Screening Of Allelopathic Plant Materials

Allelopathy is essentially a chemical defense mechanism used by plants to keep other plants out of their space. Though allelopathy forms a part of network of chemical communication between plants, it is part of plant interference along with competition for resources. Competition involves the removal or a diminution of shared resources, while allelopathy involves the addition of a chemical compound to the environment through different processes (Rice, 1984; Putnam, 1985). Allelopathic chemicals in general affect seed germination, root growth, shoot growth and/or seedling vigor in the early stages of the receiver's growth and may interfere with metabolic functions like photosynthesis, membrane permeability, biosynthesis of enzymes, lipids, protein, etc. as the receiver progress in growth processes. In aquatic systems and wetlands, screening of allelopathic plant materials for biological efficiency is relatively easier as the allelochemicals are frequently absorbed through the roots of the receiver and transported from the donor directly through water without much of resistance from the soil-interface. However, screening processes have different phases involving either larger size or larger population of target weeds in different environments. Screening is used mainly to confirm the biological efficiency of the substances to a magnitude which would at least serve as component of integrated weed management though not as a holistic weed control measure. Such a rigorous or repeated experimentation with different sets or modes of bio-assay becomes imperative as many of the allelochemicals exhibit inhibitory response on seedling germination and establishment but seldom lethal on large sized receiver plants. Screening techniques for allelopathy in aquatic systems or allelochemicals which are transported mainly through water body should include some critical points raised by Willis (1985), Putnam and Tang (1986), and Cheng (1992). They are:

• Pattern of inhibition of one species by another must be shown using suitable controls, describing the symptoms and quantitative growth reduction;

• The putative aggressor plant must produce a toxin;

• There must be a mode of toxin release from the plant to the environment and thus the target plant;

• The afflicted plant must have some means of toxin uptake, be exposed to the chemical in sufficient quantities and time to cause damage, and to notice similar symptoms;

• The observed pattern of inhibition should not be explained solely by physical factors or other biotic factors, especially competition.

The first phase or initial stage of screening include bio-assays. Those plant materials that are confirmed to possess biological activity through bio-assay need to be further studied for their dose response pattern under controlled environment. Plant materials elucidating appreciable response even at minimal doses could be further evaluated for their allelopathic potential under natural habitats.

5.1. Bio-Assays

Bioassays are an integral part in all studies of allelopathy. They have multiple uses such as evaluating allelopathic potential of different plant material, tracing activity during extraction, purification, and identification of bioactive compounds. The techniques used vary greatly and one has to standardize the procedure independently, and modify to suit the occasion and conditions. According to Rice (1984) and Putnam and Tang (1986), seed germination is used as a test in most bioassays. Though different types of bioassays are used, all of them in general include seed placed on substrate saturated with the test solution. Germination, as indicated by the emergence of the radical 2 mm beyond the seed coat is scored over a period of time. The factors that need to be kept constant are oxygen availability, osmotic potential of the test solution, pH, and temperature. The elongation of the hypocotyls or coleoptiles are often observed along with germination. Dry biomass which is easier to measure could be used as a measure of growth (Bhowmik and Doll, 1984). Sensitivity is normally higher in growth bioassays than in germination bioassays. When the quantity of allelochemicals is limited, agar cultures can be used. Pre-germinated seeds can be exposed to the agar cultures containing test solution.

Bioassay for searching of allelopathy in aquatic weeds is comparatively easier and this could be designed under both laboratory or greenhouse conditions using either part or whole plant of the aquatic weed (Kathiresan, 2000; Kannan, 2002). Whole plants of floating or submerged aquatic weeds targeted for allelopathic control can be grown in suitable containers with water containing standardized nutrients. The powder of candidate allelopathic substances or plant products are added to water either on w/v or v/v basis with appropriate untreated controls. Periodic observations at designated intervals could include reduction in root length and mass, reduction in shoot length and mass, desiccation, scorching and bleaching or mortality score similar to herbicide injury. Based on the screening data, plant products could be classified in to highly allelopathic, moderately allelopathic, and less allelopathic. For example, 55 different plant products were screened for allelopathic inhibition of water hyacinth involving whole plants as well as cut leaves at Annamalai University. In bio-assays involving whole plants, ten of them including C. amboinicus, P. hysterophorus and L. leucocephala were highly allelopathic based on fresh weight reduction (>30%) of water hyacinth within 48 hr after treatment. Another 12 including Acalypha indica Linn., Trianthema portulacastum L. and Sesbania grandiflora (L.) Pers. showed moderate allelopathy, fresh weight reduction of water hyacinth was 15-30% within 48 hr after treatment. Twelve other plant products including Croton sparsiflorus Morong, Cleome viscosa L. and Eclipta alba L. showed less allelopathy, fresh weight reduction of water hyacinth was less than 15% within 48 hr after treatment (Table 1). The remaining 21 plant species including Leucas aspera Spreng. Curcuma longa L. and Euphorbia hirta L. did not show any allelopathic effect on water hyacinth (Kannan, 2002). To measure dose dependant responses of allelopathic substances more precisely

Table 1 : Percentage reduction in fresh weight of water hyacinth (Eichhornia crassipes.) due to various plant products. (Kannan, 2002)a.

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