Archives - Allelopathy Journal

Field-grown plants were harvested at inflorescence bud stage and the fresh weight of leaves, stems and inflorescences were recorded. Allelopathic activity of aqueous extracts of leaves, stems, inflorescences, roots, petioles and seeds was determined on mustard germination. The test genotypes differed significantly both in the biological yield (approx 9 folds) and in biomass partitioning (stems > leaves > inflorescences). Allelochemicals in leaf extracts, significantly delayed and decreased the mustard germination. Significant differences were also recorded between the plant organs, leaves showed highest and roots had no activity. Both allelopathic activity of particular organs and the biomass accumulated, together with its partitioning pattern were major components of allelopathic potential.

We studied the allelopathic interference between the Zizyphus spina-christi and Prosopis juliflora. Dried leaves from both species were milled to prepare leaf extracts of 0, 5, 20, 40, 60 and 100 g/l concentrations. The leaf extracts of P. juliflora and Z. spina-christi were used to irrigate seedlings of each other. Zizyphus spina-christi leaf extract decreased the root parameters (total root length, root surface area, number of tips and root volume) of P. juliflora and the decrease was concentration dependent. However, leaf extract of P. juliflora stimulated the parameters of Z. spina-christi roots. The root length, root surface area and number of tips increased with increasing concentration, except at 100 g/l concentration. The study revealed that Z. spina-christi leaf extract was inhibitory to the growth of another plant species, it was reported first time.

In Petri dish bioassay, the allelopathic effects of different extracts (petroleum, ethyl acetate, n-butanol and aqueous) of Polygonatum odoratum rhizome were studied on the seed germination and growth of wheat, cucumber and carrot. The allelopathic inhibitory effects depended on the extracts concentration. All extracts inhibited the seed germination and shoot growth, however, petroleum extracts inhibited the root growth at higher concentrations but stimulated at lower concentrations. The ethyl acetate extracts proved most inhibitory followed by aqueous extracts, n-butanol extracts and petroleum extracts. Two rare C-methylated homoisoflavanones: 3-(4'-hydroxy-benzyl)-5,7-dihydroxy-6,8-dimethyl-chroman-4-one(1) and 3-(4'-hydroxy-benzyl)-5,7-dihydroxy-6-methyl-8-methoxy- chroman-4-one (2), were isolated and purified from the ethyl acetate extracts using silica gel and Sephadex LH-20 column chromatography. Their structures were identified using 1D and 2D NMR spectroscopy. Compound 2 was isolated from the Polygonum genus for the first time. Although the two isolated compounds did not inhibit seed germination, but were inhibitory to root and shoot growth of test spp. At 0.5 mg/ml concentration, the compound 1 inhibited the root growth of carrot and wheat by 84.6% and 88.2%, respectively, while, Compound 2 caused inhibition in carrot, cucumber and wheat by 87.9, 67.8 and 90.9%, respectively. These results indicated that P. odoratum may be allelopathic to succeeding crops.

We studied the effects of light intensities: ambient and low light on physiological and phytochemical changes in Withania. Low light grown plants accumulated more chlorophylls (Chl) and less carotenoids (Car), flavonoids and anthocyanins compared to ambient light. The ambient light adapted plants exhibited higher photochemical activities in isolated chloroplast than in low light plants. Thylakoid membrane protein analysis showed the presence of four proteins of light harvesting Chl-II (LHC-II) apoproteins and their level slightly increased in low light than in ambient light plants. HPTLC analysis revealed that the level of withaferin A increased by 5% in ambient compared to low light.

Watermelon cultivars resistant or susceptible to fusarium wilt were grown hydroponically as seedlings or grafted onto calabash or pumpkin rootstocks. To investigate disease resistance, the composition and content of amino acids in root exudates were analysed using an automatic amino acid analyser. Fifteen amino acids were detected in the exudates. Amino acid content was lowest in resistant cultivars and highest in susceptible cultivars, but there were no significant differences between the two rootstocks. The amino acid content was significantly higher in root exudates from resistant cultivars grafted on calabash rootstock than those from watermelon seedlings, whereas, susceptible cultivars grafted on pumpkin rootstock had lower amino acid content than watermelon seedlings. The effects of different amino acids concentrations (0, 0.6, 1.2, and 1.8 g·L-1) on growth and development of F. oxysporum f.sp. nevium were measured. Asparagine, methionine, leucine, glutamic acid and phenylalanine promoted the growth and development of F. oxysporum f.sp. nevium at concentrations of 1.2 g·L-1 or more; but methionine and phenylalanine inhibited the growth and development of F. oxysporum f.sp. nevium at conc of 0.6 g·L-1 or less.

We studied the changes in rhizosphere microbial community of grafted eggplants at different stages and the effect of grafting on disease resistance of eggplant. Antagonistic microbes against Verticillium dahiae from rhizosphere soil of grafted and non-grafted eggplants were screened. Compared with non-grafted eggplant, grafting changed the rhizosphere microbial community structure of eggplants, the increase of bacteria (B) and actinomycetes (A), decreased the fungi (F), increased ratios of B to F and A to F, also decreased incidence of Verticillium wilt. Of the 352 strains isolated from rhizosphere soil of grafted and non-grafted eggplants, 7 strains were inhibitory against Verticillium dahliae and their inhibition zone was > 13 mm. In greenhouse experiment, two antagonistic strains ‘B631’ and ‘F111’ showed significant inhibitory effects against Verticillium dahliae. The prevention and curative effects of strain ‘B631’ and strain ‘F111’ on Verticillium wilt were 62.50%, 53.33%, 56.25% and 46.67%, respectively. Using the taxonomic classification and Bergey’s Manual of Determinative bacteriology, ‘B631’ was identified as Serratia Bizio and ‘F111’ was identified as Trichoderma. The results from this study indicated that the rhizosphere microbial community from grafting is favourable to eggplant’s disease resistance.

The role of phenolic acids in autotoxicity of four barley (Hordeum vulgare L.) varieties was investigated using radicle growth bioassays and analytical techniques. Total phenolic content of barley plant components varied within and between varieties during the 1999-2002 growing seasons. Inhibition of barley radicle growth was positively correlated with total phenolics depending on growing season and variety. Only total phenolic content of barley stems contributed significantly to barley autotoxicity. Concentrations of five phenolic acids differed in all plant components, among barley varieties and growing seasons. Ferulic acid and vanillic acid occurred least and most frequently in barley plant tissues, respectively; p-hydroxybenzoic acid, syringic acid, and p-coumaric acids were positively associated with barley autotoxicity (r ≥ 0.31). Inhibition was significantly correlated with total phenolics, although other allelochemicals could also contribute to barley autotoxicity. Variations in total phenolics and phenolic acid composition over growing seasons may indicate a strong impact of climatic conditions on phenolic accumulation in barley plants.

The phytotoxicity of high purity helvolic acid and a related compound, fusidic acid was assessed using multiple growth and biochemical endpoints performed on tissues of several plant species. Bioassay tests for phytotoxicity of helvolic (0.25 mM) and fusidic (0.50 mM) acids were conducted on weed and crop species including : germination/growth (seeds/seedlings) of hemp sesbania [Sesbania exalta (Raf.) Rydb.], sicklepod (Cassia obtusifolia L.), mung bean (Vigna radiata L.), wheat (Triticum aestivum L.), and cucumber (Cucumis sativa L.; leaf disk electrolyte leakage of corn (Zea mays L.) and giant ragweed (Ambrosia trifida L.) leaf tissues; and greening of etiolated tissues of hemp sesbania, sicklepod, wheat (Triticum aestivum L.), mung bean, and sorghum (Sorghum vulgare L.) tissues. Fusidic acid reduced the chlorophyll accumulation in hemp sesbania (95%), wheat (70%), mung bean (60%), sorghum (60%) and sicklepod (48%); helvolic acid at this low concentration had only weak effects, except in mung bean. Neither compound affected the germination at the concentrations used, but both inhibited the seedling growth of all test species except sicklepod and helvolic acid-treated hemp sesbania. Both chemicals increased the electrolyte leakage in corn and giant ragweed leaf disks after 48-h exposure. Overall these natural products have broad-range phytotoxicity and may affect the plants directly or indirectly via antibiotic effects on soil/rhizosphere organisms.

Physiological responses of rice accessions, PI1312777 (allelopathic) and Lemont (non-allelopathic) to phosphorus supply were evaluated in hydroponics in a greenhouse experiment. Under the low phosphorus supply, the weed-suppressive potential of PI312777 was enhanced and inhibited the morphological traits of barnyardgrass; besides, the root length of PI312777 was enhanced and that of Lemont was reduced. There were no decreases in plant height and dry weight of PI312777, but these were significantly reduced in Lemont. The contents of soluble sugar and protein were increased in PI312777, but these were decreased in Lemont. Compared to control, all photosynthetic physiological indices in PI312777 were slightly decreased in low P condition but were increased in Lemont. Furthermore, the uptake efficiency of nitrogen and potassium in two rice accessions was enhanced under low P treatment, but phosphorus uptake efficiency was decreased. At low P supply, the nitrogen, phosphorus and potassium utilization efficiency was higher in allelopathic rice ‘PI312777’ than in non allelopathic rice ‘Lemont’. The differential expression of key enzymes involved in photosynthesis and nutrient absorption, was higher in allelopathic rice than in non-allelopathic rice under low P dose. This study confirmed that allelopathic rice ‘PI312777’ maintained high photosynthesis and nutrients absorption than non-allelopathic ‘Lemont’.

This study analyzed the effects of essential oil from Melaleuca alternifolia (tea tree oil) on tobacco Bright Yellow 2 cells to understand the cytological basis for its allelopathic properties. The minimum cidal concentration values of tea tree depended on duration of treatment. Cell viability was lost even at lower concentrations, if exposure to the essential oil was sufficiently long. Loss of vitality corresponded to rapid leakage of fluorescent dye 7-amino-4-chloromethylcoumarin from the vacuoles and cells. These cytological changes were monitored in real time and documented using time-lapse photography. The results have shown that tea tree oil has a major effect on membrane permeability not only in bacteria and fungi but also in plant cells. Melaleuca oils’ mode of action at the cellular level can explain the allelopathic properties (inhibition of germination and seedling growth) observed in nature. It suggested the potential use of tea tree oil for weed control.

Under lab conditions, bioassay studies were done to study the allelopathic effects of aqueous extracts (0,10,20,40%) of different parts (leaf, root, leaf + root) of common lambsquarters (Chenopodium album L.) and salinity (0, 75 and 150 mM NaCl) on germination of soybean (Glycine max L.). The different types and concentrations of C. album. extract and salinity adversely inhibited all germination traits of soybean. The aqueous leaf extracts of C. album showed the highest inhibitory effects on studied germination traits of soybean, followed by root + leaf and root extracts, respectively. The degree of inhibition increased with increasing concentrations of extract and salinity. The extracts + NaCl drastically reduced the growth more than the separate effects of these stress treatments e.g., extract and salinity alone. The highest concentrations of extract and salinity reduced the radicle length by 63 and 72% respectively, while leaf extract + NaCl at highest concentrations caused 88% reduction than control. Radicle weight and plumule length and weight showed similar response.

The allelopathic effects of 2,4-dihydroxy-1,4-benzoxazin-3-one, isolated from sugarcane leaves and its decomposition product, 2-benzoxazolinone was evaluated on germination and growth of sugarcane (interspecific hybrids of Saccharum), wheat (Triticum aestivum L.), mustard (Brassica campestris L.) and mung bean (Vigna radiata L.). The 2,4-dihydroxy-1,4-benzoxazin-3-one and 2-benzoxazolinone at concentrations > 0.3 mM significantly inhibited the germination of sugarcane and completely inhibited > 0.6 mM concentrations. Both hydroxamic acids also significantly reduced the number of leaves per plant, plant height and dry weight of sugarcane. The 0.3-0.7 mM concentrations caused 15-40 % inhibition, however, the maximum reduction was observed at 0.8 mM. Both hydroxamic acids < 0.3 mM had no adverse effects on sugarcane germination. However, they completely inhibited seed germination of wheat and mustard > 0.5 mM concentrations and caused 90% inhibition in mung bean seeds.

We studied the allelopathic effects of aqueous extracts of leaf and reproductive organs (flower and fruit) of L. camara on seed germination, seedling growth and dry matter production of radish and lettuce. The reproductive organs of L. camara were found allelopathic and the effects were concentration-dependent. GC-MS analysis showed that the reproductive and vegetative organs have similar volatile compounds but their contents were different. Fruit extracts were most stimulatory, while flower and leaf extracts had similar stimulatory/ inhibitory effects. Thus the allelopathic effects of L. camara reproductive organs were stronger than vegetative organs. Since L. camara is perennial, hence, large quantity of flowers and fruits produced more allelochemicals. The allelopathic effects of its reproductive organs makes it more competitive and invasive.

The inhibitory activity of essential oil from rhizomes of Cyperus giganteus Vahl. was investigated in bioassays on seed germination, radicle and hypocotyl elongation of 3 common weed species [Mimosa pudica Mill., Senna obtusifolia (L.) Irwin & Barneby and Pueraria phaseoloides (Roxb.) Benth] of Amazon region. The major constituent of essential oil ‘cyperotundone’ was also tested. The inhibitory effects of essential oil and cyperotundone were found concentration dependent, except on seed germination of M. pudica and P. phaseoloides. The S. obtusifolia germination was most adversely affected by the essential oil (15.0% to 20.0%). The essential oil proved more potent than cyperotundone on the radicle and hypocotyl growth. The essential oil reduced the hypocotyl and radicle growth of M. pudica by 32 and 55%, respectively. Cyperotundone was more inhibitory to the radicle (34.0%) than hypocotyl (20.0%) growth of P. phaseoloides. Cyperotundone can be considered an allelochemical and essential oil i.e. as an allelopathic agent.

Fodder galega is a forage legume which effectively competes with weeds, perhaps allelopathy contributes to this interference with neighbours. Aqueous extracts of shoots and roots of fodder galega were prepared from different growth stages and assayed on rape germination. Concentration of total phenolic compounds in aqueous extracts was also measured. Extracts of shoots of fodder galega were more suppressive to germination than root extracts. Maximum allelopathic potential of shoots was found at flowering stage. Concentration of phenolic compounds in extracts of shoots was higher than in root extracts. Maximum concentrations of phenolic compounds were found in shoots at budding and flowering stages. Our results suggested that shoots of fodder galega are the main source of its allelochemicals, specially at flowering stage. These allelochemicals may be phenolic compounds but other metabolites may also participate.

We examined the efficiency of different solvents (water, methanol, acetone) to extract potential allelochemicals from okara and their effect on the root growth of cockscomb, barnyard grass, false daisy, lettuce, sesame. Ten % methanol (10% methanol in water), designated as water extract and methanol extract drastically inhibited the root growth of plants than acetone extract. Root growth of sesame was most inhibited, followed by barnyard grass, cockscomb, lettuce and false daisy. Extracts inhibited the root growth of plants in a concentration-dependent manner. Thus okara contained water- and methanol-soluble allelochemicals inhibitory to root growth of test plants. Methanol extract was most inhibitory to shoot growth, followed by acetone extract and water extract. Shoot growth inhibition followed the order: Sesame > lettuce > barnyard grass > false daisy > cockscomb. The interactions among plant spp., extraction solvent and extract concentration were significant (P < 0.05). In cockscomb and false daisy, 30 g eq./l of water extract was most stimulatory to shoot growth, followed by 10 g eq./l and 3 g eq./l. In contrast, 100 g eq./l of water was similar to control. These results suggested that okara contained water- and methanol-soluble allelochemicals with selectivity of plants.