Inheritance of Seed Dormancy in Weedy Rice

should provide new insight into seed dormancy and resistance to preharvest sprouting. Seed dormancy contributes to the adaptability of plants in nature Rice germplasm (Oryza spp.) varies in degree of seed and is of considerable importance in agriculture. The weedy rice dormancy (Roberts, 1961a; Wu, 1978; Oka, 1988, p. 87– (Oryza sativa L.) strains LD, SS18-2, and TKN12-2 and cultivar ‘N22’ 123; Siddique et al., 1988; Kalita et al., 1994; Rao, 1994), were selected to investigate the inheritance of dormancy in controlled and cultivated rice (O. sativa L.) is well suited for cloning conditions. Initial investigations using intact seeds, caryopses, caryopdormancy genes. Rice is a diploid, has a relatively small ses with pericarp/testa removed, and excised embryos demonstrated that seed dormancy was imposed by the hull in SS18-2 and TKN12-2, genome, is the base genome for comparative genetic and by the hull and pericarp/testa in LD and N22. Seed dormancy investigations in grasses, and many genetic and molecuat 0 d after harvest (DAH) was dominant with average degree of lar resources are publicly available (Gale et al., 1996). dominance (ADD) 0.8 in the crosses with weedy strains. Dominance Also, rice is the only species in the family Poaceae for for duration of seed dormancy was incomplete when judged by days which the draft genome sequences have been published, to 50% germination. Broad-sense heritability (hb) for seed germinaand a more detailed genome sequence will be available tion was lower at 0 DAH and highest at 20 DAH in all the crosses. soon (Goff et al., 2002; Yu et al., 2002). However, before The weedy strain-derived F2 populations maintained a higher h2 b during pursuing a cloning strategy for dormancy genes, it is afterripening. The effects of three and two major genes on seed germicritical to select appropriate gene donors in the species. nation at 20 DAH were detected in the SS18-2and N22-derived F2 There are two common ways by which seed dormancy populations, respectively. A positive ADD, a high h2 b, and major gene is imposed: seed coverings (e.g., pericarp, testa, and effect for caryopsis germination at 0 DAH were detected only in the in some cases the endosperm) and the embryo itself cross with LD. Seed or caryopsis dormancy was correlated with the (Bewley and Black, 1994, p. 199–230). In grasses, the characteristics awn and black hull or red pericarp colors in the SS18-2term seed is commonly used to describe the dispersal or LD-derived F2 populations. This research demonstrates that weedy unit (Simpson, 1990, p. 3–113). An intact rice seed has rice provides ideal gene resources to elucidate mechanisms of dora hard enclosure–hull outside the caryopsis. There is mancy and to improve resistance to preharvest sprouting. abundant evidence that seed dormancy in rice is imposed through the hull, pericarp/testa, or both (Roberts, 1961b; Seshu and Dadlani, 1991). The occurrence of D distributes germination with time and embryo dormancy in rice has been suggested (Takais critical for the survival of seed-bearing plants. hashi, 1962; Nair et al., 1965), but remains uncertain In agriculture, dormancy has considerable importance because some reports indicate that rice lacks embryo as it relates to preharvest sprouting and the persistence dormancy (Roberts, 1961b; Seshu and Sorrells, 1986; of weed seeds in the soil. Preharvest sprouting is germiSeshu and Dadlani, 1991). nation in the inflorescence after maturation of the crop, Rice geneticists have studied the inheritance of dorwhen moist conditions prevail or untimely rains occur. mancy to introduce the trait in elite cultivars to impart Resistance to preharvest sprouting is correlated with resistance to preharvest sprouting. Most genetic rethe level of dormancy in dry, mature seeds (Seshu and search focused on hull-imposed dormancy in cultivated Sorrells, 1986). Dormancy is the temporary failure of a rice. Chang and Yen (1969) and Chang and Tagumpay viable seed to germinate, after a specific length of time, (1973) concluded that the seed dormancy is a quantitain a particular set of environmental conditions that allow tive trait governed by polygenes with cumulative but germination after the restrictive state has been termiunequal effects and is strongly affected by environmennated by either natural or artificial conditions (Simpson, tal conditions during seed development. Heritability for 1990, p. 3–113). Germinability is used to describe the the trait was low (0.12 to 0.42) in their field experiments. capacity of seeds in a population with dormancy for Other researchers used a Mendelian method to identify immediate, intermediate, or much delayed germination dormancy genes from rice cultivars (Takahashi, 1962; due to the internal conditions (Foley, 2001). Despite Tripathi and Rao, 1982; Tomar, 1984; Das and Bhaduri, years of research on seed dormancy, mechanisms for 1985; Seshu and Sorrells, 1986; Shenoy, 1993; Das, 1995). the regulation of germinability are basically unknown These investigators developed a few putative models (Foley, 2001; Koornneef et al., 2002). Cloning and charconsisting of one or two major genes plus minor genes to explain the inheritance of dormancy in cultivars. More acterizing genes that directly regulate germinability recently, researchers have identified ≈20 putative quantitative trait loci for seed dormancy in rice using molecuX.-Y. Gu, Plant Sci. Dep., North Dakota State Univ., Fargo, ND 58105 lar markers (Wan et al., 1997; Lin et al., 1998; Cai and USA; Z.-X. Chen, Agricultural College, Yangzhou Univ., Yangzhou, Morishima, 2000). 225008 China; and M.E. Foley, USDA-ARS, Biosciences Research Lab., Fargo, ND 58105-5674, USA. Received 13 June 2002. *CorreAbbreviations: ADD, average degree of dominance; DAH, days after sponding author ([email protected]). harvest; DAI, days after imbibition; hb, broad-sense heritability; RH, relative humidity. Published in Crop Sci. 43:835–843 (2003).