Cultivated rice (Oryza sativa L.) was domesticated from the Asian wild species O. rufipogon. Compared with cultivated rice, wild rice has spikelets/seeds with long barbed awns. In order to evaluate the role of awns in wild rice, four seed groups with different awn lengths (full, half, quarter and no awns) were prepared, and the following seed dispersal movements were investigated under simulated natural conditions as observed in the tropical Asian habitat: (1) seed detachment from the parent plant; (2) falling angle of mature seed; (3) ability to slip into small spaces; (4) horizontal movement on the ground and (5) horizontal movement in water. As a result, awns were found to enhance the detachment of matured seeds from the panicles in the initial seed dispersal step. They regulated vertical orientation in the air, and the vertical form may enable seeds to squeeze to the ground. The awned seeds also showed advantages in horizontal movements on the ground and in the water. In most of the experiments, seeds with full awns showed the best performance for seed dispersal, suggesting that wild rice keeps long awns to survive under natural conditions. Since seed awning is dominantly controlled by wild functional alleles at several loci, wild rice may be able to prevent a drastic reduction of awn length.

Weedy rice is a serious weed of cultivated rice in most of the rice-growing areas of the tropics and yet, despite its importance as a weed, limited information is available on its biology in Asia. A study was conducted to determine the seed characteristics and growth response to competition of weedy rice variants from five Asian countries (India [IWR], Malaysia [MWR], Philippines [PWR], Thailand [TWR], and Vietnam [VWR]) when grown alone or with either 4 or 12 cultivated rice (‘IR72’) plants. Seed characteristics including weight, length, width, and presence and length of awn differed between weedy rice variants. PWR had the heaviest seed (25 mg seed−1). Growth of weedy rice plants differed among variants and, generally, VWR grew the fastest. IR72 interference greatly reduced tiller and leaf numbers, leaf area, and shoot biomass of all weedy rice variants. At 8 wk after sowing, for example, shoot biomass of weedy rice variants in competition with 12 IR72 plants was 13 to 30% of that where plants were grown alone. TWR plants were shorter than those of other variants at all levels of crop interference. These results show that there are considerable differences among Asian weedy rice variants and yet, competition from the rice crop greatly suppresses weedy rice growth. This suggests that measures to ensure a competitive crop should be an important component of control strategies.