Effects on weedy rice seeds of rice field overwinter flooding

Weedy rice (Oryza sativa L.) is a difficult weed present in most of the rice growing areas throughout the world. The weediness of this species is due to several traits, including the seed longevity and dormancy. This study was aimed at evaluating the effects on weedy rice seeds of field submersion maintained over the winter and of different storage conditions and durations. The study was conducted in 2005-2007 in Vercelli area (Northwest Italy), both in field and laboratory conditions. The field experiment had the purpose to assess the number of the viable weedy rice seeds present in autumn (after harvesting) and in the spring (just before seed bed preparation), both in condition of continuous submersion of the field and in dry soil during winter. The laboratory experiment was carried out on seeds kept from autumn to spring in dry and in water conditions, and at 4 thermal regimens (-20°C constant, +4°C constant, +25°C constant and field temperature). Seed germination was assessed every two weeks in Petri dishes maintained constantly at 25°C until complete germination was attained. In the field experiment, overwinter reduction of viable seeds was by about 100% in the submerged conditions, and only about 27% in the field conditions. The results of the laboratory experiment showed that about 80% of the weedy rice seeds were able to germinate already two month after harvesting at two different overwintering conditions: field temperatures in flooded soil and +25°C in dry soil. The results obtained at the laboratory conditions seem in keeping with the finding obtained at field level. The high number of seeds germinated since the autumn under flooding condition and subsequently devitalized in winter, could explain the absence of viable seeds recorded before seed bed preparation.