The use of herbicide-resistant (HR) Clearfield rice (Oryza sativa) to control weedy rice has increased in the past 12 years to constitute about 60% of rice acreage in Arkansas, where most U.S. rice is grown. To assess the impact of HR cultivated rice on the herbicide resistance and population structure of weedy rice, weedy samples were collected from commercial fields with a history of Clearfield rice. Panicles from each weedy type were harvested and tested for resistance to imazethapyr. The majority of plants sampled had at least 20% resistant offspring. These resistant weeds were 97 to 199 cm tall and initiated flowering from 78 to 128 d, generally later than recorded for accessions collected prior to the widespread use of Clearfield rice (i.e. historical accessions). Whereas the majority (70%) of historical accessions had straw-colored hulls, only 30% of contemporary HR weedy rice had straw-colored hulls. Analysis of genotyping-by-sequencing data showed that HR weeds were not genetically structured according to hull color, whereas historical weedy rice was separated into straw-hull and black-hull populations. A significant portion of the local rice crop genome was introgressed into HR weedy rice, which was rare in historical weedy accessions. Admixture analyses showed that HR weeds tend to possess crop haplotypes in the portion of chromosome 2 containing the ACETOLACTATE SYNTHASE gene, which confers herbicide resistance to Clearfield rice. Thus, U.S. HR weedy rice is a distinct population relative to historical weedy rice and shows modifications in morphology and phenology that are relevant to weed management.

Oryza sativa (weedy red rice), the same species as cultivated rice, is a serious problem in rice production worldwide. Seed dormancy contributes to its persistence. We determined the effect of germination temperature and after-ripening period on germination capacity (GC) of red rice seeds from Arkansas rice fields in three production zones. We also determined the gene diversity (GD) of dormancy-linked loci among selected populations. The germination behaviour was evaluated at three temperatures (1°C, 15°C and 35°C) and four after-ripening periods (0, 30, 60 and 90 days) in two independent experiments. Germination response to temperature and after-ripening time differed among and within populations in each production zone. Overall, populations from the Delta and Grand Prairie were more dormant than those from White River. Regardless of ecotype or production zone, incubation at 35°C (mean GC = 84-100%) favoured the germination of seeds after-ripened for 60 days. Germination of these seeds was most variable at suboptimal temperature (15°C), with mean GC ranging from 44 to 97%; at 1°C, none of the seeds germinated. Primary dormancy was released in the majority of populations after 90 days of after-ripening. Blackhull populations generally had lower mean GC than strawhull populations, regardless of temperature, and required longer after-ripening time to release dormancy. They also showed a higher inter-and intrapopulation variation in germination and after-ripening than strawhulls and had the highest gene diversity (GD = 0.55-0.58) among test populations. Nondormant strawhulls were most distant (D = 0.63) from dormant blackhulls. Ecotype influenced genotypic clustering more than the dormancy trait.

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

Recent de-domestication of weedy species - such as in California weedy rice - can involve trait combinations indistinguishable from the crop. This underscores the need for strict seed certification with genetic monitoring and proactive field inspection to prevent proliferation of weedy plant types. In established populations, tillage practice may affect weed diversity and persistence over time. © 2017 Society of Chemical Industry.

Rice (Oryza sativa L.) cultivars show impairment of growth in response to environmental stresses such as cold at the early seedling stage. Locally adapted weedy rice is able to survive under adverse environmental conditions, and can emerge in fields from greater soil depth. Cold-tolerant weedy rice can be a good genetic source for developing cold-tolerant, weed-competitive rice cultivars. An in-depth analysis is presented here of diverse indica and japonica rice genotypes, mostly weedy rice, for cold stress response to provide an understanding of different stress adaptive mechanisms towards improvement of the rice crop performance in the field. We have tested a collection of weedy rice genotypes to: 1) classify the subspecies (ssp.) grouping (japonica or indica) of 21 accessions; 2) evaluate their sensitivity to cold stress; and 3) analyze the expression of stress-responsive genes under cold stress and a combination of cold and depth stress. Seeds were germinated at 25°C at 1.5- and 10-cm sowing depth for 10d. Seedlings were then exposed to cold stress at 10°C for 6, 24 and 96h, and the expression of cold-, anoxia-, and submergence-inducible genes was analyzed. Control plants were seeded at 1.5cm depth and kept at 25°C. The analysis revealed that cold stress signaling in indica genotypes is more complex than that of japonica as it operates via both the CBF-dependent and CBF-independent pathways, implicated through induction of transcription factors including OsNAC2, OsMYB46 and OsF-BOX28. When plants were exposed to cold + sowing depth stress, a complex signaling network was induced that involved cross talk between stresses mediated by CBF-dependent and CBF-independent pathways to circumvent the detrimental effects of stresses. The experiments revealed the importance of the CBF regulon for tolerance to both stresses in japonica and indica ssp. The mechanisms for cold tolerance differed among weedy indica genotypes and also between weedy indica and cultivated japonica ssp. as indicated by the up/downregulation of various stress-responsive pathways identified from gene expression analysis. The cold-stress response is described in relation to the stress signaling pathways, showing complex adaptive mechanisms in different genotypes.

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Shelf-life and transplant success of sodded and sprigged turfgrasses are negatively affected by harvest and post-harvest handling/storage conditions. Research was conducted to evaluate effects of several commercially available fungicides, a bio-nutritional plant extract, and a commonly used plant growth regulator on sod shelf-life and transplant success. Field studies were conducted in 2018 and 2019 on 'Latitude 36' hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt Davy]. Treatments included: the experimental bio-nutritional plant extract ACA-3434; the fungicides fluxapyroxad + pyraclostrobin, fluopyram + trifloxystrobin, and azoxystrobin; and the plant growth regulator trinexapac-ethyl. Sod was harvested, stacked to a height of 12 layers, and stored for 72 h; treated and untreated sod were stored at ambient field temperature (28 • C average). To evaluate storage refrigeration on sod establishment, an additional untreated check was stored at 4 • C refrigeration. Sod was installed on an adjacent prepared native soil, and the treatment effects on bermudagrass transplanting were measured visually, by spectral reflectance, and by root analysis. Research failed to simulate heating inside stored sod masses, and treatments failed to affect internal storage temperatures. Nevertheless, refrigeration and, in some cases, fluxapyroxad + pyraclostrobin application, increased plant health characteristics of transplanted sod. None of the treatments affected recovery of areas from which sod was harvested. Albeit often unrealistic, refrigeration of harvested sod may be the best practice to extend sod shelf-life. When applied prior to harvest, fluxapyroxad + pyraclostrobin fungicides provided slight improvements in post-installation sod health; however, little is known about the mechanism by which this occurs. Abbreviations: CI-RE, chlorophyll index-red edge; DAI, days after installation; NDVI, normalized difference vegetative index; QoI, quinone outside inhibiting; RVI, simple ratio vegetative index; SDHI, succinate dehydrogenase inhibiting. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Core Ideas Preemergence herbicides may increase grow‐in time for hybrid bermudagrass sprigs. Preemergence herbicide treatments did not reduce sod tensile strength. Preemergence herbicides are viable options for weed control during establishment. Preemergence herbicides often negatively affect establishment of sprigged hybrid bermudagrass (Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt‐Davy). However, limited research has quantified the effects of preemergence herbicides on establishment and tensile strength of warm‐season turfgrass species commonly grown as sod in the southeastern United States. Field research was conducted at Mississippi State University in 2016 and 2017 to evaluate the effects of atrazine, atrazine + S‐metolachlor, dithiopyr, flumioxazin, indaziflam, liquid‐ and granular‐applied oxadiazon, S‐metolachlor, pendimethalin, prodiamine, and simazine on grow‐in time and tensile strength of newly sprigged ‘Latitude 36’ hybrid bermudagrass. Plots (4.65 m2) were sprigged at a rate of 44 m3 ha−1 (468 US bushels ac−1), and preemergence herbicide treatments were applied at the recommended labeled rates 1 d after planting. Dithiopyr, flumioxazin, S‐metolachlor, and indaziflam increased days to reach 50% hybrid bermudagrass cover in both 2016 and 2017. Normalized difference vegetative index and relative chlorophyll index were similarly affected. No herbicide treatments reduced sod tensile strength in either year; however, prodiamine, pendimethalin, dithiopyr, flumioxazin, and S‐metolachlor unexpectedly increased sod tensile strength in 2017.