The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

On the Mechanism of Resistance to Paraquat in Hordeum glaucum and H. leporinum: Delayed Inhibition of Photosynthetic O(2) Evolution after Paraquat Application

The mechanism of resistance to paraquat was investigated in biotypes of Hordeum glaucum Steud. and H. leporinum Link. with high levels of resistance. Inhibition of photosynthetic O₂ evolution after herbicide application was used to monitor the presence of paraquat at the active site. Inhibition of photosynthetic O₂ evolution after paraquat application was delayed in both resistant biotypes compared with the susceptible biotypes; however, this differential was more pronounced in the case of H. glaucum than in H. leporinum. Similar results could be obtained with the related herbicide diquat. Examination of the concentration dependence of paraquat-induced inhibition of O₂ evolution showed that the resistant H. glaucum biotype was less affected by herbicide compared with the susceptible biotype 3 h after treatment at most rates. The resistant H. leporinum biotype, in contrast, was as inhibited as the susceptible biotype except at the higher rates. In all cases photosynthetic O₂ evolution was dramatically inhibited 24 h after treatment. Measurement of the amount of paraquat transported to the young tissue of these plants 24 h after treatment showed 57% and 53% reductions in the amount of herbicide transported in the case of the resistant H. glaucum and H. leporinum biotypes, respectively, compared with the susceptible biotypes. This was associated with 62% and 66% decreases in photosynthetic O₂ evolution of young leaves in the susceptible H. glaucum and H. leporinum biotypes, respectively, a 39% decrease in activity for the resistant H. leporinum biotype, but no change in the resistant H. glaucum biotype. Photosynthetic O₂ evolution of leaf slices from resistant H. glaucum was not as inhibited by paraquat compared with the susceptible biotype; however, those of resistant and susceptible biotypes of H. leporinum were equally inhibited by paraquat. Paraquat resistance in these two biotypes appears to be a consequence of reduced movement of the herbicide in the resistant plants; however, the mechanism involved is not the same in H. glaucum as in H. leporinum.     

Paraquat resistance in conyza

A biotype of Conyza bonariensis (L.) Cronq. (identical to Conyza linefolia in other publications) originating in Egypt is resistant to the herbicide 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat). Penetration of the cuticle by [¹⁴C]paraquat was greater in the resistant biotype than the susceptible (wild) biotype; therefore, resistance was not due to differences in uptake. The resistant and susceptible biotypes were indistinguishable by measuring in vitro photosystem I partial reactions using paraquat, 6,7-dihydrodipyrido [1,2-α:2′,1′-c] pyrazinediium ion (diquat), or 7,8-dihydro-6H-dipyrido [1,2-α:2′,1′-c] [1,4] diazepinediium ion (triquat) as electron acceptors. Therefore, alteration at the electron acceptor level of photosystem I is not the basis for resistance. Chlorophyll fluorescence measured in vivo was quenched in the susceptible biotype by leaf treatment with the bipyridinium herbicides. Resistance to quenching of in vivo chlorophyll fluorescence was observed in the resistant biotype, indicating that the herbicide was excluded from the chloroplasts. Movement of [¹⁴C] paraquat was restricted in the resistant biotype when excised leaves were supplied [¹⁴C]paraquat through the petiole. We propose that the mechanism of resistance to paraquat is exclusion of paraquat from its site of action in the chloroplast by a rapid sequestration mechanism. No differential binding of paraquat to cell walls isolated from susceptible and resistant biotypes could be detected. The exact site and mechanism of paraquat binding to sequester the herbicide remains to be determined.     

Investigations of mechanisms of resistance to bipyridyl herbicides in Arctotheca calendula (L.) Levyns

The mechanism of resistance to diquat and paraquat was investigated in a bipyridyl-herbicide-resistant biotype of Arctotheca calendula (L.) Levyns. No differences were observed in the interactions of these herbicides with Photo-system I, the active site, in thylakoids isolated from resistant and susceptible biotypes. Likewise, absorption of herbicide through the cuticle and gross translocation were identical in plants of the two biotypes. Foliar application of either 25 g ha⁻¹ diquat or 200 g ha⁻¹ paraquat rapidly inhibited CO₂-dependent O₂ evolution of leaf segments of the susceptible biotype. O₂ evolution of leaf segments of the resistant biotype was less affected by these treatments. Fluorescence imaging was used to observe visually, as fluorescence quenching, the penetration of herbicide to the active site. These experiments demonstrated that diquat appears at the active site more slowly in the resistant biotype compared to the susceptible biotype. HCO₃-dependent O₂ evolution of thin leaf slices was less inhibited by diquat in the resistant biotype than in the susceptible biotype. The mechanism of resistance to the bipyridyl herbicides in this biotype of A. calendula is not a result of changes at the active site, decreased herbicide absorption or decreased translocation, but appears to be due to reduced herbicide penetration to the active site.     

Effect of growth temperature on biomass production of nearly isonuclear triazine-resistant and -susceptible common groundsel (Senecio vulgaris L.)

Abstract. The effect of growth temperature on biomass production and photosynthesis of nearly-isonuclear triazine-resistant and -susceptible Senecio vulgaris L. bio-types was investigated. Plants were grown in growth chambers with day/night temperatures of 13/8, 20/15 and 30/25°C, and were harvested 35, 42, 49 and 56d after planting (DAP). The S biotype produced more shoot dry weight than the R × S_BC6 biotype, and the S × R_BC6 biotype produced more shoot dry weight than the R biotype at all DAP and growth temperature combinations. The S and S × R_BC6 biotypes had greater photosynthetic rates than the R X S_BC6 and R biotypes, respectively. Thus, plants containing the susceptible chloroplast genome produced more biomass and had greater photosynthetic rates than those with the resistant chloroplast genome, when in association with the same nuclear genome. There was no differential temperature effect on biomass production of isonuclear plants possessing resistant or susceptible chloroplast genomes. However, there was a large differential temperature effect on the amount of biomass produced by plants containing different nuclear genomes (R or S) in association with the same chloroplast genome. The R nuclear genome appeared to be better adapted to cooler growth temperatures while the S nuclear genome was better adapted to warmer growth conditions.     

A mechanism of chlorotoluron resistance in Lolium rigidum

Many biotypes of Lolium rigidum Gaud, (annual ryegrass) have developed resistance to herbicides; however, few have developed resistance to phenylurea herbicides. Two biotypes with different histories of herbicide selection pressure were six to eight times less sensitive to the phenylurea herbicide, chlorotoluron, than a susceptible biotype. Resistance was not due to differences in the herbicide target site as oxygen evolution by thylakoids isolated from resistant and susceptible biotypes was similarly inhibited by diuron and chlorotoluron. There was no difference in the uptake and distribution of chlorotoluron into resistant and susceptible plants. There was a twofold greater rate of chlorotoluron detoxification in resistant plants with N-demethylation being a major detoxification reaction. Resistant plants treated with a 3-h pulse of 120 M chlorotoluron recovered net carbon fixation after 42 h, half the time taken by susceptible plants. The mixed-function oxidase inhibitor 1-aminobenzotriazole (70 M) intensified the effects of chlorotoluron in resistant plants when applied in combination with the herbicide for 7 d. 1-Aminobenzotriazole also inhibited the metabolism of chlorotoluron in both resistant and susceptible plants. The cytochrome P-₄₅₀ inhibitor, piperonyl butoxide piperonyl butoxide, interacted with chlorotoluron when applied to plants growing in soil. Chlorotoluron applied with reduced plant dry weight to a greater extent than chlorotoluron alone. It appears, therefore, that enhanced detoxification is the major mechanism of resistance to chlorotoluron in the resistant biotypes studied.Abbreviations ABT 1-aminobenzotriazole - VLR1 Victorian L. rigidum biotype 1 — herbicide susceptible - VLR69 Victorian L. rigidum biotype 69 — herbicide resistant - WLR2 Western Australian L. rigidum biotype 2 — herbicide resistant M.W.M.B, was supported by an Australian Postgraduate Research Award and a supplementary scholarship from the Grains Research and Development Corporation. We are very grateful to Dr. E. Ebert, Ciba Geigy, Basal, Switzerland for providing [¹⁴C]chlorotoluron and standards of chlorotoluron metabolites. We express our gratitude to Dr. John Huppatz of the CSIRO Division of Plant Industry for providing ABT. We also thank Ciba Geigy Australia for providing technical-grade chlorotoluron and formulated phenylurea herbicides.     

Resistant Acetyl-CoA Carboxylase is a Mechanism of Herbicide Resistance in a Biotype of Avena sterilis ssp. ludoviciana

A biotype of Avena sterilis ssp. ludoviciana is highly resistantto a range of herbicides which inhibit a key enzyme in fattyacid synthesis, acetyl-CoA carboxylase (ACCase). Possible mechanismsof herbicide resistance were investigated in this biotype. Acetyl-CoAcarboxylase from the resistant biotype is less sensitive toinhibition by herbicides to which resistance is expressed. I₅₀values for herbicide inhibition of ACCase were 52 to 6 timesgreater in the resistant biotype than in the susceptible biotype.This was the only major difference found between the resistantand susceptible biotypes. The amount of ACCase in the meristemsof the resistant and susceptible is similar during ontogenyand no difference was found in distribution of ACCase betweenthe two biotypes. Uptake, translocation and metabolism of [¹⁴C]diclofop-methylwere not different between the two biotypes. In vivo, ACCaseactivity in the meristems of the susceptible biotype was greatlyinhibited by herbicide application whereas only 25% inhibitionoccurred in the resistant biotype. Depolarisation of plasmamembrane potential by 50 µM diclofop acid was observedin both biotypes and neither biotype showed recovery of themembrane potential following removal of the herbicide. Hence,a modified form of ACCase appears to be the major determinantof resistance in this resistant wild oat biotype. (Received February 10, 1994; Accepted March 11, 1994)     

Evidence for vacuolar sequestration of paraquat in roots of a paraquat-resistant Hordeum glaucum biotype

Paraquat resistance in the grass weed Hordeum glaucum Steud. has been proposed to result from herbicide sequestration away from the growing points. In the present study, we used roots as a model system to investigate cellular transport of paraquat in resistant (R) and susceptible (S) H. glaucum biotypes. Both time- and concentration-dependent kinetics of paraquat influx across the root cell plasma membrane were similar in the S and R biotype. However, compartmentation analysis indicated greater herbicide accumulation in root vacuoles of the R seedlings. In contrast, the amount of paraquat accumulated in the cytoplasm of S was double that found in R biotype. While paraquat efflux from the cytoplasm back into the external solution was similar in the two biotypes, efflux across the tonoplast from the vacuole back into the cytoplasm was 5 times slower in the R than in the S biotype. At the end of a 48-h efflux period, nearly 7-fold more herbicide was retained in the roots of the R compared with those of the S biotype. These results suggest that paraquat resistance in H. glaucum may be due to the herbicide sequestration in the vacuole.     

Differential Light Responses of Photosynthesis by Triazine-resistant and Triazine-susceptible Senecio vulgaris Biotypes

Studies were conducted to determine a physiological basis for competitive differences between Senecio vulgaris L. biotypes which are either resistant or susceptible to triazine herbicides. Net carbon fixation of intact leaves of mature plants was higher at all light intensities in the susceptible biotype than in the resistant biotype. Quantum yields measured under identical conditions for each biotype were 20% lower in the resistant than in the susceptible biotype. Oxygen evolution in continuous light measured in stroma-free chloroplasts was also higher at all light intensities in the susceptible biotype than in the resistant biotype. Oxygen evolution in response to flashing light was measured in stroma-free chloroplasts of both biotypes. The steady-state yield per flash of resistant chloroplasts was less than 20% that of susceptible chloroplasts. Susceptible chloroplasts displayed oscillations in oxygen yield per flash typically observed in normal chloroplasts, whereas the pattern of oscillations in resistant chloroplasts was noticeably damped. It is suggested that modification of the herbicide binding site which confers s-triazine resistance may also affect the oxidizing side of photosystem II, making photochemical electron transport much less efficient. This alteration has resulted in a lowered capacity for net carbon fixation and lower quantum yields in whole plants of the resistant type.     

Atrazine resistance entails a limited xanthophyll cycle activity,a lower PSII efficiency and an altered pattern of excess excitation dissipation

Atrazine-resistant (AR) weeds have a modified D1 protein structure, with a Ser₂₆₄→Gly mutation on the D1 protein, near the plastoquinone binding niche. The photosynthetic performance, the light response of the xanthophyll cycle and chlorophyll fluorescence quenching-related parameters were compared in attached leaves of susceptible (S) and AR biotypes of the C₃ dicot Chenopodium album L., Epilobium adenocaulon Hausskn., Erigeron canadensis L., Senecio vulgaris L. and Solanum nigrum L. and the C₄ dicot Amaranthus retroflexus L. grown under natural high-light conditions. No significant difference in CO₂ assimilation rate per leaf area unit was found between the S and AR biotypes of the investigated C₃ plants, whereas the AR biotype of A. retroflexus exhibited a relatively poor photosynthetic performance. The D1 protein mutant plants expressed a reduced activity of light-stimulated zeaxanthin formation. Neither the lower violaxanthin de-epoxidase activity nor the depletion of ascorbate seems to be the cause of the lower in vivo zeaxanthin formation in the AR plants. All the D1 mutant weeds had limited light-induced non-photochemical (NPQ) and photochemical (q_P) quenching capacities, and displayed a higher photosensitivity, as characterized by the ratio (1-q_P)/NPQ and a higher susceptibility to photoinhibition. Analysis of the chlorophyll fluorescence parameters showed that a lower proportion of excitation energy was allocated to PSII photochemistry, while a higher excess of excitation remained in the AR weeds relative to the S plants.     

Purification and Characterization of Acetyl-Coenzyme A Carboxylase from Diclofop-Resistant and -Susceptible Lolium multiflorum

Acetyl-coenzyme A carboxylase (ACCase) was purified >100-fold (specific activity 3.5 units mg-1) from leaf tissue of diclofopresistant and -susceptible biotypes of Lolium multiflorum. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified fractions from both biotypes contained a single 206-kD biotinylated polypeptide. The molecular mass of the native enzyme from both biotypes was approximately 520 kD. In some cases the native dimer from both biotypes dissociated during gel filtration to form a subunit of approximately 224 kD. The inclusion of 5% (w/v) polyethylene glycol 3350 (PEG) in the elution buffer prevented this dissociation. Steady-state substrate kinetics were analyzed in both the presence and absence of 5% PEG. For ACCase from both biotypes, addition of PEG increased the velocity 22% and decreased the apparent Km values for acetyl-coenzyme A (acetyl-CoA), but increased the Km values for bicarbonate and ATP. In the presence of PEG, the Km values for bicarbonate and ATP were approximately 35% higher for the enzyme from the susceptible biotype compared with the resistant enzyme. In the absence of PEG, no differences in apparent Km values were observed for the enzymes from the two biotypes. Inhibition constants (Ki app) were determined for CoA, malonyl-CoA, and diclofop. CoA was an S-hyperbolic (slope replots)-I-hyperbolic (intercept replots) noncompetitive inhibitor with respect to acetyl-CoA, with Ki app values of 711 and 795 [mu]M for enzymes from the resistant and susceptible biotypes, respectively. Malonyl-CoA competitively inhibited both enzymes (versus acetyl-CoA) with Ki app values of 140 and 104 [mu]M for ACCase from resistant and susceptible biotypes, respectively. Diclofop was a linear noncompetitive inhibitor of ACCase from the susceptible biotype and a nonlinear, or S-hyperbolic-I-hyperbolic, noncompetitive inhibitor of ACCase from the resistant biotype. For ACCase from the susceptible biotype the slope (Kis) and intercept (Kii) inhibition constants for diclofop versus acetyl-CoA were 0.08 and 0.44 [mu]M, respectively. ACCase from the resistant biotype had a Ki app value of 6.5 [mu]M. At a subsaturating acetyl-CoA concentration of 50 [mu]M, the Hill coefficients for diclofop binding were 0.61 and 1.2 for ACCase from the resistant and susceptible biotypes, respectively. The Hill coefficients for diclofop binding and the inhibitor replots suggest that the resistant form of ACCase exhibits negative cooperativity in binding diclofop. However, the possibility that the nonlinear inhibition of ACCase activity by diclofop in the enzyme fraction isolated from the resistant biotype is due to the presence of both resistant and susceptible forms of ACCase cannot be excluded.     

Germination ecology of Ambrosia artemisiifolia L. and Ambrosia trifida L. biotypes suspected of glyphosate resistance

The germination ecology of Ambrosia artemisiifolia and A. trifida glyphosate susceptible biotypes sampled in marginal areas, was compared with that of the same species but different biotypes suspected of glyphosate resistance, common and giant ragweed, respectively. The suspected resistant biotypes were sampled in Roundup Ready® soybean fields. Within each weed species, the seeds of the biotype sampled in marginal area were significantly bigger and heavier than those of the biotype sampled in the soybean fields. A. artemisiifolia biotypes exhibited a similar dormancy and germination, while differences between A. trifida biotypes were observed. A. artemisiifolia biotypes showed similar threshold temperature for germination, whereas, the threshold temperature of the susceptible A. trifida biotype was half as compared to that of the resistant A. trifida biotype. No significant differences in emergence as a function of sowing depth were observed between susceptible A. artemisiifolia and suspected resistant A. trifida biotype, while at a six-cm seedling depth the emergence of the A. artemisiifolia susceptible biotype was 2.5 times higher than that of the A. trifida suspected resistant biotype. This study identified important differences in seed germination between herbicide resistant and susceptible biotypes and relates this information to the ecology of species adapted to Roundup Ready® fields. Information obtained in this study supports sustainable management strategies, with continued use of glyphosate as a possibility.     

Physiological tests for early detection of rigid ryegrass (Lolium rigidum Goud.) resistance to fenoxaprop-P

The study aimed to determine the usefulness of isothermal calorimetry and FT-Raman spectroscopy for the early evaluation of rigid ryegrass resistance to fenoxaprop-P ethyl (active ingredient one of aryloxyphenoxypropionate herbicides). The calorimetric measurements were done on the 4-day-old seedlings of susceptible and resistant biotypes of rigid ryegrass (Lolium rigidum Goud.) for 72 h, at 20 °C. It was observed that the specific thermal power–timecurves of the susceptible and resistant biotypes growing on water (control) were qualitatively similar. Herbicides changed the shape of the specific thermal power–time curves of both biotypes. Furthermore, the total specific thermal energy was significantly higher for the seedlings of resistant biotype, growing both on water or herbicide, as compared to the susceptible ones. The analysis of the seedlings’ endosperm, conducted using FT-Raman spectroscopy, showed a weaker intensity of the bands in the spectra derived from the resistant biotype. Differences in the specific thermal power–time curves and FT-Raman spectra between susceptible and resistant biotypes growing on water indicate that the sensitive and resistant biotypes are metabolically and chemically different already in the early stages of the seedling growth. We conclude that isothermal calorimetry and FT-Raman spectroscopy are efficient tools for the early detection of rigid ryegrass resistance to fenoxaprop-P ethyl.     

Pollen Expression of Herbicide Target Site Resistance Genes in Annual Ryegrass (Lolium rigidum)

Herbicide resistance can occur either through target-site insensitivity or by nontarget site-based mechanisms. Two herbicide-resistant biotypes of Lolium rigidum Gaud., one resistant to acetolactate synthase (ALS)-inhibiting herbicides (biotype WLR1) and the other resistant to acetyl CoA carboxylase (ACCase)-inhibiting herbicides (biotype WLR96) through target-site insensitivity at the whole plant and enzymic levels, were found to express this resistance in the pollen. Pollen produced by resistant biotypes grew uninhibited when challenged with herbicide, whereas that from a susceptible biotype was inhibited. A third biotype, SLR31, resistant to ACCase-inhibiting and certain ALS-inhibiting herbicides at the whole plant level through nontarget site-based mechanisms, did not exhibit this expression in the pollen. The technique described may form the basis for a rapid screen for certain nuclear-encoded, target site-based herbicide-resistance mechanisms.     

In vitro susceptibility of Clostridium perfringens isolated from farm animals to growth-enhancing antibiotics

Minimal inhibitory concentration (MIC) determinations were carried out with seven growth-enhancing antibiotics against 95 Clostridium perfringens field isolates obtained during 1991 and 1992 from poultry, pigs and calves. All were resistant to 64 μg ml⁻¹ of the bambermycin antibiotic, flavomycin (flavophospholipol) and susceptible to avoparcin (MIC₉₀ 0.25 μg ml⁻¹), avilamycin (MIC₉₀ 0.5 μg ml⁻¹) and salinomycin (MIC₉₀≤ 0.12 μg ml⁻¹). Acquired resistance against bacitracin was detected in some isolates from poultry and bovines and resistance to tylosin and virginiamycin in some strains from all species investigated. Overall, the prevalence of resistance was comparable to the low levels recorded in 1979 in Cl. perfringens isolates from the same animal host species.     

Developmental Variability of Photooxidative Stress Tolerance in Paraquat-Resistant Conyza

Paraquat-resistant hairy fleabane (Conyza bonariensis L. Cronq.) has been extensively studied, with some contention. A single, dominant gene pleiotropically controls levels of oxidant-detoxifying enzymes and tolerance to many photooxidants, to photoinhibition, and possibly to other stresses. The weed forms a rosette on humid short days and flowers in dry long days and, thus, needs plasticity to photooxidant stresses. In a series of four experiments over 20 months, the resistant and susceptible biotypes were cultured in constant 10-h low-light short days at 25[deg]C. Resistance was measured as recovery from paraquat. The concentration required to achieve 50% inhibition of the resistant biotype was about 30 times that of the susceptible one just after germination, increased to >300 times that of the susceptibles at 10 weeks of growth, and then decreased to 20-fold, remaining constant except for a brief increase while bolting. Resistance increased when plants were induced to flower by long days. The levels of plastid superoxide dismutase and of glutathione reductase were generally highest in resistant plants compared to those of the susceptibles at the times of highest paraquat resistance, but they were imperceptibly different from the susceptible type at the times of lower paraquat resistance. Photoinhibition tolerance measured as quantum yield of oxygen evolution at ambient temperatures was highest when the relative amounts of enzymes were highest in the resistant biotype. Resistance to photoinhibition was not detected by chlorophyll a fluorescence. Enzyme levels, photoinhibition tolerance, and paraquat resistance all increased during flowering in both biotypes. Imperceptibly small increases in enzyme levels would be needed for 20-fold resistance, based on the moderate enzyme increases correlated with 300-fold resistance. Thus, it is feasible that either these enzymes play a role in the first line of defense against photooxidants, or another, yet unknown mechanism(s) facilitate(s) the lower level of resistance, or the enzymes and unknown mechanisms act together.     

Glyphosate,paraquat and ACCase multiple herbicide resistance evolved in a <Emphasis Type="Italic">Lolium rigidum</Emphasis> biotype

Glyphosate is the world’s most widely used herbicide. A potential substitute for glyphosate in some use patterns is the herbicide paraquat. Following many years of successful use, neither glyphosate nor paraquat could control a biotype of the widespread annual ryegrass (Lolium rigidum), and here the world’s first case of multiple resistance to glyphosate and paraquat is confirmed. Dose–response experiments established that the glyphosate rate causing 50% mortality (LD₅₀) for the resistant (R) biotype is 14 times greater than for the susceptible (S) biotype. Similarly, the paraquat LD₅₀ for the R biotype is 32 times greater than for the S biotype. Thus, based on the LD₅₀ R/S ratio, this R biotype of L. rigidum is 14-fold resistant to glyphosate and 32-fold resistant to paraquat. This R biotype also has evolved resistance to the acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides. The mechanism of paraquat resistance in this biotype was determined as restricted paraquat translocation. Resistance to ACCase-inhibiting herbicides was determined as due to an insensitive ACCase. Two mechanisms endowing glyphosate resistance were established: firstly, a point mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, resulting in an amino acid substitution of proline to alanine at position 106; secondly, reduced glyphosate translocation was found in this R biotype, indicating a co-occurrence of two distinct glyphosate resistance mechanisms within the R population. In total, this R biotype displays at least four co-existing resistance mechanisms, endowing multiple resistance to glyphosate, paraquat and ACCase herbicides. This alarming case in the history of herbicide resistance evolution represents a serious challenge for the sustainable use of the precious agrochemical resources such as glyphosate and paraquat.     

Effect of light and gibberellic acid on photosynthesis during leaf senescence of alstroemeria cut flowers.

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas-exchange measurements and analysis of in vivo chlorophyll fluorescence. Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA₃). The maximal photosynthesis of the leaves was approximately 6 μmol CO₂ m⁻² s⁻¹ at I 350 μmol m⁻² s⁻¹ (PAR) which is relatively low for intact C₃ leaves. Qualitatively the gas-exchange rates followed the decline in chlorophyll content for the various treatments, i.e. light and GA₃-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosynthetic rates were already strongly decreased. In vivo fluorescence measurements revealed that the decrease in CO₂ uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analysis of the fluorescence data showed a high nonphotochemical quenching under all experimental conditions, indicating that the consumption of reducing power in the Calvin cycle is very low. The chlorophyll, remaining after 9 days incubation of leaves with GA₃ in the dark should be considered as a 'cosmetic' pigment without any function in the supply of assimilates to the flowers.     

Regulation of Photosynthesis in Triazine-Resistant and -Susceptible Brassica napus

The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO₂ partial pressure (C_i), O₂ partial pressure, and leaf temperature (15-35°C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. Three categories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 35°C leaf temperatures with low C_i. When the carbon assimilation rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters were similar between the two biotypes. A second category, called feedback-limited photosynthesis, was evident at 15 and 25°C above 300 microbars C_i. The third category, photosynthetic electron transport-limited photosynthesis, was evident at 25 and 35°C at moderate to high CO₂. At low temperature, when the response curves of carbon assimilation to C_i indicated little or no electron transport limitation, the carbon assimilation rate was similar in the resistant and susceptible biotypes. With increasing temperature, more electron transport-limited carbon assimilation was observed, and a greater difference between resistant and susceptible biotypes was observed. These observations reveal the increasing importance of photosynthetic electron transport in controlling the overall rate of photosynthesis in the resistant biotype as temperature increases. Photochemical quenching of chlorophyll fluorescence (q_P) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q_P, but not when it had less than 60% q_P.     

The Effect of Light and Temperature on Competition between Atrazine Susceptible and ResistantBrassica rapa

Biotypes ofBrassica rapasusceptible (S) and resistant (R) toatrazine were grown in competitive replacement series in allpossible combinations of two light levels and three temperatureregimes in controlled growth cabinets. Photosystem II functionwas investigated in all conditions by fluorescence-inductiontechniques. There were no significant differences in the dryweight of the two biotypes when grown in pure stands. In purestands both biotypes produced more biomass under the high lightlevel. Under high light both biotypes yielded more biomass athigh temperature; in low light they did so at medium temperature.Under high light conditions at high and medium temperaturesthe susceptible biotype had a greater photon yield and relativecompetitive ability than the resistant due to the greater vulnerabilityof triazine-resistant biotypes to photoinhibition. However,surprisingly, the resistant biotype was the better competitor,and had a higher photon yield, in the high light/low temperatureregime. In low light no photoinhibition was expected and indeedthere were no significant differences in any fluorescence parametersbetween the resistant and susceptible biotypes. Nevertheless,there were differences in the whole plant performance; the susceptiblebiotype was a better competitor at low and medium temperatures,but the resistant biotype was better at high temperature. Relativelysmall variations in both light and temperature, well withinthe range encountered during British summer time, can have largeeffects on the relative competitiveness of triazine R and Sbiotypes in this species with implications for the spread ofresistance genes through semi-natural communities. In lightof predicted climate changes, interactions between climate andresistance should be studied across a wider range of herbicidetypes and weed species.Copyright 1997 Annals of Botany Company Brassica rapa; chlorophyll fluorescence; competition; light; navew; temperature; triazine resistance