Persistence of pyrazosulfuron in rice-field and laboratory soil under Indian tropical conditions

BACKGROUND: Pyrazosulfuron ethyl, a new rice herbicide belonging to the sulfonylurea group, has recently been registered in India for weed control in rice crops. Many field experiments revealed the bioefficacy of this herbicide; however, no information is available on the persistence of this herbicide in paddy soil under Indian tropical conditions. Therefore, a field experiment was undertaken to investigate the fate of pyrazosulfuron ethyl in soil and water of rice fields. Persistence studies were also carried out under laboratory conditions in sterile and non‐sterile soil to evaluate the microbial contribution to degradation. RESULTS: High‐performance liquid chromatography (HPLC) of pyrazosulfuron ethyl gave a single sharp peak at 3.41 min. The instrument detection limit (IDL) for pyrazosulfuron ethyl by HPLC was 0.1 µg mL^?1, with a sensitivity of 2 ng. The estimated method detection limit (EMDL) was 0.001 µg mL^?1 and 0.002 µg g^?1 for water and soil respectively. Two applications at an interval of 10 days gave good weed control. The herbicide residues dissipated faster in water than in soil. In the present study, with a field‐soil pH of 8.2 and an organic matter content of 0.5%, the pyrazosulfuron ethyl residues dissipated with a half‐life of 5.4 and 0.9 days in soil and water respectively. Dissipation followed first‐order kinetics. Under laboratory conditions, degradation of pyrazosulfuron ethyl was faster in non‐sterile soil (t_1/2 = 9.7 days) than in sterile soil (t_1/2 = 16.9 days). CONCLUSION: Pyrazosulfuron ethyl is a short‐lived molecule, and it dissipated rapidly in field soil and water. The faster degradation of pyrazosulfuron in non‐sterile soil than in sterile soil indicated microbial degradation of this herbicide. Copyright © 2012 Society of Chemical Industry     

Persistence and biodegradation of vinclozolin in tropical rice soils

The persistence of vinclozolin in three tropical rice soils, widely varying in their physicochemical characteristics, was compared under both non-flooded and flooded conditions. Degradation of the fungicide was more rapid in all the soils under flooded conditions than under non-flooded conditions. Kinetic analysis indicated that the degradation of the fungicide followed a first-order reaction irrespective of soil or water regime. Soil acidity and salinity significantly affected the persistence of the fungicide under non-flooded conditions. The degradation of the dicarboxymide fungicide was enhanced following repeated applications to an alluvial soil under both water regimes, with the enhancement being more marked under flooded conditions. Faster degradation of vinclozolin in mineral salts medium inoculated with non-sterile suspension from retreated alluvial soil indicates the involvement of micro-organisms. 3,5-Dichloroaniline was detected as a metabolite in the degradation of the fungicide in both soil and mineral salts medium. © 1999 Society of Chemical Industry     

Persistence of chlorfenvinphos in soils

The persistence and behaviour of chlorfenvinphos, an organophosphorus insecticide, was studied on different soils. On a peaty soil the insecticide was only very slowly degraded, there being 70% of the applied dose remaining after 21 weeks and 30% after nearly 12 months. On the sandy soils, however, persistence was much shorter; between 3 and 15% of the applied dose remained after a period of 15 weeks. Rates of loss appear to be related to soil moisture conditions; in dry seasons, although the initial rate of loss was high, subsequent rate of breakdown was slower than in wetter seasons. The results are discussed relative to the efficiency of the insecticide for controlling brassica root fly.     

Persistence of tebufenozide in aquatic ecosystems under laboratory and field conditions

The persistence and dissipation behaviour of tebufenozide, an ecdysone agonist, were investigated: (1) under laboratory conditions in aquatic models set up in glass aquaria, and (2) under field conditions in in-situ aquatic enclosures deployed in a mixed-wood boreal forest lake. Two models were set up in the laboratory study (Study I), which was conducted at constant conditions of temperature, water pH and photoperiod. In Model I, partitioning of tebufenozide from sediment, treated at a concentration of 1400 μg kg^-1, into untreated water was examined. The results showed that the chemical moved very little from the treated sediment into water. The concentration in sediment and water decreased gradually during the 90-day incubation period. Tebufenozide disappeared faster from the top layer of sediment than from the middle and bottom layers. The half-lives of disappearance were 64 days for the top layer but >90 days for the middle and bottom layers respectively. In Model II, partitioning from water, treated at a concentration of 350 μg litre^-1, into untreated sediment was investigated. The results showed that the chemical moved from treated water into sediment due to adsorption. Little vertical downward movement of the adsorbed residues from the top layer of sediment occurred into layers beneath. The adsorbed residues were also not released readily back into water. The concentration in water and sediment decreased gradually during the 90-day incubation period. The half-life of dissipation from water was 67 days. The field microcosm study (Study II), conducted under fluctuating conditions of temperature, water pH and photoperiod, involved application of tebufenozide onto aquatic enclosures at four concentrations of 0·05, 0·10, 0·26 and 0·5 mg litre^-1. This study also showed that the chemical moved downwards from the applied location and was adsorbed onto sediment. The chemical persisted longer in Study II than in Study I. Tebufenozide, being photo-labile, probably degraded faster after constant exposure to light in Study I than after exposure to fluctuating light in Study II. At 90 days after treatment in Study I, only about 55% of the applied material persisted in the sediment, and there was little accumulation. In Study II, the material not only persisted but also was accumulated in the sediment, since at 92 days post-treatment the residues were about 25 times higher than the applied concentration level. Residues in water also decreased more rapidly in Study I than in Study II, because the concentration at 90 days post-treatment was about 41% of the applied value. In Study II, however, about 65% of the applied chemical persisted in water at 92 days post-treatment. While the long persistence of tebufenozide in both the laboratory and field studies was attributable to its low vapour pressure, low water solubility, high octanol/water partition coefficient etc., the differences in the persistence characteristics observed in the two studies were due to the fluctuating environmental conditions and water pH encountered in the field study, compared with the constant environmental conditions and water pH utilized in the laboratory study. © 1997 SCI.     

Persistence of terbacil and trifluralin under different soil and climatic conditions

Soil samples were collected from forty-three field trials conducted on ten soil types to investigate the residual activity of terbacil and trifluralin at the end of the cropping season, approximately 6 months after application. The soil was bioassayed in a glasshouse using soya beans and German millet for terbacil and trifluralin respectively. At 1 kg/ha terbacil phytotoxic residues occurred in a majority of the trials, while at 2 kg/ha such a carry-over could be found in every case. Trifluralin doses of up to 1 kg/ha did not persist in toxic amounts in most soils, while application of 2 kg/ha showed residual activity in 74% of the trials and application of 4 kg/ha killed German millet in all instances. Results are also presented from the time-rate dissipation studies conducted on both herbicides by assaying soil samples collected at monthly intervals. Both organic matter and clay content of the trial sites affected the persistence of terbacil, while trifluralin residues were influenced only by the soil organic matter content. Trial sites receiving high rainfall showed considerably less residues of terbacil, but the persistence of trifluralin was not affected by rainfall to any appreciable extent.     

水直播稻田杂草发生特点与防除

江苏常州市武进区直播稻田杂草发生早，有明显的2个出草高峰。杂草发生量大，危害严重，产量损失有时可达80％以上。播后苗前施用扫茀特、直播宁等土壤处理剂是推迟出草时间，减少发生数量，控制杂草危害的有效措施。     

Persistence and metabolism of dinitroaniline herbicides in soils

The persistence, binding, and metabolism of six dinitroaniline herbicides, including trifluralin, profluralin, dinitramine, butralin, fluchloralin, and chlornidine, added to Matapeake silt loam were determined after 3, 5, and 7 months. Dinitramine was rapidly degraded during the first 5 months, while butralin and chlornidine were less persistent than fluchloralin, profluralin, and trifluralin after 7 months. The latter three herbicides were similar in persistence and binding properties. The parent herbicide was the major extractable product detected in soil at each sampling time. Degradation products were identified by cochromatography on thin-layer plates, retention times on gas-liquid and high-pressure liquid chromatography, and mass spectral analysis. Dealkylated and cyclic derivatives of the parent herbicide were detected as metabolites. The cyclic products included benzimidazole derivatives of dinitramine, trifluralin, and fluchloralin; a morpholine derivative of chlornidine; and a quinoxaline derivative of fluchloralin. A unique metabolite of butralin was derived from the parent material by the loss of one nitro substituent.     

Persistence and mobility of bromacil in orchard soils

Bromacil was applied annually for 6 or 7 years in two apple-orchard experiments. At intervals of a year after the last applications, concentrations at various depths down to more than 1 m were measured by gas-liquid chromatography. On an average, 54% of the annual dosage was still present, corresponding to a decline to 35% of the initial amount within a year. The highest concentration in the profile was found at about 10–20 cm, which indicates a much lower mobility than that found with soil thin-layer experiments. Traces of bromacil were found at depths around 1 m, especially with the higher dosages. The upper limits of the amount leaching below a depth of 1 m were estimated at less than 3% of the dosage. Persistance et mobilité du bromacile dans les sols de vergers. Du bromacile a été apporté annuellement pendant six ou sept ans dans des essais effectués dans deux vergers de pommiers. A des intervalles d'une année aprés la derniére application, les concentrations ont été mesurées par chroma-tographie gaz-liquide, á des profondeurs variées allant jusqu'a plus d'un métre. En moyenne, il a été retrouvé 54% de la dose annuelle, ce qui correspond á une diminution de 35% de l'apport initial, dans l'intervalle d'une année. Les concentrations les plus fortes dans le profil ont été trouvées aux environs de 10 à 20 cm, ce qui indique une mobilité beaucoup plus lente que ce qui a été trouvé dans des expériences avec des couches minces. Des traces de bromacile ont été retrouvées à des profondeurs avoisinant 1 m, particuliére-ment avec des dosages élevés. Les limites supérieures du lessivage à une profondeur inférieure à 1 m ont été estimées à moins de 3% de la dose. Persistenz und Mobilität von Bromacil im Boden von Obstanlagen In zwei Apfelanlagen wurde iiber sechs bzw. sieben Jahre, jährlich einmal Bromacil angewendet. In Abständen von einem Jahr nach den letzten Anwendungen wurde das Herbizid in verschiedenen Bodentiefen bis liber 1 m Tiefe gaschromatographisch nachgewiesen. Im Durchschnitt waren noch 54% der jährlichen Aufwandmenge vorhanden. Das entspricht einer Abnahme der ursprünglich ausgebrach-ten Herbizidmenge um 35% innerhalb eines Jahres. Die höchste Konzentration wurde in einer Tiefe von etwa 10–20 cm gefunden. Die Mobilität des Herbizids war, verglichen mit seiner Beweglichkeit in Versuchen mit der Boden-Dunn-schicht-Chromatographie wesentlfch geringer. Spuren von Bromacil wurden besonders bei den hoheren Aufwand-mengen in einer Tiefe von 1 m gefunden. Es wird angenom-men, dass die hochste Herbizidmenge, die liber 1 m tief transportiert wurde, weniger als 3% der Aufwandmenge entspricht.     

Degradation of chlorsulfuron and triasulfuron in alkaline soils under laboratory conditions

The degradation of chlorsulfuron and triasulfuron was investigated in alkaline soils (pH 7.1–9.4) spiked at 40 μg a.i. kg^–1 under laboratory conditions at 25 °C and a moisture content corresponding to 70% field capacity (–33 kPa), using high-performance liquid chromatography. Degradation data for the two herbicides did not follow first-order kinetics, and observed DT₅₀ values in surface soils ranged from 19 to 42 days and from 3 to 24 days for chlorsulfuron and triasulfuron respectively. Disappearance of both chlorsulfuron and triasulfuron was faster in non-sterile than in sterile soil, demonstrating the importance of microbes in the breakdown process. The persistence of chlorsulfuron increased with increasing depth, which can be attributed to the decline in the microbial populations down the profile. The DT₅₀ value for chlorsulfuron at 30–40 cm depth was nearly four times higher than that in the top-soil. The results obtained show that persistence of these herbicides in alkaline surface soils at 25 °C and at a moisture content of 70% field capacity is similar to those reported in other European and North American soils. The study shows that if these herbicides are contained in surface soil layers, the risk of residue carry-over under southern Australian conditions is small. However, the rate of their degradation in alkaline subsoils is very slow, and under conditions conducive to leaching their prolonged persistence in the soil profile is possible.     

Inhibition of biological hydrolysis of parathion in rice straw-amended flooded soil and its reversal by nitrogen compounds and aerobic conditions

The effect of nitrogen compounds and moisture regimes on the inhibition of the biological hydrolysis of parathion in rice straw-amended soil under flooded conditions was studied. Single applications of rice straw, potassium nitrate or ammonium sulphate to the flooded soil, inoculated with parathion-hydrolysing enrichment culture, inhibited the hydrolysis of the insecticide; the inhibition was reversed, however, in combined applications of rice straw with potassium nitrate or with ammonium sulphate. Aqueous extracts of rice straw-amended soils incubated under flooded undisturbed conditions as well as at 100% moisture inhibited the hydrolysis. The inhibitory factor was not formed in rice straw-amended flooded soils incubated at 50 and 75% moisture levels.     

Persistence and mobility of chlorsulfuron and metsulfuron under different soil and climatic conditions

The persistence and mobility of chlorsulfuron and metsulfuron were studied in Italy at four field locations. Chlorsulfuron was applied pre-emer-gence to wheat at 15 and 30 g a.i. ha^?1 in the autumn of 1986 and metsulfuron early post-emergence at 8 g a.i. ha^?1 in the spring of 1987. Soil samples were taken periodically at each location for 1 year. Trials were conducted until 1989 at three locations. After the wheat harvest in 1989, chlorsulfuron and metsulfuron residues in the soil were measured by bioassay in plots that had been treated annually for 3 years and also in plots that had received single treatments. Chlorsulfuron at both application rates dissipated to levels below 0.3 g a.i. ha^?1 over 12 months at three locations while, at the fourth, which received only 250 mm rain during the study, significant levels of residues were still present after 15 months. The degradation of chlorsulfuron applied at 30 g a.i. ha^?1 followed first-order kinetics, with half-lives of 149, 70, 59 and 51 days at the four locations. Chlorsulfuron mobility was high at three locations while, in the silty clay soil of the fourth, the herbicide was confined to the upper 30 cm, despite the high rainfall. Metsulfuron applied in spring was detected only in the first soil sample collection. It was present in all sampled layers at one location but, in the others, was confined to the surface layer. There was no evidence of herbicide accumulation in the soil with repeated applications. Persistance et mobilité du chlorsulfuron et du met-sulfuron dans différents sols et sous différentes conditions climatiques La persistance et la mobilité du chorsulfuron et du metsulfuron au champ ont étéétudiées en Italie sur quatre sites. Le chlorsulfuron était appliqué en pré-levée sur le blé aux doses de matière active (m.a.) 15 et 30 g ha^?1à l'automne 1986 et le metsulfuron en post-levée précoce à la dose 8 g m.a. ha^?1 au printemps 1987. Des échantillons de sol étaient prélevés périodiquement sur chaque site pendant 1 an. Les essais ont été conduits jusqu'en 1989 sur trois des sites. En 1989, après la récolte du blé. les résidus de chlorsulfuron et de metsulfuron dans le sol ont été mesurés par des essais biologiques dans des parcelles qui avaient été traitées chaque année pendant 3 ans, ainsi que dans des parcelles qui n'avaient reçu qu'un seul traitement. Le chlorsulfuron aux deux doses disparaissait jusqu'à des niveaux inférieurs à 0.3 g m.a. ha^?1 en 12 mois sur trois des sites, alors que sur le quatrième, qui n'a reçu que 250 mm de précipitations pendant l'étude, des résidus étaient présents en quantité significative après 15 mois. La dégradation du chlorsulfuron appliquéà la dose 30 g m.a. ha^?1 suivait une cinétique du premier ordre, avec des demi-vies 149, 70, 59 et 51 j sur les quatre sites. La mobilité du chlorsulfuron était élevée sur trois des sites alors que sur le quatrième oú le sol était une argile limoneuse, l'herbicide était confiné aux 30 cm superficiels en dépit de précipitations élevées. Le metsulfuron appliqué au printemps n'était détecté que dans le premier prélèvement. Sur un des sites, il était présent dans toutes les couches, mais était confinéà la couche superficielle sur les autres sites. Après des applications répétées, aucune accumulation d'herbicide n'était observée dans le sol. Persistenz und Mobilität von Chlorsulfuron und Metsulfuron in verschiedenen Böden und bei unterschiedlicher Witterung Die Persistenz und Mobilität von Chlorsulfuron und Metsulfuron wurden in Italien an 4 Standorten unter Freilandbedingungen untersucht. Chlorsulfuron wurde im Herbst 1986 bei Weizen im Vorauflauf mit 15 und 30 g AS ha^?1 und Metsulfuron im Frühjahr 1987 im frühen Nachauflauf mit 8 g AS ha^?1 angewandt. Von jeder Versuchsfläche wurden über einen Zeitraum von 1 Jahr periodisch Bodenproben entnommen. Die Versuche wurden an 3 Standorten bis 1989 durchgeführt. Nach der Weizenernte 1989 wurden mit Biotests die Rückstände von Chlorsulfuron und Metsulfuron in den Böden mit jährlicher (über 3 Jahre) sowie mit einmaliger Behandlung bestimmt. Die Chlorsulfuron-Rückstände nahmen an 3 Standorten bei beiden Aufwandmengen nach 12 Monaten auf 0,3 g AS ha^?1 ab. Am 4. Standort, wo während der Untersuchung nur 250 mm Niederschlag gefallen waren, wurden nach 15 Monaten noch signifikante Rückstände gefunden. Der Abbau von Chlorsulfuron folgte an allen Standorten bei einer Aufwandmenge von 30 g AS ha^?1 einer Kinetik erster Ordnung mit Halbwertszeiten von 149, 70, 59 und 51 Tagen. Die Mobilität von Chlorsulfuron war an 3 Standorten hoch, während es in dem schluffigen Tonboden des 4. Standortes trotz hohen Niederschlags in den oberen 30 cm verblieb. Das im Frühjahr ausgebrachte Metsulfuron wurde nur in der 1. Bodenprobenserie gefunden. An einem Standort lagen in allen beprobten Bodenschichten Rückstände vor, doch sonst waren sie auf die oberste Bodenschicht beschränkt. Bei wiederholter Anwendung konnte keine Herbizidakkumulation im Boden festgestellt werden.     

水直播稻田杂草发生与防除技术探讨

2001—2002年，进行了直播水稻田杂草发生及防除技术的研究，基本摸清和掌握了水直播稻田杂草发生的特点和草相，采用适量50％丁草胺 10％苄嘧磺隆播前或30％扫弗特 10％苄嘧磺隆播后封杀，防效达90％以上，并大面积得到了推广应用。     

Degradation of fluridone in submersed soils under controlled laboratory conditions

The experimental, aquatic herbicide fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone) was degraded in two submersed soils and in the water above those soils to one acidic metabolite (identified as 1,4-dihydro-1-methyl-4-oxo-5-[3-(trifluoromethyl)phenyl]-3-pyridinecarboxylic acid by mass spectrometry). A sandy and a silt loam soil were treated with [¹⁴C]fluridone, immersed in water, and analyzed after 1, 3, 5, 7, 9, and 12 months. Seven to fifteen percent of the ¹⁴C applied to the soils was recovered in the water on each of the various collection dates. The acidic metabolite accounted for 86 to 93% of the radioactivity in the water fraction 7 months after treatment. The metabolite was absorbed strongly by both soils and comprised about 60% of the total ¹⁴C in each soil after 12 months. The remainder of the ¹⁴C in the soils after 12 months was either the parent compound (～30%) or an undefined insoluble residue (～10%).     

Leaching potential and decomposition of fluroxypyr in Swedish soils under field conditions

The mobility and decomposition of the herbicide fluroxypyr (4-amino-3,5-dichloro-6-fluoro-2-pyridyloxyacetic acid) was studied under field conditions in a sandy soil and a clay soil. Leachate was collected in lysimeters with undisturbed soil (sand) and in tile-drained plots (clay). Soil samples to a depth of one metre were also collected in both soils to characterize the temporal depth distribution of fluroxypyr in the profiles. The herbicide was applied as the I-methylheptyl ester of fluroxypyr at two rates, 187.5 and 375.0 g a.e. ha^?1, representing the normal and double the dose of the compound used for spring cereals. Some lysimeters received supplementary watering. Only two leachate samples (one from each soil) had concentrations of fluroxypyr above the detection limit (1 μg litre^?1), i.e. 2 and 5 μg litre^?1. Both samples were collected within two months after application, when less than 2 mm of drainage had been collected. The methylheptyl ester of fluroxypyr was not found in any of the samples. Fluroxypyr levels above the detection limit in soil (5 μg kg^?1 dry soil), were never found below the topsoil (0.2 m) in the clay profile, while, in the sandy profile, levels just above the detection limit were found occasionally in deeper soil layers. Concentrations were reduced to undetectable or very low levels within three months after spraying.     

Persistence and mobility of tebufenozide in forest litter and soil ecosystems under field and laboratory conditions

A field microcosm study was conducted to determine persistence of tebufenozide, an insect growth regulator, in sandy litter and soil. Litter and soil plots (c. 4·5 m² each) were sprayed with an aqueous suspension concentrate formulation of tebufenozide at rates of 35, 70 and 140 g AI ha^-1. Samples were collected at intervals up to 408 days after spraying, and analyzed for tebufenozide residues. The data were subjected to regression analysis and half-life (DT₅₀, the time required for 50% of the initial residues to disappear) values were computed. The DT₅₀ was c. 62 days for both substrates treated with the two lower dosage rates. At the highest dosage rate, the DT₅₀ was 115 days for the litter and c. 52 days for the soil, indicating irregular variations in persistence. Downward movement in soil occurred only in trace amounts, suggesting strong adsorption. Laboratory microcosm studies were conducted to investigate the relative importance of rainfall, exposure to light and volatilization on persistence. Vertical movement occurred in litter and soil (both sandy and clay types) during rainfall. The amount moved increased with the amount of rainfall, but decreased with the rain-free period. The larger the rain droplets, the greater the downward movement. When the rainwater could move laterally along the surface of the substrate (as would occur on a slope), more lateral movement than vertical movement of tebufenozide occurred. The photolysis study indicated that disappearance of tebufenozide was directly related to the duration of exposure to radiation and radiation intensity. Volatilization of tebufenozide depended upon the ambient temperature and the duration of air passing through the substrates. Nonetheless, the amount lost by volatilization was much lower than the amount lost after rainfall or exposure to radiation, thus indicating the greater influence of rainfall and sunlight on persistence. In the laboratory microcosm studies, more tebufenozide was lost from the sandy substrates than from the clay substrates. This behaviour was attributed to the greater adsorptive capacity of the clay substrates, thus providing a greater protection against downward mobility and loss due to radiation. © 1997 SCI     

Allelopathic potential of Cambodian rice lines under field conditions

A series of field experiments was conducted during 1999 and 2000 to study the effect of six Cambodian rice lines that had been selected for their allelopathic potential on the growth of three weed species (barnyardgrass, small umbrella sedge, and water primrose). The results from 2 years' study demonstrate that powerful weed‐establishment and growth‐suppressive mechanisms were present in all of the rice lines tested. This mechanism was equally active on all three weed species studied. Across all the rice lines and across all the weed species, weed establishment was reduced by 71%, the final plant height was reduced by 49%, and the dry biomass was reduced by 80%. A tentative comparison between the effects of the Cambodian rice lines and those of previously characterized allelopathic and non‐allelopathic rice lines revealed that approximately three‐quarters of the weed growth suppression in the Cambodian lines could be attributed to resource competition and one‐quarter could be attributed to allelopathy, although this analysis did not take into account morphological differences between the two types of rice. Such weed growth‐suppressing activity could be particularly useful in subsistence farming systems where the use of selective herbicides is prohibitive or when organic rice production is the objective. The use of rice lines that suppress the growth of weeds is likely to be a potent supplement to present weed management practises and will reduce production costs and the potential for environmental pollution, as well as alleviate some of the social constraints that are associated with labor‐intensive manual weeding.     

Strategy of Echinochloa oryzicola Vasing. for survival in flooded rice

Echinochloa oryzicola Vasing. (= Echinochloa phyllopogon Stapf ex Kessenko) is an obligate weed with an elaborated survival strategy in the flooded rice of Japan. In this review various adaptive characters of the weed, which comprise the survival strategy, are discussed through the life cycle. The weed is distributed only in flooded rice. Seeds (spikelets) buried in the soil exhibit annual cycles between dormant and non‐dormant state, and non‐dormant seeds recurrently appear in spring when rice growers start to prepare seedling beds and fields for rice transplanting. The non‐dormant seeds have unique characters metabolically adapted to submerged conditions to germinate and grow by the anaerobic respiration through alcohol fermentation. The weed has seemingly perfect mimicry of the rice plants throughout its development from seedling to heading, by which the weed escapes from manual weeding. In a rice paddy, the weed starts heading coincidentally with the rice plants at the period when the growers are reluctant to walk in the rice paddy to weed. Irrespective of plant height of the rice cultivar, the weed develops a few upper leaves above the rice canopy during the heading period of rice. This phenotypic plasticity of E. oryzicola in plant height is one of the characters conferring its competitive aggressiveness in flooded rice. When weeding is begun again after heading, the dormant weed seeds escape weeding by shattering and join the soil seedbank. The dormant seeds express the gene of an enzyme catalyzing ATP synthesis through the mitochondrial oxidative phosphorylation more abundantly, and have larger oxygen absorption and enzyme activity of the aerobic respiration than the non‐dormant seeds, suggesting that the dormant seeds maintain viability by the conventional aerobic respiration in the paddy soil drained from rice harvesting in fall to the next early spring. The various adaptive characters comprising the survival strategy of E. oryzicola in flooded rice consist of those inherited from the wild progenitor and those selected by the crop cultivation pressure. It is suggested that both the mimicry of the weed and the heading coincident with the rice plants have been acquired by the large selection pressure of frequent weeding, which has been done over the past hundred years. However, today, the manual weeding is substituted with herbicides, which cannot detect the mimicry and heading photoperiodic sensitivity. As a result, the dominant species of Echinochloa weeds in flooded rice is changing from E. oryzicola to Echinochloa crus‐galli var. crus‐galli that has neither mimicry nor photoperiodic sensitivity synchronizing to that of rice, but is more competitive against rice.     

Effect of soil solarization on weeds and nematodes under tropical Indian conditions

The control of weeds and nematodes by solar heating of the soil using transparent polyethylene (PE) sheets was studied in the field during the summer of 1990. PE mulching for 32 days decreased the emergence from seed of the dominant weeds Dactyloctenium aegyptium (L.) P. Beauv., Acrachne racemosa (Roem. & Schult.) Ohwi, Trianthema monogyna L. and Cyperus rotundus L. by over 90%. Emergence of C. rotundus from tubers was increased by the mulching treatments. Mulching for 16 days also decreased weed emergence but to a lesser extent than the 32-day treatment. The main solarization effect was restricted to the 0–5 cm layer of soil. Solarization reduced the population of plant-parasitic nematodes (Tylenchus, Heterodera, Xiphinema, Hoplolaimus, Pratylenchus and Ro-tylenchus spp.) by about 90% and of saprophytic nematodes by about 60% but, after 70 days, the nematode levels had largely recovered. The mean maximum soil temperatures recorded at 5 cm were higher when mulching followed irrigation. Data are given on the soil temperatures recorded over the 32-day period. The growth of soybean was improved and seed yield increased by up to 78% following solarization. Effet de la solarisation du sol sur les mauvaises herbes et les nematodes en conditions tropicales indiennes La lutte contre les mauvaises herbes et les nématodes par solarisation du sol à l'aide de films de polyéthylène transparent (PE) a étéétudiée au champ pendant l'été 1990. Trente deux jours de solarisation diminuaient de plus de 90% la levée à partir de graines des mauvaises herbes dominantes Dactyloctenium aegyptium (L.) P. Beauv., Acrachne racemosa (Roem. & Schult.) Ohwi, Trianthema monogyna L. et Cyperus rotundus L. La levée de C. rotundusà partir de tubercules était accrue par la solarisation. Seize jours de solarisation diminuaient aussi la levée des mauvaises herbes, mais dans un moindre mesure. L'effet de solarisation s'exerçait principalement dans les 5 centimètres superficiels du sol. La solarisation réduisait d'environ 90% la population de nématodes parasites des plantes (Tylenchus, Heterodera, Xiphinema, Hoplolaimus, Pratylenchus et Rotylenchus spp.) et d'environ 60% la population des nématodes saprophytes. Après 70 jours, les populations de nematodes étaient pratiquement reconstituées. Les moyennes des températures maximales du sol enregistrées à 5 cm de profondeur étaient plus élevées quand l'installation du film polyéthylène suivait une irrigation. Des données concernant la température du sol sur 32 jours sont communiques. La croissance du soja était accrue et le rendement en grains augmentait jusqu'à 78% après solarisation. Wirkung der Bodensolarisation auf Unkräuter und Nematoden unter tropischen Bedingungen Die Bekämpfung von Unkräutern und Nematoden durch Solarisation mittels farbloser Polyethylenfolia (PE) wurde im Sommer 1990 im Freiland untersucht. Nach 32tägiger Solarisation nahm die Keimung der vorherrschenden Unkräuter Dactyloctenium aegypticum (L.) P. Beauv., Acrachne racemosa (Roem. & Schult.) Ohwi, Trianthema monogyna L. und Cyperus rotundus L. um über 90% ab. Der Austrieb von Cyperus rotundus aus Knollen wurde gefördert. Eine l6tägige Solarisation war nicht so wirkungsvoll. Die Wirkung war auf die oberste Bodenschicht bis 5 cm Tiefe beschränkt. Gegen pflanzenparasitische Nematoden (Tylenchus, Heterodera, Xiphinema, Hoplolaimus, Pratylenchus und Rotylenchus spp.) betrug die Wirkung der Solarisation etwa 90%, gegen saprophytische Nematoden rund 60%, aber nach 70 Tagen hatten sich die Nematodenpopulationen wieder weitgehend erholt. Nach Bewässerung wurde in 5 cm Tiefe eine höhere Bodentemperatur erreicht. Für die 32-Tage-Periode werden die Temperaturdaten wiedergegeben. Nach der Solarisation war das Wachstum von Sojabohne verbessert, und die Erträge stiegen um bis zu 78%.