Allelopathic Nature Research Articles

Short-Term Effect of Eucalyptus Leachates on Green Food Webs in Headwaters

Despite the reported allelopathic nature of the Eucalyptus genus, eucalypt leachates have unknown effects on the trophic base of stream green food webs. Eucalypt plantations have increased worldwide, including riparian ecosystems. We aimed to test whether short-term eucalypt leachates might alter water chemistry, periphytic algal biomass and diatoms, and herbivorous invertebrates’ (i.e., scrapers). We studied two oligotrophic and well-preserved headwaters from NW Spain. The experiment followed a before-after control-impact paired (BACIp) design, with weekly sampling before and after eucalypt leaves addition to streams. In the stream with lower discharge, the eucalypt treatments seemed to affect increases in biomass accrual (Chl-a) and diatom assemblages, disfavouring sensitive species (e.g., Eunotia minor and Achnanthidium pyrenaicum). Therefore, the ecological status was reduced from high to good. In the stream with higher discharge, invertebrate assemblages changed with scrapers having their abundances modified in comparison with the control (e.g., Habrophlebia sp. and Elmidae). Results suggest that eucalypt leaves leachates exert toxic effects on periphyton and on diatom and invertebrate assemblages, and this effect might be mediated by discharge. Alterations at the base of stream food webs may compromise their good ecological status. Further studies are necessary to identify whether this toxicity is due to allelopathic processes.

Taxonomic and community composition of epigeal arthropods in monoculture and mixed tree species plantations in a deciduous forest of Ghana

Tropical forests provide several ecosystem services and functions and support approximately two-thirds of the world’s biodiversity but are seriously threatened by deforestation. Approaches to counteract this menace have revolved around afforestation with several or a single tree species. We thus investigated how plantation forests with either a single or several tree species influenced arthropod taxonomic and community composition using pitfall traps to sample selected groups of epigeal arthropods (Araneae, Coleoptera, Orthoptera and Hymenoptera) and with environmental variables assessed simultaneously. Our results revealed 54 taxonomic groups with significantly higher taxonomic richness, activity density, and diversity in the mixed stands than in the monoculture stands. The significant differences in community composition were mainly driven by families including Lycosidae, Formicidae, Staphylinidae, Scotylidae, Hydrophilidae, Gryllidae and Scarabaeidae and were explained by distinct habitat characteristics (canopy openness, litter depth, deadwood volume, and tree height). While the diverse tree communities and heterogeneous vegetation structure offered food and habitat resources for diverse arthropod groups, the allelopathic nature coupled with homogenous stand characteristics of the Tectona grandis stands in the monoculture suppressed the growth of understorey vegetation that could otherwise serve as food and habitat resources for arthropods, which might have led to limited activities and diversity of arthropods in the monoculture plantation stands. The findings thus highlight the need to promote mixed tree plantations in degraded tropical areas, especially when restoring biodiversity is the prime management focus.

Cover crops as a weed seed bank management tool: A soil down review

This review explores ways that cover crops alter soil environmental conditions that can be used to decrease seed survival, maintain weed seed dormancy, and reduce germination cues, thus reducing above-ground weed pressures. Cover crops are grown between cash crops in rotation, and their residues persist into subsequent crops, impacting weed seeds both during and after cover crops’ growth. Compared to no cover crop, cover crops may reduce weed seedling recruitment and density via: i) reducing soil temperature and fluctuations thereof; ii) reducing light availability and altering light quality; and iii) trapping nitrogen in the cover crop, thus making it less soil-available to weeds. In addition, cover crops may provide habitat for above- and below-ground fauna, resulting in increased weed seed predation. The allelopathic nature of some cover crops can also suppress weeds. However, not all effects of cover crops discourage weeds, such as potentially increasing soil oxygen. Furthermore, cover crops can reduce soil moisture while actively growing but conserve soil moisture after termination, resulting in time-dependent effects. Similarly, decaying legume cover crops can release nitrogen into the soil, potentially aiding weeds. The multiplicity of cover crop species and mixtures, differing responses between weed species, environmental conditions, and other factors hampers uniform recommendations and complicates management for producers. But, cover crops that are managed to maximize biomass, do not increase soil nitrogen, and are terminated at or after cash crop planting will have the greatest potential to attenuate the weed seed bank. There are still many questions to be answered, such as if targeting management efforts at the weed seed bank level is agronomically worthwhile. Future research on cover crops and weed management should include measurements of soil seed banks, including dormancy status, predation levels, and germination. Highlights - Cover crops alter the weed seed bank environment, influencing survival, dormancy, and germination. - Weed seed germination may be reduced by decreased temperature and fluctuations thereof, light, and soil nitrogen. - Weed seed germination may be increased by greater soil moisture, soil nitrogen, and oxygen. - Management should maximize cover crop biomass, decrease soil nitrogen, and delay termination for the greatest potential. - Future research should include measurements of weed seed banks, including dormancy status, predation, and germination.

Effect of Elicitation with (+)-Usnic Acid on Accumulation of Phenolic Acids and Flavonoids in Agitated Microshoots of Eryngium alpinum L.

The present work was aimed at studying the potential of elicitation on the accumulation of phenolic compounds in in vitro shoot cultures of Eryngium alpinum L., a protected plant from the Apiaceae family. The study examined the influence of (+)-usnic acid on the biomass growth as well as on the biosynthesis of the desired flavonoids and phenolic acids in the cultured microshoots. The phenolic compound content was determined by HPLC-DAD. The flavonoid of the highest concentration was isoquercetin, and the phenolic acids of the highest amount were rosmarinic acid, caffeic acid and 3,4-dihydroxyphenylacetic acid, both in the non-elicited and elicited biomass. Isoquercetin accumulation was efficiently increased by a longer elicitation with a lower concentration of lichenic compound (107.17 ± 4.67 mg/100 g DW) or a shorter elicitation with a higher concentration of acid (127.54 ± 11.34 and 108.37 ± 12.1 mg/100 g DW). Rosmarinic acid production generally remained high in all elicited and non-elicited microshoots. The highest content of this acid was recorded at 24 h of elicitation with 3.125 µM usnic acid (512.69 ± 4.89 mg/100 g DW). The process of elicitation with (+)-usnic acid, a well-known lichenic compound with allelopathic nature, may therefore be an effective technique of enhancing phenolic compound accumulation in alpine eryngo microshoot biomass.

Plant Morphoecological Traits, Grass-Weed Interactions and Water Use Efficiencies of Grasses Used for Restoration of African Rangelands

Degradation characterized by depleted vegetation cover is a serious environmental problem in African rangelands. It poses a serious threat to millions of pastoralists and agropastoralists who depend on livestock as a source of livelihood. Consequently, there has been a growing global interest to consolidate efforts to restore degraded ecosystems. For example, the UN decade of Ecosystem Restoration initiative aims at uniting the world behind a common goal of preventing, halting and reversing the degradation of ecosystems. Grass reseeding using native perennial species has been identified as one of the practical ecological strategies for restoring degraded African rangelands, enhancing vegetation cover and forage production. Knowledge of the multifaceted performance of African rangeland grasses in terms of morphoecological traits, interaction with weeds and water use efficiencies is however largely limited and often elusive. Perennial grasses indigenous to African rangelands Cenchrus ciliaris L. (African foxtail grass), Enteropogon macrostachyus (Hochst. Ex A. Rich.) Monro ex Benth. (Bush rye grass) and Eragrostis superba Peyr. (Maasai love grass), were established in an African semi-arid rangeland under natural conditions to fill this knowledge gap. Morphoecological plant traits: aboveground biomass (shoot, leaf and stem) production, plant densities, basal cover, tiller densities and plant height were measured 9 months after establishment. Interaction between the target grass species and weeds and water use efficiencies (WUE) were also determined. Enteropogon macrostachyus displayed significantly higher values for plant densities, tiller densities and basal cover, indices commonly used to estimate the potential of grasses for ecological restoration. Eragrostis superba produced the highest shoot biomass and water use efficiencies. This is attributed to its higher leafy biomass fraction. Higher aboveground biomass production of E. superba demonstrate its suitability for enhancing rangeland productivity. Cenchrus ciliaris suppressed the weeds. This is linked to its aggressive and allelopathic nature. In conclusion, the three perennial grasses displayed distinct morphoecological traits. In order to achieve successful seed-based restoration of degraded African rangelands using native perennial grasses, careful selection species to maximize on their unique traits is recommended. Ultimately, this selection process should match the desired restoration outcomes and subsequent use of the rangeland.

Curcuma-based botanicals as crop protectors: From knowledge to application in food crops

Essential oils, extracts and chemical components from aromatic plants are widely used as flavors and fragrances by the food and cosmetics industries because they act highly effective against plant pathogens and pests. According to recent research studies, some botanicals derived from higher plants are efficient insecticides against particular pests, and fungicidal against necrotrophic and biotrophic plant pathogens, in addition to their use in food production. Towards substituting synthetic insecticides/fungicides with natural substitutes, which do not carry negative health effects, the essential oils, extracts and chemical components of Curcuma species have been examined against various plant pathogens and insect pests that harm food crops. The present review discusses the efficacy of essential oils, extracts and few main components derived from a wide range of Curcuma plants against a wide variety of pests and pathogens of food crops. In addition, their mechanism of action against insects and plant pathogens as well as their potential impact on crops (i.e. the allelopathic nature) are discussed. Finally, the commercialization of Curcuma-based pesticides is reviewed and challenges and knowledge gaps are highlighted.

Effect of Prosopis Juliflora on the Soil Fertility in Usilampatti zone, Tamil Nadu

The invasion of Prosopis Juliflora in the tropical and sub-tropical ecosystems reached alarming condition because of their allelopathic nature and potential threat to the diversity. In this paper, the effect of Prosopis Juliflora on soil fertility in Usilampatti area is analysed. The concentration of macronutrients and micronutrients beneath and outside the Prosopis Juliflora canopy is studied and compared. The paper also explains the soil characteristics on the basis of the pH of the soil sample.

On the suitability of Eucalyptus globulus green manure for field weed control

Eucalyptus globulus Labill. (Tasmanian blue gum), one of the most widespread wood species, is broadly grown in forestry exploitations for timber production, giving rise to abundant harvest residues devoted to obtaining bioenergy or essential oils. However, considering the need for searching new environmentally friendly strategies for weed control, a question arises: ‘would remnants of pruning and logging from eucalyptus plantations be processed and used as a green manure with herbicidal potential in agricultural fields?’ After the evident allelopathic nature of E. globulus and the phytotoxic effects observed from our previous in vitro and greenhouse studies, it is time to address the actual ability of eucalyptus residues for weed control under field conditions. In this study, the potential of E. globulus leaves for weed control was approached by field experiments conducted in weed-infested maize fields, during two growing seasons and at two different locations. Significant reductions were observed on the biomass of the dominant problematic weed species Digitaria sanguinalis (L.) Scop. and Chenopodium album L. up to 69.5 and 88.5% relative to control plots, respectively, together with general effectiveness for weed control throughout the experiments. Concomitantly, maize was not negatively affected by eucalyptus green manure. The phytotoxic effects on weeds were more significant at early stages of maize establishment since weed biomass was reduced from ca. 38–80% relative to control plots, thus alleviating weed-crop competition. Additionally, eucalyptus green manure increased pH, CECe, exchangeable cations Ca2+, Mg2+, and K+, and microbial biomass carbon, as symptoms of enhanced soil quality. Our results demonstrate that the incorporation of E. globulus harvest residues into the soil as a green manure can be a feasible and environmentally friendly practice for field weed control in organic maize-based cropping systems.

Interaction between Ailanthus altissima and Native Robinia pseudoacacia in Early Succession: Implications for Forest Management

The goal of this study was to discover the nature and intensity of the interaction between an exotic invader Ailanthus altissima (Mill.) Swingle and its coexisting native Robinia pseudoacacia L. and consider management implications. The study occurred in the Mid-Appalachian region of the eastern United States. Ailanthus altissima can have a strong negative influence on community diversity and succession due to its allelopathic nature while R. pseudoacacia can have a positive effect on community diversity and succession because of its ability to fix nitrogen. How these trees interact and the influence of the interaction on succession will have important implications for forests in many regions of the world. An additive-replacement series common garden experiment was established to identify the type and extent of interactions between these trees over a three-year period. Both A. altissima and R. pseudoacacia grown in monoculture were inhibited by intraspecific competition. In the first year, A. altissima grown with R. pseudoacacia tended to be larger than A. altissima in monoculture, suggesting that R. pseudoacacia may facilitate the growth of A. altissima at the seedling stage. After the second year, R. pseudoacacia growth decreased as the proportion of coexisting A. altissima increased, indicating inhibition of R. pseudoacacia by A. altissima even though the R. pseudoacacia plants were much larger aboveground than the A. altissima plants. In early successional sites A. altissima should be removed, particularly in the presence of R. pseudoacacia in order to promote long-term community succession.

Socio-economic impacts of Parthenium hysterophorus L. in East Shewa and West Arsi zones of Ethiopia

Parthenium hysterophorus L. is non-native invasive plant species belongs to the family Asteraceae. Currently it is a notorious weed in Ethiopia. Socio-economic impacts as well as the local people’s perception towards P. hysterophorus are poorly understood. Thus, the aim of this study was to generate information for a better understanding of the means of dispersal, source introduction and socio-economic impacts, of P. hysterophorus in East shewa and West arsi Zones of the Oromia Regional State of Ethiopia. The study revealed that parthenium was introduced mainly by following vehicles road and railways. This weed grows throughout the year. This results in its fast dispersal. Many mechanisms were assumed by respondents for fast distribution of P. hysterophorus in the study zones. Among these vehicles and wind are the major agents, which proliferate the seeds of the weed. Based on the study it grows in roadsides, grazing land, cropland, village sides and wastelands. Most of respondents also indicated that the weed first appeared on roadside and propagated to other habitats. This finding indicated that the weed high in disturbed habitats. Control methods, which are mostly practiced by local people, were tillage and hand weeding. However, these methods were not an efficient to control the weed distribution; rather it is expanding from time to time since its invasions. P. hysterophorus has a number of socioeconomic impacts that include effect on crop and livestock production, human health, soil fertility and biodiversity. This finding showed that it competes and suppress the growth of crops due its allelopathic nature. Seeds of this weed changes the normal flavor of food when mix with it. It also colonized grazing fields, thus causing animal food scarcity, animals’ health defect. This finding also reported that P. hysterophorus has health hazards on human being, which include allergic, skin itching (irritation), cough and hemorrhage. Generally, P. hysterophorus is a foremost troublesome weed and subterfuge threat on agriculture, biodiversity and on human health. Therefore, proper and targeted control methods should be employed to control the further spread and reduce the adverse impacts.Int. J. Agril. Res. Innov. & Tech. 6 (2): 5-11, December, 2016

Effect of Seed Priming with Some Plant Leaf Extract on Seedling Growth Characteristics and Root Rot Disease in Tomato

Tomato is one of the important vegetable crops. The problem of seedling establishment is found in tomato due to several soil borne diseases. One of them is root rot caused by Fusarium oxysproum. There are many chemical methods available to control this disease, but use of chemicals deplete the soil micro-environment and causes soil and water pollution and also do not fit within the framework of ‘Organic farming’. Seed priming with certain phytochemicals may be an economic and ecofriendly alternative to such chemicals. In present study we primed tomato seeds with leaf extract of six different plants (White musale, Periwinkle, Neem, Wood apple, Lantana and White cedar). Different leaf extracts of dose of 2% was taken independently for seed priming. We found that priming with White musale, Periwinkle, Neem and wood apple leaf extract had an improvement in different seed and seedling growth parameters in presence of pathogen. Priming with Lantana and white cedar leaf extract showed a reduction in some of the parameters that may be due to allelopathic nature of these plants. Seed priming with leaf extract of Wood apple exhibited maximum survival rate (76.50 %) followed by Neem (68.46 %) and White Musale (52.60 %).

Isolation and identification of Desmodium root exudates from drought tolerant species used as intercrops against Striga hermonthica

Plants from the genus Desmodium, in particular D. uncinatum, are used on sub-Saharan small-holder farms as intercrops to inhibit parasitism of cereal crops by Striga hermonthica and Striga asiatica via an allelopathic mechanism. The search for Desmodium species which are adapted to more arid conditions, and which show resilience to increased drought stress, previously identified D. intortum, D. incanum and D. ramosissimum as potential drought tolerant intercrops. Their potential as intercrops was assessed for resource poor areas of rain-fed cereal production where drought conditions can persist through normal meteorological activity, or where drought may have increasing impact through climate change. The chemical composition of the root exudates were characterised and the whole exudate biological activity was shown to be active in pot experiments for inhibition of Striga parasitism on maize. Furthermore, rain fed plot experiments showed the drought tolerant Desmodium intercrops to be effective for Striga inhibition. This work demonstrates the allelopathic nature of the new drought tolerant intercrops through activity of root exudates and the major compounds seen in the exudates are characterised as being C-glycosylflavonoid. In young plants, the exudates show large qualitative differences but as the plants mature, there is a high degree of convergence of the C-glycosylflavonoid exudate chemical profile amongst active Desmodium intercrops that confers biological activity. This defines the material for examining the mechanism for Striga inhibition.

Morpho-histological and bioherbicidal evaluation of wild-type and transformed hairy roots of goosefoot

Chenopodium murale L. wild-type roots (WR) and hairy roots (HR) exudates have been previously shown to exert allelopathic activity against test plant species lettuce, wheat and Arabidopsis. To further investigate their allelopathic nature, a comparative morpho-histological characterization and total phenolic content (TPC) evaluation were performed. Furthermore, the phytotoxic activity of WR and HR clone R5 against three weed species and their antioxidant responses were also assessed. Except for the higher degree of branching and root hair incidence in HR clones, both WR and HR showed similar anatomical features, typical for higher plants. No significant difference in total phenolic content between WR and HRs, nor their exudates was found. Root exudates of WR and R5, applied as growth medium (GM) wherein these C. murale roots were cultured, displayed selective phytotoxic activity depending on the target weed species, ranging from suppression of germination to inhibition of seedling growth, that were linked with alterations in antioxidant enzyme activities. R5 greatly inhibited germination and seedling growth of Capsella bursa-pastoris, while WR inhibited those of Vicia angustifolia and Amaranthus retroflexus. Such phytotoxic properties (of WR and R5) qualify them as a promising natural resource in the management of weeds. Differences in allelopathic activity between HR and WR should be searched for in their allelochemical profile and the content of each allelopathic substance. The lack of anatomical abnormalities in hairy roots renders them an efficient tool for functional-genomic studies of C. murale root genes.

DIFFERENTIAL RESPONSE OF MAIZE AND MUNGBEAN TO TOBACCO ALLELOPATHY

SUMMARYSeedling emergence and stand establishment of crops following the allelopathic crops are often affected. When the crops are grown in rotation, they are influenced by allelopathy of preceding crop. In this study, consisting of three independent experiments, allelopathic effect of tobacco, rich in several allelochemicals, on the subsequent maize and mungbean crops was investigated. In first experiment, maize and mungbean were sown in field previously occupied by tobacco or was fallow. In second experiment, maize and mungbean were planted in pots after the harvest of tobacco. Whereas in third experiment, maize and mungbean seeds were soaked in water or 2 and 4 mM nicotine and were then sown in soil-filled pots. Stand establishment, leaf emergence and growth of maize were significantly improved when grown after tobacco, whereas in mungbean these characters were suppressed when grown after tobacco. Likewise external application of nicotine improved the emergence uniformity, seedling dry weight and chlorophyll contents in maize and suppressed these parameters in mungbean. Tobacco allelopathy also increased the total soil nitrogen, zinc, iron, nicotine and total phenolics. Nicotine-induced change in chlorophyll contents and α-amylase activity seems the possible reason of differential response of maize and mungbean to tobacco allelopathy. When grown in rotation with tobacco, stand establishment and growth of maize were improved, whereas mungbean stand and growth were suppressed. In crux, allelopathic nature of the crops must be considered while making the crop rotations.

Unravelling the beneficial role of microbial contributors in reducing the allelopathic effects of weeds

The field of allelopathy is one of the most fascinating but controversial processes in plant ecology that offers an exciting, interdisciplinary, complex, and challenging study. In spite of the established role of soil microbes in plant health, their role has also been consolidated in studies of allelopathy. Moreover, allelopathy can be better understood by incorporating soil microbial ecology that determines the relevance of allelopathy phenomenon. Therefore, while discussing the role of allelochemicals in plant-plant interactions, the dynamic nature of soil microbes should not be overlooked. The occurrence and toxicity of allelochemicals in soil depend on various factors, but the type of microflora in the surroundings plays a crucial role because it can interfere with its allelopathic nature. Such microbes could be of prime importance for biological control management of weeds reducing the cost and ill effects of chemical herbicides. Among microbes, our main focus is on bacteria--as they are dominant among other microbes and are being used for enhancing crop production for decades--and fungi. Hence, to refer to both bacteria and fungi, we have used the term microbes. This review discusses the beneficial role of microbes in reducing the allelopathic effects of weeds. The review is mainly focused on various functions of bacteria in (1) reducing allelopathic inhibition caused by weeds to reduce crop yield loss, (2) building inherent defense capacity in plants against allelopathic weed, and (3) deciphering beneficial rhizospheric process such as chemotaxis/biofilm, degradation of toxic allelochemicals, and induced gene expression.

Lichen Photobionts Show Tolerance against Lichen Acids Produced by Lichen Mycobionts

In order to determine the allelopathic nature of lichen acids produced by lichen mycobionts, we compared lichen photobionts with other photosynthetic organisms in terms of inhibition of photosynthetic electron transport around photosystem II (PSII) by representative lichen acids. Whereas at the thylakoid level we found no clear difference in tolerance against lichen acids between lichen photobionts and other species, at the cellular level lichen photobionts showed strong tolerance as compared with other species. These findings suggest the presence of a lichen acid-specific exclusion or detoxification mechanism in lichen photobiont cells.

Fast-growing trees for cogongrass ( Imperata cylindrica) suppression and enhanced colonization of understory plant species on a phosphate-mine clay settling area

Native plants rarely occur in cogongrass ( Imperata cylindrica) dominated phosphate-mine clay settling areas (CSA) in central Florida. This is primarily due to the allelopathic nature and strong competitiveness of cogongrass and frequent fires in these grasslands. This study examined the performance of fast-growing 2.5-year-old cottonwood ( Populus deltoides) and 2–3.5-year-old eucalypts ( Eucalyptus grandis and Eucalyptus amplifolia) in suppressing cogongrass in an old CSA near Lakeland, Florida. Understory vegetation was studied in two-row cottonwood and one-row, two-row, and four-row E. grandis cultures in the commercial planting and a clone-configuration-fertilizer study (SRWC-90). Cogongrass was still dominant in two-row cottonwood and the first four treatments of study SRWC-90. A total of 57 herbs and 26 shrubs, mostly native, were present in the understory. More herbs and shrubs occurred in the commercial planting than in the cogongrass-dominated study SRWC-90. Stand age and proximity to natural areas positively affected species recruitment. Cogongrass importance value index (IVI) decreased with increasing stand density, cottonwood basal area in the commercial planting and with both E. grandis and E. amplifolia basal area in study SRWC-90. Fast-growing trees with good survival on intensively prepared CSAs produce early and permanent canopy closure and appear to suppress cogongrass.

Bracken effects on inorganic nitrogen leaching from an upland podzol

AbstractLeaching of inorganic N species to stream waters from upland areas of the UK is increasing, reflecting the increases in atmospheric deposition of nitrogen species due to increased levels of vehicular emissions. Bracken cover in UK uplands is also increasing overall (Taylor, 1986), and the architectural nature of bracken has been shown to both increase deposition of atmospheric constituents to soils and change their chemical and physical nature. We have tested the nutrient status of upland podzols from the Lake District, Cumbria, UK, from under both moorland grass and bracken. The results show reduced levels of base cations and nitrate throughout the profile and decreases in the organic matter content of surface horizons as a consequence of bracken encroachment. We also report increased ammonium concentrations in lower soil horizons under bracken, possibly leading to increased leaching to stream waters. These results suggest either increased leaching of soil nutrients due to bracken encroachment or increased storage of nutrients within the bracken rhizome system. Results also suggest changes in either microbial activity or the microbial community of podzols, possibly due to the allelopathic nature of bracken. This may have caused changes in the soil nutrient dynamics, possibly accounting for some of the changes seen. We hypothesize that increased bracken cover within the British uplands has the potential to increase leaching of inorganic nitrogen to upland streams.

Allelopathic substances and interactions ofDelonix regia (Boj) Raf

A unique pattern of weed exclusion was found under the canopy ofDelonix regia, which was planted in many places as an ornamental tree in the south of Taiwan. A quadrat method was employed to examine the botanical composition between the area underneathD. regia and its adjacent control grassland. The number of species and coverage of understory species were significantly lower in the area of theD. regia than that of the grassland, indicating the growth of understory species was suppressed byD. regia. A series of aqueous extracts of leaves, flowers, and twigs ofD. regia were bioassayed against three species to determine their phytotoxicity, and the results showed highest inhibition in the flowers. A water-culture experiment indicated the aqueous extract of flowers ofD. regia on two local understory species (Isachne nipponensis andCentella asiatica) inhibited growth of both species by more than 70%. The phytotoxicities of fallen leaves and flowers ofD. regia were not significantly affected by temperature. When the plant material was subjected to temperatures above 70°C, however, phytotoxicity was decreased, indicating that the allelopathic nature ofD. regia could easily be decomposed by fire. By means of paper, thin-layer, and high-performance liquid chromatography, and UV-visible spectrophotometry, responsible phytotoxins present in leaves, flowers, and twigs ofD. regia were identified as 4-hydroxybenzoic, chlorogenic, 3,4-dihydroxybenzoic, gallic, 3,4-dihydroxycinnamic, 3,5-dinitrobenzoic, and L-azetidine-2-carboxylic acids, and 3,4-dihydroxybenzaldehyde. The findings of bioassays and the number and amount of responsible allelopathic compounds found inD. regia are well correlated, thus permitting the conclusion that the exclusion of understory plants under the canopy ofD. regia trees was due primarily to the allelopathic effect of the fallen flower, leaves, and twigs of theD. regia. A possible mechanism of action is discussed.

In situ experiments on interrelationships between six brackish macrophyte species

Variations in total biomass and root: shoot ratios of four macrophyte species, Potamogeton filiformis Pers., Potamogeton perfoliatus L., Potamogeton pectinatus L. and Ranunculus baudotii Godr. were investigated at natural densities, in situ, in pots with three types of sediment, sand, mud and a 50/50% mixture of these. Higher growth rates, lower percentage root and rhizome biomass at harvest, and more reproductive parts were produced on muddy substrate compared with sand in all species tested, except for R. baudotii, where the highest growth rates were found in the 50/50% mixture. Further inter- and intraspecific competitive effects of these and two more species ( Zannichellia major (Hartm.) Boenn. and Ruppia cirrhosa (Petagna) Grande) were examined using de Wit replacement series. P. pectinatus was unaffected by the presence of P. filiformis, but suppressed by P. perfoliatus. Bilateral negative relationships exist between P. pectinatus and Ranunculus baudotii, as well as Ruppia cirrhosa, and between P. filiformis and Ruppia cirrhosa. Possibly, the negative effect of Ruppia cirrhosa on the two Potamogeton species could be of an allelopathic nature. Interspecific relationships were generally weaker than intraspecific relationships in combinations of P. pectinatus with Z. major, P. filiformis with Z. major, and P. filiformis with P. perfoliatus.