Coexistence of ALS-inhibitors resistant Amaranthus species in North-Eastern Italy.

After the first case of Amaranthus retroflexus resistant to ALS-inhibitors in 2003 in soybean fields, the evolution of resistance in Amaranthus spp. has been rather slow in Italy. The recent discovery of some soybean fields infested with several species of Amaranthus, including A. tuberculatus (syn: A. rudis), is particularly concerning because this species has been proven to inherit herbicide resistance from other amaranth species due to unidirectional gene flow. A. tuberculatus represents one of the most dramatic cases of weed invasion documented in the mid-western USA. The discovery of more than one species of resistant Amaranthus in soybean fields in North-eastern Italy presents a complex situation. From the resistance management point of view, the rapid and precise determination of parental species and hybrid progeny is a key preliminary step. This is often difficult because of the intrinsic variability of the genus and the interspecific hybridization. Furthermore, botanical keys are based on morphological characteristics of the flowers, so many species can only be correctly identified late in the season.This study aims to determine the parental species and possible hybrid-derived individuals of Amaranthus through the observation of specific traits of the inflorescences and pollen size. The shape and size characteristics of the tepals and the bracts/tepals length ratio allowed the specimens to be ascribed to A. tuberculatus, A. retroflexus and A. hybridus species. The pollen size can provide information regarding cyto-morphological changes, such as hybridization events. Inflorescences 6-7 cm long were collected and stored in glacial acetic acid:ethanol (3:1 by vol.) for 4 days at room temperature. The pollen grains were obtained squashing 5-6 individual male florets on a microscope slide and then observed under fluorescence microscope. The first analyses of pollen size are in progress.