Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: January 7, 2005
Publication Date: March 23, 2005
Citation: Gealy, D.R. 2005. Gene movement between rice (Oryza sativa) and weedy rice (Oryza sativa): a U.S. temperate rice perspective. In: Gressel, J. editor.Crop Ferality and Volunteerism. Boca Raton, Florida: CRC Press. p. 323-354. Technical Abstract: The rice industry in the U.S. is young by world standards, and weedy (red) rice (Oryza sativa) poses serious threats to rice in the southern U.S. through its competitiveness and its ability to reduce the economic value of milled rice. Red rice may become a further threat to herbicide-resistant rice systems because rice and red rice (O. sativa) are the same species and can intercross with each other, potentially resulting in gene flow. This crossing can be potentially damaging in rice cropping systems through development of herbicide-resistant red rice or volunteer rice plants that remain resistant to herbicides in rotational crops. Outcrossing may also affect ecosystems outside of the rice production fields if sufficient numbers of these crosses could move or develop offsite and establish self sustaining feral populations. This manuscript addresses production practices and weed problems in temperate U.S. rice systems; the development and management considerations for newly developed herbicide-resistant rice cultivars; the frequency, causes and consequences of outcrossing between rice and red rice; some useful visual traits and DNA markers that can aid in identification of crosses; the ease with which traits such as seed shattering and dormancy can be inherited, thus enhancing the potential weediness of crosses; and several practical examples of the natural outcrossing that has occurred between these species. Taking into consideration numerous studies conducted over many years, the maximum outcrossing rates between rice and red rice were usually less than 0.2%, were greatest when flowering periods were synchronized, were usually greatest in the direction of prevailing winds, were usually undetectable (essentially zero) at distances greater than 10 m; and were often greatest when pollen was transferred from the anthers of taller red rice plants to the stigmas of the shorter rice plants. Potential gene flow problems in the U.S. may be mitigated somewhat because nearly all rice and weedy relatives of rice in the U.S. are confined to agricultural fields, first generation crosses can be far less fertile than normal red rice, and wild relatives such as O. rufipogon, which have much greater outcrossing rates, are not present in the U.S.