Variability of hormetic dose responses of the antiauxin PCIB on Lactuca sativa in a plant bioassay

Some herbicidal agents that damage plants at high doses stimulate their growth at low doses. This phenomenon of hormesis constitutes an alternative possible use of herbicidal agents that is, however, compromised by the apparent variability of the phenomenon. However, studies demonstrating and quantifying this apparent variability are lacking. The auxin‐inhibitor PCIB [2‐(p‐chlorophenoxy)‐2‐methylpropionic acid] was therefore investigated to determine whether interassay variability of stimulatory effects exceeds those of inhibitory effects and which hormetic quantity is most variable. Reparameterisation of the dose–response model used to estimate the hormetic dose range allowed including this feature as an explicit parameter. In bioassays with lettuce and root length as response variable, the variability of PCIB effects was evaluated in 33 complete dose–response assays. In a meta‐analysis, the occurrence of PCIB hormesis proved highly reproducible. However, the variability of effects was dose‐dependent and increased with decreasing dose. The response before the maximum stimulatory response and the absolute magnitude of hormesis proved most variable. Comparing the frequency distribution of effective doses demonstrated there was a risk of a previously hormetic dose causing a loss of hormesis or inhibitory effects in a subsequent experiment. Therefore, selecting a hormetic dose that will induce hormesis under any circumstances will be a major challenge.     

Conyza sumatrensis: A new weed species resistant to glyphosate in the Americas

Recent reports of weed‐control failures after the use of glyphosate led to suspicion about the selection of resistant biotypes of Conyza at locations in west and north Paraná, Brazil. Plants were collected, identified as Conyza sumatrensis and subsequently evaluated for possible resistance to glyphosate in four stages of weed development. The experiments were carried out in a greenhouse by combining biotypes, stages of development and a range of glyphosate doses. All the suspected biotypes were collected from locations in Cascavel, Toledo, Assis Chateaubriand, Tupãssi and Campo Mourão with a history of glyphosate use in burndown and in glyphosate‐resistant soybean for at least the four previous years and were compared to a susceptible biotype (São Jorge do Ivaí) with no previous history of herbicide use. The doses of glyphosate ranged from 0 to 5760 g ae ha^?1. The biotypes were considered as resistant if two combined criteria were present (resistance factor > 1 and the rate required to achieve 80% control is >720 g ha^?1). The results provided evidence that there is a marked difference in the level of control of older plants and also confirmed the presence of some resistant biotypes. For applications at the first stage of development, two biotypes that were resistant to glyphosate were identified (Cascavel‐1 and Tupãssi‐6). For applications in the second stage of development, beyond the biotypes that were found in the first stage, three other biotypes were considered as resistant: Toledo‐5, Assis Chateaubriand‐7 and Floresta‐10. However, for applications at the third and fourth stages, all the biotypes were considered as resistant.     

Using a selectivity index to evaluate logarithmic spraying in grass seed crops

BACKGROUND: Grass seed crops are minor crops that cannot support the development of selective herbicides for grass weed control in grass seed crops. An option is to screen for selective herbicides with the use of logarithmic spraying technology. The aim of this paper is to assess selectivity of various herbicides in grass seed crops by using dose–response curves. RESULTS: Six grass species were subjected to logarithmic spraying with 11 herbicides and with Poa pratensis L. as a weed. The ratio between the doses that caused 10% of damage to the crop and 90% of damage to the weed was used as a selectivity index. Compounds with selectivity indices above 2 can be safely used in a crop. The two ACCase herbicides clodinafop‐propargyl and fenoxaprop‐P‐ethyl and a mixture of the two ALS herbicides mesosulfuron and iodosulfuron could be used selectively to control P. pratensis in Festuca rubra L., although the selectivity indices in no instances were greater than the desired 2.0. CONCLUSION: The logarithmic sprayer can be a rapid screening tool for identifying compounds with favourable selectivity indices. Good experimental design is needed to alleviate rates being systematically distributed and confounded with growth rate and soil fertility gradients. Copyright © 2009 Society of Chemical Industry     

Herbicides can stimulate plant growth

Low dose stimulations by toxicants have long been observed. Great controversies exist concerning the interpretation of these observations, spanning from believing that they are a general stress response occurring for all chemicals, to simply being an experimental artefact resulting from poorly growing control plants or from biomass allocation between plant parts. This study investigates the growth response and biomass allocation pattern of barley exposed to 10–15 doses of eight different herbicides. The results show that the globally most widely used herbicide, glyphosate, together with the sulfonylurea, metsulfuron‐methyl, can induce a real stimulation in biomass growth of approximately 25% when applied at doses corresponding to 5–10% field rate. The other six herbicides tested did not induce consistent hormesis, thereby undermining the theory of hormesis being a general stress response. Biomass allocations between plant parts did take place, but were not the cause of the hormetic growth stimulations. The results demonstrate that plant physiological responses to low herbicide doses cannot be extrapolated from our knowledge of effects of higher, commercially used, doses. Other physiological mechanisms seem to be triggered in the low dose‐range, and the investigation of these mechanisms poses new challenges for agronomists, environmentalists and plant physiologists.     

Selection for resistance to the herbicide imazethapyr in somaclones of soyabean

Cultivars of soyabean [ Glycine max (L.) Merr.] resistant to the herbicide imazethapyr were identified by suspending the roots of 5-day-old seedlings in nutrient culture containing 2.5 mg a.i. L^–1 imazethapyr and then comparing the inhibitory effect on root length and shoot dry weight. The four most resistant cultivars were subsequently screened as regenerating tissue cultures in a medium containing 2.0 mg a.i. L^–l imazethapyr to select somaclonal cells with increased resistance. Surviving portions of cultures were regenerated to give shoots, the plants isolated, allowed to flower and seed. These progeny were then used for further seed multiplication and seedlings from this latter generation were exposed to imazethapyr in vivo and callus and cell suspension cultures derived from these seedlings were exposed to imazethapyr in vitro . A reduction in the inhibitory effect of the imazethapyr was noted in the somaclone seedlings and tissue cultures. However, measurement of acetolactate synthase (ALS) activity showed no differences among the parent cultivars and in the selected somaclones in this trait.     

Modelling herbicide dose and weed density effects on crop:weed competition

The effects of a range of herbicide doses on crop:weed competition were investigated by measuring crop yield and weed seed production. Weed competitivity of wheat was greater in cv. Spark than in cv. Avalon, and decreased with increasing herbicide dose, being well described by the standard dose–response curve. A combined model was then developed by incorporating the standard dose–response curve into the rectangular hyperbola competition model to describe the effects of plant density of a model weed, Brassica napus L., and a herbicide, metsulfuron‐methyl, on crop yield and weed seed production. The model developed in this study was used to describe crop yield and weed seed production, and to estimate the herbicide dose required to restrict crop yield loss caused by weeds and weed seed production to an acceptable level. At the acceptable yield loss of 5% and the weed density of 200 B. napus plants m^–2, the model recommends 0.9 g a.i. metsulfuron‐methyl ha^–1 in Avalon and 2.0 g a.i. in Spark.     

Early physiological and biochemical responses of soyabean to neighbouring weeds under resource‐independent competition

The mechanisms by which weeds compete with crop plants are poorly understood. To gain insight into these mechanisms, we characterised early physiological responses of soyabean to neighbouring weeds using a biological weedy system that generated a consistent far‐red‐enriched light environment and excluded direct resource competition. Neighbouring weeds decreased superoxide dismutase activity in unifoliate leaves. This coincided with increased hydrogen peroxide (H₂O₂) and oxidized ascorbate levels, while the steady‐state level of superoxide, catalase activity and lipid peroxidation remained unchanged. These responses suggested increased leaf production of singlet oxygen (¹O₂), which was demonstrated by detection of increased Singlet Oxygen Sensor Green (SOSG) fluorescence within 3 h after staining of unifoliate leaves. This finding was further supported by increased ratios of the photosensitiser protochlorophyllide to both chlorophyllide a, and total chlorophyll in the dark as well as enhanced sensitivity to cell death by a ¹O₂‐generating compound in the light. These responses coincided with dramatic changes in photosynthesis, carbon partitioning and biomass allocation with a persistent decline in leaf sucrose level and biomass production at later growth stages. This study provides direct experimental evidence that under resource‐independent competition, far‐red‐enriched light reflected by neighbouring weeds can alter the balance between ROS production and detoxification and thereby generate an oxidative stress signal in soyabean leaves.     

Resistance levels of sulfonylurea‐resistant Schoenoplectus juncoides (Roxb.) Palla with various Pro197 mutations in acetolactate synthase to imazosulfuron,bensulfuron‐methyl,metsulfuron‐methyl and imazaquin‐ammonium

An investigation, using herbicidal pot tests in a greenhouse condition, was conducted to determine the whole‐plant dose–response relationships to several acetolactate synthase (ALS)‐inhibiting herbicides of sulfonylurea (SU)‐resistant Schoenoplectus juncoides with various Pro₁₉₇ mutations in ALS that was collected from Japanese rice paddy fields. All the tested SU‐resistant accessions with a Pro₁₉₇ mutation were highly resistant to two commonly used SU herbicides (imazosulfuron and bensulfuron‐methyl), but were much less resistant to another SU herbicide, metsulfuron‐methyl, and were substantially not resistant to imazaquin‐ammonium. These cross‐resistance patterns have been known previously in fragments of S. juncoides and other weed species and were comprehensively confirmed in this study with a whole set of Pro₁₉₇ mutations. The analyses of resistance levels, based on ED₉₀ values, newly showed that different accessions with a common amino acid substitution in ALS1 showed similar responses to these herbicides (confirmed with four amino acid substitutions), that the rankings of resistance levels that were conferred by various Pro₁₉₇ mutations in ALS1 differed among the SU herbicides and that the resistance levels of the ALS2‐mutated accessions were higher than, lower than or similar to those of the corresponding ALS1‐mutated accessions, depending on the compared pair, but the deviation patterns were generally similar among the SU herbicides in each compared pair. The final finding might suggest that the abundance of ALS2 is not as stable as that of ALS1. In addition, as a result of these new findings, together with expected further research, a suggested possibility is that substituting amino acids at Pro₁₉₇ generally could be estimated by plotting each accession's ED₉₀ values of imazosulfuron and bensulfuron‐methyl in a two‐dimensional graph.     

Non‐target‐site mechanism of glyphosate resistance in Italian ryegrass (Lolium multiflorum)

In Shizuoka Prefecture, Japan, glyphosate‐resistant Lolium multiflorum is a serious problem on the levees of rice paddies and in wheat fields. The mechanism of resistance of this biotype was analyzed. Based on LD₅₀, the resistant population was 2.8–5.0 times more resistant to glyphosate than the susceptible population. The 5‐enolpyruvyl‐shikimate‐3‐phosphate synthase (EPSPS) gene sequence of the resistant biotype did not show a non‐synonymous substitution at Pro106, and amplification of the gene was not observed in the resistant biotype. The metabolism and translocation of glyphosate were examined 4 days after application through the direct detection of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) using liquid chromatograph‐tandem mass spectrometer (LC‐MS/MS). AMPA was not detected in either biotype in glyphosate‐treated leaves or the other plant parts. The respective absorption rates of the susceptible and resistant biotypes were 37.90 ± 3.63% and 41.09 ± 3.36%, respectively, which were not significantly different. The resistant biotype retained more glyphosate in a glyphosate‐treated leaf (91.36 ± 1.56% of absorbed glyphosate) and less in the untreated parts of shoots (5.90 ± 1.17%) and roots (2.76 ± 0.44%) compared with the susceptible biotype, 79.58 ± 3.73%, 15.77 ± 3.06% and 4.65 ± 0.89%, respectively. The results indicate that the resistance mechanism is neither the acquisition of a metabolic system nor limiting the absorption of glyphosate but limited translocation of the herbicide in the resistant biotype of L. multiflorum in Shizuoka Prefecture.     

Impact of Brassica juncea biofumigation on viability of propagules of pernicious weed species

Biofumigation may be a promising tool for depletion of persistent weed seedbanks/bud banks. This technique is based on the incorporation of chopped glucosinolate‐rich plant biomass into the soil, upon which isothiocyanates with herbicidal properties are released. To gain acceptance by farmers and foster its implementation, the biofumigation process should be further optimised. This study elucidated the impact of biological (species), technical (burial depth, ground cover) and pedohydrological (temperature and moisture content) factors on efficacy of Brassica juncea biofumigation under (semi‐)natural conditions. In a first experiment (field experiment), seeds and vegetative propagules of various weed species were buried at different depths and exposed to different doses of fresh fine‐chopped B. juncea biomass in the presence or absence of a plastic ground cover. In a second experiment (container experiment), buried seeds of ten species were subjected to biofumigation at diverging soil organic matter content, soil moisture content and soil temperature. In a third experiment (dose–response Petri dish bioassay), unburied seeds of eight species were subjected to various doses of rehydrated B. juncea powder. Biofumigation efficacy was determined by analysing viability of treated and untreated propagules. In general, efficacy of biofumigation increased with decreasing burial depth and increasing B. juncea dose. Biofumigation was highly effective (mortality >85%) against small‐seeded species but less effective (mortality 0%–20%) against hard‐seeded and large‐seeded species at 200 t ha^?1. Vegetative propagules of Sonchus arvensis, Equisetum arvense and Convolvulus sepium were highly sensitive (mortality >90%) to biofumigation. Efficacy was most pronounced under moist warm incubation conditions, in the presence of a plastic ground cover.     

Effect of herbicides and doses on short- and long-term control of Eleusine tristachya

In Argentina, Eleusine tristachya has been recently reported as a problematic weed that can occur at high densities in spring and summer in fallows and in maize and soyabean. The reason for the increase in E. tristachya populations is that once the weed is established, it is difficult to eliminate because it produces a high number of seeds and plant regrowth occurs after herbicide treatments. The aim of this study was to determine the effect of post-emergence application of herbicides (glyphosate, haloxyfop-methyl and clethodim) at the seedling, vegetative and reproductive stages on the short-term (biomass 30 days after treatment—30 DAT) and long-term (tiller number and height and seed production—regrowth at 330 DAT) control of E. tristachya selected biotypes in outdoor pot experiments. Data fitted to a log-logistic model. For all the herbicides considered, at the seedling and vegetative stages, short-term control was achieved with low ED₅₀ and ED₉₀ values, whereas at the reproductive stage, ED₉₀ values were three- to sixfold (glyphosate), six- to 52-fold (haloxyfop-methyl) and five- to 13-fold (clethodim) higher. Long-term control at the recommended dose or lower was not possible at the reproductive stage as tiller regrowth and seed production occurred with all herbicides. It is advisable to control Eleusine tristachya when plants are small at the beginning of the growing season using the recommended herbicide dose. A delayed application will produce tiller regeneration the following year, and consequently, control would only be achieved applying an overdose, which can cause risks to health and the environment. We concluded that a management programme based on the combination of glyphosate with post-emergence graminicides applied at early stages will be effective to control future infestations.     

Intraregional and inter‐regional variability of herbicide sensitivity in common arable weed populations

The question on intraregional versus inter‐regional variability in herbicide sensitivity for weed populations is of major importance, both in extrapolation of model parameters and in herbicide zonal approval procedures. We hypothesised that inter‐regional variability in herbicide sensitivity for field populations would be the same as intraregional variability for regions with similar climatic conditions. Seeds of field weed populations were collected in a Danish, German and Polish region. Herbicide sensitivity was tested in dose–response experiments in the glasshouse with flufenacet and iodosulfuron (Apera spica‐venti), florasulam and tribenuron (Tripleurospermum inodorum), diflufenican, diflufenican + flurtamone and pendimethalin (Viola arvensis). ED₅₀ values and variance components of the ED₅₀ values were estimated to describe the influence of region, year and population. The regions accounted for a maximum of 26% of the variance and always less than the variance accounted for by individual populations. Sensitivity indices (SI₅₀) were calculated as the ratio between ED₅₀ of the test population and a reference population. There was considerable intraregional variability in SI₅₀ values and SI₅₀ values from a single region did not consistently differ from other regions. The large intraregional variability in herbicide sensitivity between populations, with no evidence of resistance, is of interest both for zonal evaluation of herbicides and resistance research. For practical weed management, we conclude that dose–response functions can be transferred between the study regions, for example for the common use in decision support systems with proper insurance for the control of less sensitive populations.