Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2015
Publication Date: 7/21/2015
Publication URL: http://handle.nal.usda.gov/10113/61168
Citation: Gealy, D.R., Burgos, N.R., Yeater, K.M., Jackson, A.K. 2015. Outcrossing Potential between U.S. Blackhull Red Rice and Indica Rice Cultivars. Weed Science. 63:647-657. DOI: 10.1614/WS-D-14-00150.1.
Interpretive Summary: Weedy red rice is a major weed problem in rice in Arkansas and four other southern states. Natural outcrossing or cross-pollination between red rice and commercial rice cultivars can sometimes form new rice-red rice hybrid biotypes that are difficult for farmers to control in their rice fields. Indica rice, which is a type of rice more commonly grown in tropical regions of Asia, has now begun to be used in the U.S. in crop breeding programs and in organic rice production. However, we know very little about its potential for cross-pollinating with red rice plants that commonly infest U.S. rice farms. In a two-year study, we used DNA marker analysis to show that a red rice line with black-colored hulls outcrossed to four indica rice varieties at a rate averaging 0.0086% and ranging from a low of 0.002% for the variety TeQing to a high of 0.0173% for the variety '4484'. Red rice outcrossing to a commercial (non-indica) U.S. variety, Kaybonnet, was almost four times greater than it was for the indica varieties. The differences in outcrossing observed in our study appeared to occur primarily because varieties such as Kaybonnet flowered at about the same time as the red rice, whereas indica varieties such as TeQing, flowered many days later than the red rice. Outcrossing from flowers on the rice varieties to flowers on the red rice plants, which were much taller, did not occur in our studies. This was probably due to the 'uphill climb against gravity' that is required for rice pollen to reach the flowers on the taller red rice plants. Among all of the rice varieties tested, the seed production of red rice was highest when this weed was grown next to the Kaybonnet rice plants. This demonstrated that the indica varieties were better competitors against red rice than was this U.S. variety. Scores of different red rice biotypes with a diverse range of flowering times are known to infest southern U.S. rice fields. Thus, although the outcrossing from the particular red rice biotype used in this study to the indica varieties was less than that for the commercial U.S. variety, our experiments demonstrated that red rice plants which overlap in flowering time with indica rice varieties could be a new potential pollen source for outcrossing and the formation of rice-red rice hybrid weed biotypes.
Technical Abstract: Weedy red rice is a major weed pest of rice in the southern U.S. Outcrossing between red rice and commercial tropical japonica rice cultivars has resulted in new weed biotypes that further hinder the effectiveness of weed management. In recent years, indica rice has been used increasingly as a germplasm source for breeding and for reduced-input systems in the U.S., but little is known about its outcrossing potential with U.S. weedy red rice biotypes. In a two-year study, SSR marker analysis was used to show that blackhull red rice (PI 653424) outcrossing to four, late-maturing indica cultivars averaged 0.0086% and ranged from 0.002% for ‘TeQing’ to 0.0173% for ‘4484’ (PI 615022). Rates of outcrossing to a tropical japonica cultivar standard, ‘Kaybonnet’ (0.032%), were substantially greater than for the indicas. These differences in outcrossing were due largely to synchronization of flowering times between rice and red rice, with Kaybonnet and TeQing exhibiting the greatest and least synchronization, respectively. Outcrossing rates also may have been affected by rice-red rice flower density differences within the rice plots. Outcrossing from cultivated rice to the red rice (as pollen recipient), which was taller than all rice cultivars, was undetectable in these studies, and environmental conditions (like temperature, humidity, solar radiation, and rainfall) were not strongly correlated with the outcrossing rates observed. Grain yields of the original blackhull red rice line were greatest in the Kaybonnet plots demonstrating that the indica cultivars were superior competitors against this weed. Collectively, these results suggest that red rice biotypes which flower synchronously with rice cultivars are a potential source of pollen for outcrossing and gene flow in rice fields in the southern U.S.