Seed Dormancy Imposed by Covering Tissues Interrelates to Shattering and Seed Morphological Characteristics in Weedy Rice

such as rice to weedy or red rice (Oryza spp.) (Oard et al., 2000; Gealy et al., 2003; Zhang et al., 2003). SystemSeed dormancy, a major adaptive trait in plants, facilitates the atic research is required to understand how seed dorsurvival of weeds and provides for resistance to preharvest sprouting (PHS) in cereal crops. Seventeen weedy strains and 24 cultivars of mancy and other characteristics such as awns, hull and rice (Oryza sativa L.) were evaluated for germinability to screen pericarp/testa colorations, herbicide resistance, and shatfor donors of dormancy genes. Extremely dormant genotypes were tering influence fitness in weed/cultivar hybrid-derived identified from the weedy strains. These genotypes displayed hulloffspring. and pericarp/testa-imposed dormancy. Three dormant weedy strains, Weedy strains are hypothesized to be intermediate LD, TKN12-2, and SS18-2, were crossed and backcrossed with the between wild species and cultivars, and thus weeds are nondormant breeding line EM93-1 to determine the relationship beconsidered as part of the primary gene pool for crop tween dormancy and the shattering, awn, hull color, and pericarp/ breeding (Harlan et al., 1973). It is assumed that weedy testa color characteristics. All these characteristics interrelated to the races harbor genes for tolerance to various adverse concovering-imposed dormancy; the weedy forms of the characteristics ditions, but their breeding potential has not been well significantly reduced germination in the BC1F1 populations. Moreover, multiple linear regression analyses revealed significant effects of interexplored (Harlan, 1965; Oka, 1988). New genetic and action between the characteristics on dormancy in the populations. genomic approaches will facilitate discovery of novel The interrelation and interaction reflect the importance of combined genes in nondomesticated germplasm for major adeffects of dormancy and other weedy characteristics in the adaptation vances in crop improvement (Tanksley and McCouch, of weedy populations to agroecosystems, and suggest that domestica1997). We are utilizing weedy rice as a genetic system tion and breeding activities have eliminated dormancy alleles at loci to explore fundamental and applied aspects of seed dornear the genes for shattering and the morphological characteristics mancy because weedy strains generally have a higher from improved cultivars. level of dormancy compared with cultivars (Oka, 1988; Tang and Morishima, 1997). Additionally, rice is a model system with a small genome size (430 Mbp) that has W are organisms adapted to human disturbeen sequenced (Goff et al., 2002; Yu et al., 2002). bances (Harlan, 1965). Many characteristics conWeedy rice occurs in areas with and without wild rice tribute to the adaptation and persistence of weeds (Baker, relatives (Oryza spp.) (Oka, 1988). Previous research 1974). For example, shattering enables weed seeds to has characterized the level and type of seed dormancy escape harvest, and dormancy promotes survival in the for several weedy and cultivated genotypes (Gu et al., soil seed bank. Other characteristics, such as grain type 2003). Seed dormancy was associated with the presence and awns in wild oat (Avena fatua L.) (Johnson, 1935; of an awn, and black hull or red pericarp/testa colorSimpson, 1992) and dark seed coat color in proso millet ations in two weedy strain-derived F2 populations, but (Panicum miliaceum L.) contribute to weed persistence the degree of association varied with the cross or gene(Khan et al., 1996), but their adaptive significance has tic background (Gu et al., 2003). In this research, we not been extensively examined. screened additional weedy strains, and traditional and Outcrossing frequently occurs between conspecific improved cultivars for dormant genotypes. Three strongly weeds and crops, sometimes yielding fertile hybrids dormant weedy strains were crossed and backcrossed (Harlan et al., 1973; Langevin et al., 1990). Outcrossing with a breeding line to synchronize the genetic backfacilitates gene flow from crop to weed populations and ground of dormancy genes. Thus, the objectives of this initiates hybridization-differentiation cycles (Ladizinresearch were to use primary segregation populations sky, 1985; Oka, 1988). It is these cycles that allow popu(BC1F1s) to establish the co-adaptive relationships belations of weeds to generate types that are better adapted tween dormancy and other weedy characteristics, and to compete with or mimic improved cultivars. For examdemonstrate how domestication and breeding have reple, researchers are now concerned with gene flow besulted in a reduction of seed dormancy. tween herbicide-resistant cultivars and conspecific weeds MATERIALS AND METHODS X.-Y. Gu, Plant Sci. Dep., North Dakota State Univ., Fargo, ND 58105, and Agricultural College, Yangzhou Univ., Yangzhou 225009, Plant Materials and Cultivation China; S.F. Kianian, Plant Sci. Dep., North Dakota State Univ., Fargo, Seventeen weedy strains and 24 cultivars of Asian rice (O. ND 58105; M.E. Foley, USDA-ARS, Biosciences Research Lab., sativa L.) and four cultivars of African rice (O. glaberrima Fargo, ND 58105-5674. Received 2 June 2004. Crop Breeding, GenetSteud.) were screened for strongly dormant and nondormant ics & Cytology. *Corresponding author ([email protected]). genotypes. The weedy strains originated from Bhutan (BT1Ac, Published in Crop Sci. 45:948–955 (2005). doi:10.2135/cropsci2004.0339 © Crop Science Society of America Abbreviations: DAR, days of afterripening; PHS, preharvest sprouting; QTL, quantitative trait loci. 677 S. Segoe Rd., Madison, WI 53711 USA 948 Published online March 28, 2005