Allelopathic potential of indigenous Bangladeshi rice varieties

A series of experiments was conducted in the laboratory and greenhouse of the Subtropical Field Science Center, University of the Ryukyus, Japan, from April to October 2015 to assess the allelopathic potential of 50 indigenous Bangladeshi rice varieties by using the donor–receiver bioassay, equal compartment agar method (ECAM), plant residue extract method and pot culture method. Lettuce (Lactuca sativa L.), cress (Lepidium sativum L.), radish (Raphanus sativus L.), barnyard grass (Echinochloa crus‐galli L. Beauv.) and jungle rice (Echinochloa colona L.) were used as the test plants. The highest inhibition effect was given by Boterswar, while the stimulating effect was given by Kartikbalam and Panbira in the donor–receiver bioassay and ECAM tests. Boterswar, Goria, Biron and Kartiksail were selected as the highest allelopathic‐potential varieties by the donor–receiver bioassay and ECAM. In the methanol extract test, Boterswar gave the strongest inhibitory effect on both barnyard grass and jungle rice, while Kartiksail gave the highest inhibitory effect on the jungle rice shoot. The growth parameters and total dry matter of barnyard grass in the greenhouse pot experiment were significantly reduced as a result of the application of aqueous extracts of the selected rice varieties, which was similar to the results of the laboratory experiments. The varieties of Boterswar, Goria, Biron and Kartiksail were selected as the most allelopathic among the 50 indigenous Bangladeshi rice varieties. These rice varieties could be used for the isolation and identification of allelochemicals and to further develop new varieties that are tolerant to weeds.     

Evaluation of rice allelopathy in hydroponics

The inhibitory activity of water extracts from the shoots and roots of three rice cultivars, Taichung native 1 (TN1) and IAC165 (both allelopathic rice) and AUS196 (non-allelopathic rice), grown in hydroponics was evaluated. The release of germination inhibitors by allelopathic rice plants into hydroponic solution was also determined with freshly collected solution and XAD-4 resin desorbate. The degree of the inhibition was quantified in terms of root growth in Echinochloa colona, Echinochloa crus-galli, Echinochloa crus-galli var. oryzicola, Triantema portulacastrum and Lactuca sativa. The allelopathic activity of rice was species specific, and depended on source and concentration. Root length of all test species was inhibited by the different concentrations of shoot extract of allelopathic and non-allelopathic rice. However, of the three cultivars, TN1 showed higher inhibition than IAC165 and AUS196 in all test species. Water extracts of shoots and roots significantly inhibited root growth in E. crus-galli but the shoot extract gave a greater inhibitory effect on E. crus-galli than the root extract. Root exudate of TN1 inhibited root elongation of E. crus-galli from 2 weeks after transplanting (WAT) and the inhibition continued for 4 WAT. The results confirmed the previous finding of a laboratory bioassay that the TN1 had allelopathic activity and produced allelochemicals that inhibit growth of some weed species.     

Compounds from the subterranean part of Johnsongrass and their allelopathic potential

The allelopathic effects of the exotic invasive weed, Johnsongrass, on Lactuca sativa, a native plant in China, were evaluated and the phytotoxins were investigated under laboratory conditions. The crude extracts (chloroform and ethyl acetate fractions) that were obtained from the ethanol extract of the subterranean parts of Johnsongrass inhibited the germination speed and growth of the roots and shoots of the test plant. The four compounds, ethyl p‐hydroxybenzoate, diosmetin, apigenin, and luteolin, were isolated from the extract of the subterranean parts of Johnsongrass first, along with three other compounds (reported previously), p‐hydroxybenzaldehyde, p‐hydroxybenzoic acid, and dhurrin, and they all were evaluated on L. sativa. At the concentration of <0.5 mmol L^?1, ethyl p‐hydroxybenzoate, dhurrin, and the mixture of the compounds delayed the germination speed of the seeds of L. sativa at 24 h of incubation, while apigenin delayed the germination speed of the seeds of L. sativa at 72 h of incubation. Ethyl p‐hydroxybenzoate demonstrated the strongest delaying effect among the phenols and a similar effect was found with apigenin among the flavonoids, wheras ethyl p‐hydroxybenzoate induced a drastic inhibition of the germination at 2 mmol L^?1. In contrast, p‐hydroxybenzaldehyde and luteolin had no effect on the germination at any concentration at any stage. All the substances inhibited the shoot and root growth of L. sativa at 3 mmol L^?1. Increasing the concentration increased the inhibition of the growth of L. sativa. The inhibitory activity of ethyl p‐hydroxybenzoate and p‐hydroxybenzaldehyde was greater than that of the other compounds. This result suggested that the isolated phytotoxins might contribute to the successful invasion by Johnsongrass.     

Allelopathic potential of Acacia melanoxylon on the germination and root growth of native species

Water extracts that were obtained from the flowers and phyllodes of Acacia melanoxylon were used to determine their allelopathic potential in relation to the germination and seedling growth of the native species, cocksfoot (Dactylis glomerata), perennial ryegrass (Lolium perenne), and common sorrel (Rumex acetosa), as well as a general biotest specie, lettuce (Lactuca sativa), in laboratory bioassays. The flowers and phyllodes of A. melanoxylon were soaked separately in distilled water in a ratio of 1:1 (w/v) for 24 h in order to prepare the aqueous extracts. Distilled water was used as the control. The seeds of the target species were germinated in Petri dishes and counted daily for up to 7 days. The A. melanoxylon flower extract (100%, 75%, and 50%) decreased the seed germination of D. glomerata, R. acetosa, L. perenne, and L. sativa. The flower extract caused the most reduction in the germination index and germination speed in D. glomerata, L. perenne, and L. sativa. The mean LC₅₀ value of the A. melanoxylon flower and phyllode extracts in relation to the germination inhibition of L. perenne was 43% and 41%, respectively, 40% and 38%, respectively, in R. acetosa, and 53% and 41%, respectively, in L. sativa. All four concentrations of the flower extract proved to be more phytotoxic than the phyllode extract, reducing the root length of all four species, while the phyllode extract decreased the root length of L. perenne and R. acetosa at the 100% concentration. The L. perenne and D. glomerata grass seeds were more sensitive regarding germination, as compared to L. sativa and R. acetosa. The flower aqueous extract of A. melanoxylon was more phytotoxic, as compared to the phyllode aqueous extract, even at the lowest concentration (25%).     

Plant growth inhibitory activity and active substances with allelopathic potential of cogongrass (Imperata cylindrica) rhizome

Cogongrass (Imperata cylindrica [L.] Raeusch.) is a perennial rhizomatous grass that belongs to the Poaceae and is widely distributed in tropical, subtropical and temperate regions over the world. The grass is strongly invasive and is suggested to have allelopathic potential. However, limited information is available on allelopathic substances in cogongrass rhizomes. Therefore, the allelopathic potential and substances in cogongrass rhizomes were investigated. The extracts of cogongrass rhizomes inhibited the root and shoot growth of barnyard grass, ryegrass, timothy, cress, lettuce and alfalfa. After bioassay‐guided separations of the extracts, four growth inhibitory substances, 5‐methoxyflavone, 5,2′‐dimethoxyflavone, methyl caffeate and abscisic acid, were isolated. 5‐Methoxyflavone, 5,2′‐dimethoxyflavone and methyl caffeate significantly inhibited the root and shoot growth of cress at concentrations of ≥0.03–0.3 m mol L^?1 and their concentrations required for 50% growth inhibition were 0.079–0.24, 0.23–1.1 and 0.59–0.88 m mol L^?1, respectively. The other isolated substance, abscisic acid, has been reported to have strong growth inhibitory activity. Its concentrations required for 50% growth inhibition on cress were 0.31–0.61 μ mol L^?1. The present study suggests that cogongrass rhizomes might have allelopathic potential and that those four substances might contribute to that potential.     

Effect of naringenin on the growth and lignin biosynthesis of gramineous plants

Lignin biosynthesis is essential for plant growth. 4‐Coumarate CoA ligase (4‐CL, EC6.2.1.12) is involved in the monolignol synthesis and occupies a key role in regulating carbon flow into the phenylpropanoid metabolism pathway. Naringenin, one of the metabolites in this pathway, is known as a potent in vitro inhibitor of 4‐CL. The growth of rice (Oryza sativa L. cv. Koshihikari), maize (Zea mays L. cv. Yellow corn) and Echinochloa oryzicola Vasing seedlings at the 2nd leaf stage was inhibited after continuous root application with 0.1 mmol L^?1 naringenin for 1 week, although naringenin did not kill these gramineous plants. The highest inhibition of fresh weight increase was observed in maize, followed by rice and E. oryzicola. The symptoms in these plants were root browning, delay of leaf/root development and shoot dwarfing. Naringenin treatment increased the contents of 4‐CL substrates, cinnamic acid, 4‐coumaric acid, caffeic acid and ferulic acid from 1.2 to 7.2 times and from 1.2 to 3.5 times in shoots and roots, respectively, except for ferulic acid in E. oryzicola roots. It also caused a slight decrease of the lignin content and alteration of lignin constitutions in rice plants. These results suggested that the monolignol pathways after 4‐CL towards lignin has the possibility to be the novel action sites of plant growth retardants, although further investigations are needed to clarify the mode of action.     

Potential allelopathic effects of Mikania micrantha on the seed germination and seedling growth of Coix lacryma‐jobi

The allelopathic potential of Mikania micrantha H.B.K. to affect the seed germination and seedling growth of Coix lacryma‐jobi L. was investigated. Water‐soluble allelopathic substances were found in the water extracts of M. micrantha. The effect of the water extracts on the seed germination and seedling growth of C. lacryma‐jobi was concentration‐dependent. The water extracts from the different plant parts (leaf, stem, and root) of M. micrantha differed in their effect on the germination and seedling growth of C. lacryma‐jobi, with the effect of the leaf extract being the least inhibitory. The malondialdehyde (MDA) content in the C. lacryma‐jobi seedlings increased by 64%, 45%, and 52% of the control with increasing concentrations of the extracts of the root, stem, and leaf (80, 400, and 400 g L^?1, respectively). The extract from the M. micrantha roots significantly increased the catalase (CAT) activity of the C. lacryma‐jobi seedlings (48% and 54% of the control at the concentrations of 20 g L^?1 and 80 g L^?1, respectively). The extracts from the leaves and stems at low concentrations increased the CAT activity, but at high concentrations, the extracts decreased the CAT activity. The extracts from the roots, stems, and leaves at concentrations of 80, 400, and 400 g L^?1 also significantly decreased the peroxidase (POD) activity of the C. lacryma‐jobi seedlings to 27%, 52%, and 34% of the control, respectively. These results indicate that the water extracts of M. micrantha could inhibit the seed germination and seedling growth of C. lacryma‐jobi through the regulation of anti‐oxidase activity, such as POD and CAT in the cells. The growth inhibition of the C. lacryma‐jobi seedlings is probably related to injury after oxidization of the cell membranes with the increase of MDA content.     

Field evaluation of allelopathic plant extracts alongside herbicides on weed management indices and weed–crop regression analysis in maize

Synthetic herbicides are posing problems owing to the development of weed resistance and emerging debate on their associated health hazards and ecological threats. Allelopathic manipulations are evolving as applicable substitutes for weed management in agroecosystems. In order to assess the efficacy of potential allelopathic water extracts from different plant species, field experiments were conducted during 2010 and 2011. Sorghum bicolor L., Helianthus annuus L., Brassica napus L., Oryza sativa L., Zea mays L. and Morus alba L. aqueous extracts in different combinations alone or along with a reduced dose of herbicide were evaluated for weed suppression in a maize crop. A weedy check and two herbicidal treatments, S‐metolachlor + atrazine (pre‐emergence) and atrazine alone (early postemergence), were included for comparison. Sorghum, brassica or sunflower tank‐mixed with 25% of the recommended dose of atrazine significantly suppressed the total weed density and dry biomass, along with a concomitant decrease in the crop resistance indices and treatment efficacy indices over the control. The weed density and dry biomass that were recorded at 60 days after sowing showed a strong negative correlation, while the leaf area index, crop growth rate, dry matter accumulation and net assimilation rate predicted a strong positive correlation, with the stover and grain yield of maize.     

Screening of weed extracts for antifungal properties against Colletotrichum lagenarium,the causal agent of anthracnose in cucumber

The antifungal activity of the leaf extracts from 203 weed species was investigated by performing a bioassay using cucumber plants and Colletotrichum orbiculare. The leaf extracts from four families, namely, Urticaceae, Onagraceae, Commelinaceae, and Solanaceae, showed a relatively stronger inhibition of the anthracnose lesions in cucumber plants when compared with the other families investigated in the study. A remarkable inhibition of anthracnose infection in cucumber leaves was observed with the extracts from the following 19 weed species: Boehmeria nipononivea and Boehmeria longispica, Persicaria scabra, Ranunculus japonicus and Ranunculus sceleratus, Cardamine flexuosa, Oenothera biennis, Aeschynomene indica, Indigofera pseudo‐tinctoria, Torilis scabra, Calystegia japonica, Solanum americanum, Bidens pilosa, Gnaphalium japonicum, Kalimeris yomena, Bromus catharticus, Cynodon dactylon, Alopecurus aequalis, and Scirpus tabernaemontani. In particular, it is noteworthy that the extracts from C. dactylon, K. yomena, and S. americanum completely inhibited anthracnose infection in cucumber.     

Use of phytotoxic rice crop residues for weed management

There is a general perception among Cambodian rice (Oryza sativa) farmers that, after harvesting, rice crop residues that are incorporated into the field benefit the growth of the subsequent rice crop. However, the effect of this action upon weed establishment and growth has not yet been considered. A series of pot and field trials were conducted to determine whether such action could inhibit weed establishment and/or growth. The pot studies first evaluated the response of the test plant (rice line ST‐3) and three weed species, barnyardgrass (Echinochloa crus‐galli), small umbrella sedge (Cyperus difformis), and water primrose (Ludwigia octovalves), to the residue of 16 rice lines and the field trials were later conducted to evaluate the response of the same test plants to the residue of seven putatively allelopathic rice lines and one non‐allelopathic rice line. The residue of all the studied rice lines, depending on how long they had been incorporated into the soil, reduced the establishment and growth of all three weed species, as well as the rice crop. However, if the residue's incorporation was delayed by 2 weeks or only a proportion of the residue was incorporated, the rice crop could withstand the growth‐inhibiting effect, while the inhibition of the establishment and growth of the three weed species was retained. These responses of rice and the weeds to rice crop residues might provide a basis for a weed management strategy, particularly in the resource‐poor rice‐production systems of Cambodia.     

Weed suppression by Passiflora edulis and its potential allelochemicals

Passion fruit (Passiflora edulis) is grown in the tropics for its edible fruits and for its many ethno‐pharmacologic and pharmacological properties. Our study revealed that the plant contains a strong allelopathic potential. In a bioassay, aqueous extracts of P. edulis strongly suppressed germination and growth of lettuce, radish and two major paddy rice weeds, Echinochloa crusgalli and Monochoria vaginalis. In glasshouse and field experiments, P. edulis also strongly inhibited the growth of paddy rice weeds. Application of 2 t ha^?1 dry plant material of P. edulis reduced weed biomass by 70% and increased rice yield by 35% compared with the unweeded control. Ten newly identified substances in P. edulis extracts, including coumarin, long‐chain fatty acids and lactones, may be responsible for the inhibitory activity of P. edulis. Coumarin and the lactones showed greater inhibition of germination and growth of E. crusgalli than the fatty acids. The authors suggest that P. edulis may be used as a natural herbicide to reduce the dependency on synthetic herbicides.     

Comparison of the allelopathic effects and uptake of Fallopia japonica phytochemicals by Raphanus sativus

Fallopia japonica (Japanese Knotweed) is a prolific invader possessing several potentially allelopathic chemicals. The aim of this study was to determine whether four of these compounds inhibit the germination and growth of radish (Raphanus sativus) seedlings. Seeds were planted in potting soil and grown under glasshouse conditions for 2 weeks and the phytochemicals were gradually added to the soil over the course of the study. For all phytochemicals, there was no significant reduction in germination, plant heights, leaf widths or leaf lengths. However, plants treated with resveratrol, emodin and (–)‐epicatechin had significantly shorter root lengths than the control. Seedlings were dried and extracts were prepared with methanol. Extracts were tested via fluorescence spectrophotometry for uptake of the chemicals. Polydatin was detected at the highest concentration, with resveratrol and emodin detected at much lower concentrations. (–)‐Epicatechin was below the limits of detection. These results suggest that F. japonica may negatively impact the growth of other species and that multiple chemicals work together to give the plant a competitive advantage.     

Geelnervigheid,een virusziekte van luzerne

Samenvatting Door middel van de erwtebladluis (Acyrthosiphon pisum Harris) werd het topvergelingsvirus van de erwt op luzerne (Medicago sativa L.) overgebracht. De besmette planten vertoonden na verloop van tijd het verschijnsel van geelnervigheid, te weten een regelmatige, lichtgroene tot gele nerftekening op de bladeren. Dit verschijnsel blijkt veelvuldig in luzernepercelen in het zuidwesten van het land (het gebied waar topvergeling in erwt algemeen voorkomt) op te treden. Het gelukte uit vier van vijf van verschillende plaatsen van West-Brabant en Zuid-Beveland afkomstige monsters geelnervige luzerne het topvergelingsvirus te isoleren.Uit waarnemingen bij planten, groeiende in het veld en in de kas, is gebleken, dat de omstandigheden een grote invloed hebben op het te voorschijn komen van het ziektebeeld. In het voor- en najaar treedt de geelnervigheid doorgaans zeer duidelijk op; in de zomermaanden verdwijnt dit verschijnsel veelal tijdelijk bij de besmette planten en onderscheiden deze zich niet van virusvrije luzerne.In een veldproef bleek het topvergelingsvirus, gezien het voorkomen van geelnervigheid in de loop van het seizoen, vrij snel in luzerne verspreid te worden.Summary Tip yellows (leaf roll) virus of pea was transmitted to lucerne (Medicago sativa L.) by means of the pea aphid (Acrythosiphon pisum Harris). After some months the infected lucerne plants showed a regular lightgreen to yellow discoloration of the veins and the adjacent tissue (fig. 1). No differences in growth and in shape of the leaves between infected and healthy plants were found to exist.This disease—for which we have chosen the name of vein yellowing of lucerne—is very prevalent in lucerne fields in the southwestern part of The Netherlands where tip yellows is often found in peas. We succeeded in isolating tip yellows virus from four out of five samples of lucerne showing vein yellowing collected in western Brabant and Zuid-Beveland.In carrying out transmission tests, it was found that the pea aphid often hardly sucks on lucerne in the greenhouse. Although it generally is present in large numbers in lucerne fields, it refuses to feed on lucerne after having been reared on peas for a prolonged period. This behaviour of the vector impedes the experiments. It may be due to the fact that the different generations of the insect differ in host plant requirements (Evans & Gyrisco, 1956) but also to the fact that morphologically indistinguishable forms of the insect exist, which may differ in host plant preferences (Hille Ris Lambers, 1947).Furthermore, in our experiments seldom a 100% infection of pea plants was obtained, even when using pea aphids which fed on a tip yellows diseased pea plant for a period of 48 hours or longer, and using five to ten aphids per test plant.Observations on infected plants growing in the field and in the greenhouse indicate that the environment has great influence on the appearance of the vein yellowing symptom. Generally, vein yellowing occurs very strikingly during the spring-time and the late autumn but disappears in the summer months.According to observations on the rate of occurrence of vein yellowing at different times of the year in a field experiment at Wageningen (fig. 2), it appears that tip yellows virus is capable of spreading fast in a lucerne field.The experiments and observations described in the present paper confirm the idea expressed byde Fluiter & Hubbeling (1955) that lucerne fields are important as a reservoir of the tip yellows virus of pea in The Netherlands.     

Allelopathic Potential of Soliva sessilis Ruiz & Pav. on Wheat

Soliva sessilis (Burweed) is a winter annual weed found in wheat crop of Pakistan. It is thought to have an allelopathic effect on wheat crop. In this regard, an experiment was conducted in 2019 in the department of Weed Science and Botany at the University of Agriculture Peshawar, to assess the allelopathic potential of S. sessilis on various wheat varieties grown in Peshawar, Pakistan. Physiologically matured roots and leaves of S. sessilis were dried, ground and stored. Three common wheat varieties were selected for the research including Atta Habib, Pirsabaq-15, and Shahkar-13. The treatments of the experiment comprised of the aqueous extracts of the roots and leaves of S. sessilis, hot water extracts, methanol extracts, litter, and mulches. A mass of 5?g of S. sessilis was taken for the different extracts for 24?h, however, the methanol extract was kept for 48?h. The effect of the treatments was highly significant on wheat seed germination, radicle length, plumule length, fresh biomass as well as dry biomasses of the three tested wheat varieties. The variety Atta Habib was discovered as the most susceptible variety to the treatments of aqueous extracts, hot water extracts, and leaves’ litter. In contrary, the variety Pirsabaq-15 exhibited a remarkable increase in the various data collection parameters. The leaves of S. sessilis were observed more toxic as compared to the roots. The mulch of S. sessilis indicated stimulatory effect on the wheat varieties. For the interaction effect between treatments and varieties, the leaves and roots aqueous extract and their litter showed inhibitory effect. In conclusion, the leaves and roots of S. sessilis are potentiality allelopathic to wheat crop plants and further study is needed to assess the types of these allelochemicals present in the leaves and roots of the weed.

     

Population dynamics of Rumex obtusifolius under contrasting lucerne cropping systems

The population dynamics of Rumex obtusifolius L. was analysed in a lucerne:winter cereal crop rotation by means of a matrix population model that takes into consideration two crop rotation periods: the lucerne (Medicago sativa L.) cropping period and the cereal cropping period. Several transition matrices based on life-cycle stages were calculated for each cropping period using experimental data and were used in the construction of a model that analyses the population dynamics of R. obtusifolius under different harvest dates and lengths of lucerne cropping periods. Model projections showed that populations of R. obtusifolius increased during the lucerne cropping period regardless of harvest date and decreased during the cereal cropping period. Under a late harvest date, populations decreased at each crop rotation when lucerne was grown for 3 years, remained close to the equilibrium when lucerne was left to grow for 5 years, and increased for longer lucerne cropping periods. In contrast, populations of R. obtusifolius decreased even with a lucerne cropping period of 9 years under an early harvest date. The significance of these results in relation to the biology and the non-chemical control of the species is discussed.     

Effet de la densité et de la proportion des plantes lors de renvahissement de la luzerne par Taraxacum officinale Weber

Des expériences ont été conduites en salle climatisée pour mesurer l'influence de la densité et de la proportion des plantes sur l'envahisse-ment de la luzerne (Medicago sativa L.) établie par le pissenlit (Taraxacum officinale Weber). La durée de l'interférence et la densité de la luzerne ont permis d'expliquer la variation de la biomasse aérienne chez la luzerne cultivée en mélange avec le pissenlit. La densité du pissenlit a été toutefois un facteur important permettant d'expliquer la variation de la biomasse racinaire de la luzerne. La biomasse aérienne et racinaire du pissenlit a varié en fonction de la durée de la compétition, de la densité du pissenlit, de la densité de la luzerne et de la photopériode. Les équations de surface de réponse et hyper-boliques Iinéarisées ont permis de déterminer que la compétition dépendait de la densité plutôt que des proportions de chaque espèce dans le mélange. Effect of plant density and proportion on invasion of lucerne by Taraxacum officinale Weber Experiments were conducted in a growth chamber to establish the effect of plant density and proportion on invasion by dandelion (Taraxacum officinale Weber) of established lucerne (Medicago sativa). Duration of competition and lucerne density accounted for the variation in shoot biomass of lucerne growth in mixture with dandelion. The dandelion density explained a large proportion of the variation in lucerne root biomass. Dandelion root and shoot biomass varied according to duration of competition, dandelion density, lucerne density and photoperiod. Response surface and linearized hyperbolic equations suggested that competition was related more to density than to the proportion of each species in the mixture. Einfluβ der Pflanzendichte und-verteilung auf die Verunkrautung von Lucerne durch Taraxacum officinale Weber In Klimakammerversuchen zum Einfluß der Pflanzendichte und-verteilung auf die Verunkrautung von Luzerne (Medicago sativa L.) durch Löwenzahn (Taraxacum officinale Weber) erwiesen sich die Konkurrenzdauer und die Luzernendichte als wichtige Faktoren für die Bildung der Sproßbiomasse der Luzerne in Mischbeständen mit Löwenzahn, dessen Dichte für die Wurzelbiomasse der Luzerne ausschlaggebend war. Sproß- und Wurzelbiomasse der Löwenzahns variierten mit der Konkurrenzdauer, der Unkraut- sowie der Luzernendichte und der Photoperiode. Die Pflanzendichte erwies sich für die Konkurrenz wichtiger als das Mischungsverhältnis der Arten.     

Screening of 170 Peruvian plant species for allelopathic activity by using the Sandwich Method

Peru is one of the 20 botanically extremely diverse countries in the world, with >17 000 flowering plants, of which 30% are endemic. So far, no systematic research has been conducted on the screening of the allelopathic plants. In this study, the allelopathic activity of 170 species from 61 families of Peruvian plants that were collected from the three main regions of Peru – the Costa (Pacific coastline), the Sierra (Andean mountains), and the Selva (Amazonian rainforest) – was evaluated. The allelopathic activity was determined by the Sandwich Method, which can evaluate the activity of leaf leachates. The species that were found to be highly inhibitory in this screening, under the criterion of >90% inhibition of the radicle of lettuce (Lactuca sativa) seedlings, were Aristeguietia ballii and Diplostephium foliosissimum (Asteraceae) and Spondias mombin (Anacardiaceae). All of these species are native plants from Peru. This study gives a strong clue regarding the potential of isolating potent allelochemicals from these plants in the future.     

Performance of Bangladesh indigenous rice in a weed infested field and separation of allelopathy from resource competition

This study aimed to identify the potential allelopathic indigenous rice (Oryza sativa L. ssp. indica) varieties from Bangladesh using a performance study in a weed‐infested field and to assess the extent of allelopathic interference relative to resource competition in a glasshouse experiment. Six varieties – namely, “Boterswar,” “Goria,” “Biron” and “Kartiksail” as the most allelopathic, “Hashikolmi” as weakly allelopathic and “Holoi” as nonallelopathic – were raised following a nonweed control method. The infestation levels of weed species were calculated using Simpson's Diversity Index (SDI), which ranged from 0.2 to 0.56. However, a significant correlation coefficient (0.87, P < 0.001) was obtained from these field data compared with the root inhibition percentage from the laboratory bioassay, and the “Boterswar” variety was the most allelopathic. The interactions between the allelopathic variety “Boterswar,” weakly allelopathic variety “Hashikolmi” and Echinochloa oryzicola via a target (rice)‐adjacent (E. oryzicola) cogrowth culture were determined in a hydroponic arrangement. The relative competitive intensity (RCI) and the relative neighbor effect (RNE) values showed that the crop–weed interaction was facilitation for “Boterswar” and competition for “Hashikolmi” and E. oryzicola in rice/E. oryzicola cogrowth cultures. The allelopathic effects of “Boterswar” were much higher than the resource competition in rice/E. oryzicola cogrowth cultures. The converse was observed for “Hashikolmi.” Moreover, the mineral content of E. oryzicola was severely affected by “Boterswar”/E. oryzicola cogrowth cultures’ exudate solution. Therefore, the allelopathic potential of “Boterswar” variety might be useful for developing the weed‐suppressing capacity of rice, which will likely have a significant influence on paddy weed control.     

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.