Phytototoxicity of Phenolic Allelochemicals to Aquatic Weeds with Reference to Conservation

Phytototoxicity of phenolic allelochemicals to aquatic weeds with reference to conservation, restoration and management of aquatic ecosystems

Pandey D. K. and Mishra N.

Physiology Section, National Research Centre for Weed Science (ICAR),

Maharajpur, Jabalpur (M.P.) 482004, India

Phone: 0761-353101 (O); 353934

Fax: 0761-332329

E-mail:

The study was aimed at investigating phytotoxicity of allelochemicals viz. o-, m- and p- anisic acids; o-, m- and p- coumaric acids; caffeic acid, caffeine anhydrous, catechol, chloremphenicol and cinnamic acid to six floating aquatic weeds Eichhornia crassipes Mart Solmns., Salvinia molesta Mitchell, Pistia stratiotes L., Azolla nilotica Decne., Spirodela polyrhiza L. Schleid. and Lemna pausicostata Hegelm.; and four submerged weeds - Hydrilla verticillata L. f. Royle, Ceratophylum demersum L., Najas graminea Del. and Chara sp. The aquatic weeds were allowed to grow in solutions of the test allelochemicals in a quarter strength standard nutrient medium in plastic containers outdoors. Biomass of the weeds were monitored after 5 and 10 days. Toxicity of the allelochemicals varied with species. The chloramphenicol was lethal to Chara sp. at relatively very low concentration (one- fiftieth the lethal level for other aquatic weeds) and this has implications on management of the weed using the allelochemical (an antibiotic) as a circumstantial allelochemic herbicide for an aquatic ecosystem especially in fisheries. Alternatively, it may be an idea that an organism releasing the allelochemical at substantial concentration into the medium could be found out for facilitating management of the weeds especially Chara sp. in aquatic ecosystems. Accumulation of the allelochemicals may affect conservation, restoration and management of aquatic ecosystems.

Introduction

Allelopathy (a term first coined by Prof. Hans Molisch, a German Plant Physiologist in 1937) is a chemical interaction among plants including microbes. It involves release of a wide variety of chemicals by a plant into its environment, which are taken up by a sensitive plant resulting in growth promotion or inhibition (Rice, 1984). In addition to release of the allelochemicals by a living plant, residue of plants including microbes, which becomes part of soil or aquatic systems can also liberate the allelochemicals as such and/or consequent to death and decay involving various microbial and chemical interactions. The allelochemicals may be released into the environment variously including volatilisation or exudation from above and/or below ground plant parts. Allelopathy is a simple phenomenon with complex consequences. A plant is allelopathic when at least in one instance in nature or in a natural ecosystem or under field conditions it has unequivocally shown allelopathy (Rice, 1984). An allelochemical is a plant constituent other than a growth regulator and a nutrient that has been decisively shown to have caused or is implicated in allelopathy in one or more instances, usually active at a concentration below 10-4M. Allelopathic potential of plant is the magnitude of its capacity to cause allelopathy. Allelopathy can be and should be unequivocally proved when it occurs by showing presence of allelochemicals at physiologically effective concentration from the substratum and by showing allelopathic effects of the allelochemicals at a concentration found in the substratum under controlled conditions (Putnam and Tang, 1986). In general, allelochemicals may promote growth of the target species at a lower concentration and may inhibit them at higher concentrations. Thus, allelopathy and allelochemicals may play important roles in restoration, conservation and management of ecosystems including aquatic ecosystems (Rice, 1984, Inderjit and Keating, 1999).

Allelochemicals may comprise several classes of plant constituents. These involve secondary metabolites belonging to various chemical classes including simple acids and esters, polyacetylenes, long chain fatty acids, alkaloids, benzoic acid derivatives, cinnamic acid derivatives, coumarins, mono-, di- and triterpenoids, sesquiterpe lactones, quinones and sulfated compounds, etc. (Kanchan and Jayachandra, 1980, Putnam, 1983; Rice, 1984; Macias, 1995). Among these, phenolic compounds comprise a wide range of plant substances possessing in common an aromatic ring bearing one or more hydroxyl substituents. Phenolic substances are water soluble, frequently occur combined with sugar as glycosides and are usually located in the cell vacuole. Function of some classes of phenolic compounds has been well established. For example, lignins are structural material of cell wall; the anthocyanins as flower pigments, the purpose of other classes is still speculative (Harborne, 1988). The phenolic compounds have been attributed to have a role in regulation of growth, and their adverse effects on insect feeding (Rosenthal and Berenbaum, 1991). The phenolics have been widely implicated in allelopathy including in aquatic ecosystems.

The secretion and leaching of organic compounds from living aquatic plants, both macro- and micro-phytes have been frequently observed (Gopal and Goel, 1993). Since most phenolic compounds are water soluble, their release from dead and decaying tissues from terrestrial and aquatic origin may result in accumulation at physiologically significant concentrations, which may show interference in aquatic plants. The effect of phenolics on growth and metabolism of aquatic macrophytes are poorly understood (Stom and Roth, 1981). The phenolics at high concentrations may cause injury to algae and aquatic spermatophytes. Phenolic compounds have been suggested to be principal active agents in preventing germination in a marshy environment (McNaughton, 1968). Several phenolic plant constituents including p-hydroxybenzoic acid, vanillic acid, syringic acid, ferulic acid, p-coumaric acid, salicylic acid, protocatechuic acid and eugenol have been reported from aquatic plants (McNaughton, 1968; Drost and Doll, 1980; Jangaard et al., 1971; Sanchez et al., 1973; Cheng and Riemer, 1989; Gopal and Goel, 1993). Investigations have reported phytotoxicity of allelochemicals including phenolics on several aquatic weeds (Pandey et al., 1993; Pandey, 1994; Pandey, 1996).

Several species have been reported to show allelopathic interactions in aquatic ecosystems. For example, Eleocharisacicularis or Sagittaria subutata eliminated Potamogeton sp. Eleocharis sp. showed allelopathic inhibition of several other macrophytes (Osborn et al., 1954) including Najas guadalupensis (Yeo, 1980). A compound in leachates of Eleocharis was found toxic to Potamogeton pectinatus and Hydrilla verticillata (Ashton et al., 1985). Salicylic acid had the maximum potential to inhibit the sprouting of Hydrilla tubers and subsequent growth of the shoots. Hydrilla verticillata inhibited Ceratophyllumdemersum and Ceratophyllum muricatum (Kulshreshtha and Gopal, 1983). Myriophyllum suppressed Najas (Agami and Waisal, 1985). Aqueous extracts of Azolla and Potamogeton illinoensis stimulated the growth of Lemna pausicostata at lower concentration but reduced the growth at higher concentrations (Sutton and Portier, 1989). Ceratophyllum demersum also contained allelopathic compounds that negatively influenced other macrophytes (Phillips et al., 1978). These findings imply that at high density such plants in aquatic ecosystems may exert chemical regulation on the growth of other plants. Dead and decomposition of plant and plant parts, and macrophytes and aquatic-terrestrial plant interactions may also interact with species dynamics in aquatic ecosystems (Gopal and Goel, 1993).

Phenolics are secondary metabolites derived from the aromatic amino acids synthesised through shikimic acid pathway (Harborne, 1988, Hopkins, 1999). Terrestrial and aquatic plants release several phenolics directly from their living tissues and/or indirectly after death and decomposition of the tissues (Rice, 1984). It is likely that these may get washed or dissolved in water and find their way into aquatic substratum and thus may affect the aquatic ecosystems. Allelochemicals have been shown to be inhibitory to aquatic weeds (e.g., Pandey et al., 1993; Pandey, 1994; Pandey, 1996). However, detailed studies as regard to the response of the aquatic weeds to a range of physiological concentrations of the allelochemicals have not been undertaken. The study may help in understanding the effect of accumulation of the allelochemicals in the aquatic ecosystems on species survival, population dynamics and may have implications of practical significance for management of the species. Present investigation reports phytotoxicity of some of the phenolic allelochemicals on six floating and four submerged weeds with reference to conservation, restoration and management of aquatic ecosystems.

Materials and methods

Six floating species Eichhornia crassipes Mart Solms., Salvinia molesta Mitchell, Pistia stratiotes L., Azolla pinnata Decne., Spirodela polyrhiza L. Schleid. and Lemna pausicostata Hegelm.; and four submerged species Hydrilla verticillata L. f. Royle, Ceratophyllum demersum L., Najas gramineaDel. and Chara sp. maintained at the centre were used in the experiments. Solutions of the allelochemicals viz. o-, m- and p- anisic acids; o-, m- and p- coumaric acids; caffeic acid, caffeine anhydrous, catechol, chloremphenicol and cinnamic acid were prepared in a quarter strength nutrient medium. The nutrient medium (Einhellig et al., 1985) consisted of KH2PO4, 690 mg; KNO3, 515 mg; Ca(NO3)2.4H2O, 1180 mg; MgSO4.7H2O, 500 mg; H3BO3, 2.86 mg; ZnSO4.7H2O, 0.22 mg; Na2MoO4.2H2O, 0.12 mg; CuSO4.5H2O, 0.08 mg; MnCl2.4H2O, 3.62 mg; and FeCl3.6H2O, 5.4 mg with the final pH adjusted at 6.5 with HCl or NaOH. The concentrations of the allelochemicals used were control 0.01, 0.05, 0.10, 0.50, 1.00, 2.50 and 5.00 mM. The controls contained the nutrient medium only. Five hundred ml or one litre or a suitable volume of the treatment medium was poured into a plastic container or glass beakers with side walls covered with black carbon sheet. Pre-weighed aquatic weeds were placed in the medium. The containers with aquatic weeds were incubated outdoors in natural light. Effect of allelochemicals on test plants was monitored and biomass of the test species was measured 5 and 10 days after initiation of the treatments. Evapotranspiratory loss of water was replenished daily. All the experiments were repeated three times. From the 0, 5 and 10 days biomass, % change in biomass over original value were calculated and the data was analysed for ANOVA.

Results and discussion

Results of effects of allelochemical o-, m- and p-anisic acid and o-, m- and p-coumeric acid, caffeic acid, caffeine anhydrous, chloramphenicol and cinnamic acid on biomass of the ten aquatic weeds studied have been shown in Table 1-10. The percentage changes in biomass over original value in response to the allelochimicals 5 and 10 days after initiation of the treatment show effect on growth of the test species. Ortho- and m-anisic acids were lethal to the aquatic weeds at and above 5 mM. However, p-anisic acid was lethal at 2.5 mM. Ortho-coumaric acid was lethal at varying concentrations in different species ranging from 1-5 mM. Meta-coumaric acid was lethal at 5 mM. However, p-coumaric acid was lethal at 0.5-5 mM, depending on the weed species. Caffeic acid was consistently lethal to all species at 5 mM except to floating weeds Lemna pausicostata, Azolla pinnata and Spirodella polyrhiza, which withstood this concentration. Caffeine anhydrous was lethal to submerged weed Eichhornia crassipes and floating weed Chara sp. at 5 mM and to other floating and submerged species at 10 mM. Catechol was lethal to all floating and submerged weeds at 5 mM except for floating weed Pistia stratiotes and submerged weed Chara sp. which were killed at 10 and 1 mM, respectively. Chloramphenicol was lethal to floating weeds Eichhornia crassipes, Pistia stratiotes and Salvinia molesta, and submerged weeds Hydrilla verticillata and Najas sp. at 0.5 mM and to floating weed Lemna pausicostata, Azolla pinnata and Spirodella polyrhiza, and submerged weeds Najas graminea and Ceratophyllum demersum at 1.0 mM. However, it was lethal to submerged weed Chara sp. at and above 0.01 mM. Cinnamic acid was lethal to all floating and submerged weeds at and above 5.0 mM except for Azolla pinnata, which was killed at 1.0 mM. None of the allelochemicals tested showed toxicity in the dark for about one day. Light seems to exacerbate manifestation of the toxicity in all cases. At lethal dose, toxicity in the treated plants appeared the very next day after initiation of the treatment in o- and m-anisic acid. In p-anisic acid the toxicity appeared after 3 days. Ortho- and p-coumaric acid showed toxicity in two days. Meta-coumaric acid showed toxicity after a day at the lethal dose. Caffeic acids caused toxicity in about two days. Caffeine anhydrous was toxic in about three days. Catechol and chloramphenicol showed toxicity in about two days. Cinnamic acid showed toxicity in about three days. Different allelochemicals caused death and decay of different weeds in different durations ranging from 5-10 days.

Toxicity symptoms did not vary much with the allelochemical. The symptoms of toxicity of different allelochemicals at lethal dose involved dull green colour, chlorosis, desiccation of leaves and above water plant parts, and flaccid roots. The flaccid roots and desiccation of above water plant parts implicate involvement of root dysfunction derived desiccation and associated physiological changes including massive damage to cellular membrane integrity and loss of macromolecules decisively killing the treated plants. Similarly, in submerged species the symptoms including dull green colour, chlorosis, flaccid plant body and fragmentation implicate damage to macromolecules and excessive loss of cellular membrane integrity resulting in death of the treated plants at the lethal dose.

Among all the allelohemicals, chloramphenicol was lethal to the species studied at much lower concentrations. Chara sp. was most sensitive to this allelochemical with lethal dose at 0.01 mM. The allelochemical was also lethal to floating weeds Eichhornia crassipes, Salvinia molesta,Pistia stratiotes, and submerged weed Hydrilla verticillata at 0.5 mM. The lethal dose for floating weeds Lemna pausicostata and Spirodella polyrhiza, and submerged weeds Najas graminea and Ceratophyllum demersum was at 1.0 mM. The chloramphenicol was lethal to Chara sp. at relatively very low concentration (one fiftieth the lethal level for other aquatic weeds) and this has implications on management of the weed using the allelochemical (an antibiotic) as a circumstantial allelochemic herbicide for an aquatic ecosystem especially in fisheries. Alternatively, it may be postulated that an organism releasing the allelochemical at substantial concentration into the medium could be found out for facilitating management of the weeds especially Chara sp. in aquatic ecosystems.

Phenolics are widely occurring plant constituents including in aquatic species (McNaughton, 1968; Drost and Doll, 1980; Jangaard et al., 1971; Sanchez et al., 1973; Cheng and Riemer, 1989; Gopal and Goel, 1993). Accumulation of phenolics and other allelochemicals in an aquatic ecosystem due to allelopathy and/or by washing or dissolution of allelochemicals as a result of death and decay or decomposition of the plants or plant parts and their residue (Rice, 1984; Gopal and Goel, 1993; Pandey et al., 1993; Pandey, 1994, 1996) may affect conservation, restoration and management of aquatic ecosystems. Different species may have different sensitivities to the allelochemicals. Accumulation of the allelochemicals at higher concentration may influence growth, population dynamics and survival of more sensitive species first and then those that are less sensitive ones. Thus, the allelochemical accumulation in an ecosystem may affect species distribution, shifts and succession, and the biodiversity. Aquatic ecosystems with accumulation of allelochemicals may necessitate their management for effective restoration. Allelopathy and/or use of allelochemicals have potential, that are yet to be explored for application , for selective management of plants in an aquatic ecosystem. Exploration of such a potential may be rewarding by facilitating invasive and noxious weed management in an aquatic ecosystem in an eco-friendly approach.

REFERENCES

Agami M and Waisel Y (1985) Inter-relationships between Najas marina L. and three other species of aquatic macrophytes. Hyrobiologia 127, 169-173.

Ashton FM, Ditomassco JM and Anderson LWJ (1985) Spikerush (Eleocharis spp.): A source of allelopathics for the control of undesirable aquatic weeds. Journal of Aquatic Plant Management 22, 52-56.

Cheng TS and Riemer DN (1989) Characterization of allelochemicals in American eelgrass. Journal of Aquatic Plant Management 27, 84-89.

DrostDC and Doll JD (1980) The allelopathic effect of yellow nutsedge (Cyperus esculentus) on corn (Zea mays)and soybeans (Glycine max). Weed Science 28, 229-233.

Einhellig FA, Leather GR and Hoffs LL (1985) Use of Lemna minor L. as a bioassay in allelopathy. Journal of Chemical Ecology 11, 65-72.

Gopal B and Goel U (1993) Competition and allelopathy in aquatic plant communities. The Botanical Review 59, 155-210.

Harborne JB (1998) Introduction to ecological biochemistry, 3rd edition. Academic Press, New York.

Hopkins WG (1999) Introduction to plant physiology, 2nd edition. John Wiley and Sons Inc., New York.

Inderjit and Keating KI (1999) Allelopathy: procedures, processes and promises for biological control. Advances in Agronomy 67, 141-231.

Jangaard NO, Sckeri MM and Schieferstein RH (1971) The role of phenolics and abscisic acid in nutsedge tuber dormancy. Weed Science 19, 17-20.

KanchanSD and Jayachandra (1980) Allelopathic effect of Parthenium hysterophorus. IV. Identification of inhibitors. Plant and Soil 55, 67-75.

Kulshreshtha M and Gopal B (1983) Allelopathic influence of Hydrilla verticillata (L. F.) Royle on the distribution of Ceratophyllum species. Aquatic Botanty 16, 207-209.

Macias FA (1995) Allelopathy in search of natural herbicide models. In “Allelopathy: organisms, processes and applications” edited by Inderjit, Dakshini KMM and Einhellig FA, pp 310-329. American Chemical Society, WashingtonDC.

McNaughton SJ (1968) Autotoxic feedback in relation to germination and seedling growth in Typha latifolia. Ecology 49, 367-369.

Osborn ET, Moran WT, Greene KT and Bartley TR (1954) Weed control investigation on some important plants which impede flow of western irrigation waters. Joint Lab Report SI-2: 16-17. USDA Bureau of Reclamation Engineering Lab. USA ARS Field Crops Branch.

Pandey DK (1994) Inhibition of salvinia (Salvinia molesta Mitchell) by parthenium (Parthenium hysterophorous L.). I. Effect of leaf residue and allelochemicals. Journal of Chemical Ecology 20, 3111-3122.

Pandey DK (1996) Relative toxicity of allelochemicals to aquatic weeds. Allepothay Journal 3, 229-234.

Pandey DK, Kaurav LP and Bhan VM (1993) Inhibitory effect of parthenium (Parthenium hysterophorous L.) residue on growth of water hyacinth (Eichhornia crassipes Mart Solm.). I. Effect of leaf residue. Journal of Chemical Ecology 19, 2651-2662.

Phillips GL, Eminson D and Moss B (1978) A mechanism to account for macrophyte decline in progressively eutrophicated freshwaters. Aquatic Botany 4, 103-126.

Rice EL (1984) Allelopathy, 2nd edition. Academic Press, Florida.

RosenthalGA and Berenbaum MR (1991) Herbivores: their interactions with secondary metabolites. Vol. I. The chemical participants, 2nd edition. Academic Press, San Diego.

Sanchez Tames R, Gesto MDV and Vieitez E (1973) Growth substances isolated from tubers of Cyperus esculentus var. aureus. Physiologia Plantarum 28, 195-200.

Stom DI and Roth R (1981) Some effects of polyphenols on aquatic plants. I. Toxicity of phenols in aquatic plants. Bulletin on Environmental Contamination and Toxicology 27, 332-337.

Sutton DL and Portier KM (1989) Influence of allelochemicals and aqueous plant extracts on growth of duckweed. Journal of Aquatic Plant Management 27, 90-95.