Weedy rice germination as affected by overwinter temperatures and water conditions

Weedy rice (Oryza sativa L.) is one of the major weed issues in rice production worldwide. A large number of weedy rice populations have dormant seeds that do not germinate until the next spring. Overwinter flooding of rice fields may promote weed seed depletion by attracting waterbirds and favoring suicidal germination during autumn. Information on dynamics of weedy rice seed dormancy during overwinter flooding can be usefully exploited for a rational control of the weed. The objectives of the study were to investigate the effect of temperature and water on seed germination and to assess the depletion of viable seeds present on soil surface after overwinter flooding. The study was conducted in 2005-2008 with laboratory and field trials. In the laboratory trial, seeds of two weedy rice populations (awned and awnless), collected in the field, were maintained at constant -20 8C, +5 8C, +25 8C, or at field conditions. At each temperature regime, the seeds were stored either dry or immersed in water. Amounts of seeds were tested for germinability every 2 weeks up to about 200 days. The field trial was carried out at two locations in Northwestern Italy. At each site, 3 paddy fields of at least 3 ha each were overwintered either in dry conditions, continuously flooded for up to 3 years or for up to 4 years. After harvesting and at the end of winter flooding, soil cores (12 cm diameter, 2-4 cm depth) were taken in different areas from each field and weedy rice seeds present on core top were counted. Constant temperatures of seed storage resulted in variable germination patterns with no consistent correla- tions with populations and watering conditions. In simulated field conditions, both populations showed a consistent trend to lose dormancy during storage, espe- cially in the case of storage in water. After 150 days of storage, germinability ranged from 78% (dry) to 90% (water) in the awnless population and from 26% (dry) to 85% (water) in the awned one. A fast increase of germinability was observed in all conditions starting from about 180 GDD (min. 88C, max. 258C). In the field study, the weedy rice infestation recorded before seed dispersal averaged from 10.8 to 23.5 plants/m2 at the two sites, and seed rain ranged from about 960 to 2100 seeds/m2. Overwinter flooding always resulted in a reduction of the number of seeds on soil surface greater than 90%. In fields kept dry overwinter, a reduction ranging from 25 to 28% of the seed rain was also observed. The results obtained in laboratory conditions may in part explain those achieved in the field, as flooding may induce a significant number of seeds to germinate, thus exposing the seedlings to the rigors of winter.