Influence of cutting time and stubble height on regrowth capacity of Juncus effusus and Juncus conglomeratus

Infestation by Juncus effusus and Juncus conglomeratus causes a serious reduction in forage quality along the western coastline in Norway. Timing of treatments, for example cutting to reduce photosynthate return to the weeds, is crucial for successful and cost‐effective weed control. However, the effect may depend on changes in regrowth capacity over the growing season. A growth chamber experiment was conducted to investigate the effect of cutting time and stubble height on the regrowth capacity of J. effusus and J. conglomeratus from western and eastern Norway. Tussocks were collected from extensively managed pastures at 4‐ and 6‐week intervals in 2009 and 2010, respectively. The plants were potted, cut at 1 or 5 cm and exposed to three different temperatures (7.5, 12.5 and 17.5°C) in 2009 and to 17.5°C only in 2010. The regrowth ability was evaluated after 6 weeks by measuring stem number and dry biomass from old mowed and new shoots. Regrowth increased more with temperature for the eastern than for the western ecotype of J. effusus. Cutting to 1 cm stubble height gave substantially less regrowth than 5 cm cutting, both for dry biomass and number of shoots in both years. Regrowth varied significantly with date of cut and was lowest in late summer (mid‐July–August). This suggests that weeding strategies based on low cutting and mowing rush tussocks in late summer will be most effective, due to the low regrowth capacity.     

Nematicidal activity of Ochradenus baccatus against the root‐knot nematode Meloidogyne javanica

Ochradenus baccatus is a widely distributed shrub in desert regions of the Middle East and North Africa. This plant's nematicidal activity against the root‐knot nematode Meloidogyne javanica was evaluated because it has been found to contain exceptionally high levels of glucosinolates. In in vitro assays with aqueous extracts of the plant, 100% of second‐stage juveniles were immobilized after exposure to 4% root‐core extract for 48 h; 8% root‐core extract suppressed their hatching by 87%, whereas stem, flower and root bark showed lower activity. Incorporation of root core or bark into the soil, as fresh or dry powder at 1 and 0·5% (w/w), respectively, reduced the number of nematodes recovered from the soil by 95–100%, whereas the flower and stem were much less effective. Results from further pot experiments indicated that only the root bark consistently contains nematicidal compounds which are effective in soil, whereas the nematicidal activity of the root core in soil was inconsistent. The presence of non‐volatile lipophilic and lipophobic nematicidal compounds in the root bark was suggested by extraction with different polar solvents, but these compounds do not seem to be isothiocyanates – glucosinolate‐hydrolysed compounds with nematicidal activity. Very poor host status of Ochradenus baccatus to M. javanica, M. incognita and M. hapla, but with root‐penetration rates of juveniles similar to those in tomato roots, suggest that this plant may be used as a cover plant or trap plant to reduce nematode populations in the soil.     

Sugarbeet yield response to competition from Sinapis arvensis or Lolium perenne growing at three different distances from the beet and removed at various times during early growth

A sugarbeet field experiment was conducted in 1999 and 2000 to measure beet yield where Sinapis arvensis or Lolium perenne were growing in the crop row at 2, 4 or 8 cm from the beet plants. The weeds were removed by cutting once in the growing season in either late May, mid‐June or early July. The number of neighbouring beet plants to every target beet plant was recorded. Projected leaf cover of a subset of the data with non‐cut weeds was analysed by using image analysis to investigate whether this could be used to predict beet yield loss early in the growing season. Increasing the distance between beet and weed from 2 to 8 cm increased the beet yield significantly by an average of 20%, regardless of weed species. The dry weight of non‐cut and re‐growing weeds at harvest time decreased when cutting was postponed to the period between mid‐June and early July. The number of neighbours described a sigmoidal yield decline of the single beet plants. Results from image analysis showed that approximately 33 g of beet yield was lost in October/November for each per cent relative projected leaf cover of the weeds in May, despite variation in growing conditions. The results are discussed in relation to potentials for robotic in‐row weed control.     

Effects of selective cutting and timing of herbicide application on growth and development of Cirsium arvense in spring barley

Recently, a new implement for controlling weeds in cereals (CombCut) has been developed. It cuts weeds in growing cereals without damaging them by using the physical differences (in height, stem thickness, straw stiffness and branching pattern) between crops and weeds. To evaluate and compare the effects of selective cutting with different timings of herbicide application on Cirsium arvense in spring barley, a randomised block experiment was conducted in Sweden in 2015–2017, in a field with a naturally occurring C. arvense population. Treatments consisted of control (C), herbicide application at 4–5‐leaf stage of C. arvense (H1), herbicide application at 8–10‐leaf stage (H2) and selective cutting at 10‐leaf stage (S). The treatments were performed in 2015 and repeated in 2016 in the same plots, and a final evaluation was performed in 2017. Compared to the control, S, H1 and H2 were equally efficient in reducing above‐ground biomass production of C. arvense and increasing spring barley grain yield per unit area. The number of C. arvense shoots per area was, however, higher in S compared to H1 and H2. No differences in control effects on shoot number were observed between H1 and H2. Our study indicates that (i) selective cutting (S) reduces C. arvense equally efficient as herbicide application and (ii) early herbicide spraying is as efficient as spraying later in the season.     

Utilization of Parthenium hysterophorus for the remediation of lead‐contaminated soil

A greenhouse experiment was conducted to study the role of plant growth regulators, a chelating agent, and plant growth‐promoting bacteria in lead (Pb) phytoextraction and their subsequent effect on the weed plant, Parthenium hysterophorus. Gibberellic acid (GA₃) and indole‐3‐acetic acid (IAA) were used as the foliar spray. Ethylenediamine tetra acetic acid (EDTA) was applied in split doses. Bacillus and Rhizobium strains were used as a single culture and as co‐cultures. The accumulation of Pb in different parts of the plant was analyzed by using an atomic absorption spectrophotometer. The amount of lead translocation and accumulation in the stems and leaves was significantly higher in the hormonal and EDTA treatments, while the microbial treatments showed no significant difference in the amount of Pb translocation into the stems and leaves, when compared to the control. The EDTA increased the amount of translocation into the shoots, but the dry biomass declined and subsequently reduced the total Pb phytoextraction. The GA₃ treatment showed the maximum total Pb accumulation, along with a higher dry biomass. The microbial co‐inoculated plants showed a significant increase in their dry biomass but the Pb accumulation did not increase like with the GA₃ and IAA treatments. These findings encourage the use of the GA₃ application for Pb phytoextraction by P. hysterophorus. One important feature of this weed plant is its unpalatable nature to herbivores, which could help in reducing the entrance of Pb into the food chain. Gibberellic acid is environmentally friendly compared to EDTA; therefore, more investigation of GA₃ and P. hysterophorus is required.     

Evaluation of the suppression ability of rice (Oryza sativa) on Monochoria vaginalis by measuring photosynthetic photon flux density below rice canopy

Field experiments involving eight cultivars were conducted in 1998 and 16 cultivars in 1999 to study the ability of rice (Oryza sativa L.) to suppress Monochoria vaginalis (Burm. f ) Kunth through light competition. Dry weights of M. vaginalis shoots in early season culture exceeded those in normal season culture of any rice cultivars. The relative photosynthetic photon flux density (R‐PPFD), which was calculated as the ratio of the photosynthetic photon flux density (PPFD) below the rice canopy to that measured above the rice canopy, varied according to rice cultivar. A strong linear correlation was observed between the mean R‐PPFD at 29–35 days after transplanting (DAT) (r² = 0.80; p < 0.01 in 1998; r² = 0.63, p < 0.001; and r² = 0.93, p < 0.001 in 1999), or 36–42 DAT (r² = 0.66, p < 0.05 in 1998; r² = 0.72, p < 0.001; and r² = 0.97, p < 0.001 in 1999), and the dry weight of M. vaginalis shoots at approximately 60 DAT. Data from the three experiments could be pooled into one regression line because intercepts and regression coefficients were not significantly different. The r² values of the combined regression were highest when R‐PPFD was expressed as the mean of measurements taken during 14 days (from 29 to 42 DAT; r² = 0.81, p < 0.001). The shortest period for measuring mean R‐PPFD in order to obtain a meaningful relationship with M. vaginalis shoot dry weight was 7 days (from 29 to 35 DAT; r² = 0.78, p < 0.001). For that same period, relationships between M. vaginalis shoot dry weight at 60 DAT and rice tiller number or leaf area index (LAI) at ground level were weak. However, there were negative relationships between M. vaginalis shoot dry weights at 60 DAT and rice LAI measured 20 cm above the ground, plant heights or rice shoot dry weight, but these coefficients of determination were smaller than those calculated by R‐PPFD for the same period. Thus, the ability of rice to suppress M. vaginalis can be evaluated more accurately by measuring mean R‐PPFD below the rice canopy for 7 days (from 29 to 35 DAT) than by measuring rice LAI, plant height and shoot dry weight.     

Interaction of Moniliophthora perniciosa biotypes with Micro‐Tom tomato: a model system to investigate the witches' broom disease of Theobroma cacao

The miniature tomato (Solanum lycopersicum) cultivar Micro‐Tom (MT) has become an important platform to investigate plant–pathogen interactions. In the case of the witches' broom disease of Theobroma cacao (cacao), the existence of Moniliophthora perniciosa isolates pathogenic to Solanaceae (S‐biotype) may enable the use of MT to circumvent limitations of the cacao host, whereas the availability of a non‐infective cacao C‐biotype allows the evaluation of contrasting responses of MT. Infection of MT by the S‐biotype led to stem swelling and axillary shoot growth to form broom‐like symptoms similar to the biotrophic phase in cacao, but the infected tissues did not progress to necrosis. Conversely, inoculation with the C‐biotype did not cause typical symptoms, but reduced plant height, appearing as a non‐host interaction. Histopathological characterization of the S‐biotype infection of MT by light and electron microscopy indicated limited germ tube penetration, preferentially through wounds at the base of trichomes or actively through the epidermis. No intracellular mycelium was observed, corroborating the lack of the necrotrophic stage of the pathogen. The analysis of gene expression during the interaction between the S‐ or C‐biotype with MT indicated that expression of plant defence‐associated genes differs for kinetics and intensity between a compatible or incompatible M. perniciosa–MT interaction. The pattern of spore germination and low rate of mycelia penetration suggests that the S‐biotype is not a fully adapted tomato pathogen, but possibly a case of broken non‐host resistance, and evidence suggests the occurrence of a non‐host MT response against the C‐biotype.     

Cutting weeds with a CO2 laser

Stems of Chenopodium album . and Sinapis arvensis . and leaves of Lolium perenne . were cut with a CO₂ laser or with a pair of scissors. Treatments were carried out on greenhouse-grown pot plants at three different growth stages and at two heights. Plant dry matter was measured 2 to 5 weeks after treatment. The relationship between dry weight and laser energy was analysed using a non-linear dose–response regression model. The regression parameters differed significantly between the weed species. At all growth stages and heights S. arvensis was more difficult to cut with a CO₂ laser than C. album . When stems were cut below the meristems, 0.9 and 2.3 J mm⁻¹ of CO₂ laser energy dose was sufficient to reduce by 90% the biomass of C. album and S. arvensis respectively. Regrowth appeared when dicotyledonous plant stems were cut above meristems, indicating that it is important to cut close to the soil surface to obtain a significant effect. When cutting L. perenne plants with 2-true leaves at a height of 2 cm from the soil surface with a laser, the biomass decreased significantly compared with plants cut by scissors, indicating a delay in regrowth. This delay was not observed for the dicotyledonous plants nor for the other growth stages of L. perenne .     

Effect of phenology on susceptibility of Norway spruce (Picea abies) to fungal pathogens

Ceratocystis polonica and Heterobasidion parviporum are important fungal pathogens in Norway spruce (Picea abies). Tree susceptibility to these pathogens with respect to phenology was studied using artificial fungal inoculations at six stages of bud development, and assessed by measuring phloem necroses in the stems of 2‐ and 8‐year‐old trees. Tree capacity for resistance was assessed by measuring phloem nonstructural carbohydrates at each stage. Phloem necroses were significantly larger in trees with fungal versus control inoculations and increased significantly over time. Changes in nonstructural carbohydrates occurred in the trees; a significant decline in starch and a slight but significant increase in total sugars occurred over time. These results suggest that susceptibility to fungal pathogens and carbohydrate levels in the stems of the trees were related to fine‐scale changes in bud development. A trade‐off may occur between allocation of starch (the major fraction of the stem carbohydrate pool) to bud development/shoot growth versus defence of the stem. Previous tests of plant defence hypotheses have focused on herbivory on plants growing under different environmental conditions, but the role of phenology and the effect of pathogens are also important to consider in understanding plant resource allocation patterns.     

Growth pattern of Juncus effusus and Juncus conglomeratus in response to cutting frequency

Increasing abundance of Juncus effusus (soft rush) and Juncus conglomeratus (compact rush) in pastures and meadows in western Norway has caused reductions in forage yield and quality in recent decades. Understanding plant development and regrowth following cutting is essential in devising cost‐effective means to control rushes. In a field experiment in western Norway, we investigated development of above‐ and below‐ground fractions of rush from seedlings to three‐year‐old plants, including the impact on vigour of disturbing growth by different cutting frequencies during the period 2009–2012. Each year, the plants were exposed to one or two annual cuts or left untreated and five destructive samplings were performed from March to early December. Juncus effusus showed significantly more vigorous growth than Juncus conglomeratus in the last two years of the study period. The above‐ground:below‐ground biomass ratio of both species increased mainly in spring and early summer and was reduced in late summer and autumn. Removal of aerial shoots also reduced the below‐ground fraction of both species. One annual cut in July effectively reduced biomass production in both species by 30–82%, which was only a slightly smaller reduction than with two annual cuts, in June and August. Mechanical control measures such as cutting can thus effectively reduce rush vigour when performed late in the growing season.     

Effects of leaf position on reflectance,transmittance and absorption of red and far‐red light in tomato,Chenopodium album and Amaranthus retroflexus leaves

Leaf optical properties can play an important role in determining the red/far‐red light ratio, a signal of impending competition, in plant canopies. Knowledge of leaf optical properties and factors affecting them is important in understanding of the impacts of red/far‐red ratio in agroecosystems. Effects of leaf position on the plant stem on their optical properties at 660 and 730 nm were studied in tomato and two weeds Chenopodium album and Amaranthus retroflexus. Leaf position on stem strongly influenced leaf optical properties. Reflectance and transmittance were generally lower for the C. album and A. retroflexus leaves at higher positions on the stem, except for reflectance at 730 nm in C. album, which did not change. Reflectance was not affected in tomato. Transmittance generally decreased for leaves at higher positions. Red/far‐red ratios of reflected (R_ratio) and transmitted (T_ratio) light generally decreased in all species, except R_ratio in tomato, where it increased slightly at higher positions. These effects were greater in A. retroflexus compared with C. album and tomato. Changes in these ratios were partly explained by chlorophyll content and leaf mass per area. The results show that leaf position on plant stem influences leaf optical properties in tomato and two weeds and this effect differed between species. These influences and the differences among species could modify red/far‐red ratios in canopies comprising these species, which could influence their growth and inter‐plant interactions.     

‘Universal’ spray droplet adhesion model – accounting for hairy leaves

The adhesion of a spray droplet upon initial contact with a leaf surface is extremely important to spray efficacy and is dependent on dynamic interactions between droplets (formulation, size, velocity) and leaf (micro‐topography, surface chemistry, veininess, hairiness and orientation). A ‘universal’ spray droplet adhesion model has previously been developed, using 50% aqueous acetone contact angles as a measure of leaf surface properties; this model satisfactorily predicts initial adhesion over a range of formulation surface tensions, droplet sizes and velocities. However, it failed to fit data from hairier leaves. This study investigates initial spray droplet adhesion on hairy leaves. Two categories of hairy leaves were identified by how the droplets penetrate the leaf hairs, Wenzel (hairy) and Cassie–Baxter (super hairy). For the Wenzel‐type, a simple constant accounted for the increased droplet shatter caused by the hairs. For the Cassie–Baxter‐type, a cushioning factor was introduced to account for the absorption of kinetic energy at impact by the hair mat. The cushioning factor was estimated by measuring the relative height of the hair mat. By including these two parameters, the new model successfully predicted the mean adhesion of non‐hairy, hairy and super‐hairy plants (R² = 0.96). This model and the underlying principles determining hairy leaf adhesion developed in this article will help develop spray formulations effective at targeting hairy‐leaved weed and crop species.     

Impact of wetting agents,time of day and periodic energy dosing strategy on the efficacy of hot water for weed control

To further increase the cost‐effectiveness and sustainability of hot water weed control, the further optimisation of hot water efficacy is needed. Therefore, three dose–response experiments were carried out to evaluate the impact of wetting agents (an ethoxylated triglyceride, an esterified rapeseed oil and a liquid linseed oil‐based soap), time of day (2, 4, 6, 8, 10 and 12 h after sunrise) and treatment frequency (1–6 treatments in a 12 week period) on the hot water sensitivity of prevalent weed species (Lolium perenne, Festuca rubra, Taraxacum officinale and Plantago major) on public pavements. Generally, wetting agents did not improve hot water sensitivity. Taraxacum officinale with large planophile leaves showed a higher sensitivity than grasses with an erectophile growth habit. Festuca rubra was more sensitive than L. perenne due to its high leaf dry matter content. The plant species were most sensitive to hot water in the afternoon. This variation in sensitivity during the day was related to the variation in leaf thickness and dry matter content. In general, hot water weed control was highly efficient when conducted four times in a 12 week period at an energy dose of 589 kJ m^?2.     

Alteration of photosynthetic performance and source–sink relationships in wheat plants infected by Pyricularia oryzae

Two experiments were carried out to assess the changes associated with photoassimilate production and partitioning in the source–sink relationship of flag leaves and spikes of wheat plants infected with Pyricularia oryzae, the causal agent of blast. Flag leaves and spikes were inoculated at 10 and 20 days after anthesis (daa) with a conidial suspension of P. oryzae. Analysis of chlorophyll a fluorescence using maximal photosystem II quantum efficiency (F_v?F_m), fraction of energy absorbed that is used in photochemistry (YII), quantum yield of non‐regulated energy dissipation (Y(NO)) and quantum yield of regulated energy dissipation (Y(NPQ)), showed an impairment of the photosynthetic performance in both infected flag leaves and spikes, coupled with reduced concentrations of chlorophyll a  + b and carotenoids. Compared to non‐inoculated controls, there was lower capacity for CO₂ fixation by RuBisCO in the infected flag leaves. Similarly, in the infected flag leaves and grains (obtained from infected spikes), there were lower concentrations of soluble sugars, while the hexoses‐to‐sucrose ratio increased in infected flag leaves. Compared to non‐inoculated controls, infected flag leaves showed lower sucrose phosphate synthase (SPS) activity and lower expression of the sucrose synthesis (SuSy) gene, while higher expression and activity of acid invertases also occurred. At the advanced stages of fungal infection, the concentration of starch in grains decreased but remained high for the infected flag leaves. There were reductions in ADP‐glucose pyrophosphorylase activity and the expression of ADP‐glucose pyrophosphorylase genes and a down‐regulation of β‐ and α‐amylase expression at the advanced stages of fungal infection on flag leaves and spikes. In conclusion, the effect of blast on both grain quality and yield can be associated with alterations in both production and partitioning of carbohydrates during the grain filling process.     

Apple scab: numerical optimization of a new thermal time scale and application for modelling ascospore release in southern France

Adequate protection of apple trees during the primary contamination period is a cornerstone for management of apple scab. Correct timing of spring treatments is fundamental and thus, much effort has been devoted to forecasting ascospore release by Venturia inaequalis. Most models rely on degree‐day accumulation starting from a biofix date established yearly on the basis of biological observations. Here, the potential of using a single calendar date as a biofix and new types of time scales were explored, with the help of numerical optimization with field‐collected data. Using data acquired between 1996 and 2008, the daily rate of development for V. inaequalis primary inoculum was assessed by fitting generic time scale functions, a method that requires the smallest number of assumptions about the effect of temperature on the biological phenomenon. An optimal calendar biofix was established for Provence and use of non‐linear functions relating pseudothecial development rate to temperature for accumulating thermal time was compared with the usual linear response in standard degree‐day models. A model was then constructed using four additional years of data for validation. The predictive value of the model was further improved by adjusting the time scale with ‘accelerating rules’ to take into account the positive influence of rainy days on pseudothecial maturation prior to ascospore release. However, ‘halting rules’ inserted in the time scale to account for dry days during the ascospore release period strongly reduced the predictive value of the model for southern France, suggesting the possible occurrence of strains adapted to dry conditions.     

Effect of anaerobic soil disinfestation amendment type and C:N ratio on Cyperus esculentus tuber sprouting,growth and reproduction

Anaerobic soil disinfestation (ASD) is a cultural technique primarily targeted for control of soilborne plant pathogens, but can also impact weed propagules. A repeated pot study was conducted to evaluate ASD treatment impact on sprouting and growth of introduced Cyperus esculentus (yellow nutsedge) tubers using dry molasses‐based and wheat bran‐based amendment mixtures at four carbon‐to‐nitrogen (C:N) ratios (from 10:1 to 40:1) and compared with a non‐amended control. The mean percentage of sprouted tubers recovered after ASD treatment was lower for wheat bran‐based (42%) than dry molasses‐based (65%) amendments, and tuber production was 1.6‐fold higher in dry molasses‐based than wheat bran‐based treatments. The highest percentage of sprouted tubers (79%) and the highest mean production of large tubers (threefold higher than wheat bran‐based and 1.7‐fold higher than molasses‐based amendments) were observed for the non‐amended control. Tuber sprouting was significantly lower from all ASD treatments (regardless of amendment C:N ratio) compared with the non‐amended control at a 15 cm burial depth. New tuber production was lowest at C:N ratios of 10:1 and 20:1 and more than twofold higher in the non‐amended control. Wheat bran‐based amendments reduced above‐ground C. esculentus biomass compared with the non‐amended control and ASD treatments with molasses‐based amendments, and reduced below‐ground biomass compared with molasses‐based amendments. Above‐ground biomass was highest at amendment C:N ratio of 10:1, and below‐ground biomass was highest at amendment C:N ratio of 40:1 and the non‐amended control. ASD treatment with wheat bran‐based amendments at lower C:N ratios reduced tuber sprouting and reproduction compared with the non‐amended control, but not at rates high enough to use as a primary weed management tactic.     

Generation and evaluation of movement protein‐specific single‐chain antibodies for delaying symptoms of Tomato spotted wilt virus infection in tobacco

This study investigated whether single‐chain antibodies (scFvs) specific for a viral movement protein could accumulate in the plant cell cytosol and restrict viral systemic infection in plants. Nine chicken scFv fragments against the Tomato spotted wilt virus (TSWV) movement protein (NS_M) were isolated by phage display. Soluble scFvs were produced in bacteria and the NS_M binding activity of purified scFvs was confirmed. The nine scFv genes were cloned into a plant expression vector enabling recombinant protein accumulation in the plant cell cytosol. Immunoblot analysis demonstrated that two of the nine chicken scFvs accumulated to high levels (5·9 and 8·0% of total soluble protein). Bioassays of viral infection using transgenic tobacco plants producing NS_M‐specific chicken scFvs showed delayed symptom development when compared to non‐transgenic control plants, indicating that expression of antibodies recognizing the TSWV movement protein is a potential strategy for generating resistant plants.     

In search of new herbicidal inhibitors of the non‐mevalonate pathway

The first five steps of the non‐mevalonate pathway have been tested in high‐throughput screening (HTS) campaigns, using enzymes of plant origin. Hit rates were in general relatively low, which could be attributed to the high polarity and charged nature of substrates and active sites of these enzymes. Still, for all the enzymes, apart from IspF (2‐methylerythritol 2,4‐cyclodiphosphate synthase), inhibitors could be identified with activities below 100 μM, and these were followed up to identify structure–activity relationships (SARs). For the enzyme IspD (2C‐methyl‐D‐erythritol 4‐phosphate cytidyltransferase), inhibitors with IC₅₀ down to 35 nM were identified that also showed herbicidal activity.     

Glyphosate uncouples gas exchange and chlorophyll fluorescence

BACKGROUND: Changes in chlorophyll fluorescence have often been advocated as a sensitive biomarker of plant stress, assuming that any kind of plant stress serious enough to affect plant growth will also affect photosynthesis. Glyphosate affects photosynthetic electron transport indirectly by inhibiting sink processes. The question is how fast this inhibition can be observed on CO₂ assimilation and ultimately on chlorophyll fluorescence? RESULTS: Experiments measuring CO₂ assimilation, conductance and chlorophyll fluorescence using four Kautsky curve parameters on barley (Hordeum vulgare L.) exposed to increasing doses of glyphosate showed a total cessation of CO₂ fixation and conductance without significant changes in chlorophyll fluorescence. The decrease in CO₂ fixation and conductance was significant 1 day after spraying and corresponded well to the decrease in biomass 5–7 days after spraying. CONCLUSION: A total cessation of CO₂ assimilation can take place without affecting chlorophyll fluorescence. Hypotheses concerning what happens to the energy from the photosynthetic apparatus that is not used for CO₂ assimilation are discussed. The results question the use of chlorophyll fluorescence as a universal indicator of stress on photosynthetic processes. Also, they demonstrate that changes in gas‐exchange parameters are more sensitive biomarkers for glyphosate toxicity compared with chlorophyll fluorescence. Copyright © 2010 Society of Chemical Industry