Argemone mexicana L. a weed native to Central America (Mexico) is one of invasive alien plant species in Tanzania with allelopathic behaviour [25] . A. mexicana is a common herb plant found in most places by road sides, agricultural fields and natural ecosystem in Tanzania. A. mexicana belonging to the family papaveraceae is a widely distributed plant throughout the tropical and subtropical regions of the world. A. mexicana is an annual herb, up to 150 cm tall with a slightly branched tap root [26] . The stem is erect, branched, usually prickly, pale bluish-green and exudes an unpleasant smelling yellow sap when cut. Leaves are alternate, without petioles, more or less sheathing the stem, up to 15 cm long, deeply lobed with irregularly toothed, spiny margins; greyish white veins are conspicuous on the bluish green upper surface of the leaves [27] .

It has varying physiology of seed production and germination whereby it can produce seed at an average of 60 to 90 capsules per plant with 300 to 400 seeds in each capsule [28] . Seeds are dormant when shed and after ripening for several weeks or months [29] . Most seeds fall around the base of the parent plant where they form a carpet of seedlings. Most seeds do not normally germinate the year after shedding. Instead they enter the seed bank and seedlings establish, even in well maintained field, probably for many years [25] [29] . Dispersal occurs in surface water and in mud adhering to farm machinery and the feet of man and livestock [30] . Chemical investigations of this plant have revealed the presence of alkaloids, amino acids, phenolics and fatty acids [31] . Therefore some of chemicals in A. mexicana might have allelopathic potential that may affect other plants in their vicinity.

Interaction between plants can be facilitated through release of allelochemicals from donor plants which then influence germination, growth, development, and establishment of receptor plants [16] . These processes play an important role on the determination of vegetation pattern and it is an invasive strategy used by many invasive plant species. In natural ecosystems, allelochemicals produced by invasive plants can inhibit the growth of competing native species through direct or indirect means, thereby providing the invader with a competitive advantage [3] [32] [33] . Phenolic compounds including p-hydroxybenzoic acid, vanillic acid and salicylic acid (Figure 1) are the major allelochemicals of A. mexicana [34] . Phenolic compounds are amongst the most widespread plant secondary metabolites which are of great significance in plant metabolism like defence against ultraviolet radiation or aggression by pathogens [21] . Phenolic allelochemicals have been observed in both natural and managed ecosystems, where they cause ecological and economic problems, such as decline in crop yield due to soil sickness, regeneration failure of natural forests, and replanting problems in orchards [15] .

Allelochemicals changes membrane permeability and inhibit plant nutrient uptake [15] . Cell membrane permeability can be increased by phenolic allelochemicals resulting to spill of cell contents, increased lipid peroxidation and hence slow growth or death of plant tissue [16] . Inhibition of plants from absorbing nutrients from surroundings affects the normal growth of plants. Phenolic allelochemicals may inhibit cell division, elongation, and sub microscopic structure and consequently interferes with the normal growth and development of the whole plant [17] [54] . Disruption of amino acid metabolism is another important mode of action for some allelochemicals [15] [20] . On another hand, phenolic allelochemicals can change the activity and functions of certain enzymes [15] . Some of phenolic allelochemicals reduce or inactivate the physiological activity of plant hormones which may then inhibit the normal physiological process of plants [21] . Hence, the contribution of phenolic compounds to allelopathy is probably not due to a single substance; there are a series of physiological and biochemical changes in plants induced by phenolic compounds. Hence phenolic compounds present in A. mexicana may affect germination, seedling growth, fresh weight and chlorophyll contents of target species.

Many allelopathic chemicals have more dramatic effects on seed germination than on the growth and viability of matured plants [20] . The allelopathic effects on seed germination are related to the types and concentrations of allelochemicals, species of recipient plants and environmental conditions [55] . Seed germination inhibition by allelochemicals is associated with changes in physiological and biochemical processes necessary for seed germination. Disruption of mitochondrial respiration is one of the mechanisms used by seed to inhibit seed germination [20] . During seed germination, there is a rapid increase in glycolytic activity (Glycolysis) linked to an increased rate of respiration [56] . Glycolytic activity is necessary to mobilize stored carbohydrates to provide the seed with the reducing power, ATP, and carbon products required for the biosynthesis of the roots and aerial parts of the emerging seedling [20] . Therefore, allelochemicals may disrupt activity of metabolic enzymes that are involved in glycolysis [57] . However, all kinds of allelochemicals can affect seed germination through affecting seed cell membrane permeability, cell division and differentiation, protein synthesis, gene expression, and hormone synthesis and equilibrium [55] . Paul and Begum [58] reported that aqueous extracts of A. mexicana inhibited germination of Lentil (Lens culinaris). Therefore A. mexicana may similarly affect the germination of several plant species growing in the ecosystem.

Root growth is characterized by high metabolic rates and, for this reason; roots are highly susceptible to environmental stresses such as allelochemicals in soils [18] . Normal plant cell growth and morphogenesis requires regulation of the concentrations of hormones such as auxins and gibberellins [59] . Some of allelochemicals disrupts hormone equilibrium. For instance they may inhibit polar auxin transport leading to a disturbance in normal auxin levels and resulting in the induction of lateral roots and the suppression of ageotropic growth [60] . The contact of phenolic acids with the root cell membrane leads to depolarization, an efflux of ions, and a reduction of hydrolic conductivity, water uptake and net nutrient uptake [61] . Moreover, allelochemical inhibit root elongation and cell division and enlarge thickness of roots due to the inhibition of root longitudinal growth [51] . The allelochemical also initiates a series of reactive oxygen species (ROS) in root meristem which inactivate enzymes, damage vital cellular organelles in plants, and destroy membranes by inducing the degradation of pigments, proteins, lipids and nucleic acids which ultimately results in death of the root system [62] [63] . It were also significantly decreased with the increase of both root and leaf extracts concentration of A. mexicana [34] [64] -[66] . Therefore, A. mexicana is known to possess allelochemicals that might induce reduced root and shoot growth in different plant species growing in their vicinity. However, the extent of such damage in different plant species requires scientific studies to substantiate.

Decrease in plant growth caused by allelochemicals subsequently decreases fresh and dry weights. Allelochemicals reduce plant water potential [37] and inhibits minerals and ion uptake by plants [20] and reduces fresh weight. Interference with plant water balance is one of the mechanisms of action of p-hydroxybenzoic acid causing a reduction in plant growth. Studies have shown that soybean growth was reduced and water potential lowered with high concentrations of p-hydroxybenzoic acid [37] . Allelochemicals inhibits protein and carbohydrates synthesis, which in-turn reduce plant growth and weight [15] . Moreover, Einhellig and Rasmussen [67] found that decrease in biomass of treated soybean by phenolic acids were associated with reduced chlorophyll content in leaves. Similarly, Alagesaboopathi [65] also reported the decrease in fresh and dry weights of sorghum upon treatment with different concentrations of A. mexicana leaf aqueous extracts. Thus, allelochemicals in A. mexicana may inhibit growth of plants.

Allelochemicals inhibits photosynthesis and oxygen evolution through interactions with components of photosystem II (PSII) [38] [68] . Allelochemicals can affect the performance of the three main processes of photosynthesis: stomatal control of carbon dioxide supply, thylakoid electron transport (light reaction), and the carbon reduction cycle (dark reaction) [38] . Chlorophyll is among the molecules in photosystem involved in light reaction which can be affected by allelochemicals. Chlorophyll is the most important pigment for photosynthesis [69] . The chlorophyll a and b are essential pigments for the conversion of light energy (solar radiation) to stored chemical energy [70] . Photosynthetic potential of a plant is determined by chlorophyll contents and any changes are expected to bring about change in photosynthesis [71] [72] . Chlorophyll also gives estimation of the plant nutrient status because much of leaf nitrogen is incorporated in chlorophyll [72] -[74] . The amount of chlorophyll per unit leaf area in plant is an important indicator of the overall plant healthy condition [72] . Healthy plants capable of maximum growth are generally expected to have larger amounts of chlorophyll than unhealthy ones [73] . Allelochemicals may reduce chlorophyll accumulation by inhibiting chlorophyll synthesis and/or stimulating chlorophyll degradation [75] . Several studies reported the decrease in chlorophyll contents with increase in concentration of allelopathic phenolics (vanillic acid, o-hydroxyphenyl acetic, p-hydroxybenzoic acid, camminic acid, ferulic and p-coumaric acids) in rice cabbage, Chinese fir, Echinochloa crus-galliand Chenopodium album seedlings [19] [52] [76] [78] .