Factors Affecting the Outcrossing Rate between ClearfieldTM Rice and Red Rice (Oryza sativa)
Vinod K. Shivrain, Nilda R. Burgos, Marites A. Sales, Andy Mauromoustakos, David R. Gealy, Kenneth L. Smith, Howard L. Black, and Melissa Jia*
The commercialization of imazethapyr-resistant (ClearfieldTM, CL) rice in the southern United States has raised serious concernsabout gene flow to red rice, producing imazethapyr-resistant red rice populations. Our objectives were to determine the impact of planting date, CL cultivars, and red rice biotypes on outcrossing rate; and to investigate the relative contribution of flowering time of CL rice and red rice biotypes, together with air temperature and relative humidity (RH), on outcrossing rate. Field experiments wereconducted at Stuttgart, Rohwer, and Kibler, AR, from 2005 to 2007, at three or four planting times from mid-April to late May. ‘CL161’ (inbred cultivar) and ‘CLXL8’ (hybrid) rice were planted in nine-row plots, with red rice planted in the middle row. Twelve red rice biotypes were used. The flowering of red rice and CL rice, air temperature, and RH were recorded. Red rice seeds were collected atmaturity. To estimate outcrossing rate, resistance to imazethapyr was evaluated in subsequent years and confirmed using rice microsatellite markers. CLXL8 rice flowered 2 to 4 d earlier than CL161 rice, and flowering was completed within 1 wk in all plantings. The flowering duration of most red rice biotypes ranged from 4 to 17 d. Flowering synchrony of red rice biotypes and CL rice ranged from 0to 100% at different plantings. In general, CLXL8 had greater flowering overlap and higher outcrossing rate with red rice than did CL161 rice. The outcrossing rate of red rice biotypes ranged from 0 to 0.21% and 0 to 1.26% with CL161 and CLXL8 rice, respectively. The outcrossing rate differed within each planting date (P , 0.05). Outcrossing was generally lower in mid-May and late May than inmid-April and late April planting times. Flowering synchrony and outcrossing rate were not correlated (r2 , 0.01). Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature of . 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was , 54%, outcrossing was less (0.12%) than when RH was $ 54% (0.38%).With CLXL8 rice, a minimum RH of $ 54%, from mid-morning to noon, increased outcrossing with red rice. To fully understand the interaction effects of these factors on outcrossing with red rice, controlled experiments are needed. Nomenclature: Imazethapyr; Red rice, Oryza sativa L.; Rice, Oryza sativa L. ORYSA. Key words: Acetolactate synthase (ALS), biosafety, flowering synchrony, gene flow,herbicide-resistance, imazethapyrresistant rice, pollen flow, relative humidity.
Weedy red rice, a noxious weed in cultivated rice, can cost producers nearly $300 ha21 in economic losses (Burgos et al. 2008), mainly because of low harvest efficiency when cultivated rice lodges as a result of red rice infestation (personal observation), contamination of rice grain (Ottis et al. 2005), and rice yieldlosses from competition with red rice (Shivrain et al. 2009). Limited chemical control options are available for red rice in conventional rice culture (Burgos et al. 2006). The majority of red rice in the southern United States is traditionally controlled by crop rotation between rice and soybean. Recently, herbicide-resistant (HR) CL rice was released, which provides the option for selectivecontrol of red rice in rice (Burgos et al. 2008). Red rice is an annual species belonging to Oryza sativa. Oryza also includes other annual (Asian wild rice [O. nivara Sharma & Shavri], African wild rice [O. barthii A. Chev.], and black swamp rice [O. meridionalis Ng.]) as well as perennial (brownbeard rice [O. rufipogon Griff], long stamen brown rice [O. longistaminata A. Chev & Roehr.], and...