Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2006
Publication Date: January 22, 2007
Citation: Gealy, D.R., Black, H.L., Moldenhauer, K.K. 2007. A multi-year comparison of rice-barnyardgrass root interactions in weed suppressive and non-suppressive rice cultivars using stable 13c isotope analysis. Proceedings of Southern Weed Science Society Vol. 60. Technical Abstract: Weed-suppressive (primarily indica-based) rice cultivars can aid with the control of barnyardgrass (Echinochloa crus-galli) more effectively than do traditional long-grain rice cultivars in drill-seeded rice systems in the southern U.S. In earlier tests, weed-suppressive cultivars often have reduced the aboveground biomass and/or population density of barnyardgrass (as well as % visual control) and produced high rice grain yields. However, little is known about the underground interactions of barnyardgrass and rice roots in these systems, in part because of the technical difficulties in efficiently extracting these roots from soil and separating them. Thus, experiments were conducted in the field in 2001, 2002, 2003, and 2004 to estimate the relative levels of barnyardgrass and rice roots extracted from field plots that had been planted to two or three suppressive (PI 312777, Teqing, or CL XL8) or non-suppressive (Kaybonnet, Lemont, or Francis) rice cultivars and over seeded with barnyardgrass. The experiment was a randomized complete block design with three replications. Rice was drill-seeded using a nine-row planter with nine rows and 18 cm between rows. Plot size was approximately 2 m by 3 m. Visual control ratings for barnyardgrass (above-ground) were recorded mid- to late-season after barnyardgrass had reached physiological maturity. Four soil cores (10-cm diam. by 15-cm deep) were randomly sampled from between rice rows in the middle part of each plot near harvest time. A mixture of rice and barnyardgrass roots was extracted from the soil cores using water pressure and agitation, and captured on a screen as a single root mass. Roots were dried to a constant dry weight, weighed, and ground to a fine powder. C3 plants (e.g. rice) and C4 plants (e.g. barnyardgrass) discriminate differentially against photosynthetic fixation of a naturally-occurring stable isotope, 13C. Thus, 13C isotope depletion analysis was used to estimate the proportion of rice and barnyardgrass roots present in samples. Standard concentration curves were generated each year by mixing known ratios of barnyardgrass and rice roots that had been extracted from pure stands of these species. The rice:barnyardgrass root ratios in the non standard root samples were extrapolated from the standard curves. The most suppressive rice cultivars (PI 312777 or CL XL8) provided greater above-ground suppression (visual control ratings) of barnyardgrass than did the non-suppressive cultivars, Kaybonnet or Lemont, in all four years. Barnyardgrass root densities usually were greater in non-suppressive than in suppressive rice plots, consistent with the results from the visual ratings of above ground weed control. In years with heavy barnyardgrass infestation pressure, the visual control levels were generally lower than in the years with lighter weed pressures and the barnyardgrass root densities tended to be inversely related to these trends. Breeding selections (RU 9701151 or STG96L-26-093) developed from crosses between commercial and suppressive cultivars generally did not reduce root densities or visual control of barnyardgrass plants to the same degree as did the suppressive cultivars. Rice root densities obtained from these soil cores were similar for the suppressive and non-suppressive cultivars. This was somewhat unexpected because previous experiments have indicated that suppressive rice lines typically produce greater root mass than non-suppressive commercial cultivars. Overall, our results demonstrated the utility of using 13C isotope depletion analysis to quantify root interactions between barnyardgrass and rice and to relate these to above-ground plant interactions in the field. Due to the substantial time commitment and costs involved in extraction, preparation, and analysis of samples for this isotope analysis method, the scope and objectives of potential experiments must be considered judiciously.