The transfer of the imazethapyr-resistant gene from Clearfield™ (CL) rice to red rice is an ecological risk. Flowering synchronization and genetic compatibility between cultivated rice and red rice could influence gene transfer. We examined the (1) variability in maximum outcrossing rate between 12 red rice biotypes and ‘CL161’ rice during their peak flowering overlap in the field and (2) genetic compatibility of red rice biotypes with CL161 rice. Experiments were conducted at Stuttgart, AR, and Fayetteville, AR, from 2005 to 2007. To evaluate the flowering synchrony of red rice and CL161 rice as well as its impact on outcrossing rate, field experiments were conducted at four planting times from early April to late May. The red rice biotypes were planted in the middle row of nine-row CL161 plots and flowering was monitored. Outcrosses were evaluated in subsequent years by herbicide response and simple-sequence-repeat marker assays. To determine compatibility, manual crosses were performed between 12 red rice biotypes and CL161 rice in the greenhouse. The flowering duration of all red rice types ranged from 5 to 16 d after the onset of flowering in contrast to 6 d in CL161 rice. Ten of the twelve types of red rice had ≥ 70% overlap in flowering time with CL161 rice in at least one planting date. The maximum field outcrossing rate between red rice biotypes and CL161 ranged from 0.03 to 0.25%. The field outcrossing rate between red rice biotypes differed (P < 0.01), but flowering synchronization was not directly related to outcrossing rate. Manual crosses resulted in seed sets of 49 to 94%. The majority of red rice biotypes had similar compatibility with CL161 rice. Thus, other factors must contribute to hybridization rates in the field. Follow-up experiments should investigate other plant factors and environmental influence on hybridization rate.
Nomenclature: Imazethapyr; red rice, Oryza sativa L. ORYSA; rice Oryza sativa ‘CL161’.