Author
 |
Gealy, David |
|
Submitted to: Government Publication/Report
Publication Type: Proceedings Publication Acceptance Date: 9/26/2005 Publication Date: 9/1/2006 Citation: Gealy, D.R. 2006. Dynamics of pollen dispersal and confinement in u.s. rice. Proceedings of Workshop on Confinement of Genetically Engineered Crops During Field Testing, Riverdate, MD, September 13-14, 2004. pp. 96-98. Interpretive Summary: Technical Abstract: Rice (Oryza sativa L.) is arguably, the single most important food crop in the world. In the U.S. it is grown primarily in lowland irrigated areas of Arkansas, Louisiana, Mississippi, Missouri, Texas, and California. Although the U.S. produces only about 3.5 million acres of rice it is typically the world’s third or fourth largest exporter of this crop. Rice has the smallest genome, genetically is the simplest of all major cereal crops (diploid with 12 pairs of chromosomes), and has served as a useful model for crop genomics research because its genes and gene functions have a high degree of applicability to the other cereal crops. Great improvements due to rice breeding, fertility research, mechanization, pest management research, and herbicide-based weed control have been achieved in the U.S. in the last half century. However, weeds remain a major limitation to optimum rice production and economic returns. Barnyardgrass and its close relatives are the most prevalent and economically significant weed species in U.S. rice. Growers apply herbicides to essentially 100% of all rice acres largely because of these weeds. In the southern U.S., weedy red rice (O. sativa L.) is considered to be the most troublesome weed, when present, because it acts like a crop mimic of rice and has traditionally been uncontrollable in rice. California cropping systems are essentially free from red rice due to a highly effective clean seed program and exclusive use of water seeding. Rice and weedy red rice are considered to be the same species and can readily intercross with one another. Several distinctive U.S. red rice plant types are common, especially awnless strawhull, awned blackhull, and awned strawhull. Most of these are tall-statured with medium-grain seed size and shape. Flowering dates for red rice types range from being slightly earlier than most commercial cultivars (often strawhull awnless types) to several weeks later than any modern cultivar (often blackhull awned types). Numerous other red rice types can also be found in low numbers, including short-statured long-grain types, and short-statured awned types, suggesting that these may have been derived originally from natural crosses between red rice and long-grain commercial rice (most prevalent rice in the southern U.S.) or between awned red rice and semi-dwarf commercial rice (introduced into the southern U.S. in the 1970s). Both transgenic (glufosinate-resistant and glyphosate-resistant) and non-transgenic (imidazolinone-resistant ‘Clearfield’ rice) herbicide-resistant rice cultivars have been developed in recent years. With the subsequent marketing of non-transgenic ‘Clearfield’ rice, outcrossing between herbicide-resistant and non-resistant rice and between rice and weedy red rice has been increasingly scrutinized. ‘Clearfield’ rice was first grown commercially in the U.S. in 2002 It has been rapidly adopted by growers due to effective red rice control, and was planted on about 15% and 25% of the rice area of the southern U.S. in 2004, and 2005, respectively. ‘Clearfield’ rice is not grown in California. To date no pest-resistant transgenic rice cultivars have been grown commercially in the U.S. Rice and red rice are primarily self-pollinating because most stigmas (female) are fertilized by pollen (male) produced in the same flower, and pollen shedding usually occurs slightly before or concurrent with flower opening. This is in stark contrast to corn which produces male and female organs on different flowers, easily facilitating cross pollination. Maximum outcrossing between adjacent rice and red rice plants appears to average about 0.2 to 0.7% under field conditions, based on a large number of published, controlled experiments. However, outcrossing is highly variable. In small-scale field tests, the apparent outcrossing rates are sometimes zero, espec |
