h) with a 12/12 h light/dark regimen. Two types of dormancy were recognized in Xanthium species, and these types showed different responses to field storage and season of recovery. In all species except Bathurst burr innate dormancy was observed in upper seeds and was overcome by high temperature and exposure to light. Storage in soil reduced levels of innate dormancy but season of recovery had no significant effect. Induced dormancy was observed in lower seeds following storage in soil over summer and this was overcome by storage in soil over winter. It could also be overcome in Noogoora burr, Californian burr, and Hunter burr by high temperature and exposure to light. Removal of seeds from the burr increased the rate of germination, but the expression of innate and induced dormancy did not appear to be affected. The responses of the other species were not evident in Bathurst burr. Seeds of Bathurst burr germinated rapidly but a large proportion of seeds remained dormant and responses to field storage and germination conditions were variable. Differences in burr morphology between Bathurst burr and other species indicated that germination responses of Bathurst burr might be modified by the burr. Innate and induced dormancy were expressed most strongly in the least widespread species, namely Hunter burr, Californian burr and South American burr. while Noogoora burr and Bathurst burr had the most rapid germination rate. It is suggested that the success of Noogoora and Bathurst burr as weeds might be due in part to their rapid germination rate coupled with a delay in imbibition caused by the burr.