Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/3/2009
Publication Date: 7/1/2009
Publication URL: handle.nal.usda.gov/10113/32084
Citation: Shivrain, V.K., Burgos, N.R., Salesa, M.A., Mauromoustakos, A., Gealy, D.R., Smith, K.L., Black, H.L., Jia, M.H. 2009. Factors affecting the outcrossing rate between Clearfield rice and red rice (Oryza sativa). Weed Science. 57:394-403. Interpretive Summary: Problem: The commercialization of imazethapyr-resistant (trade name ‘Clearfield’; CL) rice in the southern United States has raised concerns about the potential escape of the herbicide resistance gene into weedy red rice because the resulting imazethapyr-resistant red rice populations could be very difficult to manage. Thus, we determined the effects of planting date, CL variety, red rice biotype, overlapping flowering periods of rice and red rice biotypes, and air temperature and relative humidity (RH) on outcrossing rates. Accomplishment: Field experiments were planted in three different geographic locations at three or four dates from mid-April to late May. ‘CL161’ (inbred cultivar) or ‘CLXL8’ (hybrid) rice were planted in plots with one of 12 red rice biotypes. 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 were as high as 0.21% and 1.26% with CL161 and CLXL8 rice, respectively. Outcrossing was generally lower in May plantings than in April plantings. The greatest overlap of flowering times between red rice and rice did not necessarily result in the most outcrossing. Outcrossing with CL161 was primarily influenced by the particular red rice biotype used. An evening air temperature above 24 C favors outcrossing with CL161. With CLXL8 rice, a minimum RH of 54%, from mid-morning to noon, increased outcrossing with red rice. Contribution: This work resulted in important insights into the degree to which overlapping flowering periods and the temperature and relative humidity of air in rice fields can affect outcrossing rates between herbicide-resistant rice and red rice.
Technical Abstract: The commercialization of imazethapyr-resistant (Clearfield[TM], CL) rice in the southern United States has raised serious concerns about 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 were conducted 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 at maturity. 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 0 to 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. Outcrossing was generally lower in mid-May and late May than in mid-April and late April planting times. Flowering synchrony and outcrossing rate were not well correlated. Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature above 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was below 54%, outcrossing was less (0.12%) than when RH was 54% or above (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.