Biological responses of a non‐target aquatic plant (Nasturtium officinale) to the herbicide,tribenuron‐methyl

To assess its response to the herbicide, tribenuron‐methyl, samples of Nasturtium officinale were exposed to 0, 0.01, 0.05, 0.1, and 0.5 mg L^?1 of tribenuron‐methyl for 1, 2, 4 and 7 days. The influence of this herbicide on the relative growth rate, electrolyte leakage, lipid peroxidation, photosynthetic pigmentation, protein content, and performance of anti‐oxidant enzymes, such as superoxide dismutase (SOD), catalase, and ascorbate peroxidase (APX), was examined. The results indicated that tribenuron‐methyl, applied at 0.5 mg L^?1, affected plant growth negatively. It also was determined that chlorophyll a is the most responsive photosynthetic pigment to tribenuron–methyl exposure. Under stress conditions, the anti‐oxidant enzymes were up‐regulated compared to the control. The SOD activity was significantly stimulated, while the activity of APX was inhibited. A significant correlation was found between lipid peroxidation and SOD activity. The exposure period and herbicide concentration had significant effects on the biological responses against tribenuron‐methyl stress. These results may be useful for clarifying the effect of herbicides on non‐target aquatic plants.     

Differential effects of phytotoxic metabolites from <Emphasis Type="Italic">Alternaria tagetica</Emphasis> on <Emphasis Type="Italic">Tagetes erecta</Emphasis> cell cultures

Alternaria tagetica, a fungus that causes early blight in marigold (Tagetes erecta), produces two groups of phytotoxic metabolites: one hydrophilic and the other lipophilic that show phytotoxic activity when tested by the leaf-spot assay in T. erecta. We evaluated the cellular effects of the phytotoxic culture filtrate of A. tagetica and the hydrophilic and lipophilic fractions, then determined whether the filtrate or the fractions differentially induced pathogenesis-related mechanisms in the plant. The culture filtrate and the phytotoxic fractions had adverse effects on cell viability, fresh mass, and the number of cells, and induced ROS accumulation, lipid peroxidation and DNA damage on T. erecta cell suspension cultures, and these effects are related to pathogenic mechanisms attributed to phytotoxins. However, although exposure of marigold cells to the phytotoxic culture filtrate of A. tagetica triggered programmed cell death, the hydrophilic and the lipophilic phytotoxic fractions induced death that was more related to a toxic effect leading to necrosis. This study presents a complementary perspective in the search for the roles of metabolites, including phytotoxins, produced by phytopathogenic fungi during plant infection.     

Phytotoxic activity of Chinese violet (Asystasia gangetica (L.) T. Anderson) and two phytotoxic substances

Chinese violet (Asystasia gangetica (L.) T. Anderson) is a perennial invasive weed belonging to Acanthaceae. Leaves of this weed have been suggested to possess phytotoxic activity. However, phytotoxic substances in this weed have not yet been reported. Therefore, the present study investigated phytotoxic activity of Chinese violet extracts and phytotoxic substances. The extracts of Chinese violet leaves inhibited the root and shoot growth of cress, lettuce, alfalfa, barnyard grass, ryegrass, and timothy, where the level of inhibition increased with increasing extract concentrations. Bioassay‐guided separations of the extracts led to isolation of two phytotoxic substances, indole‐3‐carboxaldehyde and (6R,9S)‐3‐oxo‐α‐ionol. Indole‐3‐carboxaldehyde significantly inhibited the root and shoot growth of cress at concentrations ≥100 and 30 μmol L^?1, respectively, and concentrations of the substance required for 50% growth inhibition were 210 and 127 μmol L^?1 for cress roots and shoots, respectively. The other substance, (6R,9S)‐3‐oxo‐α‐ionol, was reported to have strongly inhibited cress roots and shoots. The present results suggest that Chinese violet contains two phytotoxic substances indole‐3‐carboxaldehyde and (6R,9S)‐3‐oxo‐α‐ionol, and those substances may play an important role in the phytotoxic activity of Chinese violet.     

Effects of spermidine and salinity stress on growth and biochemical response of paraquat‐susceptibe and ‐resistant goosegrass (Eleusine indica L.)

This study aimed to investigate the interaction effect of spermidine (Spd) and salinity stress on growth, photosynthetic rate, antioxidant system and free polyamines (PAs) contents of goosegrass (Eleusine indica L.) seedlings. E. indica was raised in a growth chamber under normal and toxic salt stress (100 mM of NaCl) and sprayed with 0 and 1.00 mM of Spd. The degree of growth inhibition caused by salt stress was lower in a paraquat‐resistant (R) biotype compared to a paraquat‐susceptible (S) biotype. Salt stress significantly elevated the accumulation of malondialdehyde, electrolyte leakage and proline and resulted in the degradation of chlorophyll; reduction in chlorophyll fluorescence; and a decrease in photosynthetic rate, relative water content and biomass. Spd‐treated plants maintained higher activities of antioxidant enzymes (catalase, superoxide dismutase and peroxidase), a greater rate of photosynthesis and lower osmotic pressure than untreated plants in the S biotype. Endogenous Spd content was reduced significantly in response to salt stress in both biotypes, but free PAs content in the S biotype was remarkably enhanced with exogenous Spd application under normal or salinity stress conditions. The result indicated that the S biotype was more sensitive to salinity than the R biotype; meanwhile, exogenous Spd maybe play an important role in protecting S biotype plants from salt stress.     

Phytotoxic potential of Chrysopogon aciculatus (Retz.) Trin. (Poaceae)

The creeping weed, Chrysopogon aciculatus (Retz.) Trin., is widely known for its use as folk medicine, while its phytotoxic potential has not been examined. Therefore, we carried out an investigation into the phytotoxic potential of C. aciculatus to identify phytotoxic substances. C. aciculatus extracts showed inhibitory effects on shoot and root growth of cress, lettuce, rapeseed, and Italian ryegrass. Inhibition was both species‐dependent and concentration‐dependent. Two substances, (9S,10E,12Z)‐9‐hydroxyoctadeca‐10,12‐dienoic acid (9‐HO‐ODDEA) and rhizopycnin A, were isolated using chromatographies and characterized by spectral analysis. 9‐HO‐ODDEA retarded shoot and root growth of cress at concentrations higher than 1.0 and 0.3 mM, respectively, while on cress seedling by rhizopycnin A, the inhibition began from 1.0 mM. The concentrations needed for 50% inhibition of the shoot and root growth of test plants ranged from 1.71–2.31, and 0.71–0.72 mM for 9‐HO‐ODDEA and rhizopycnin A, respectively. These results indicate that these substances may contribute, to a certain extent, to the phytotoxic activity of C. aciculatus.     

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.     

Phytotoxic activity of middle-chain fatty acids II: peroxidation and membrane effects

Pelargonic acid (PA), an aliphatic 9-carbon monocarboxylic acid, is a phytotoxic burn-down compound. In the light peroxidizing activity can be measured as ethane and propane formation with cress or tobacco seedlings. This effect is strong at low pH (4–5), and saturated acids with 9–10 carbon atoms represent the optimum chain length. Methyl or ethyl esters are inactive, and safeners have no influence. In contrast to the peroxidative herbicides like acifluorfen methyl neither photosynthesis nor protoporphyrin IX is involved, although peroxidation requires light. Chlorophyll is necessary since etiolated seedlings show little peroxidation. Singlet oxygen quenchers like eugenol markedly reduce peroxidation. Membrane leakage of a similar rate is observed in light as well as in darkness. PA was described as a penetration enhancer intercalating with membranes. Our data corroborate that conclusion. Accordingly, the herbicidal mode of action of pelargonic acid is due first to membrane leakage in dark or light and second to peroxidation driven by radicals originating in the light by sensitized chlorophyll displaced from the thylakoids.     

Programmed cell death pathways induced by early plant‐virus infection are determined by isolate virulence and stage of infection

Programmed cell death (PCD) pathways caused by Turnip mosaic virus (TuMV) infection before symptom appearance were studied by light microscopy and electrolyte leakage following sap inoculation of Brassica carinata (Ethiopian mustard) TZ‐SMN‐44‐6 plants. Leaf responses to inoculation with avirulent (TuMV‐avir) and virulent (TuMV‐vir) isolates, and mock‐inoculation, were compared at 2, 20 and 52 h after inoculation (hai). The phenotypes induced were localized resistance (TuMV‐avir) and systemic susceptibility (TuMV‐vir). No visible TuMV symptoms were recorded in any inoculated plants during the 2–52 hai sampling period, but appeared as chlorotic spots in inoculated leaves at 5 days after inoculation. With TuMV‐vir alone, they were followed by systemic infection (mosaic). Dead cell number, deformation, percentage area and percentage integrated intensity, and conductivity of electrolyte leakage data, were analysed to examine their possible roles in stimulating cell death pathways. At 2 hai, dead cell number and percentage area were significantly greater for TuMV‐avir than TuMV‐vir infection or mock‐inoculation. Overall, isolate TuMV‐vir caused significantly greater cell deformation than TuMV‐avir, whereas wounding by mock‐inoculation had negligible effects. By 52 hai, isolate TuMV‐avir caused significantly greater electrolyte leakage than isolate TuMV‐vir or mock‐inoculation. This suggests both isolates triggered morphological changes consistent with apoptotic‐like PCD and necrosis‐like PCD that depended upon isolate virulence and stage of infection, respectively. These findings highlight how quantification of dead cell deformation and electrolyte leakage offer a new understanding of compatible and incompatible plant responses to early virus infection in plants.     

Involvement of lipoxygenase,lipoxygenase pathway volatiles,and lipid peroxidation during the hypersensitive reaction of pepper leaves to Xanthomonas campestris pv. vesicatoria

Lipoxygenase activity and protein, production of lipid-derived volatiles, and lipid peroxidation levels were determined in pepper (Capsicum annuum L., cv. Early Calwonder-10R) leaves during the hypersensitive reaction induced by avirulent race 2 of Xanthomonas campestris pv. vesicatoria. Lipoxygenase activity increased during the collapse phase of the hypersensitive reaction (8 to 12 h after inoculation), and an increase in electrolyte leakage occurred. However, Western blot analysis revealed that lipoxygenase proteins decreased during the same period. When only one longitudinal half of a pepper leaf was inoculated with the avirulent bacterium race, a significant increase in lipoxygenase activity was observed in both inoculated and noninoculated leaf halves, 10 h after inoculation. In addition, lipoxygenase protein decreased in inoculated leaf halves, but remained unchanged in noninoculated ones. The evolution of some volatile compounds derived from the lipoxygenase pathway [(E,E)-2,4-hexadienal, 1-hexanol, 3-hexen-1-ol, 2,4-hexadienal and 2,4-eptadienal] and carotenoid degradation (α- and β-ionone) increased in the incompatible interaction during the collapse phase of the hypersensitive reaction. The level of the oxidative index (A₂₃₅/A₂₀₅) of leaf lipid extracts, determined to estimate lipid peroxidation, significantly increased in the advanced stage of the hypersensitive reaction. Furthermore, determination of the oxidative index in neutral lipid, glycolipid and phospholipid fractions showed that the oxidative index was significantly increased only in the glycolipid fraction. Lipoxygenase activity and protein, electrolyte leakage, volatiles and lipid peroxidation were not changed in pepper leaves inoculated with the virulent race 1 of X. campestris pv.vesicatoria during the time interval considered (2–12 h after inoculations). The hypothesis that a lipoxygenase with chloroplastic location is induced in the incompatible interaction, and which is responsible for the increase in lipid peroxidation is advanced.     

Studies into the action of the diphenyl ether herbicides acifluorfen and oxyfluorfen. Part I: Activation by light and oxygen in leaf tissue

The effect of acifluorfen and oxyfluorfen on chlorophyll bleaching, lipid peroxidation and photosynthesis in pea leaf discs was studied. Both her- bicides induced light-dependent bleaching and lipid peroxidation, the level of damage being greater at higher light intensities. Photosynthetic carbon dioxide fixation was only partially inhibited in treated leaf discs incubated in darkness, thus indicating that these herbicides did not inhibit photo- synthesis as a primary mode of action. Leaf discs maintained in darkness showed no visible signs of injury, and light-dependent herbicide-induced damage was reduced by incubating discs under nitrogen, orpre-incubating them with the electron-transport inhibitor monuron. It is suggested that acifluorfen and oxyfluorfen are activated by a light-dependent process, which requires photosynthetic electron transport.     

The allelopathic effect of three Amaranthus spp. (pigweeds) on wheat (Triticum durum)

The allelopathic effect of Amaranthus retroflexus L., Amaranthus blitoides S. Wats, and Amaranthus gracilis Desf. on wheat Triticum durum L. was investigated under laboratory, glasshouse and field conditions. Laboratory experiments showed that fresh shoot or root extract of the three weed species reduced germination, coleoptile length, root length and root dry weight of wheat seedlings. The inhibitory effects were rate dependent, with low concentrations of shoot extract promoting shoot growth of wheat. Fresh plant extracts were more phytotoxic than dried plant extracts and shoot extracts had higher detrimental effects than root extracts. In pot experiments, dried shoot extract of A. gracilis increased shoot and root dry weights of wheat seedlings. For A retroflexus and A. blitoides the extracts of these and dried shoots (8 g kg^?1), which had been added to soil mixtures significantly reduced ger-mination and growth of wheat seedlings. Addition of up to 16 g kg^?1 of A. gracilis residues promoted shoot growth of wheat and had no significant harmful effect on root growth. Roots appeared more sensitive to allelopathic effect than shoots. Under field conditions, incorporation of A. retroflexus or A. blitoides residues in the soil reduced height, grain and straw yield of wheat, whereas A. gracilis residues stimulated plant height and increased yield.     

Studies on the site of uptake by plants of chlortoluron and terbutryne

Experiments were done to observe the pattern of early root development of radish (Raphanus raphatnistrum L.) and perennial ryegrass (Lolium perenne L.), the mobility of chlortoluron following application to the soil surface, the effect of protecting the subterranean shoots of four plant species on their response to chlortoluron and terbutryne and the relative quantities of ¹⁴C-labelled chlortoluron taken up by radish and Avenu fatua from root and shoot zone exposure. Both chlortoluron and terbutryne appear to be able to enter the plants examined, Alopecurus myosuroides, Stellaria media, perennial ryegrass and radish, through roots and shoots. It is suggested that shoot uptake is relatively more important for plants like perennial ryegrass than for those whose roots develop more quickly and invade the soil above the seed, such as radish. The quantities of radioactive chlortoluron taken up from soil containing 400 ng g^?1 showed that less than 3 ng per plant could reduce A. fatua fresh weight by 17–40% while over 30 ng per plans had little effect on radish. By comparison 2 kg ha^?1 chlortoluron applied to the soil surface of pots which were sub-irrigated for 3 weeks gave a concentration of 170 ng g^?1 in the layer of soil 10–12 mm from the surface. It is suggested that for shallow germinating species with herbicides of physical and phytotoxic properties similar to chlortoluron, the solvent action of rainfall, together with diffusion, is enough to allow the transport of toxic quantities to the target plant although any leaching action is likely to increase activity.     

Effectiveness of the resistance inducer prohexadione‐Ca against fireblight in shoots of apple trees inoculated with Erwinia amylovora1

Due to the lack of effective, non‐phytotoxic and publicly acceptable materials for controlling fireblight in pome fruit trees, novel strategies against Erwinia amylovora are being sought. Resistance‐inducing compounds, such as prohexadione‐Ca, represent promising alternatives. Prohexadione‐Ca is the active substance of the bioregulator Regalis, currently being introduced in several European countries and overseas. Prohexadione‐Ca reduces shoot elongation due to inhibition of gibberellin biosynthesis. Furthermore, it leads to significant changes in the spectrum of flavonoids and their phenolic precursors in pome fruits, which causes reduced susceptibility to fireblight and other pathogens. In 2002 and 2003, container‐grown apple trees of the cultivars ‘Idared’ and ‘Freedom’ were treated with different dosages of prohexadione‐Ca two weeks before inoculation with E. amylovora. The effect of prohexadione‐Ca against shoot blight was determined by measuring the lengths of necrotic lesions and symptoms on vascular bundles caused by the pathogen. Treatments with prohexadione‐Ca turned out to be much superior to the ones with streptomycin, kasugamycin and a bacterial antagonist, which were used for comparison. Acibenzolar‐S‐methyl (Bion), another resistance‐inducing compound, was included in some of the experiments and gave intermediate results. The simultaneous control of excessive shoot growth and shoot infections by fireblight is seen as a major advantage of using prohexadione‐Ca in pome fruit trees.     

Quantification of momilactone B in rice hulls and the phytotoxic potential of rice extracts on the seed germination of Alisma plantago‐aquatica

The occurrence of herbicide‐resistant weeds has boosted interest in the use of crop allelopathy as a potential alternative to herbicides for weed control in rice (Oryza sativa). The phytotoxic compounds that are released by rice could help to enhance its competitive ability and improve weed management. This study aimed to screen rice genotypes for phytotoxic activity, quantify the amount of momilactone B in various rice tissues, and identify the potential parental lines for quantitative trait locus analysis. Therefore, a total of 41 cultivars from germplasm collections was evaluated for their effects. Significant differences were found among the rice cultivars in their ability to reduce the germination, root growth, and root dry weight accumulation of Alisma plantago‐aquatica. The leaf extract was the most inhibitory to germination. Out of the five cultivars that were tested, momilactone B was detected in four of them: Marateli, Kizilirmak, Karadeniz, and Kiziltan. Karadeniz and Kiziltan were identified as the rice cultivars with a high momilactone B content in the tissues and therefore they could be used in breeding programs to enhance the phytotoxic potential of rice. The development of a rice cultivar with proven allelopathic characteristics could provide an environmentally friendly and low‐cost approach for the control of A. plantago‐aquatica.     

Chemical control of weedy rice in precise hill‐direct‐seeded rice in South China

Precise hill‐direct‐seeded rice, which is both cost‐ and labor‐saving, is based on the direct seeding of rice by using a precision rice hill‐drop drilling machine. Weedy rice (Oryza sativa f. spontanea), also known as “red rice”, is a major weed in precise hill‐direct‐seeded rice, causing an ≤80% yield loss and a reduction in grain quality. The aim of this study was to evaluate the control efficiency of weedy rice by pretilachlor (a pre‐emergence herbicide) and fenclorim (a safener) and their safety for precise hill‐direct‐seeded rice in two consecutive years. The amount of rice seed germination was accelerated by soaking the seeds in the safener at 0.67 g ai L^?1 for 1 h before sowing. The pre‐emergence pretilachlor treatments were applied 2 days after sowing cultured rice. The inhibition of the shoot fresh weight of the cultured rice was reduced by 3.3, 6.4 and 7.4% with 450, 900 and 1350 g ai ha^?1 of pretilachlor at 32 days after sowing (DAS) and that of the root fresh weight was reduced by 2.6, 4.9 and 8.1%, respectively. With fenclorim and pretilachlor in a precise hill‐direct‐seeded rice field in 2010 and 2011, the weedy rice control efficiency at 32 DAS was reduced by 100 and 98.0%, respectively. The pre‐emergence pretilachlor treatments that were applied at 2 DAS were much more efficient in the weedy rice control and less inhibitory to the cultured rice growth. The rice yield was increased by 26.1–26.7% in the mechanical precise hill‐direct‐seeded rice, relative to the manual‐seeding rice, with the application of fenclorim and pretilachlor.     

Photochemical damage and comparative performance of superoxide dismutase and ascorbate peroxidase in sugarcane leaves exposed to paraquat-induced oxidative stress

The physiological responses of sugarcane (Saccharum officinarum L.) to oxidative stress induced by methyl viologen (paraquat) were examined with respect to photochemical activity, chlorophyll content, lipid peroxidation and superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. Thirty-day-old sugarcane plants were sprayed with 0, 2, 4, 6 and 8 mM methyl viologen (MV). Chlorophyll fluorescence was measured after 18 h and biochemical analyses were performed after 24 and 48 h. Concentrations of MV above 2 mM caused significant damage to photosystem II (PSII) activity. Potential and effective quantum efficiency of PSII and apparent electron transport rate were greatly reduced or practically abolished. Both chlorophyll and soluble protein contents steadily decreased with MV concentrations above 2 mM after 24 h of exposure, which became more pronounced after 48 h, achieving a 3-fold decrease. Insoluble protein contents were little affected by MV. Oxidative stress induced by MV was evidenced by increases in lipid peroxidation. Specific activity of SOD increased, even after 48 h of exposure to the highest concentrations of MV, but total activity on a fresh weight basis did not change significantly. Nondenaturing PAGE assayed with H₂O₂ and KCN showed that treatment with MV did not change Cu/Zn-SOD and Mn-SOD isoform activities. In contrast, APX specific activity increased at 2 mM MV but then dropped at higher doses. Oxidative damage induced by MV was inversely related to APX activity. It is suggested that the major MV-induced oxidative damages in sugarcane leaves were related to excess H₂O₂, probably in chloroplasts, caused by an imbalance between SOD and APX activities, in which APX was a limiting step. Reduced photochemical activity allowed the early detection of the ensuing oxidative stress.     

Paraquat-induced production of hydrocarbon gases

Paraquat dichloride (1,1′-dimethyI-4,4′-bipyridylium dichloride) induced the production of the hydrocarbon gases propane, ethane and ethylene in the xanthophycean microalga Bumilleriopsis filiformis. Formation of these gases was dependent on the light intensity and photosynthetic electron transport. Diquat dibromide (1,1′- ethylene-2,2′-bipyridylium dibromide), having a higher midpoint potential than paraquat dichloride, induced an even higher evolution of the gases. A total lack of oxygen inhibited the production of gas, while a low oxygen concentration (4–6% by volume) stimulated paraquat-induced gas formation. Culturing algae autotrophically, in a medium with a higher iron content, also increased the production of gases in the presence of the herbicide. Formation of these gases is an indication of the peroxidation of membrane lipids which, according to previous results, is correlated with a decrease of photosynthetic electron-transport activity.     

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.