Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: January 15, 2009
Publication Date: February 1, 2009
Citation: Gealy, D.R. 2009. Outcrossing interactions of early-flowering strawhull and late-flowering blackhull red rice (Oryza sativa) with commercial rice varieties. Weed Science Society of America Meeting Abstracts. Vol 49: No.200.
Red rice continues to be a challenging weed pest in southern U.S. rice, in part due to the diversity of biotypes and its significant outcrossing potential with herbicide-resistant rice. The outcrossing interactions between red rice and true-breeding rice cultivars are generally well established, but little is known about such interactions with hybrid cultivars. A field experiment was planted at Stuttgart, AR on June 7, 2006 to evaluate outcrossing between early- (71 DAE) or late-flowering (84 DAE) red rice biotypes and four hybrid rice cultivars in comparison to true-breeding cultivar standards. The 18-row plots were 3.2-m wide by 4.6-m long and were isolated 3 to 5 m from adjacent plots. Red rice at a reduced planting density occupied 4 of the rows, each flanked by several rows of rice. This resulted in a heavy, uncontrolled red rice infestation (17%) intended to maximize natural outcrossing with rice. StgS (PI 653423) and #8 (PI 653425) red rice began flowering 3-6 days before and 4-10 days after hybrid rice flowering, respectively and flowering of rice and red rice overlapped about 1 week in most plots. In 2007, red rice and rice seeds harvested from plots in 2006 were planted in 12-m-long plots to identify putative red rice x rice hybrids. These were identified, either as survivors of three imazethapyr applications (for red rice seedlings originally obtained from imidazolinone-resistant rice plots), or by various phenotypic traits depending on the rice and red rice parents (e.g. tall plants, pubescent leaves, red pericarp, seed shape). DNA was extracted from leaves of putative crosses. True rice–red rice hybrids were confirmed using molecular markers (RM234, RM232, RM5, RM253, and Rid12) which differentiated between rice and red rice alleles. Outcrossing rates for the hybrid cultivars were typically greatest with rice serving as the female, averaging >5 times the rates in the reverse direction. Outcrossing rates also averaged >10 times more with StgS than with #8 as the red rice parent. With StgS red rice as the pollen donor, the outcrossing rate for only one of the hybrid cultivars tested was greater than that for a true-breeding standard, imidazolinone-resistant CL121 (1.1% vs. 0.35%). With rice as the pollen donor, however, outcrossing rates for three of the hybrid cultivars with StgS were greater than for the standard (0.20%, 0.34%, and 0.23% vs. 0.0045%). In the past, the impacts of outcrossing from true-breeding cultivars in red rice-infested fields have typically been greatest when non-shattering, herbicide-resistant cultivars were the predominant pollen donors for highly shattering red rice plants, which automatically reseeded the fields. Hybrid cultivars, however, have been observed to shatter more than traditional true-breeding cultivars in some situations. Thus, production of red rice hybrids from red rice as a major pollen donor could potentially increase if the use of shatter-prone hybrid cultivars continues.