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Weedy red rice (Oryza sativa) is a problematic weed in cultivated rice. About 50% of US rice is produced in Arkansas and 60% of these fields have some red rice infestation. Red rice populations are morphologically and phenologically diverse. We hypothesise that red rice in Arkansas has high genetic diversity, which underlies its wide phenotypic diversity, and that some alleles from cultivated rice have been introgressed into red rice during more than a century of coexistence. We tested 137 red rice accessions from four ecological zones in Arkansas and 36 cultivars that have been grown in Arkansas in the past century. Twenty-seven rice microsatellite primers, distributed across 12 chromosomes, were used to generate molecular markers. The overall NeiÕs genetic distance (GD) of red rice accessions was 0.70. Rice grown in the last century had an overall GD of 0.26. The awnless strawhull red rice was genetically distant from blackhull (GD = 0.55) and brownhull (GD = 0.60) red rice types. NeiÕs GD between blackhull and brownhull red rice was 0.42. Brownhull and blackhull formed one genotypic cluster, whereas the majority of strawhull red rice formed another cluster. Within blackhull red rice, the GD was 0.76, whereas for awnless strawhull it was 0.68, 0.75 for awned strawhull and 0.80 for brownhull types. The gene diversity of blackhull and strawhull correlated with zone of origin. A quarter of the red rice accessions share common alleles with cultivated rice. A diverse complex of weedy populations has evolved in a region devoid of other weedy and wild Oryza species.

The commercialization of imazethapyr-resistant (Clearfield™, 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 (P < 0.05). 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 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.

Red rice plants are vectors of gene flow back to cultivated rice and other weedy populations. The progeny of red rice hybrids from cultivated rice mother plants have higher chances of persistence than those from red rice mother plants. Gene flow mitigation strategies should consider this scenario.

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