The agronomic kingdom like India is facing the difficulties of scarcity of food, fodder, fuel, fiber etc. The fundamental concerns behind this are population eruption, reduction of cultivable land, degradation of soil richness, pollution of land, water and air. The altering ecological scenario comprising global warming, raised levels of CO2 and reduction of ozone have intensified the complications. As an outcome of this the efficiency of agronomic as well as forest ecological unit has listed excessive reduction. Despite of all the above stated factors the mainstay of degradation in soil fertility and decrease of crop efficiency is burgoing population of native weeds and successful invasion of numerous exotic weeds.
To governor and cope successfully with this problem of invasion and encroachment of weeds in cropland and forest ecosystem, allelopathic investigation is needed to defend the green shield of mother earth. The research and development in allelopathy is of extreme urgency for the improvement of agriculture, forestry and the global environment (Inderjit et al., 1999; Kohli et al., 2001 and Reigosa and Pedrol, 2002), because allelopathy majorly deals with invasive/ exotic and native weeds, allelopathic crops that keep hampering agricultural practices and bring environment degradation (Inderjit, 2005). According to Kohli (2001) and Narwal (2003) allelopathy will provide the best alternative solution, to get rid of above mentioned threat in Indian agriculture and forestry.
1.1. History of allelopathy:
Molish in 1937 first coined the term allelopathy, which refers to biochemical interactions between all types of plants including microorganisms. The term allelopathy was derived from the Greek word, which means mutual harm. This term cover both the detrimental and beneficial reciprocal biochemical interactions. Molisch, (1937) and Muller, (1970) used the term allelopathy to refer to the deleterious effects that one higher plant has on another through the production of chemical retardants that escape into the environment. De Moral and Cates, (1971) defined allelopathy as the inhibition of germination, growth and metabolism of one plant due to the release of organic chemicals by another. However, the term allelopathy usually includes interspecific (antibiotic) and intraspecific (autotoxic) chemical coactions. Putnam and Duke, (1978) used the term to refer to the detrimental effects of higher plants of one species (donor) on the germination, growth and development of plants of another (receptors) species. Rice in 1984 also defined allelopathy as any direct or indirect harmful or beneficial effects of plants, including microbes, on another plant through the release of chemicals that escape into the environment. It is common for one plant to harm another plant grown in its vicinity a phenomenon called allelopathy.
Earlier allelopathy was misinterpreted for resource completion between two or more plants present in the same vicinity. It is difficult to separate and prove allelopathic effects from completion and so Willis, (1985) suggested six criteria are which were required to prove the effect due to allelopathy. These criteria were inhibition pattern of a species, toxic production by plant, mode of toxin release in the environment, affected metabolism of receptor plant etc. On the other hand completion involves a reduction in some environmental factor which was required for the growth of other species in the same area.
Consequently, many other scientists also gave different definitions as per their advance research in the field. Akobundu, (1987) defines allelopathy as The detrimental effects of chemicals or exudate produced by one living plant species on the germination, growth and development of other plant species or microorganism sharing the same habitat. At the same time, Putnam, (1988) described allelopathy as a positive plant response mediated through chemicals produced by another plant, instead of only negative or inhibitory effect, he also considered that release of chemicals not only from the living, but even from dead plant parts can affect nearby surroundings. So allelopathy is both beneficial and detrimental effects of chemicals derived from living or dead plant parts over another in present surroundings (Molich, 1937).
Paracelsus stated that in 16th century, concerning the dose response and its effect on a plant by a phrase like All things are poison and are not poisonous. Only the dose makes a thing not a Poison (Duke et al., 2010) which explains that in allelopathy, positive or negative of a plant on another plant depends on dosage of the chemicals received by the target plant (Inderjit et al., 2005). First and foremost, the concept of allelopathy arose from the poisonous or detrimental effect of animal or plant chemicals on other animals and humans. It led to thinking on whether plants effect on other plants or not? (Willis, 2007). Recently International allelopathy society has accepted the definition of allelopathy as any process which involves the production of secondary metabolites by plants, algae, bacteria, fungi or viruses that influence the growth and development of the agricultural and biological/ natural system (Duke, 2010). Reviewing the allelopathy definition, it becomes clear that any substance can have an inhibitory effect with specific concentration and it can be beneficial at a lower concentration or vice a versa.
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms from the same community. (Cyanobacteria, 2019) .
Allelochemicals were doubted in 19 th century in agriculture because of many observations of soil sickness of farmlands. Allelochemicals refer mostly to be the secondary metabolites produced by plants and are by-products of primary metabolites. Some of them are accumulated at various stages of growth, while some depend upon the time of day or season. Allelochemicals selectively inhibit the growth of soil microorganisms or other plants or both. Although many allelochemicals are strictly defensive substance, other is offensive compounds that act directly in weed aggressiveness, compaction and the regulation of plant diversity. Sometimes a single chemical produced by one organism is harmful to another organism but beneficial to third organism (Inderjit et al., 2005; Narwal, 2012).
Allelochemicals most often impart plant resistance to insects, nematodes and pathogens; besides following their release into an environment, it may regulate the distribution and vigor of plants. Most often plants come in contact with the allelochemicals in soil and their effect on crop plants may be modified by soil moisture, soil temperature and other soil factors. It is expected that soon many allelochemicals may be used commercially as heerbicides and nematocides or as boiregulators. (Rawat et al., 2017)
1.2.1. Mechanism of action of allelochemicals:
It is difficult to understand the exact mechanism of action of allelochemicals. But Rice, (1984) and Mandava (1985) stated that the allelochemicals act through positive or negative impact on i) cell division and cell elongation, ii) phytohormone induced growth, iii) membrane permeability, iv) mineral uptake, v) stomatal opening and photosynthesis vi) respiration, vii) protein synthesis and change in lipid and organic acid metabolism viii) inhibition and stimulation of specific enzymatic activities.
1.2.2. Impact of allelochemicals:
In nature the impact of allelochemicals is centralized on a fine-tuned regulatory process in which these chemicals act together on one or more of the above mentioned process (Rizvi et al., 1999). They further divided the effect of allelochemicals on target plant into a direct and an indirect type. The effect through the alternation of soil properties, nutritional status and an altered population or activity of micro-organism and nematodes represent the indirect action. While in direct action involves the biochemical/ physiological effects of allelochemicals on various important processes of plant growth and metabolism (Latif, 2017).
After release, allelochemicals cause both inhibitory and stimulatory effects and various factors like concentration, flux rate, age, metabolic state and environmental conditions determine their toxicity. Their amount and production vary in quality and quantity with age, cultivars, plant organ and time of the year. The receiver plants show varied types of responses to the allelochemicals released from donor plants (Cheng et al., 2015).
1.2.3. The response of recipients plants to allelochemicals:
Biological activities of receiver plants in response to allelochemicals are known to be concentration dependent. These responses are characteristically, stimulation or attraction at low concentration and inhibition or repellence as the concentration increase (Lovett, 1989). When receiver plants are exposed to higher concentrations of allelochemicals, their growth and development are adversely affected. These effects include inhibition or retardation of seed germination, reduced root- shoot growth, swelling or necrosis of root tips, curling of the root axis, discoloration, lack of root hairs, increases the number of seminal roots, reduced dry weight accumulation and lowered reproductive capacity.
These allelochemicals also inhibit the growth of recipient soil microorganisms. The allelochemicals often regulate the distribution and vigor of plants. The existence and distribution of allelochemicals in plants and microorganisms had been well documented. These are generally stored in plant cells in bound form and released into the environment from the special glands on the stems and leaves (Putnam and Duke, 1978; Cheng and Cheng, 2015).