Search

Late watergrass is a competitive weed of rice that is well adapted to both aerobic and anaerobic environments. Cultural controls such as a stale-seedbed and alternating from wet- to dry-seeding have been proposed as management options. However, the effects of these systems on its emergence and early growth are unknown. The objective of this study was to modify a previously developed population-based threshold model (PBTM) to predict emergence and early growth under field conditions. In 2013, a series of experiments were conducted at the California Rice Experiment Station (CRES) in Biggs, CA, to evaluate emergence and early growth of multiple herbicide–resistant and -susceptible late watergrass at four burial depths (0.5, 2, 4, and 6 cm) under three irrigation regimes: continuously flooded (CF), daily flush (DF), and intermittent flush (IF). Resistant plants emerged at a significantly higher rate under the IF treatment (P < 0.05). Both biotypes showed decreasing emergence with increasing depth, and no plants emerged from the 4- or 6-cm depths in the CF treatment. Using the Gompertz growth curve, resistant plants had greater predicted growth rates (k), lower predicted maximum heights (hmax), and a shorter time to predicted maximum growth rate (tm) than susceptible plants under the CF and DF treatments. Under the IF treatment, the susceptible plants had greater k, lower hmax, and shorter time to predicted tm. Information about burial depth and irrigation was incorporated into a previously developed PBTM for late watergrass, and validated at the CRES in a field with a susceptible late watergrass population in 2013 and 2014, under two irrigation systems, CF and IF. Model fit was best in the CF treatments (average Akaike information criteria [AIC] = 199.05) compared to the IF treatments (average AIC = 208.6).

The repetitive use of ALS inhibitors for smallflower umbrella sedge (Cyperus difformis L.) control has selected for herbicide-resistant (R) populations that threaten the sustainability of rice (Oryza sativa L.) production and demand alternative control measures be developed. A better understanding of seedling recruitment patterns at the field level is required to optimize the timing and efficacy of control measures. Therefore, a population-based threshold model was developed for optimizing germination prediction in multiple acetolactate synthase (ALS)-R and ALS-susceptible (ALS-S) C. difformis biotypes and applied to field-level emergence predictions. Estimated base temperatures (Tb) ranged from 16.5 to 17.6 C with no clear pattern between biotypes; such values are higher than Tb values of other important rice weeds, as well as for rice. Germination rates increased linearly from 16 to 33.7 C. ALS-R seeds germinate faster due to smaller median thermal times to germination (θT(50)) while also displaying lower germination synchronicity across water potentials. Interestingly, ALS-R biotypes were capable of germinating under lower moisture availability, as indicated by their lower (more negative) base water potential values (Ψb(50)) for seed germination; Ψb(50) values ranged from −0.24 to −1.13 MPa. In-field soil germination measurements found thermal times to emergence varied across three water regimes (daily water, flooded, or saturated). Seedling emergence under the daily water treatment was fastest; however, total seedling density was lower than for the other water regimes. In order to optimize springtime C. difformis seedling emergence, soil moisture should be kept around field capacity, as germination is hindered at lower moisture contents. By predicting when most of the seed population germinates, the thermal-time model can address issues regarding the optimal timing for herbicide applications, thereby allowing for improved C. difformis management in rice fields.

Author(s): Morran, Sarah; Moretti, Marcelo L; Brunharo, Caio A; Fischer, Albert J; Hanson, Bradley D | Abstract: BACKGROUND:In California specialty cropping systems such as vineyards and orchards, Echinochloa colona is present as a summer annual weed. It is able to germinate throughout the growing season whenever favorable conditions are present, and management relies heavily on glyphosate applications. Glyphosate-resistant (GR) E. colona biotypes are present in the state, but the levels of resistance observed suggest that there may be differences in mechanisms of resistance among populations. RESULTS:Echinochloa colona lines collected from different regions of California's Central Valley presented resistance levels ranging from 1.4 to 4.3-fold compared to susceptible lines. No differences in the absorption and translocation of [14 C]-glyphosate were observed among lines. Resistant lines accumulated eight-fold less shikimic acid after treatment with 435 and 870 g a.e. ha-1 glyphosate compared to the most susceptible line. Sequencing of a region of the EPSPS gene revealed three single nucleotide changes leading to amino acid substitutions at Proline 106, including Pro106Leu, Pro106Thr and Pro106Ser. CONCLUSION:These results indicate that an altered target site in EPSPS is contributing to resistance in these lines and resistance has evolved independently, multiple times in the Central Valley of California. Additional research is needed to further understand the genomic contributions of resistance loci in this polyploid weed species. © 2018 Society of Chemical Industry.

Author(s): Kanapeckas, Kimberly L; Tseng, Te-Ming; Vigueira, Cynthia C; Ortiz, Aida; Bridges, William C; Burgos, Nilda R; Fischer, Albert J; Lawton-Rauh, Amy | Abstract: Weed evolution from crops involves changes in key traits, but it is unclear how genetic and phenotypic variation contribute to weed diversification and productivity. Weedy rice is a conspecific weed of rice (Oryza sativa) worldwide. We used principal component analysis and hierarchical clustering to understand how morphologically and evolutionarily distinct US weedy rice populations persist in rice fields in different locations under contrasting management regimes. Further, we used a representative subset of 15 sequence-tagged site fragments of expressed genes from global Oryza to assess genome-wide sequence variation among populations.Crop hull color and crop-overlapping maturity dates plus awns, seed (panicle) shattering (g 50%), pigmented pericarp and stature variation (30.2% of total phenotypic variance) characterize genetically less diverse California weedy rice. By contrast, wild-like hull color, seed shattering (g 50%) and stature differences (55.8% of total phenotypic variance) typify genetically diverse weedy rice ecotypes in Arkansas.Recent de-domestication of weedy speciesn-nsuch as in California weedy ricen-ncan 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.

Cyperus difformis L. (CYPDI) and Schoenoplectus mucronatus (L.) Palla (SCHMU) are major weeds of California (CA) rice, where resistance to acetolactate synthase (ALS)-inhibitors was identified in several CYPDI and SCHMU populations that have also evolved resistance to photosystem II (PSII)-inhibiting herbicides. The mechanism of ALS resistance in these populations remains to be clarified but this information is crucial in a weed management program, especially in a scenario where resistance to multiple herbicides has been identified. ALS activity assays are commonly used to diagnose resistance to ALS-inhibitors, but protocols currently available are burdensome for the study of CYPDI and SCHMU, as they require large amounts of plant material from young seedlings and have low yields. Our objective was to investigate the ALS resistance mechanism in suspected ALS-resistant (R) CYPDI and SCHMU biotypes using a modified ALS activity assay that requires less plant material. ALS enzymes from suspected R biotypes were at least 10,000-fold less sensitive to bensulfuron-methyl than susceptible (S) cohorts, indicating ALS resistance that is likely due to an altered target-site. Protein concentration (mgg-1 tissue) did not differ between R and S biotypes within each species, suggesting that R biotypes do not over produce ALS enzymes. CYPDI biotypes had up to 4-fold more protein per mg of tissue than SCHMU biotypes, but up to 7-fold more acetoin per mg-1 protein was quantified in SCHMU, suggesting greater ALS catalytic ability in SCHMU biotypes, regardless of their herbicide resistance status. Our optimized protocol to measure ALS activity allowed for up to a 3-fold increase in the number of assays performed per g of leaf tissue. The modified assay may be useful for measuring ALS activity in other weed species that also produce small amount of foliage in early growth stages when protein in tissue is most abundant.

Determining the mechanisms of herbicide resistance in weeds allows for the development and implementation of applied management practices aimed to control and to prevent further spread of herbicide-resistant populations in crop fields. This research was conducted to determine propanil resistance and cross-resistance to other photosystem II (PSII) inhibitors in ricefield bulrush biotypes and to elucidate the mechanism of propanil resistance. To this end, propanil-resistant (R) and propanil-susceptible (S) biotypes were selected from field-collected populations after propanil spraying at the field rate, and whole-plant, dose–response experiments were conducted to evaluate cross-resistance to PSII inhibitors and interactions between propanil and the insecticides malathion and carbaryl. In addition, thepsbAgene from R and S biotypes was sequenced for amino acid alterations following polymerase chain reaction (PCR) amplification. Plant survival data indicated the R biotype displayed a 14-fold increase in propanil resistance relative to the susceptible (S) biotype. In addition, the propanil-R biotype also had increased resistance to the PSII-inhibitors bromoxynil, diuron, and metribuzin but was more susceptible to bentazon than were propanil-S plants. Synergism between propanil and the insecticides carbaryl and malathion was greater in the S biotype than it was in the R biotype, indicating that, unlike propanil resistance in weedy grasses, enhanced degradation of the herbicide molecule is not a mechanism of resistance for propanil in ricefield bulrush. A Val219to Ile substitution in the propanil-R chloroplast D1 protein was identified following sequencing of thepsbAgene. This research suggests a single-point mutation at the target site causes resistance to propanil, diuron, metribuzin, and bromoxynil but increasing susceptibility to bentazon in propanil-R ricefield bulrush, a novel Val219–Ile feature. To our knowledge, this is the first instance of propanil resistance in weeds because of a mechanism other than enhanced herbicide metabolism. Tank-mixing bentazon and propanil, where permitted, can control both propanil-R and propanil-S biotypes.

Smallflower umbrella sedge is a prolific C3weed commonly found in rice fields in 47 countries. The increasing infestation of herbicide-resistant smallflower umbrella sedge populations threatens rice production. Our objectives for this study were to characterize thermal requirements for germination of smallflower umbrella sedge seeds from rice fields in California and to parameterize a population thermal-time model for smallflower umbrella sedge germination. Because the use of modeling techniques is hampered by the lack of thermal-time model parameters for smallflower umbrella sedge seed germination, trials were carried out by placing field-collected seeds in a thermogradient table set at constant temperatures of 11.7 to 41.7 C. Germination was assessed daily for 30 d, and the whole experiment was repeated a month later. Using probit regression analysis, thermal time to median germination [θT(50)], base temperature for germination (Tb), and SD of thermal times for germination [σθT(50)] were estimated from germination data, and model parameters were derived using the Solver tool in Microsoft Excel®. Germination rates increased linearly below the estimated optimum temperatures of 33.5 to 36 C. EstimatedTbaveraged 16.7 C, whereasθT(50)equaled 17.1 degree-days and σθT(50)was only 0.1 degree-day. The estimatedTbfor smallflower umbrella sedge is remarkably higher than that ofjaponicaandindicatypes of rice, as well asTbof important weeds in theEchinochloacomplex. Relative to the latter, smallflower umbrella sedge has lower thermal-time requirements to germination and greater germination synchronicity. However, it would also initiate germination much later because of its higherTb, given low soil temperatures early in the rice growing season in California. When integrated into weed growth models, these results might help optimize the timing and efficacy of smallflower umbrella sedge control measures.

Summary Timely detection of herbicide resistance at an early stage of crop cultivation is essential to help farmers find alternative solutions to manage herbicide resistance in their fields. In this study, maximum quantum yield of PS II [Fv/Fm = (Fm – Fo)/Fm] was measured at the 4–5 leaf stage to discriminate between herbicide-resistant and susceptible biotypes of Echinochloa species. The differences in Fv/Fm between herbicide-resistant and susceptible Echinochloa spp. were consistent with the whole-plant assay based on I50 (herbicide doses causing a 50% inhibition of Fv/Fm) and GR50 (herbicide doses causing a 50% reduction in plant fresh weight) values and R/S ratios (herbicide resistance index), regardless of the mode of action of the tested herbicides. A PS II inhibitor caused the fastest inhibition of Fv/Fm, compared with ACCase and ALS inhibitors, after herbicide treatment. The required time for discrimination between herbicide-resistant and susceptible Echinochloa spp. was 64 h after PS II inhibitor treatment, much shorter than those of ACCase and ALS inhibitor-treated plants, which required 168 and 192 h respectively. The leaf chlorophyll fluorescence assay provided reliable diagnostics of herbicide resistance in Echinochloa spp. with significant time savings and convenient measurement in field conditions compared with the conventional whole-plant assay.

BACKGROUND Propanil-resistant (R) Cyperus difformis populations were recently confirmed in California rice fields. To date, propanil resistance in other weed species has been associated with enhanced aryl acylamidase (AAA)-mediated propanil conversion into 3,4-dichloroaniline. Our objectives were to determine the level of propanil resistance and cross-resistance to other PSII inhibitors in C. difformis lines, and to elucidate the mechanism of propanil resistance. RESULTS The propanil-R line had a 14-fold propanil resistance and increased resistance to bromoxynil, diuron and metribuzin, but not to atrazine. The R line, however, displayed a fourfold increased susceptibility to bentazon. Interestingly, susceptible (S) plants accumulated more 3,4-dichloroaniline and were more injured by propanil and carbaryl (AAA-inhibitor) applications than R plants, suggesting that propanil metabolism is not the resistance mechanism. psbA gene sequence analysis indicated a valine-219-isoleucine (Val219 Ile) amino acid exchange in the propanil-R chloroplast D1 protein. CONCLUSION The D1 Val219 Ile modification in C. difformis causes resistance to propanil, diuron, metribuzin and bromoxynil but increased susceptibility to bentazon, suggesting that the Val219 residue participates in binding of these herbicides. This is the first report of a higher plant exhibiting target-site propanil resistance. Tank mixing of bentazon and propanil, where permitted, can control both propanil-R and propanil-S C. difformis and prevent the spread of the resistant phenotype. © 2016 Society of Chemical Industry.

In California specialty cropping systems such as vineyards and orchards, Echinochloa colona is present as a summer annual weed. It is able to germinate throughout the growing season whenever favorable conditions are present, and management relies heavily on glyphosate applications. Glyphosate-resistant (GR) E. colona biotypes are present in the state, but the levels of resistance observed suggest that there may be differences in mechanisms of resistance among populations.Echinochloa colona lines collected from different regions of California's Central Valley presented resistance levels ranging from 1.4 to 4.3-fold compared to susceptible lines. No differences in the absorption and translocation of [These results indicate that an altered target site in EPSPS is contributing to resistance in these lines and resistance has evolved independently, multiple times in the Central Valley of California. Additional research is needed to further understand the genomic contributions of resistance loci in this polyploid weed species. © 2018 Society of Chemical Industry.

Johnsongrass is a common weed of corn in Chile, which is most often controlled by nicosulfuron, an acetohydroxyacid synthase (AHAS)-inhibiting herbicide. Recurrent nicosulfuron use has resulted in selection for resistant johnsongrass biotypes. We conducted studies to determine nicosulfuron resistance levels in two johnsongrass biotypes from Chile and to investigate if this resistance was target-site mediated. Whole-plant resistance to nicosulfuron was 33 and 46 times higher in resistant (R) than in susceptible (S) plants grown from seed and rhizomes, respectively. The nicosulfuron concentrations for 50% inhibition of AHAS enzyme activity in vitro were more than 11 times higher in R than in S plants. Sequencing analysis of theAHAScoding sequence revealed a Trp-574-Leu substitution in both R biotypes. This study shows that resistance to nicosulfuron in the two R biotypes is conferred by an altered target site. We also report the first consensus sequence of the johnsongrassAHASgene corresponding to the known mutation sites conferring resistance to AHAS-inhibiting herbicides.

Weed evolution from crops involves changes in key traits, but it is unclear how genetic and phenotypic variation contribute to weed diversification and productivity. Weedy rice is a conspecific weed of rice (Oryza sativa) worldwide. We used principal component analysis and hierarchical clustering to understand how morphologically and evolutionarily distinct US weedy rice populations persist in rice fields in different locations under contrasting management regimes. Further, we used a representative subset of 15 sequence-tagged site fragments of expressed genes from global Oryza to assess genome-wide sequence variation among populations.Crop hull color and crop-overlapping maturity dates plus awns, seed (panicle) shattering (50%), pigmented pericarp and stature variation (30.2% of total phenotypic variance) characterize genetically less diverse California weedy rice. By contrast, wild-like hull color, seed shattering (50%) and stature differences (55.8% of total phenotypic variance) typify genetically diverse weedy rice ecotypes in Arkansas.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.

© Weed Science Society of America, 2016. Over the last 10 yr, California has experienced a series of ever-worsening droughts. Rice, traditionally a flooded crop, has come under increasing scrutiny with respect to its water use, leading to proposals to evaluate alternative irrigation systems. For growers, weed competition is one of the most limiting factors to maintaining high yields, so understanding the shifts among species in weed communities under the proposed alternative irrigation systems is vital. A field study was conducted from 2012 to 2014 to compare weed population and growth dynamics with three irrigation systems: (1) a conventional water-seeded control system (WS-Control), with a permanent flood of 10 to 15 cm from planting until 1 mo prior to harvest; (2) a water-seeded alternate wet and dry system (WS-AWD), with the field flooded from planting until canopy closure, after which floodwater was allowed to subside and the field was reflooded when the soil volumetric water content reached 35%; and (3) a drill-seeded alternate wet and dry system (DS-AWD), with rice drill seeded and then flush irrigated to establish the crop, after which the field was flooded until canopy closure and then underwent an alternate wet and dry (AWD) treatment similar to WS-AWD. In the AWD treatments, there were two drying periods, neither of which occurred after the heading stage. The dynamics of major weed species were evaluated using plant density counts (2012) and relative cover and biomass (2013 and 2014). Grasses (sprangletop and watergrass species) dominated the DS-AWD system; sedges, broadleaves, and grasses dominated both WS systems. The WS-AWD system increased smallflower umbrella sedge relative cover at canopy closure, relative dry weight at harvest, and percent frequency when compared with the WS-Control system. Yields did not differ across treatments when weeds were controlled (P > 0.05); in the absence of herbicides, yields in the WS-AWD were equivalent to the WS-Control (ranging from 40 to 65% of the herbicide-treated yields) and zero in the DS-AWD due to weed pressure. Nomenclature: bearded sprangletop, Leptochloa fusca (L.) Kunth N. Snow; ducksalad, Heteranthera rotundifolia (Kunth) Griseb.; redstem, Ammannia coccinea Rottb.; ricefield bulrush, Schoenoplectus mucronatus (L.) Palla; smallflower umbrella sedge, Cyperus difformis L.; rice (Oryza sativa L.).

Invasive and endemic weeds pose recurring challenges for California land managers. The evolution of herbicide resistance in several species has imposed new challenges in some cropping systems, and these issues are being addressed by UC Cooperative Extension farm advisors, specialists and faculty. There are currently 24 unique herbicide-resistant weed biotypes in the state, dominated by grasses and sedges in flooded rice systems and, more recently, glyphosate-resistant broadleaf and grass weeds in tree and vine systems, roadsides and glyphosate-tolerant field crops. Weed scientists address these complex issues using approaches ranging from basic physiology and genetics research to applied research and extension efforts in grower fields throughout the state. Although solutions to herbicide resistance are not simple and are affected by many biological, economic, regulatory and social factors, California stakeholders need information, training and solutions to address new weed management problems as they arise. Coordinated efforts conducted under the Endemic and Invasive Pests and Disease Strategic Initiative directly address weed management challenges in California's agricultural industries.

BACKGROUND: Echinochloa colona is an annual weed affecting field crops and orchards in California. An E. colona population carrying a mutation in the EPSPS gene endowing resistance to glyphosate, the most widely used non-selective herbicide, was recently identified in the Northern Sacramento Valley of California. Plants from this population, from a suspected glyphosate-resistant (GR) population, and from one susceptible (S) population collected in the Northern Sacramento Valley of California, were used to generate three GR and one S selfed lines to study possible mechanisms involved in glyphosate resistance. RESULTS: Based on the amount of glyphosate required to kill 50% of the plants (LD50), GR lines were 4–9-fold more resistant than S plants and accumulated less shikimate after glyphosate treatment. GR and S lines did not differ in glyphosate absorption, translocation or metabolism. A different target-site mutation was found in each of two of the GR lines corresponding to Pro106Thr and Pro106Ser substitutions; the mutations were found in different homoeologous EPSPS genes. No mutation was found in the third GR line, which exhibited 1.4-fold higher basal EPSPS activity and a fivefold greater LD 50 than S plants. Quantitative RT-PCR revealed that GR lines had similar or lower EPSPS expression than S plants. CONCLUSION: It is demonstrated that individuals with different glyphosate resistance mechanisms can coexist in the same population, individuals from different populations may carry different resistance mechanisms and different mechanisms can act in concert within single E. colona plants. However, other plant factors or resistance mechanisms appear to modulate plant expression of EPSPS sensitivity to glyphosate. © 2014 Society of Chemical Industry Supporting information may be found in the online version of this article.

Cyperus difformis L. is one of the worst weeds of rice world-wide and has evolved resistance to acetolactate synthase (ALS)-inhibiting herbicides in rice fields of California. Propanil use was intensified to control the widespread resistant biotypes. Rice growers have recently experienced poor control, suggesting resistance to this photosystem II-inhibiting herbicide may have evolved in C. difformis populations. The objectives of this study were to detect the presence of propanil resistance, to establish resistance levels, and to investigate involvement of enhanced herbicide detoxification as mechanism of resistance through the use of metabolic inhibitors. Four C. difformis populations collected in rice fields from the Sacramento Valley of California were confirmed resistant to propanil. This is the first case of such resistance outside the Poaceae and the first time C. difformis exhibits resistance to an herbicide mechanism of action other than ALS inhibition. Carbaryl and malathion applied individually in mixture with propanil had minor effects on herbicide toxicity suggesting metabolic detoxification was not a resistance mechanism. A resistant biotype produced more than 80% biomass after a propanil (6.7 kg a.i. ha−1) and carbaryl (1.9 kg a.i. ha−1) or propanil and malathion (1.0 kg a.i. ha−1) treatment compared to

The efficacy of crop management is highly sensitive to the timing of operations. This study tested the hypothesis that using site-specific, real-time temperatures to predict weed emergence at the regional scale can improve the timing of weed management in stale-seedbed and drill-seeded rice (Oryza sativa) relative to the use of regional emergence averages that incorporate the spatiotemporal variability. First, thermal models of emergence for smallflower umbrella sedge (Cyperus difformis) and watergrass (Echinochloa ssp.), two of the most problematic weeds in California’s direct-seeded rice system, were developed from field-scale observations made across 3 sites and 2 years. The models predicted smallflower umbrella sedge and watergrass emergence in an independently collected dataset with accuracy [root mean square error (RMSE) = 21% emergence and 1.3 d; model efficiency index (EF) = 0.80; and RMSE = 14% emergence and 2.2 d; EF = 0.88, respectively]. Subsequently, in order to quantify the degree to which spatially and temporally precise temperatures affect predicted emergence at the regional scale, the models were applied to a daily regional temperature dataset precise to 2 km × 2 km. For each species, the number of days to emergence was simulated for 48 dates (April 15–June 1), 9 years (2003–2011), and 193 locations in the Sacramento Valley rice growing region (83,376 total emergence predictions per species). The variability of the resulting emergence predictions due to the intra-annual, inter-annual and spatial heterogeneity of temperatures was measured with a linear model. Each of the spatiotemporal effects affected the emergence predictions (P

A suspected glyphosate-resistant (R) junglerice population was collected from a glyphosate-R corn field near Durham in northern California where glyphosate had been applied at least twice a year for over 6 yr. Based on the amount of glyphosate required to reduce growth by 50% (ED50), the R population was 6.6 times more R than the susceptible (S) standard population. Based on the glyphosate concentration that inhibits EPSPS by 50% based on shikimate accumulation (I50) in leaf discs, R plants were four times more R than S plants. By 3 d after treatment with 0.42 kg ae ha−1glyphosate, the S population had accumulated approximately five times more shikimate than the R population. No differences in [14C]-glyphosate uptake and translocation were detected between R and S plants. However, partial sequencing of theEPSPSgene revealed a mutation in R plants causing a proline to serine change at EPSPS position 106 (P106S). Our results reveal the first case of a P106S target site mutation associated with glyphosate resistance in junglerice.

Summary 1. Opportunities for diversifying the management of weedy populations may be enhanced through accurate predictions of seedling emergence, because the timing and success of control measures often hinges on the timing of weed emergence. We used population-based threshold models to establish the temperature, moisture and oxygen conditions for optimum germination of herbicide-resistant and -susceptible Echinochloa phyllopogon, a weed of temperate paddy rice, and applied them to predict emergence from field soil. 2. We combined hydrothermal time for germination, accounting for within-population variation in base water potentials (Ψb), with thermal time for early seedling growth to predict the quantity and proportional size of the emergence flushes that constitute final recruitment. 3. Emergence in field soils was reduced by moisture stress and flooding, especially for the resistant population. In all populations, germination rates increased between 9� 5 and 31 °C, Ψb was

a b s t r a c t Weed control is a primary concern in direct-seeded rice, particularly for herbicide-resistant weed species which stand to threaten the long-term sustainability of California rice systems. In a four-year field study we evaluated the potential for improved weed control using no-till stale seedbed practices in water- seeded (WS) and drill-seeded (DS) rice establishment systems. In addition, as the agronomic performance of alternative tillage and crop establishment methods is not well understood, we assessed the produc- tivity of these systems and estimated economic optimum nitrogen (EON) rates based on yield response to nitrogen (N) trials. Establishment system treatments included: water-seeded conventional tillage (WS conventional), water-seeded conventional tillage stale seedbed (WS stale), water-seeded no-till stale seedbed (WS no-till stale), drill-seeded conventional tillage (DS conventional), and drill-seeded no-till stale seedbed (DS no-till stale). Compared to the WS conventional system, WS stale and WS no-till stale treatments significantly reduced sedge weed biomass by 59 and 95%, respectively. Although redstem (Ammannia spp.) was not controlled, alternative WS systems reduced grass weed biomass by more than 99% when present. Within DS systems, no-till stale seedbed practices significantly reduced watergrass (Echinochloa spp.) biomass by 75% in the first two years but did not improve watergrass control during the second half of the study. Grain yields were not different for conventional and alternative rice establish- ment systems each year when N was applied at 168 kg N ha −1 and weeds were fully controlled. However, yields were significantly lower for alternative establishment systems compared to the WS conventional system when no N fertilizer was applied, likely as a result of greater soil N losses. The response of grain yield to N rate was significantly different among systems and estimated EON rates indicated that WS stale and WS no-till stale systems required an increase of 30-35 kg N ha −1 to maximize yields and returns to N compared to the WS conventional system. Results from this experiment demonstrate that alternative tillage and crop establishment systems can lead to improved weed control while remaining viable from an agronomic and economic standpoint in California. Provided N rates are close to optimal and WS and DS establishment methods are selected to target weed species of concern, these findings suggest that no-till stale seedbed practices should be considered as a component of integrated weed management strategies in direct-seeded rice moving forward.

BACKGROUND: Quinclorac (3,7-dichloro-quinoline-carboxylic acid) is a selective herbicide widely used to control annual grasses and certain broadleaf weeds. Echinochloa phyllopogon (Stapf) Koss. is the most noxious grass weed in California rice fields and has evolved resistance to multiple herbicides with different modes of action. A quinclorac-resistant (R) E. phyllopogon biotype found in a Sacramento Valley rice field where quinclorac has never been applied was investigated. RESULTS: Resistant to susceptible (S) GR50 (herbicide rate for 50% growth reduction) ratios ranged from 6 to 17. The cytochrome P450 inhibitor malathion (200 mg L−1) caused R plants to become as quinclorac susceptible as S plants. Quinclorac rapidly (6 HAT) stimulated ethylene formation in S plants, but only marginally in R plants. Malathion pretreatment did not reduce ethylene formation by quinclorac-treated S and R plants. Activity of β-cyanoalanine synthase (β-CAS) in tissue extracts was 2-3-fold greater in R than in S plants, and incubation of shoot extracts with 1 mM malathion reduced β-CAS activity by 40% in both biotypes. CONCLUSION: Resistance to quinclorac in R E. phyllopogon involved at least two mechanisms: (a) insensitivity along the response pathway whereby quinclorac induces ethylene production; (b) enhanced β-CAS activity, which should enable greater HCN detoxification following quinclorac stimulation of ethylene biosynthesis. This unveils new resistance mechanisms for this multiple-resistant biotype widely spread throughout California rice fields. Copyright © 2011 Society of Chemical Industry

Resistance to the thiocarbamates has been selected in early watergrass populations within the rice-growing region of California. To elucidate the processes contributing to the spread of resistance among rice fields, we characterized the genetic diversity and differentiation of thiobencarb-resistant (R) and thiobencarb-susceptible (S) populations across the Central Valley using microsatellite markers. A total of 406 individuals from 22 populations were genotyped using seven nuclear microsatellite primer pairs. Three analytical approaches (unshared allele, Shannon–Weaver, and allelic-phenotype statistics) were used to assess genetic diversity and differentiation in the allohexaploid species. Low levels of genetic variation were detected within populations, consistent with other highly selfing species, with S populations tending to be more diverse than R populations.FSTvalues indicated that populations were genetically differentiated and that genetic differentiation was greater among S populations than R populations. Principal coordinate analysis generated two orthogonal axes that explained 88% of the genetic variance among early watergrass populations and differentiated populations by geographical region, which was associated with resistance phenotype. A Mantel test revealed that genetic distances between R populations were positively correlated with the geographical distances separating populations. Taken together, our results suggest that both short- and long-distance seed dispersal, and multiple local and independent evolutionary events, are involved in the spread of thiobencarb-resistant early watergrass across rice fields in the Sacramento Valley. In contrast, resistance was not detected in early watergrass populations in the San Joaquin Valley.

Weeds are the major biotic constraint to rice production. Field observations have suggested that certain fertilizer regimes could enhance infestations of particular weed species emerging with rice. The study objective was to determine the effect of surface-applied calcium phosphate on weed growth in flooded California rice systems. In field and pot studies, triple superphosphate (TSP) applied to the soil surface increased weed emergence. Surface-applied TSP increased the number of sedge and broadleaf weeds, including smallflower umbrella sedge, blue-flowered ducksalad, redstem, ricefield bulrush, waterhyssop, and California arrowhead. A laboratory study measured germination of smallflower umbrella sedge and ricefield bulrush in response to the application of phosphorus (P) and calcium (Ca), which comprise 20 and 15% of TSP, respectively. Calcium stimulated smallflower umbrella sedge germination and had no effect on ricefield bulrush germination. Phosphorus did not stimulate either smallflower umbrella sedge or ricefield bulrush germination. Results indicate that surface applications of calcium phosphate increase the growth of certain weed species and that Ca may stimulate germination of smallflower umbrella sedge. By incorporating preplant applications of calcium phosphate into the soil profile, growers can reduce weed pressure from certain species. Alternatively, surface applications of calcium phosphate may be useful to stimulate weed emergence in stale-seedbed management. Nomenclature: Blue-flowered ducksalad, Heteranthera rotundifolia (Kunth) Griseb.; California arrowhead, Sagittaria montevidensis Cham. & Schlecht.; redstem, Ammannia coccinea Rottb.; ricefield bulrush, Schoenoplectus mucronatus (L.) Palla; waterhyssop, Bacopa spp. L.; smallflower umbrella sedge, Cyperus difformis L.; rice, Oryza sativa L.

Assessing belowground plant competition is complex because it is very difficult to separate weed and crop roots from each other by physical methods. Alternative techniques for separating crop and weed roots from each other are needed. This article introduces a stable isotope method that can quantify the amounts of roots of rice and barnyardgrass intermixed in flooded field soils. It relies on the biological principle that rice, a C3 (photosynthetic pathway) species, discriminates more effectively than barnyardgrass, a C4 species, against a relatively rare isotopic form (13C) of CO2. This results in different 13C: 12C isotope ratios (expressed as δ13C) in root tissues of the two species. δ13C values for monoculture barnyardgrass and rice grown in a standard flood-irrigated system were highly stable over 4 crop-years, averaging −13.12 ± 0.80 (SD) and −28.5 ± 0.11 (SD)‰, respectively, based on analysis by an isotope ratio mass spectrometer. Standard concentration curves relating measured δ13C values to set proportions of rice:barnyardgrass root biomass were described by linear regressions, typically with r2 values of 0.96 or greater. Quantities of intermixed rice and barnyardgrass roots sampled 0 to 5 cm deep from soil between rice rows were estimated by extrapolation from standard curves based on δ13C values. About 50% more barnyardgrass root tissue was detected in plots of Lemont long-grain rice than in weed-suppressive PI 312777 indica rice, demonstrating the feasibility of using this stable carbon isotope method in flooded rice systems.

Outdoor pot experiments were conducted in California to quantify differences in rice and Schoenoplectus mucronatus susceptibility to drought and to identify morphological and physiological traits that would favor rice over S. mucronatus under drought. Plants were grown in flooded soil for approximately 5 weeks, and then subjected to different drought periods after which pots were re-flooded. Chlorophyll fluorescence assays revealed that rice and S. mucronatus Fv/Fm first became Ψ leaf ) had decreased to approximately −4 MPa and −2 MPa, respectively. Thus, by suffering less photosynthetic damage from drought, rice had better recovery after re-flooding than S. mucronatus . When drought reduced Ψ leaf to −3 MPa, S. mucronatus re-growth was nearly suppressed but that of rice was unaffected. Rice plants depleted soil moisture 1.6 faster than S. mucronatus due to larger and deeper roots and a high water-spending strategy (when Ψ leaf decreased from approximately −0.5 MPa to −2.5 MPa, 13 δ increased from −27.8 to −27.4 and from −28.1 to −26.0 for rice and S. mucronatus , respectively). Rice under interspecific competition sustained its Ψ leaf by extracting more water from greater depths, while causing severe moisture stress and photosynthetic damage to S. mucronatus . Thus temporary drought enhanced rice competitiveness over S. mucronatus , supporting the concept of using brief drought as a tool for S. mucronatus suppression in rice. The Ψ leaf developed by the end of the drought period predicted rice yields ( R 2 = 0.77, P S. mucronatus to recover from drought upon irrigation resumption ( R 2 = 0.62, P S. mucronatus never fully recovered from drought. Rice yields were only reduced after Ψ leaf reached values below approximately −2.5 MPa. Longer drought (∼20 d) delayed maturity and reduced rice yields by approximately 60–80%. The dry-down approach could help suppress weeds similar to S. mucronatus in organic rice where premium prices can compensate for lower grain yield.

Echinochloa phyllopogon (late watergrass) is a major weed of California rice (Oryza sativa) that has evolved cytochrome P450-mediated metabolic resistance to different herbicides with multiple modes of action. E. phyllopogon populations from Sacramento Valley rice fields have also recently shown resistance to the herbicide clomazone. Clomazone is a proherbicide that must be metabolized to 5-ketoclomazone, which is the active compound that inhibits deoxyxylulose 5-phosphate synthase, a key enzyme of the nonmevalonate isoprenoid pathway. This study evaluated the differential clomazone metabolism within strains of the same species to investigate whether enhanced oxidative metabolism also confers clomazone resistance in E. phyllopogon. Using reverse-phase liquid chromatography-tandem mass spectrometry techniques in the multireaction monitoring mode, we elucidated that oxidative biotransformations are involved as a mechanism of clomazone resistance in this species. E. phyllopogon plants hydroxylated mostly the isoxazolidinone ring of clomazone, and clomazone hydroxylation activity was greater in resistant than in susceptible plants. The major clomazone metabolites resulted from monohydroxylation and dihydroxylation of the isoxazolidinone ring. Resistant plants accumulated 6- to 12-fold more of the monohydroxylated metabolite than susceptible plants, while susceptible plants accumulated 2.5-fold more of the phytotoxic metabolite of clomazone, 5-ketoclomazone. Our results demonstrate that oxidative metabolism endows multiple-herbicide-resistant E. phyllopogon with cross-resistance to clomazone through enhanced herbicide degradation and lower accumulation of the toxic metabolite in resistant versus susceptible plants.

Basic factors contributing to the rapid evolution and broad distribution of acetolactate synthase (ALS)-inhibiting herbicide resistance in smallflower umbrella sedge L. have not yet been investigated. The objectives of this study were to examine patterns of cross-resistance to ALS herbicides and genetic diversity within and among smallflower umbrella sedge populations in California rice fields to provide insight into the processes contributing to resistance spread. Twelve different patterns of herbicide cross-resistance were found across the 56 populations sampled. The frequency of populations with at least one resistant individual in the North, Central and South Sacramento Valley, and the San Joaquin Valley were 76, 86, 67, and 50%, respectively. Analysis of the genetic diversity of 29 populations using 73 sequence-related amplified polymorphism molecular markers revealed little genetic diversity within populations, with estimates of Nei's gene diversity index,h, ranging from 0 to 0.049, and Shannon's information index (I) ranging from 0 to 0.079. Hierarchical analyses of molecular variance indicated that the majority of genetic variation was partitioned among populations, rather than within populations or among regional groups. No isolation by distance was evident. Unweighted pair group method with arithmetic averages analysis indicated that population clustering was not region specific. The results suggest that resistance to ALS-inhibiting herbicides in smallflower umbrella sedge populations from California rice fields appears to have evolved independently multiple times rather than spread from a single population where resistance originated. Consequently, prevention and management of smallflower umbrella sedge in California rice fields should emphasize in-field strategies that focus on decreasing the selection pressure caused by ALS-inhibiting herbicides.

Summary Cyperus difformis (smallflower umbrella sedge) is an economically important weed of rice in California where its control has recently been complicated by the evolution of herbicide resistance. Knowledge of the mating system of this weed is needed to elucidate the dynamics of resistance evolution and to design mitigation strategies that delay its occurrence. The aim of this study was to estimate the outcrossing rate of C. difformis using molecular and phenotypic markers. Outcrossing rates were estimated in natural field populations using sequence-related amplified polymorphism (SRAP) molecular markers and in glasshouse and field experiments using resistance to the acetolactate synthase-inhibiting herbicide bensulfuron-methyl as a phenotypic marker. Using SRAP markers, the multilocus (tm) and average single-locus (ts) outcrossing rates varied from 0.014 to 0.025 and from 0.008 to 0.012, respectively, among natural weed populations in rice fields. Using resistance to bensulfuron-methyl as a genetic marker, the average C. difformis outcrossing rate estimated was 0.009 in the glasshouse and 0.0084 in the field. These results indicate that C. difformis is a highly self-fertilising species. Therefore, the primary mechanism by which genes for herbicide resistance can be transferred among C. difformis populations in different rice fields is probably seed dispersal. Weed management should emphasise prevention of seed production and dispersal to preclude the further spread and evolution of resistance in C. difformis.

BACKGROUND: Late watergrass [Echinochloa phyllopogon (Stapf.) Koss.] is a major weed of Californian rice that has evolved P450-mediated metabolic resistance to multiple herbicides. Resistant (R) populations are also poorly controlled by the recently introduced herbicide clomazone. The authors assessed whether this cross-resistance was also P450 mediated, and whether R plants also had reduced sensitivity to photooxidation. Understanding mechanism(s) of resistance facilitates the design of herbicide management strategies to delay resistance evolution. RESULTS Ratios (R/S) of R to susceptible (S) GR50 were near 2.0. [14C]Clomazone uptake was similar in R and S plants. Clomazone and its metabolite 5-ketoclomazone reduced chlorophyll and carotenoids in S more than in R plants. The P450 inhibitors disulfoton and 1-aminobenzo-triazole (ABT) safened clomazone in R and S plants. Disulfoton safened 5-ketoclomazone only in S plants, while ABT synergized 5-ketoclomazone mostly against S plants. Paraquat was more toxic in S than in R plants. CONCLUSION: Cross-resistance to clomazone explains failures to control R plants in rice fields, and safening by P450 inhibitors suggests that oxidative activation of clomazone is needed for toxicity to E. phyllopogon. Clomazone resistance requires mitigation of 5-ketoclomazone toxicity, but P450 detoxification may not significantly confer resistance, as P450 inhibitors poorly synergized 5-ketoclopmazone in R plants. Responses to paraquat suggest research on mechanisms to mitigate photooxidation in R and S plants is needed. Copyright © 2008 Society of Chemical Industry

BACKGROUND: A Chenopodium album L. biotype surviving in atrazine-treated Serbian corn fields (VC) was compared against atrazine-susceptible (S) and atrazine-resistant (R) standards. RESULTS: Atrazine (2 kg ha(-1)) killed S and VC shoot biomass 15 days after treatment (DAT), but R was only suppressed by 42% and survived 8 kg ha(-1). Atrazine at 2 kg ha(-1) only inhibited VC height by 60% as against 100 and 0% for S and R respectively. Chlorophyll fluorescence (Fv/Fm) and transpiration were insensitive to atrazine in R, but were inhibited by 90 and 100% in S and by 50 and 60% in VC respectively. Decline of Fv/Fm after 2 kg ha(-1) atrazine was stabilized at 3 DAT for the VC biotype. CONCLUSION: A toxicity mitigation mechanism could have facilitated VC survival in an atrazine-treated field. Further knowledge on this mechanism is needed to establish if surviving VC plants are indicators of atrazine resistance evolution in these Serbian corn fields. Variables related to foliar function provided better detection of weed mechanisms to survive herbicide action than the usual shoot biomass measurements. (c) 2007 Society of Chemical Industry.

Field research and grower interviews were used to evaluate the potential of minimum tillage for California rice systems. We found that by tilling only in the fall (instead of both the fall and spring), rice farmers can control herbicide-resistant weeds when combined with a stale rice seedbed, which entails spring flooding to germinate weeds followed by a gly-phosate application to kill them. Our results indicated that yield potentials are comparable between water-seeded minimum- and conventional-till systems. We also found that rice growers can reduce fuel costs and plant early. However, minimum tillage may require more nitrogen fertilizer to achieve these yields.

Echinochloa is a major weed that grows almost everywhere in farmed land. This high prevalence results from its high adaptability to various water conditions, including upland and paddy fields, and its ability to grow in a wide range of climates, ranging from tropical to temperate regions. Three Echinochloa crus-galli accessions (EC-SNU1, EC-SNU2, and EC-SNU3) collected in Korea have shown diversity in their responses to flooding, with EC-SNU1 exhibiting the greatest growth among three accessions. In the search for molecular components underlying adaptive diversity among the three Echinochloa crus-galli accessions, we performed de novo assembly of leaf transcriptomes and investigated the pattern of differentially expressed genes (DEGs). Although the overall composition of the three leaf transcriptomes was well-conserved, the gene expression patterns of particular gene ontology (GO) categories were notably different among the three accessions. Under non-submergence growing conditions, five protein categories (serine/threonine kinase, leucine-rich repeat kinase, signaling-related, glycoprotein, and glycosidase) were significantly (FDR, q < 0.05) enriched in up-regulated DEGs from EC-SNU1. These up-regulated DEGs include major components of signal transduction pathways, such as receptor-like kinase (RLK) and calcium-dependent protein kinase (CDPK) genes, as well as previously known abiotic stress-responsive genes. Our results therefore suggest that diversified gene expression regulation of upstream signaling components conferred the molecular basis of adaptive diversity in Echinochloa crus-galli.

Resistance to herbicides in the most important weeds threatens the sustainability of California rice. Weed-competitive rice cultivars could be a low-cost and safe nonchemical addition to an integrated weed management program. Trade-offs between competitiveness and productivity and inconsistent trait expression under weedy and weed-free conditions could complicate the breeding of competitive rice cultivars. A 2-year competition experiment was conducted in the greenhouse involving eight rice cultivars and two weed competition regimes (presence or absence of late watergrass) to examine the effects of rice weed-suppressive ability and tolerance to weed competition (weed tolerance) on rice yield. Competition reduced average rice yield from 32 to 48%, and watergrass biomass from 44 to 77%. Path analysis suggested that enhancing rice weed-suppressive ability and weed tolerance while minimizing possible productivity trade-offs should promote early (12 d after seeding) growth and light-capture traits foll...

The mechanism of resistance to quinclorac was investigated in a smooth crabgrass biotype [ Digitaria ischaemum (Schreb. ex Schweig) Schreb. ex Muhl] from Tulare County, California. Quinclorac (8.96 kg a.i. ha −1 ) had no effect ( P = 0.18) on the resistant (R) biotype, but reduced fresh weight of a susceptible (S) biotype by 93%. After treatment with 4.48 kg a.i. quinclorac ha −1 , the S biotype produced about three times more ethylene than the R biotype and accumulated cyanide in tissues. Similar amounts of endogenous cyanide resulting from treatment with KCN reproduced quinclorac phytotoxicity. Pre-treatment with the ACC synthase inhibitor AVG reduced quinclorac phytotoxicity by 37% and ethylene production by 89%. These data suggest a target site-based mechanism of resistance involving stimulation of ACC synthesis and accumulation of cyanide. Also, the R biotype had four times more β-cyanoalanine synthase activity than the S biotype, suggesting a higher ability to detoxify cyanide.

Late watergrass is a major weed of rice in California, and several populations showed resistance to multiple herbicides of differing modes of action. Bispyribac-sodium, fenoxaprop-ethyl, and thiobencarb were used in this study as inducers and substrates of cytochrome P-450 monooxygenases (P450s). To examine the role of P450s in herbicide metabolism by a herbicide-resistant (R) and a herbicide-susceptible (S) late watergrass (Echinochloa phyllopogon) biotype, seedlings were grown in absence and presence of the above three herbicides as inducers. The efficacy of the inducer treatments was confirmed by monitoring changes in P450 content and activity. Herbicide induction increased P450 content and activity in both R and S biotypes. This increase was higher in the R biotype. The induced P450s exhibited specificity for the different herbicide substrates. Our study suggests that induction of P450s is the mechanism involved in the multiple-herbicide resistance observed in late watergrass.

A biotype of green foxtail found in Spain exhibited cross-resistance among acetyl-CoA carboxylase (ACCase)–inhibiting herbicides. Field doses that totally inhibited shoot fresh weight in the susceptible (S) biotype were determined for six aryloxyphenoxypropionates (clodinafop, diclofop, fenoxaprop-P, fluazifop-P, haloxyfop-P, and propaquizafop) and six cyclohexanediones (clefoxydim, clethodim, cycloxydim, sethoxydim, tepraloxydim, and tralkoxydim). The resistant (R) biotype showed cross-resistance to all herbicides except fenoxaprop-P, propaquizafop, clefoxydim, and tepraloxydim. There were no differences in the absorption, translocation, and metabolism of [14C]diclofop between the S and R biotypes. On the basis of herbicide dose that inhibited ACCase activity by 50% (I50values), ACCase of the R biotype was 5.8-, 13.9-, 20.0-, 102.4-, 416.7-, and 625.0-fold less sensitive to clethodim, haloxyfop, diclofop, fluazifop, cycloxydim, and sethoxydim, respectively, than that of the S biotype. Two multifunctional ACCase isoforms (ACCase I and ACCase II) were purified partially and separated. ACCase II was highly resistant to diclofop acid in both biotypes, with I50values ranging between 92 and 95 μM. However, the I50values observed for ACCase I revealed that the R biotype was 30.8-fold less sensitive to diclofop than the S biotype. These results suggest the mechanism of resistance in green foxtail to diclofop relates to an altered ACCase I isoform.

The development of optimal weed management strategies that rely, in part, on crop interference will require an understanding of how weeds compensate for limitations in above- and belowground resources. Trade-offs in the leaf morphology and biomass partitioning of rice and late watergrass were investigated under glasshouse conditions in 1999 and 2000. Both species responded to shade with increased height, reduced biomass, greater partitioning of biomass to leaves, and greater leaf area ratios. At the lowest light level (18% sunlight), plants of both species showed little response to nitrogen (N). However, height, tillers, biomass, and leaf area increased for plants grown at 50% and full sunlight as N increased from 0 to 224 kg N ha−1. Late watergrass exhibited more plasticity in specific leaf area and root weight ratio than rice in response to shade. This plasticity contributed to the ability of late watergrass to maintain a higher percent of its tillers and total dry weight than rice when sunlight was reduced by 50%. These results support the hypothesis that except at low light levels, limited N further reduces the growth of shaded late watergrass plants. Thus, weed management strategies that limit the plasticity of late watergrass by manipulating light and N availability are likely to be more effective than strategies that rely on manipulating a single resource.

Genotypic differences between upland rice (Oryza sativa L.) varieties in the resistance to acid soil conditions were investigated in terms of the contents of Ca and Al in the apoplast of root tips. Apoplastic Ca and Al contents are assumed to be directly related to the inhibition of root cell elongation, which is the most typical symptom associated with cultivation in acid soils. The apoplastic content of Al eluted with BaCl2 was significantly different between the tolerant (Oryzica Sabana 6: SB6) and susceptible (Oryzica 1: OZ1) varieties when the Al concentration in the growth solution was in the range of 30 to 100 µM. The apoplastic Al content in SB6 was lower, probably reflecting the lower root CEC. On the other hand, the apoplastic Ca content showed significant genotypic differences when Ca was eluted with chelating solutions (Na-citrate and Na-EDTA) rather than with a BaClz solution. The apoplastic Ca content of OZ1 was much higher than that of SB6 in the absence of Al, but it decreased by ...

Late watergrass is a serious weed of California rice that has evolved resistance to molinate, thiobencarb, fenoxaprop-ethyl, and bispyribac-sodium. To obtain an insight into the origin and spread of resistant (R) late watergrass in California rice fields, we evaluated similarities in morphological traits and amplified fragment length polymorphism (AFLP) fingerprints among 15 R strains compared with susceptible (S) strains. All strains were derived by inbreeding from accessions collected in rice fields of the Sacramento Valley, CA. In the field, R plants were shorter than S plants; they also had narrower and shorter flag leaves and thinner culms. Spikelets also appeared smaller and more slender in R plants. There was greater morphological similarity among the 15 R strains than among the eight S strains. The mean coefficients of variation for morphological traits were much smaller among R strains, which in a cluster analysis (Ward's method) were grouped morphologically apart at early clustering sta...

Resistance to herbicides and the lack of viable control options have led to an interest in increasing the role of crop competition as a weed management tool in water-seeded rice production. Weed-suppressive rice cultivars have been suggested as a tool that could improve weed control and reduce the reliance of growers on herbicides. Field studies were conducted at Biggs, CA, in 1999 and 2000 with six to eight semidwarf rice cultivars to identify water-seeded rice traits related to the suppression of watergrass growth. Cultivars S-201 and M-302 were the most suppressive in both years. The dry weight (DW) of watergrass grown with the most suppressive cultivar was only 16% in 1999 and 57% in 2000 of the DW of watergrass grown with the least suppressive cultivar. Rice leaf area and root DW in weed-free plots were linearly related to watergrass DW in both years. Weed-suppressive traits were not inversely correlated with rice yields in monoculture; competitive cultivars also had high yields. This study ...

Summary Co-ordinating herbicide applications with the suppressive ability of the crop has the potential to improve weed control and optimize herbicide use in water-seeded rice. However, the successful integration of herbicide applications and crop development depends on the timing and duration of competition between rice and weeds. The critical period of competition between rice and Echinochloa species was examined in field and glasshouse experiments from 1996 to 2000. In 1999 and 2000, Echinochloa species seeded 30 days after rice in field experiments did not survive and rice yields were not reduced when plots were kept weed-free for 30 days or longer. In a basin experiment conducted in 1998, E. phyllopogon seeded with the crop was unaffected by light competition alone but the relative importance of shading by the crop increased when E. phyllopogon was seeded after rice. Management strategies that delay the germination or growth of Echinochloa species might confer a competitive advantage to rice and reduce the need for herbicide applications. However, yields in the field experiments were reduced by at least 18% after only 30 days of competition in both years, suggesting that it may be difficult to integrate currently available herbicides with crop growth in water-seeded rice.

Water-seeded and continuously flooded rice in California is mostly grown as a continuous crop and weeds are the most serious rice production problem. Echinochloa phyllopogon (Stapf) Koss and Cyperus difformis L. are adapted to this aquatic system and compete with rice, causing heavy economic losses. Flooding cannot fully suppress these weeds. Heavy reliance on few available herbicides resulted in the evolution of herbicide resistance in populations of both weeds in California rice fields. Resistance to bensulfuron-methyl (ALS inhibitor) is widespread among C. difformis populations. E. phyllopogon has evolved resistance to several herbicides, including bispyribac-sodium (ALS inhibitor), which has not yet been commercially used. A resistant (R) E. phyllopogon was also much more tolerant to bensulfuron-methyl than a susceptible (S) biotype. Understanding the patterns and mechanisms of cross-resistance in key weeds of rice to bensulfuron-methyl and bispyribac-sodium is relevant for the successful deployment of this new herbicide and for the management of herbicide resistance in California rice. Whole-plant bioassays were conducted to compare responses between E. phyllopogon and C. difformis to bispyribac-sodium and bensulfuron-methyl and to detect the involvement of cyt P-450 monooxygenases in E. phyllopogon resistance to bensulfuron-methyl using the cyt P-450 inhibitors piperonyl butoxide and malathion (previous studies had already shown cyt P-450-mediated resistance to bispyribac-sodium). ALS activity was assayed on leaf extracts from young R and S plants of both species for a range of bispyribac-sodium and bensulfuron-methyl concentrations. The dose–response studies confirmed cross-resistance in R E. phyllopogon ; minimum and maximum ratios (R/S) of the GR50 values of resistant to susceptible plants were 9 and >25.5 for bispyribac-sodium and bensulfuron-methyl, respectively. cyt P-450 contributed to bensulfuron-methyl resistance in R E. phyllopogon. C. difformis was also cross-resistant (R/S ratios: >10 for bispyribac-sodium and >26 for bensulfuron-methyl). ALS assays demonstrated that, unlike R E. phyllopogon, cross-resistance in R C. difformis was due to reduced ALS sensitivity. C. difformis ALS was more sensitive to bispyribac-sodium ( I 50 =138.87 nM ) than to bensulfuron-methyl ( I 50 =6724.56 nM ) . Also, ALS inhibition in R and S E. phyllopogon was higher with bispyribac-sodium. Thus, binding differences between both herbicides at the target site are suggested. This study reveals that cross-resistance between bensulfuron-methyl and bispyribac-sodium in both weeds involves degradation enhancement through monooxygenases and target-site alteration.

When savannas in Latin America are brought into cultivation, rice (Oryza saliva L.) can be sown with the perennial grasses palisadegrass [Brachiaria brizantha (Hochst. ex A. Rich) Stapfl and signalgrass (B. decumbens Stapf) to harvest a grain crop while establishing a pasture to suppress weeds and provide grazing in subsequent years. However, these Brachiaria spp. can reduce upland rice yields. Rice cultivars need to be competitive with Brachiaria spp. to maintain yields but must allow Brachiaria spp. sufficient growth for pasture establishment. Field studies were conducted during 1994 and 1995 on a Typic Haplustox oxisol soil in the Eastern Plains of Colombia to evaluate the competitiveness of upland rice cultivars and to identify rice traits for competitiveness. Ten (1994) and 14 (1995) upland rice cultivars were grown with and without signalgrass in 1994 and palisadegrass in 1995. Rice cultivars differed substantially in their competitiveness. Rice yield losses ranged from 18 to 55%, and Brachiaria aboveground biomass ranged from 1.4 to 3.2 Mg ha -1 dry mass. Competition for light was critical; rice photon flux density interception, leaf area index [≥45 d after emergence (DAE)], and number of tillers (≥60 DAE) were correlated with competitiveness. No tradeoff between high yield potential and competitiveness was detected in these upland rice cultivars. Early maturity of rice is a desired characteristic for the rice-Brachiaria spp. association. The development of more-competitive cultivars appears to be a viable approach for reducing herbicide dependency and improving profitability of Latin American rice-pasture intercropping systems.

Summary The e•ects of weed shoot morphology on competitiveness for light in rice (Oryza sativa L.) havenot been well described quantitatively and are di†cult to study empirically. A rice:weed modelwas used to analyse the e•ects of weed leaf area densities (LAD; m 2 m –3 ), leaf angles (as leaf lightextinction coe†cients, k leaf ) and maximum heights (H m , m) on growth and competition with rice.Weed morphologies were hypothetical but empirically based, as follows: LADs were skewed tothebottom orconical, k leaf values varied from 0.2 (erectophile)to 0.8 (planophile), andH m valueswere 0.5H R ,1H R and 1.25H R , where H R was rice maximum height. Other parameters were equalto those of rice. Growth and competitiveness were evaluated using mature seed dry weights(g m –2 ). Short weeds and weeds with conical LADs were weakly competitive, regardless of othertraits. For other weed types, interference with rice was positively related to H m , LAD skewnessand more planophile leaves. All three traits were critical determinants of weed interference but nosingle morphological trait guaranteed competitiveness. All else being equal, weeds with highlyskewed LADs produced the most seed dry weight. Planophile leaves were particularly beneficialfor short weeds, giving over five times more seed dry weight than erectophile leaves.Keywords: crop:weed competition, leaf area distribution, height, leaf angle, growth traits.

The repeated use of a limited number of herbicides has led to the development of resistant weed species in California rice (Oryza saliva L.). Competitive rice cultivars could help reduce herbicide dependency and decrease selective pressure for resistance. We conducted field experiments to determine if differences in competitive ability existed between two semidwarf cultivars of water-seeded rice. Cultivars M-202 and A-301 were grown on Stockton clay soil at two seed rates (84 and 168 kg ha 1 ) in 1996 and four seed rates (56, 112, 168, and 224 kg ha -1 ) in 1997. Molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) and propanil [N-(3,4-dichlorophenyl)propanamide] were used to control a mixed infestation of watergrass [Echinochloa oryzoides (Ard.) Fritsch, E. phyllopogon (Stapf) Koss] in 1996 and 1997, respectively. Molinate (0 and 4.5 kg a.i. ha -1 ) was applied in 1996, and propanil (0, 1, 2 and 4 kg a.i. ha -1 ) was applied in 1997. M-202 was taller, produced more leaf dry weight, had greater light interception, and reduced watergrass biomass more than A-301 in both years of the study. The more competitive M-202 also had higher yields than A-301 under weedy and weed-free conditions. Watergrass growth was not affected by rice seed rates in either year. This study suggests that herbicide rates could be reduced and weed control could be improved if more competitive cultivars were developed for water-seeded rice.