Allelopathy refers to the beneficial or harmful effects of one plant on another plant, both crop and weed species, by the release of chemicals from plant parts by leaching, root exudation, volatilization, residue decomposition and other processes in both natural and agricultural systems  . The failure of most crops in agro forestry systems has been attributed to allelopathic effects of tree species. A number of allelochemicals are present in cells and tissues of plants   and this phenomenon is as a result of phytochemicals exuded by trees. These chemicals are largely classified as secondary metabolites (such as alkaloids, isoprenoids, phenolics, flavonoids, terpenoids and gluconolactes, etc.)  .
According to  , this process can be defined as the studies involving any process with secondary metabolites which are produced by plants, algae, bacteria and fungi, which may affect the growth and development of agricultural production and biological systems. When released, the allelochemicals may influence the growth and development of surrounding biological systems  and can even be used to control undesirable plants  in agriculture , in rotating systems such as Helianthus annuus that when grown before soybean (Glycine max (L.) Merr.) reduces the amount of weed species  , terpenes, and phenols  that can combat herbivorous insects and fungi, besides influencing the growth of other plants  .
First widely studied in forestry systems, allelopathy has been found to have effect on many aspects of plant ecology including occurrence, growth, plant succession, and the structure of plant communities, dominance, diversity, and plant yield. Initially, many of the forestry species evaluated had negative allelopathic effects on food and fodder crops, but in the 1980s research was begun to identify species that had beneficial, neutral, or selective effects on companion crop plants. Jatropha curcas L. is a perennial poisonous shrub belonging to the family of Euphorbeaceae. The plant originates from Central America to tropical and subtropical countries and mainly grows in Asia and Africa. Common names include Barbados nut, purge nut, physics nut, black vomit nut, or curcas bean. The Hausa people of Northern Nigeria called it “Bini-da-zugu”. J. curcas has gained popularity as biodiesel plant both in developed and developing countries of the world. Its seed contains predominantly crude fat (oil), protein and fibres  . Jatropha oil is non-edible and is mainly used as biodiesel energy   . The potentials of J. curcas as a biodiesel source have resulted in the increased cultivation and domestication of the plant because of government interest in renewable energy. For this reason, Jatropha is an attractive crop and it is being contributed to augmenting the income of farmers by improved agronomic procedures like selection of the suitable intercropping systems and by increasing the efficiency of rural agricultural processes  . Throughout the world, including India, Jatropha has been cultivated along with other crops; in particular, it is cultivated along with groundnut, sunflower, green gram, green chilli, sesame etc.
Allelochemicals have a variety of chemical structures and actions. However, their effects on biochemical and physiological processes of target plants have not been investigated thoroughly  . Therefore, it is necessary to evaluate the phytotoxic effects of J. curcas on agricultural crops before its introduction into the agroforestry system especially with the common practice among local farmers to combine trees and or shrubs with annuals or perennials.
In the present work, an attempt has been made to evaluate the allelopathic effect of Jatropha curcas aqueous leaf extracts on the seed germination and shootgrowth of four (4) pepper species locally grown in Keffi Local Government Area of Nasarawa state.
2. Materials and Methods
2.1. Study Area
A laboratory study was carried out at the Department of Plant Science and Biotechnology laboratory, Nasarawa State University Keffi, Nigeria. Longitude 8.8558˚N and Latitude 7.8694˚. The temperature range is between 25˚C - 30˚C and at about 602 mm of precipitation fall annually. It is in the northwest of Lafia, the state capital of Nasarawa State, Nigeria  .
2.2. Sample Collection and Preparation
Fresh and matured leaves of J. curcas were collected from the botanical garden of the Department of Plant Science and Biotechnology, Nasarawa State University. The procedure for the preparation of aqueous leaf extracts was adopted from  . The leaf samples were dried under shade in the laboratory, powdered (2 mm mesh sieve) and used for bioassay treatment. Seeds of Bell pepper (Capsicum annum), Cayenne pepper (C. frutescens), Bird-eye pepper (C. frutescens) and Tatase (C. annum) were obtained from the Institute for Agricultural Research Samaru, Zaria, Kaduna State, Nigeria.
2.3. Preparation of Aqueous Leaf Extract
The dried leaves were ground to a fine powder in a mortar (2 mm mesh sieve). Using this powder, aqueous extracts were prepared by the method of  . Different amounts; 100 g/500ml, 80 g/500ml, 60 g/500ml, 40 g/500ml, 20 g/500ml and 0 g/500ml (control) of ground leaf sample were dissolved in autoclaved distilled water in 1000 ml conical flask. These gave a percentage leaf extract in 100 ml of water of 20%, 16%, 12%, 8%, 4%, and 0% (control) respectively.
2.4. Bioassay Studies
The seeds of crops were primed in distilled water for 1hour. Bioassay studies were carried out following the method of  . Ten seeds of crop (each pepper species) were placed on Whatman No1 filter paper in petriplates (9 cm × 2 cm). Petriplates were moistened with 2 ml/plate of leaf extract, distilled water (control) and incubated under laboratory condition. Seeds from each pepper were subjected to the five (5) treatments. Percentage Inhibition and shoot length were measured after 15 days. (Five seedlings were randomly harvested from each petriplate to determine average shoot length).
2.5. Data Analysis
The data were analysed by one way analysis of variance (ANOVA). Different means were considered statistical significant at p < 0.05.
2.6. Treatment and Concentrations Used
1) Control (To) seeds supplied with autoclaved distilled water.
2) T1 seeds supplied with 4% extract solution.
3) T2 seeds supplied with 8% extract solution.
4) T3 seeds supplied with 12% extract solution.
5) T4 seeds supplied with 16% extract solution.
6) T5 seeds supplied with 20% extract solution.
Table 1 shows percentage inhibitory effect of Jatropha curcas L. aqueous leaf extract on germination of four pepper species studied. The treatments (T3-T5) recorded 100% inhibition in all the cases.
Table 2 shows the allelopathic effect of aqueous leaf extract of Jatropha curcas L., on the shoot length of Cayenne pepper (Capsicum annum). Based on the average length of shoots per treatment, T0 recorded 4.04 mm, T1 2.04 mm, T2 1.88 mm while highest inhibitory effect was recorded at higher concentrations (T3-T5).
Also, Table 3 shows the effect of aqueous leaf extract of Jatropha curcas L., on the shoot length of Red pepper (Capsicum frutescense L.). Based on the average length of shoots per treatment, T0 recorded 5.24 mm, T1 1.47 mm, T2 1.31 mm, and T3 1.35 mm while the highest inhibitory effect was recorded at higher concentrations (T4-T5).
The effect of aqueous leaf extract of Jatropha curcas L., on the shoot length of bell-pepper (Capsicum annum L.) is as shown in Table 4. Based on the average length of shoots per treatment; T0 recorded 3.82 mm, T1 2.59 mm, T3 1.66 mm and also the highest inhibitory effect was recorded at higher concentrations
Table 1. Percentage inhibitory effect of Jatropha curcas L. leaf aqueous extract on shoot length of cayenne pepper, red pepper, bell pepper and bird-eye pepper.
Table 2. Effects of aqueous leaf extract of Jatropha curcas L. on the shoot length of cayenne pepper (Capsicum annum).
Table 3. Effect of aqueous leaf extract of Jatropha curcas L. on the shoot of red pepper (Capsicum frutescens).
Table 4. Effect of aqueous leaf extract of Jatropha curcas L. on the shoot length of bell-pepper (Capsicum annum).
Also the effect of aqueous leaf extract of Jatropha curcas L., on the shoot length of Bird-eye pepper (Capsicum frutescense L.) is as shown in Table 5 based on the average length of shoots per treatment; T0 recorded 4.20 mm, T1 1.41 mm, T2 1.29 mm while the highest inhibitory effect was recorded at higher concentrations (T3-T5).
The bioassay study of leaf aqueous extracts of Jatropha curcas L. on four pepper
Table 5. Effect of aqueous leaf extract of Jatropha curcas L. on the shoot length of bird-eye pepper (Capsicum frutescents).
species (Capsicum spp) commonly grown in Keffi, Nasarawa State showed a gradual inhibition to seed germination. The inhibition of seed germination and shoot length of pepper in all cases was found to be concentration dependent and this was more profound at higher concentrations except for Red (Capsicum frutescens) which recorded 75.00% (inhibition) at 12% concentration.
Based on the inhibitory effect of leaf aqueous extract as determined on shoot length, the highest inhibitory effect was found to increase as the concentration increases (T3-T5) in all case respectively. This result agrees with the findings of  who reported inhibitory effect of leaf aqueous extract of J. curcas on crops to be concentration dependent. Similarly,  reported that in all extracts of sunflower allelopathicity increased with increase in concentration. This result to a significant extent corroborated the findings of  , which showed that aqueous extracts of J. curcas at low concentration had little effect on seed germination and seedling growth, while higher concentration inhibited these processes. Investigation on the phytochemical screening of J. curcas (leaves, root and stem bark) extracts revealed the presence of saponins, steroids, tannins glycosides, alkaloids and flavonoids  These phenolic compounds could be responsible for this inhibition on germination and shoot length. Reference  suggested that the plant part that had strong effect on germination, plumule and radicle length was the leaves. This suggests that more allelochemicals may be found in the leaves than the roots of J. curcas. This was also supported by the findings of  , where preliminary screening showed that leaf extract had the strongest allelopathic effect on seed germination, thus was selected for detailed experiments. Similarly,  reported that bioassay study and analysis of different extracts of leaves and roots of J. curcas revealed the main allelopathic substance as Azeliac acid. This compound was also reported to have inhibitory effect on the germination and shoot growth of test crops more pronounced at high concentration.
The study revealed that Jatropha curcas L. leaf aqueous extracts inhibited the germination and shoot lengths of pepper (Capsicum spp). Also allelopathy is a concentration dependent phenomenon. In line with the results of this study, it is suggested that the presence of azelaic acid provides a competitive advantage to Jatropha curcas as defence mechanism and thus, can inhibit the growth of neighbouring crops in intercropping system. Based on this, Jatropha plant is recommended as not suitable for intercropping with pepper and this information is highly valuable for pepper cultivation. It is therefore recommended that further investigations on the endogenous activities of Azelaic acid in seeds of crops be carried out.
 Ashrafi, Z.Y., Sadeghi, S., Mashhadi, H.R. and Hassan, M.A. (2008) Allelopathic Effects of Sunflower (Helianthus annus) on Germination and Growth of Wild Barley (Hordeum spontaneum). Journal of Agricultural Technology, 4, 219-229.
 Gilani, S.A., Kikuchi, A., Shinwari, Z.K., Khattak, Z.I. and Watanabe, K.N. (2000) Phytochemical, Pharmacological and Ethnobotanical Studies of Rhazyastricta Decne. Phytotherapy Research, 21, 301-307. https://doi.org/10.1002/ptr.2064
 Nazir, T., Uniyal, A.K. and Todria, N.P. (2007) Allelopathic Behaviour of three Medicinal Plant Species on Traditional Agricultural Crops of Garhwal Himalaya, India. Agroforestry Systems, 69, 183-187. https://doi.org/10.1007/s10457-006-9023-8
 Appleton, B., Berrier, R., Harris, R., Alleman, D. and Sawson, L. (2009) The Walnut Tree: Allelopathic Effects and Tolerant Plants. Virginia Cooperative Extension.
 Pasqualeto, A., Costa, A.L.M., Silva, A.A. and Sidiyma, C.S. (2007) The Occurrence of Soybean Crop (L.) Menrril Weeds) Emsucesao to Safrinha Cultures in the No-Tillage System.
 Corsato, J.M., Fortes, A.M.T., Santorum, M. and Leszczynski, R. (2010) Efeito Alelopatico do Extrato Aquoso de Folhas de Girasol sobre a Germinacao de soja e picao-preto. Semina: Cienciasagrarias, 31, 353-360.
 Achten, W.M.J., Maes, W.H., Aerts, R., Verchot, L., Trabucco, A., Mathijs, E., Singh, V.P. and Muys, B. (2009) Jatropha: From Global Hype to Local Opportunity. Journal of Arid Environments, 74, 164-165.
 Ullah, F., Ullah, A., Wazir, S.M. and Shinwar, Z.K. (2014) Phytotoxic Effects of Safflower Yellow Exposure on Seed Germination and Early Seedling Growth of Canola (Brassica napus L.). Pakistan Journal of Botany, 46, 1741-1746.
 Husain, M.I., Gonzlez, I. and Reigosa, M.J. (2010) Phytotoxic Effects of Allelochemicals and herbicides on Photosynthesis, Growth and Carbon Isotope Discrimination in Lactuca sativa. Allelopathy Journal, 26, 157-174.
 Akwa, V.L., Binbol, A.L., Samaita, K.L. and Marcus, N.D. (2007) Geographical Perspective of Nasarawa State. 3rd Edition, Onaivi Printing and Publishing Company Ltd. Keffi, Nasarawa State, Nigeria, 34-35.
 Hassan, A., Mukhtar, F.B. and Mohammed, I.M. (2013) Allelopathic Effects of Jatropha curcas L. leachate on Germination and Early Seedling Growth of Five (5) Agricultural Crops in Kano, Nigeria. Bajopas, 6, 53-56.
 Abugre, S. and Sam, S.Q.J. (2010) Evaluating the Allelopathic Effect of Jatropha curcas Aqueous Extract on Germination, Plumule and Radicle Length of Crops. International Journal of Agriculture and Biology, 12, 769-772.
 Igbinosa, O.O., Igbinosa, E.O. and Aiyegoro, O.A. (2009) Antimicrobial Activity and Phytochemical Screening of Stem Bark Extracts from Jatropha curcas (Linn). African Journal of Pharmacy and Pharmacology, 3, 58-62.
 Maharjan, S., Shrestha, B.B., Jha, P.K. and Academy, R.M. (2007) Allelopathic Effects of Aqueous Extract of Leaves of Parthenium hysterophorus L. on Seedling Growth of some Cultivated and Wild Herbaceous Species. Scientific World, 5, 33-39. https://doi.org/10.3126/sw.v5i5.2653