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Weedy rice (Oryza spp.) is one of the most troublesome weeds affecting rice (Oryza sativa L.) production in many countries. Weedy rice control is difficult in rice fields, because the weed and crop are phenotypically and morphologically similar. Weedy rice can be a source of genetic diversity for cultivated rice. Thus, this study aimed to characterize the morphological diversity of weedy rice in southern Brazil. Qualitative and quantitative traits of 249 accessions from eight rice-growing mesoregions in Rio Grande do Sul (RS) and Santa Catarina (SC) states were analyzed. For each accession, 24 morphological descriptors (14 qualitative and 10 quantitative) were evaluated. All 249 accessions from RS and SC are of indica lineage. Considering all the phenotypic traits evaluated, the accessions separated into 14 distinct groups. One of the largest groups consisted of plants that were predominantly tall with green leaves, intermediate shattering, and variable flowering time. Distinct subgroups exist within larger clusters, showing discernible phenotypic diversity within the main clusters. The variability in flowering time was high (77 to 110 d after emergence), indicating high potential for flowering synchrony with rice cultivars and, consequently, gene flow. This indicates the need to remove escapes when planting herbicide-resistant rice. Thus, weedy rice populations in southern Brazil are highly diverse, and this diversity could result in variable response to weed management.

Background: Weedy rice (Oryza sativa L.) is the main weed of rice crop. The high genetic variability of weedy rice contributes to the high phenotypic diversity between biotypes and different responses to environmental stress. Objective: The present study aimed to evaluate the response of imidazolinone-susceptible and -resistant weedy rice populations to increased atmospheric [CO2]. Methods: The experiment was arranged in a complete randomized design with six replications. The treatments included two [CO2] concentration (700 and 400 μmol mol-1) and three treatments: resistant genotype (IMI-resistant) treated with imazethapyr; resistant genotype without imazethapyr, and a susceptible genotype without imazethapyr. Results: The IMI-resistant and -susceptible weedy rice responded similarly to [CO2] enrichment. Enhanced [CO2] increased competitive ability of the weedy rice populations tested, by means of increased plant height. Weedy rice seed production also increased with enhanced [CO2] by means of increased photosynthesis rate and reduced transpiration (increased water use efficiency). Increased seed production also means increased weed persistence as it increases the soil seedbank size. The application of imazethapyr on IMI-resistant weedy rice did not alter its response to [CO2]; conversely, increased [CO2] did not change the resistance level of weedy rice to imazethapyr. High [CO2] increased spikelet sterility, but this beneficial effect was negated by the overall increase in production of filled grains. Conclusions: Enhanced [CO2] concentrations increases weedy rice growth, photosynthesis rates, seed production and spikelet sterility; the imidazolinone application does not affect the response of weedy rice to enhanced [CO2] affects weedy rice response to imidazolinone herbicide.

The use of dietholate seed treatment, as well as the use of preemergence clomazone, can affect the selectivity of the imidazolinones chemical group of postemergence herbicides applied in crops Clearfield™. The aim was to evaluate the morphophysiological effects of a formulated mixture of imazapyr + imazapic applied in postemergence in combination with seed treatment with dietholate and applying in preemergence of clomazone on Avaxi CL rice hybrid cultivar. The experiment was conducted in the agricultural year 2012/13, in a greenhouse. The experimental design was completely randomized in a factorial design with four replications, in which factor A consisted of seed treatment with dietholate and applied clomazone herbicide in preemergence on rice and factor B consisted of eight doses of the mixture of imazapyr + imazapic herbicides. The results showed that the rice seed treatment with dietholate combined with the spray of herbicide clomazone in preemergence affects the selectivity of postemergence formulated mixture of imazapic + imazapyr applied on Avaxi CL rice. Physiological characteristics such as photosynthetic rate, transpiration rate and stomatal conductance on Avaxi CL hybrid are negatively affected. RESUMO: O uso de dietholate no tratamento de sementes, bem como a utilização de clomazone em pré-emergência, pode afetar a seletividade de herbicidas do grupo químico das imidazolinonas aplicados em pós-emergência em cultivares Clearfield®. O objetivo deste trabalho foi avaliar os efeitos morfofisiológicos da mistura formulada do herbicida imazapyr + imazapic aplicada em pós-emergência em combinação com o tratamento de sementes com dietholate e a aplicação em pré-emergência de clomazone no cultivar híbrido de arroz Avaxi CL. O experimento foi conduzido no ano agrícola de 2012/13, em casa de vegetação. O delineamento experimental utilizado foi inteiramente casualizado, em esquema fatorial, com quatro repetições, no qual o fator A foi composto pelo tratamento de sementes com dietholate e aplicação em pré-emergência do arroz do herbicida clomazone, e o fator B, por oito doses da mistura formulada dos herbicidas imazapyr + imazapic. Os resultados mostraram que o tratamento de sementes de arroz com dietholate combinado com a aplicação de clomazone em pré-emergência afeta a seletividade do herbicida composto pela mistura formulada de imazapyr + imazapic aplicada em pós-emergência no híbrido de arroz Avaxi CL. Características fisiológicas, como taxa fotossintética, taxa de transpiração e condutância estomática, no híbrido Avaxi CL são afetadas negativamente.

We aimed with this study to evaluate the effectsof residual activity in soil of formulated mixtures of the herbicidesimazethapyr + imazapic and imazapyr + imazapic on ryegrass and on rice cultivars, IRGA 424 and BRS Querencia. Two experiments wereconducted under greenhouse in randomized blocks design with four replications insplit-plot and split-split-plot designs, respectively for the first (E1) andsecond (E2)experiments. ForE1, main plots allocated herbicides (imazethapyr + imazapic andimazapyr + imazapic), and subplots the doses [(0, 0.5, 1.0, 1.5 and 2.0 foldthe label dose of imazethapyr + imazapic (1L·ha-1)and imazapyr + imazapic (140 g·ha-1)plus 0.5% of the adjuvant Dash?)]; for E2, sub-subplotsallocated rice cultivars (BRS 424 and IRGA Querencia). Phytotoxicity, plantheight and shoot dry weight (E1 and E2) and plant stand (E2) were evaluated. Results showed thatthe annual ryegrass and rice cultivars IRGA 424 and BRS Querencia are sensitiveto imazethapyr + imazapic and imazapyr + imazapic, serving as bioindicators ofits residual activity in soils following Clearfield? rice. Imazapyr+ imazapic, applied post-emergence in irrigated rice at doses of 1.0, 1.5 and2.0× the label dose, present longer residualactivity in soil compared to imazethapyr + imazapic for ryegrass andnon-Clearfield? rice.