Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2011
Publication Date: 10/1/2011
Publication URL: http://handle.nal.usda.gov/10113/60892
Citation: Gealy, D.R., Gealy, G.S. 2011. 13Carbon isotope discrimination in roots and shoots of major weed species of southern U.S. rice fields and its potential use for analysis of rice-weed root interactions. Weed Science. 59(4):587-600. Interpretive Summary: Measuring competitive interactions between rice and weed roots has been difficult in the past because roots of weed and crop become tightly intertwined. A method involving the naturally-occurring stable carbon isotope (13C) was previously developed to assess interactions between roots of barnyardgrass and rice, but the suitability of this approach for other troublesome rice weed species is not known. d13C, a measurement of the proportion of 13C and the predominant 12C isotope present in roots and leaves, was compared in rice and ten troublesome weed species. Rice root d13C levels averaged ~-28‰ indicating that these roots are highly 13C-depleted compared to the ‘tropical’ (i.e. C4 weed type) weed species, whose d13C levels ranged from -10‰ to -17‰. d13C values of roots and shoots were consistent year to year and in different environments. The findings indicate that numerous C4 weeds will be suitable for 13C root competition studies that me be useful in the development and evaluation of ‘weed-suppressive’ rice varieties
Technical Abstract: Assessing below ground plant interference in rice has been difficult in the past because thorough, accurate separation of the intertwined roots of weed and crop is extremely challenging. A d13C depletion method has been developed to assess interactions between roots of barnyardgrass and weed-suppressive rice in flooded fields, but the suitability of this approach for other rice weed species is not known. Thus, d13C (an expression of 13C:12C ratios) levels in roots and leaves of rice were compared to those of ten troublesome weed species grown in monoculture in greenhouse and/or field. Species included the C4 tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf signalgrass, fall panicum, and large crabgrass; the C4 sedge: yellow nutsedge; and the C3 weed species: red rice, gooseweed, and redstem. Rice root d13C levels averaged ~-28‰ indicating that these roots are highly 13C-depleted. Root d13C levels ranged from -12‰ to -17‰ among the tropical grass weeds, and were -10‰ in yellow nutsedge, indicating that these weed species were much less 13C-depleted than rice, were C4 plant types, and were suitable for 13C discrimination studies with rice. Among the C4 weed species, bearded sprangletop and yellow nutsedge were most and least 13C-depleted, respectively. d13C values for all species tested were strikingly consistent from year to year and in different environments. Root d13C levels in pot-grown plants averaged only 0.5‰ lower in a greenhouse than in the field. Typically, shoots of rice were more 13C-depleted than roots, whereas the reverse was true for the C4 weed species. These relationships between d13C levels in roots and shoots were consistent within species, suggesting the possibility of using shoot d13C values as a proxy for root d13C values in calibration curves if pure monoculture root standards are unavailable.