G. Marshall

Biology and control of Phalaris minor Retz. (littleseed canarygrass) in wheat

Abstract Phalaris minor Retz. (littleseed canarygrass) is an important winter season weed of several crops across many continents. P. minor is a prolific and competitive weed especially in wheat crops. This review considers its distribution, biology and agro-ecology. Special importance is attached to considering the value and limitations of cultural and chemical control methods together with a crop management blueprint. While the use of selective herbicides is critical to maintain economic returns of wheat production, there are issues associated with their continuous use. Undoubtedly the development of herbicide-resistant biotypes of P. minor is an epidemic in India of economic, cultural and scientific importance. Consideration is given to the nature of the resistance problem and management approaches designated to minimise the impact of resistance and perhaps avoid further spread of the epidemic. Future research strategies are discussed to address the nature of this important grass weed problem. These include the importance of agronomic and physiological research to understand the basis of weed behaviour.

Absorption, localisation, translocation and activity of glyphosate in barnyardgrass (Echinochloa crus-galli (L) Beauv) : influence of herbicide and surfactant concentration

The influence of sub-lethal and lethal doses of glyphosate (5 µg and 10 µg per plant) applied to the fourth leaf of barnyardgrass (Echinochloa crus-galli) was examined over a treatment period of up to 14 days. Assessments were undertaken on plant growth, chlorophyll fluorescence, absorption and translocation of [14C]glyphosate. Electronic autoradiography and image analysis were used to examine the distribution of [14C]glyphosate over the duration of the study. Major sinks affecting glyphosate distribution included the emerging fifth leaf, the roots and ‘shoot’ (meristem area). At 10 µg per plant, chlorophyll fluorescence declined over the treatment period; in the source and sink leaves effects were particularly evident at 5 DAT. Absorption and translocation of [14C]glyphosate (5 and 10 µg per plant) was rapid during 1–2 DAT, remaining relatively constant thereafter. Approximately 70% of the application was absorbed and, of this, 70% was translocated. The concentration of glyphosate increased in the sinks (the emerging fifth leaf, the roots and shoot (meristem) area) to a maximum at 3 DAT, thereafter declining. This decline was coincident with a decrease (2–3 DAT) in the level of photosynthesis (fluorescence) in the source and sink leaves of plants treated with 10 µg glyphosate. Incorporation of the surfactant MON 0818 at 0.5, 1.0 or 2.0 ml litre−1 enhanced herbicidal activity, absorption, translocation and sink accumulation of [14C]glyphosate (5 µg per plant), with absorption and translocation greatest at 0.5 ml litre−1 at 5 DAT. Herbicidal activity at 12–14 DAT, however, was greatest at the 1.0 ml litre−1 concentration. © 2000 Society of Chemical Industry

Resistance of barnyardgrass (Echinochloa crus‐galli) to atrazine and quinclorac

Two populations of Echinochloa crus-galli (R and I) exhibited resistance to quinclorac. Another population (X) exhibited resistance to quinclorac and atrazine. The R and I populations were collected from monocultures of rice in southern Spain. The X population was collected from maize fields subjected to the application of atrazine over several years. The susceptible (S) population of the same genus was collected from locations which had never been treated with herbicides. The quinclorac ED50 value (dose causing 50% reduction in shoot fresh weight) for the R and I biotypes were 26- and 6-fold greater than for the S biotype. The X biotype was 10 times more tolerant to quinclorac than the S biotype and also showed cross-resistance to atrazine, being 82-fold more resistant to atrazine than the R, I and S biotypes. Chlorophyll fluorescence and Hill reaction analysis supported the view that the mechanism of resistance to atrazine in the X biotype was modification of the target site, the DI protein. Quinclorac at 20 mg litre-1 did not inhibit photosynthetic electron transport in any of the test biotypes. The quinclorac I50 values (herbicide dose needed for 50% Hill reaction reduction) of the S population was over 50000-fold higher than the atrazine I50 value for the same S population, indicating that quinclorac is not a PS II inhibiting herbicide. Propanil at doses greater than 0·5 kg ha-1 controlled all the biotypes.

Effect of ABT on the activity and rate of degradation of isoproturon in susceptible and resistant biotypes of Phalaris minor and in wheat

The effect of the monooxygenase inhibitor, 1-aminobenzotriazole (ABT) on isoproturon phytotoxicity and metabolism was studied in resistant (R) and susceptible (S) biotypes of Phalaris minor and in wheat (Triticum aestivum). Addition of ABT (2·5, 5 and 10 mg litre-1) to isoproturon (0·25, 0·5, 1, 2 and 4 mg litre-1) in the nutrient solution significantly enhanced the phytotoxicity of isoproturon against the R biotype. Isoproturon at 0·25 mg litre-1 reduced the dry weight (DW) of the S biotype by 77%, whereas the R biotype required 4·0 mg litre-1 for similar reduction. Addition of 10 mg litre-1 of ABT to the 0·25 mg litre-1 isoproturon caused 71 and 82% reduction in DW of R and S biotypes, respectively. Wheat was more sensitive to the mixture of isoproturon and ABT than the R biotype of P. minor. Reduced concentrations of ABT in the mixture from 10 to 2·5 mg litre-1 increased the DW of the R biotype more than that of the S biotype. The R biotype metabolised [14C]isoproturon at a faster rate than the S biotype. ABT (5 mg litre-1) inhibited the degradation of [14C]isoproturon in both biotypes of P. minor and in wheat. In the presence of ABT, about half of the applied [14C]isoproturon remained as parent herbicide in all the three species after two days. The metabolites were similar in the R and S biotypes and wheat as determined by co-chromatography with reference standards and mass spectroscopy (MS). ABT inhibited the appearance of the hydroxy and monomethyl metabolites and their conjugates in all the test plants. These results suggest that the activity of the enzymes responsible for the degradation of isoproturon is greater in the R than in the S biotype of P. minor, resulting in its rapid detoxification. Incorporation of the monooxygenase inhibitor ABT into the nutrient solution greatly inhibited the degradation of [14C]isoproturon in the R biotype and increased its phytotoxicity. Both hydroxylation and N-dealkylation reactions were found to be sensitive to ABT; inhibition of hydroxylation was greater than that of demethylation. Since ABT could not completely suppress isoproturon degradation, it is possible that more than one monooxygenase is involved.

Herbicide-tolerant crops - Real farmer opportunity or potential environmental problem

Classical breeding, in-vitro selection and genetic engineering techniques have produced herbicide-tolerant crops. Commercial adoption of these cultivars in North America provides tolerance predominantly to non-selective herbicides for novel weed-control strategies. Of special value is the integration of these crops in minimum tillage situations, maintaining of a wide range of herbicides with different modes of action to provide a variety of opportunities for weed-control management. Associated with these special crops are a series of environmental issues which at present limit the rate of commercial development in Europe. Unlike the successful performance of herbicide tolerance in the crops, these strategic issues are much more difficult to resolve for technical, political, ethical and moral reasons. The primary concerns are the feasibility of controlling the volunteer crop and the opportunity for indiscriminate introgression of the herbicide-tolerance gene into agricultural and natural ecosystems. It is unlikely that these questions will be resolved without scaling-up field experiments to include the detection of herbicide-tolerance genes. The economic and management implications of herbicide-tolerant crops require special consideration in view of the necessity to integrate conventional and transgenic crops in new cropping systems.

Molecular Characterization of Herbicide Resistance in Echinochloa Spp.

Echinochloa spp. are weeds of maize and rice that cause serious yield losses and require to be controlled. Two main herbicides used in these crops are atrazine and quinclorac. Triazine herbicides inhibit photosystem II and quinclorac enhances ethylene production in susceptible plants (Devine et al., 1993; Grossmann and Kwiatkowski, 1993). Development of resistance to atrazine in weeds is well documented (Gronwald, 1994), but quinclorac resistance has not been reported in Echinochloa spp. We describe the characterisation of atrazine and quinclorac resistance in Echinochloa spp. Classification and identification of Echinochloa spp. is notoriously difficult but establishing the identity of herbicide susceptible/wild biotypes of Echinochloa is essential when undertaking comparative mode of action studies with putative herbicide-resistant biotypes of the same species. Accordingly, we employed molecular techniques to make genetic comparisons of test biotypes of Echinochloa, free from the vaguaries of morphological variation and subjective assessment (Carretero, 1981).

Recent developments in flax breeding relevant to production technologies

Abstract This review describes the existing and future objectives in flax breeding for traditional and non-traditional fibre outlets. Specific objectives include improved fibre yield and quality together with resistance to lodging and disease. New biotechniques must now be evaluated to determine their value as adjuncts to classical breeding and selection techniques. These biotechniques include tissue culture methods for somaclonal variation, in vitro selection for disease and herbicide resistance together with anther and microspore culture for haploid production. In addition, a range of molecular techniques are now available to provide gene mapping, isolation, cloning and transformation.