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Glyphosate is a popular herbicide used to control goosegrass [Eleusine indica (L.) Gaertn.], one of the most troublesome weeds in cotton (Gossypium hirsutum L.) fields. However, high selection pressure has led to some populations being difficult to control in cotton fields in China. In this study, levels of glyphosate resistance were quantified and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) amplification was confirmed. In addition, distribution of the EPSPS gene among the chromosomes was also investigated using fluorescence in situ hybridization (FISH) methodology. One population (AHCZ) was confirmed to be glyphosate resistant with a resistance index of 3.4 and significantly reduced shikimate accumulation compared with the susceptible population. All examined AHCZ individuals exhibited no mutations in the EPSPS gene. Expression and copy numbers of EPSPS in the AHCZ population were 5.7 and 15.4 times higher, respectively, than in the susceptible population. A positive correlation was identified between signal intensities of primary anti-EPSPS antibody and copy numbers of the EPSPS protein, as indicated by immunoblot analysis. FISH results revealed that, in mitotic metaphase chromosomes, signals were observed dispersed across two sets of homologous chromosomes in a resistant individual (copy number = 31), while in susceptible individuals, signals were only partially detected in one set of homologous chromosomes. In interphase nuclei, EPSPS signals were brightest in resistant individuals compared with susceptible individuals. In conclusion, one E. indica population from a cotton field in the Anhui Province has evolved resistance to glyphosate, and EPSPS gene amplification was confirmed as the resistance mechanism.
Eclipta, widespread in tropical, subtropical, and temperate regions, is one of the main malignant broadleaf weeds and thrives in moist and dryland fields. Field rates of acetolactate synthase (ALS) inhibitors have failed to control eclipta in some farmlands in China. One ALS inhibitor–resistant population (R) collected from Jiangsu province in China was confirmed in the greenhouse in our preliminary work. Whole-plant assays revealed that this R population was highly resistant to four sulfonylureas (pyrazosulfuron-ethyl, 134-fold; bensulfuron-methyl, 172-fold; metsulfuron-methyl, 30-fold; and tribenuron-methyl, 195-fold), two triazolopyrimidines (pyroxsulam, 98-fold; penoxsulam, 30-fold), and one pyrimidinylthio-benzoate (bispyribac-sodium, 166-fold) and was moderately resistant to two imidazolinones (imazethapyr, 10-fold; imazapic, 19-fold). ALS enzyme-activity assays showed insensitivity of the ALS from the R population (resistance index values ranged from 12 to 293) to all of the above ALS inhibitors in vitro. Chromatograms from ALS gene sequence analysis detected a homozygous Pro-197-Ser amino acid substitution in the R population. These results confirmed that the Pro-197-Ser substitution results in broad-spectrum cross-resistance to ALS inhibitors in the eclipta R population. To our knowledge, this study is the first to report broad cross-resistance to ALS inhibitors in eclipta and to obtain the full-length ALS gene sequence.
A 4-yr field experiment was conducted to determine the merits of combining cultural and chemical controls to manage foxtail barley in reduced-tillage systems. Factors studied were crop row spacing, seeding rate, and application rate and timing of glyphosate within a spring wheat-flax cropping sequence. Glyphosate applied preseeding at 400 or 800 g/ha killed foxtail barley seedlings but only suppressed established perennial plants. Glyphosate applied postharvest at 800 g/ha killed 60 to 70% of established plants. Combinations of preseeding and postharvest glyphosate gave the greatest reductions in foxtail barley biomass and seed production and resulted in the greatest increases in crop yield. Including flax in the rotation allowed use of grass herbicides such as quizalofop or sethoxydim that effectively controlled foxtail barley seedlings and provided some suppression of perennial plants. An increase in wheat seeding rate from 75 to 115 kg/ha reduced foxtail barley growth and increased wheat yield in 3 of 4 yr. Increasing the flax seeding rate from 40 to 80 kg/ha or reducing wheat and flax row spacing from 30 to 20 cm provided little benefit in managing foxtail barley or increasing crop yield. A multiyear approach combining agronomic practices and timely use of herbicides should allow growers to effectively manage foxtail barley in annual cropping systems using conservation tillage.
The increasing use of ACCase-inhibiting herbicides has resulted in evolved resistance in key grass weeds infesting cereal cropping systems worldwide. Japanese foxtail is one of the most important grass weed species in wheat in China. Most populations have evolved resistance to fenoxaprop-p-ethyl, which is one of the most common ACCase-inhibiting herbicides in wheat. The seeds of two Japanese foxtail populations were collected from wheat fields where farmers complained that control could not be effectively obtained with fenoxaprop-p-ethyl. Seeds from one susceptible population were collected from an area along a roadside where ACCase inhibitors had not been used to be used for validating cross-resistance and elucidating the mechanism of resistance. The experimental results showed that the two populations, Aloja-JS10-R1 and Aloja-JS10-R2, expressed high resistance to fenoxaprop-p-ethyl, with resistance indexes (RIs) of 29.2 and 27.9. These populations also expressed high cross-resistance to clodinafop-propargyl with RIs of 12.8 and 14.7, and moderate cross-resistance to clethodim and pinoxaden with RIs ranging from 2.6 to 11.4. Comparison of the ACCase carboxyl-transferase (CT) domain sequences of the susceptible and resistant populations with blackgrass revealed that tryptophan at position 2027 of the ACCase gene was substituted by cysteine in population Aloja-JS10-R1, and isoleucine at position 1781 of the ACCase gene was substituted by leucine in populations Aloja-JS10-R2. The study confirmed Japanese foxtail resistance to the ACCase inhibitor fenoxafop-p-ethyl, cross-resistance to other ACCase inhibitors, and the resistance mechanism being conferred by specific ACCase point mutations at amino acid position 1781 and 2027.
The spread of buffalobur in China poses a serious threat to existing ecosystems, and control and eradication of this species have become increasingly important. Studies were carried out to ascertain the seed production, morphological characterization, dormancy behavior, and methods for breaking dormancy of buffalobur. The results showed that a single buffalobur plant could produce 1,600 to 43,800 seeds with an average weight of 3.0 mg. Average seed length, width, and thickness were 2.5, 2.0, and 1.0 mm, respectively. Newly ripened buffalobur seeds were innately dormant and exhibited combinational dormancy, which involves a hard seed coat (physical dormancy, PY), a partial dormant embryo (physiological dormancy, PD), and a dark requirement to germinate. PY of buffalobur seeds could be broken by dehusking or acid scarification by 14 M H2SO4 for 15 min, with germination rates of 55 or 50%, respectively. PD was effectively broken by KNO3 or gibberellic acid (GA3). The optimum concentration for KNO3 was between 20 and 40 mM, which resulted in over 70% seed germination. When presoaked with GA3 at 30 C in dark for 24 h, maximum germination (> 98%) was obtained at 2.4 mM, the corresponding germination speed (85%) and germination index (16) were also highest at this concentration. Synergistic effects were observed in seed germination when H2SO4 and GA3 were combined. The most rapid and effective combination in breaking dormancy was when the seeds were immersed in H2SO4 (14 M) for 20 min and presoaked with 2.4 mM GA3 for 24 h. Germination index for this combination was over 35, and 95% of the seeds germinated within 7 d. Knowledge gained in this study will be useful in increasing germination of buffalobur and facilitating further laboratory studies.
Buffalobur is a noxious and invasive weed species native to North America. The influence of environmental factors on seed germination and seedling emergence of buffalobur were evaluated in laboratory and greenhouse experiments. The germination of buffalobur seeds occurred at temperatures ranging from 12.5 to 45 C, with optimum germination attained between 25 and 35 C. Buffalobur seeds germinated equally well under both a 14-h photoperiod and continuous darkness; however, prolonged light exposure (≥ 16 h) significantly inhibited the seed germination. Buffalobur seed is rather tolerant to low water potential and high salt stress, as germination was 28 and 52% at osmotic potentials of −1.1 MPa and salinity level of 160 mM, respectively. Medium pH has no significant effect on seed germination; germination was greater than 95% over a broad pH range from 3 to 10. Seedling emergence was higher (85%) for seeds buried at a soil depth of 2 cm than for those placed on the soil surface (32%), but no seedlings emerged when burial depth reached 8 cm. Knowledge of germination biology of buffalobur obtained in this study will be useful in predicting the potential distribution area and developing effective management strategies for this species.
A field study was conducted to determine the combined effects of row spacing, plant density, and herbicides on weed management and dry bean (Phaseolus vulgaris) yield. In weed-free dry bean, a reduction in row spacing from 69 to 23 cm increased yield by 19% and an increase in density from 20 to 50 plants/m2 increased yield by 17%. In the presence of weeds, narrow rows and high plant densities increased dry bean yield, but without herbicides, yields remained low. However, when combined with herbicides, narrow-row and high-density production practices resulted in better weed control and higher dry bean yield than that attained in a wide-row and low-density production system. Herbicide combinations, often at reduced rates, controlled weeds as well or better than the full rate of any individual herbicide. Ethalfluralin applied preplant incorporated followed by reduced rates of imazethapyr or bentazon postemergence (POST) consistently controlled weeds. Imazamox exhibited the potential to provide a total POST weed control option in dry bean production. Information gained in this study will be used to develop improved weed management programs appropriate for either wide- or narrow-row dry bean production systems.
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