Introduction
Prevention of man mosquito contact is indispensible for protection from mosquito-borne diseases and use of repellents have been advocated for the same. Insect repellents date back to ancient times, with the use of tars, smokes, plant oils and other modalities (Peterson and Coats, 2001). In recent years, new synthetic repellents have been formulated and advocated. However, continuous and indiscriminate use of these synthetic repellents causes adverse effects on the user (Mandal, 2011). There has been a paradigm shift towards botanicals to overcome the problems associated with the use of synthetic compounds. Plant products have been used traditionally to repel and kill mosquitoes in many parts of the world. Thousands of plants have been tested as potential sources of insect repellents (King, 1954; Jacobson, 1990; Sukumar et al., 1991). Plant based products have been used as repellents either as topical applicant (Samuel et al., 2012a) or fumigant and many plant species have been screened for their repellent and insecticidal property against mosquitoes (Sukumar et al., 1991; Shalan et al., 2005; Sakthivadivel and Daniel, 2008; Govindarajan and Sivakumar, 2011; Kamaraj et al., 2011; Samuel et al., 2011; 2012a,b,c,d; Ghosh et al., 2012; Ravindran et al., 2012).

Repellent properties are reported in plants viz., Eucalyptus maculate citriodon against Anopheles gambiae and Anopheles funestus (Trigg, 1996), Pelargonium reniforme against Anopheles arabiensis (Govere et al., 2000), Curuma longa, Cymbopogon winterianus and Ocimum americanum against Aedes aegypti, Anopheles dirus and Culex quinquefasciatus (Tawatsin et al., 2001), Mentha piperita against Anopheles annularis, Anopheles culicifacies and Culex quinquefasciatus (Ansari et al., 2000), Litsea cubeba, Melaleuca leucadendron, Melaleuca quinquenervia, Viola odorata and Nepeta cataria against Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus (Amer and Mehlhorn, 2006). Cymbopogon winterianus and Tagetes minuta showed good repellency effect against Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus (Tyagi et al., 1997; ICMR, 2000; Tawatsin et al., 2001), Ocimum selloi against Anopheles braziliensis (Paula et al., 2003) and neem against Aedes, Culex, Anopheles and Armigeres mosquitoes (Sharma et al., 1993; Dua et al., 1995; Mishra et al., 1995; Pandian and Devi, 1998).

Interest in antimosquito products of plant origin is being revived because of the drawbacks associated with the continued application of synthetic compounds, some of which have led to widespread development of insecticide resistance. Some people mainly in rural areas burn plant materials using glowing charcoal to produce smoke which repels or kills mosquitoes. Therefore, in the present study, evaluations were carried out by burning leaves of five plants viz., Azadirachta indica, Citrus medica, Murraya koenigii, Ocimum tenuifloreum and Ricinus communis using glowing charcoal for their repellent activity against mosquitoes.

1 Results
In the present study, the repellent activity of selected local plants viz., Azadirachta indica, Citrus medica, Murraya koenigii, Ocimum tenuifloreum and Ricinus communis was evaluated. The results indicated that amongst the five plants utilized, Azadirachta indica, Murraya koenigii, Citrus medica exhibited more repellence followed by Ricinus communis and Ocimum tenuifloreum with a range between 25 to 75% repellent activity (Table 1). Maximum repellence was provided by Azadirachta indica leaves in house 1 with a protection time of six hours. In house 2 it was Azadirachta indica and Murraya koenigii and in house 3, Murraya koenigii and Citrus medica with six hours, whereas in house 4 it was Azadirachta indica, Ricinus communis and Ocimum tenuifloreum with four hours (Figure 1). House 5 which served as negative control provided forty five minutes protection time and in the case of house six (positive control) no repellence was observed. The present study also indicated that the incidence of mosquito bites significantly reduced after usage of plant products.

Table 1 Details of plant leaves utilized for the repellent study

Figure 1 Repellency effect of plant leaves against mosquitoes

2 Discussion
Mosquitoes are a serious threat to public health transmitting several dangerous diseases for over two billion people in the tropics. There has been a large increase in the insecticide resistance of this vector and has become a global problem. Insecticides residues in the environment, as a result of chemical insecticide usage, have turned the researcher’s attention towards natural products (Murty and Jamil, 1987). In the past years, the plant kingdom has been of great interest as a potential source of insecticidal products. Many species in the plant kingdom synthesize a variety of secondary metabolites which play a vital role in defense of plants against insects/mosquitoes. Plants may be alternative source for mosquito repellent agents since they constitute a rich source of bioactive chemicals (Wink, 1993). Plant products can be used, either as an insecticide for killing larvae or adult mosquitoes or as repellents for protection against mosquito bites, depending on the type of activity they posses (ICMR, 2003). Products of secondary plant metabolisms may be responsible for the chemical communication between plants and insects. Allelochemicals have been considered as potential natural insecticides and can be used for insect/mosquito management in integrated control (Jilani and Su, 1983). Phytochemicals obtained from plants are usually less environmentally harmful than synthetic chemicals and it has renewed the interest in the research on phytocompounds, considering them as an ecologically safe alternative for synthetic insecticides (Isman, 2006). A review of botanical phytochemicals with mosquitocidal potential published by Shaalan et al. (2005) demonstrates identification of novel effective mosquitocidal compounds from botanicals containing active phytochemicals. Phytochemicals obtained from plants with proven mosquito control potential can be used as an alternative to synthetic potential insecticides or along with other insecticides under the integrated vector control.

Repellents have an important place in protecting man from the bites of insect pests. An effective repellent will be useful in reducing man vector contact and in the interruption of disease transmission. Mosquito repellents may be one of the most effective tools for protecting human from vector-borne diseases and nuisance caused by mosquitoes (Curtis et al., 1990; Barnard, 2000). Repellent compounds should be non toxic, non irritating and long lasting (Kalyanasundaram and Babu, 1982). Repellents are substances that act locally or at a distance, deterring an arthropod (insect/mosquito) from flying to, landing on or biting human or animal skin (Blackwell et al., 2003; Choochote et al., 2007). Usually, insect repellents work by providing a vapour barrier deterring the arthropod (insect/mosquito) from coming into contact with the surface (Brown and Hebert, 1997) and sometimes, applied on to the skin for protection. Repellents of plant origin do not pose hazards of toxicity to humans and domestic animals and are easily biodegradable. Natural products are safe for humans when compared to that of synthetic compounds (Sharma et al., 1993; Ansari and Razdan, 1996). Repellents are used as personal protection methods against biting arthropods with a major aim of avoiding nuisance (Trigg and Hill, 1996). Repellents properly utilized are an inexpensive means of reducing or preventing arthropod-borne diseases and biting nuisance while acting as a wide range of vectors (Gupta and Rutledge, 1994).

Smoke is still, the most widely used common method of repelling biting insects that is used throughout the world. Fresh and dried plants are frequently added to fires to enhance the repellent properties of the smoke (Roemantyo, 1990). Several field evaluations, where plants were burned to repel mosquitoes, have shown good reduction in mosquito landings (Palsson and Jaenson, 1999a,b). Most households in the developing world rely on personal protection measures of limited effectiveness, such as burning mosquito coils or leaves (Hanson et al., 2003). Mosquito coils were traditionally made with finely ground Chrysanthemum cinrariaefolium flowers mixed with coconut husks or sawdust (Chadwick, 1985) and also neem kernels and leaves are burned to repel mosquitoes along with mosquito coils (Konradsen et al., 1997). Plants of terrestrial origin have also been reported to be a source of mosquito repellents (Hwang et al., 1985; Thangam and Kathiresan, 1992). Pandian et al. (1989) observed the repellent activity of herbal smoke on the biting activity of Culex quinquefasciatus. Thangam and Kathiresan (1992) stated that smoke from burning various dry materials has been used since early times to deter insects especially mosquitoes. Hwang et al. (1985) observed that the bundles of dried Artemisia vulgaris were burned to repel biting insects since it contains insect repellents that can be released from the plant by combustion. The smoke of the leaves of Vitex negundo and Leucas aspera were found more toxic to the filarial vector mosquito, Culex quinquefasciatus than the synthetic mosquito mats which contain 4% d-allethrin (Pandian et al., 1994). Anopheles karwari was repelled by coconut husks, ginger and betel nut leaves. Culicines were repelled by mango wood, coconut husks, ginger and beetle nut leaves (Vernede and Marnix, 1994).

The repellent activity of plants might be due to the presence of certain chemicals that are able to irritate the olfactory senses of the mosquitoes. These chemicals can be grouped into three major categories viz., alkaloids, phenolics and terpenoids. Alkaloids are basically insecticidal at low concentration, nevertheless they can be used as a repellence. They are non volatile and release insecticidal smoke when the plant materials or the mosquito coil containing the active ingredients are burnt. They repel the mosquitoes through direct toxicity (Sears, 1996). Mosquito repellent chemicals present in the plant contain phytochemicals like, methone, limonene, beta pinene, alpha pinene and linaliol (Eun, 2001). Burning wood and adding repellent plants to it probably works in several ways. First, the smoke may disguise human kairomones and disrupts convention currents essential in mosquito host location. Secondly, burning may, release repellent irritant molecules and the molecules released by the plants also may be insecticidal (Charlwood and Jolley, 1984). Therefore, the use of plants in mosquito control is an alternative pest control method for minimizing the noxious effects of some pesticidal compounds on the environment (Fatope et al., 1993). Botanicals have widespread insecticidal properties and will obviously work as a new weapon, and in future may act as suitable alternative product to fight against vector mosquitoes (Ghosh, 2012). It may concluded that natural products from plants of insecticidal and medicinal values have higher efficiency in reducing mosquito menace due to their repellent toxicity. Further in-depth laboratory studies and field bioassays are needed as the present study indicated that there is scope to use local plants to control and repel mosquitoes.

3 Materials and Methods
3.1 Collection of plants
Five plants viz., Azadirachta indica, Citrus medica, Murraya koenigii, Ocimum tenuifloreum and Ricinus communis were collected in and around Tambaram, Chennai, Tamil Nadu, India. The plants were selected based on available literature, abundant availability, medicinal and insecticidal properties. The details of collected plants utilized for the present study are presented in Table 2. The collected plant leaves were brought to the laboratory, washed with dechlorinated water, shade dried under room temperature and was then powdered individually.

Table 2 Repellent activity of plants against mosquitoes

3.2 Bioassay
Powdered plant parts burned on glowing charcoal produce smoke acts as a repellent mosquitocide (Kazembe and Nkomo, 2010). In the present study, the same methodology was adopted. The repellent activity was studied under natural conditions in the field. Field observations were undertaken in Meenakshi Avenue, Old Perungaluthur, Chennai, Tamil Nadu, India based on the assessment of mosquito density status in the natural/domestic habitats during dusk which involved direct observations. A total of six houses were selected of which four were used for treatment. The house treated with burning of charcoal only served as negative control and the other house as positive control did not receive any treatment including charcoal. In each of the four houses selected for treatment, the dried powdered leaves of each plant (250 g) were burnt on glowing charcoal. The burning of five plant leaves was changed in a cyclic manner to avoid bias during each trial. A total of five trials were carried out. The experimental set up was kept out of reach of children and use of mosquito repellents was discouraged during the period of study. Prior to the start of the experiment, pre treatment questionnaires were distributed to people to evaluate the status of nuisance caused by the mosquitoes to the people residing in the houses where the experiment was conducted and likewise a post treatment questionnaire after the experiment.

Acknowledgements
Authors are thankful to Star College Scheme of Department of Biotechnology (DBT), Government of India for the financial assistance rendered in carrying the research work.

References
Amer A., and Mehlhorn H., 2006, Repellency effect of forty one essential oils against Aedes, Anopheles and Culex mosquitoes, Parasitology Research, 99: 478-490
http://dx.doi.org/10.1007/s00436-006-0182-3
http://dx.doi.org/10.1007/s00436-006-0184-1
PMid:16642384

Ansari M.A., and Razdan R.K., 1996, Operational feasibility of malaria control by burning neem oil on kerosene lamp in Bell Akbarpur village, Ghaziabad District, India, Indian Journal of Malariology, 33: 81-87
PMid:8952172

Ansari M.A., Vasudevan P., Tandon M., and Razdan R.K., 2000, Larvicidal and mosquito repellent action of peppermint (Mentha piperita) oil, Bioresource Technology, 71: 267-271
http://dx.doi.org/10.1016/S0960-8524(99)00079-6

Barnard D.R., 2000, Repellents and toxicants for personal protection: W.H.O. – Geneva

Blackwell A., Stuart A.E., and Estambale B.A., 2003, The repellent and antifeedant activity of oil of Myria gale against Aedes aegypti mosquitoes and its enhancement by the addition of salicyluric acid, Proceedings of the Royal College of Physicians of Edinburgh, 35: 209-214

Brown M., and Hebert A.A., 1997, Insect repellents: an overview, Journal of American Academic Dermatology, 36: 243-249
http://dx.doi.org/10.1016/S0190-9622(97)70289-5

Chadwick P.R., 1985, Mosquito coils protect against bites, Parasitology Today, 1(3): 90-91
http://dx.doi.org/10.1016/0169-4758(85)90052-3

Charlwood J.D., and Jolley D., 1984, The coil work against mosquito in New Guinea, Transactions of the Royal Society of Tropical Medicine and Hygiene, 78: 678
http://dx.doi.org/10.1016/0035-9203(84)90238-4

Choochote W., Chaithong U., Kamsuk K., Jitpakdi A., Tippawangkosol P., Tuetun B., Champakaew D., and Pitasawat B., 2007, Repellent activity of selected essential oils against Aedes aegypti, Fitoterapia, 78: 359-364
http://dx.doi.org/10.1016/j.fitote.2007.02.006
PMid:17512681

Curtis C.F., Lines J.D., Lu B., and Renz A., 1990, Natural and synthetic repellents in appropriate technology in vector control, pp.75-92

Dua V.K., Nagpal B.N., and Sharma V.P., 1995, Repellent action of neem cream against mosquitoes, Indian Journal of Malariology, 32: 47-53
PMid:7589727

Eun L.S., 2000, Mosquito larvicidal activity of pipernonaline, a piperine alkalid derived from long pepper, Piper longum, Journal of American Mosquito Control Association, 16(13): 245-247

Fatope M.O., Ibrahim H., and Takeda Y., 1993, Screening of higher plants reputed as pesticides using the brine shrimp lethality assay, International Journal of Pharmacognosy, 31(2): 250-254
http://dx.doi.org/10.3109/13880209309082949

Ghosh A., Chowdhury N., and Chandra G., 2012, Plant extracts as potential mosquito larvicides, Indian Journal of Medical Research, 135: 581-598
PMid:22771587

Govere J., Durrheim D.N., Du T.N., Hunt R.H., and Cotzee M., 2000, Local plants as repellents aginst Anopheles arabiensis in Mpumalanga Province, South Africa, Central African Journal of Medicine, 46: 213-216
PMid:11317593

Govindarajan M., and Sivakumar R., 2011, Mosquito adulticidal and repellent activities of botanical extracts against the malarial vector, Anopheles stephensi Liston (Diptera: Culicidae), Asian Pacific Journal of Tropical Biomedicine, 941-947

Gupta R.K., and Rutledge L.C., 1994, Role of repellents in vector control and disease prevention, American Journal of Tropical Medicine and Hygiene, 50: 82-86
PMid:8024088

Hanson K., Odan C., Lines J., Meek S., Bradley D., and Mills A., 2003, The economics of malaria control, Malaria Cusortium, London

Hwang Y.S., Wu K.H., Kumamoto J., Akelrod H., and Mulla M.S., 1985, Isolation and identification of mosquito repellent in Artemisia vulgaris, Journal of Chemical Ecology, 11(9): 1297-1306
http://dx.doi.org/10.1007/BF01024117
PMid:24310391

ICMR, 2000, Air-borne toxicity of plant extracts against mosquitoes, Indian Council of Medical Research, Annual Report, New Delhi

ICMR, 2003, Prospects of using herbal products in the control of mosquito vectors, ICMR Bulletin, 33: 1

Isman M., 2006, Botanical insecticides, deterrents and repellents in modern agriculture and an increasingly regulated world, Annual Review of Entomology, 55: 45-66
http://dx.doi.org/10.1146/annurev.ento.51.110104.151146
PMid:16332203

Jacobson M., 1990, Glossary of plant derived insect deterrents, Boca Raton, CRC

Jilani G., and Su H.C.F., 1983, Laboratory studies on several plant materials as insect repellents for protection of cereal grains, Journal of Economic Entomology, 76: 154-157

Kalyanasundaram M., and Babu C.J., 1982, Biologically active plant extracts as mosquito larvicides, Indian Journal of Medical Research, 76: 102-106
PMid:7185750

Kamaraj C., Rahuman A.A., Bagavan A., Elango G., Zahir A.A., and Santhoshkumar T., 2011, Larvicidal and repellent activity of medicinal plant extracts from Eastern Ghats of South India against malaria and filariasis vectors, Asian Pacific Journal of Tropical Biomedicine, 698-705

Kazembe T.C., and Nkomo S., 2010, Mosquitocides of Ageratum conyzoides, Boscia salicifolia and Grewia monticola, Indian Journal of Traditional Knowledge, 9(2): 394-397

King W.V., 1954, Chemicals evaluated as insecticides and repellents at Orlando, Florida, U.S. Department of Agriculture, Agricultural Handbook No. 69, Washington, D.C.

Konradsen F., Vanderhoek W., Amerasinghe P.H., Amerasinghe F.P., and Fonseka K.T., 1997, Household responses to malaria and their costs: a study from rural Srilanka, Transactions of the Royal Society of Tropical Medicine and Hygiene, 91(2): 127-130
http://dx.doi.org/10.1016/S0035-9203(97)90194-2

Mandal S., 2011, Repellent activity of Eucalyptus and Azadirachta indica seed oil against the filarial mosquito Culex quinquefasciatus Say (Diptera: Culicidae) in India, Asian Pacific Journal of Tropical Biomedicine, 1(2): S109-S112
http://dx.doi.org/10.1016/S2221-1691(11)60135-4

Mishra A.K., Singh N., and Sharma V.P., 1995, Use of neem oil as a mosquito repellent in tribal villages of Mandala district, Madhya Pradesh, Indian Journal of Malariology, 32: 99-103
PMid:8936291

Murty U.S., and Jamil K., 1987, Effect of South Indian vetiver oil (Vetiveria zizinioides (L) Nash) against the immatures of Culex quinquefasciatus Say (Diptera: Culicidae), Journal of Entomology, 2(1): 8-9

Palsson K., and Jaenson T.G., 1999a, Plant products used as mosquito repellents in Guinea Bissau, West Africa, Acta Tropica, 72: 39-52
http://dx.doi.org/10.1016/S0001-706X(98)00083-7

Palsson K., and Jaenson T.G., 1999b, Comparison of plant products and pyrethroid treated bed nets for protection against mosquitoes (Diptera: Culicidae) in GuineaBissau, West Africa, Journal of Medical Entomology, 36(2): 144-148
PMid:10083750

Pandian R.S., and Devi T.S., 1998, Repellent action of plant oils on mosquito, Insect Environment, 4: 58

Pandian R.S., Revathy C., and Manoharan C.A., 1994, Toxicity evaluation of herbal smoke and synthetic mosquito mat on Culex quinquefasciatus Say, Geobios, 21: 166-168

Pandian R.S., Dwarakanath S.K., and Martin P., 1989, Repellent activity of herbal smoke on the biting activity of mosquitoes, Journal of Ecobiology, 1(2); 87-89

Paula J.P., Carnerio M.R., and Pumgartten F.J.R., 2003, Chemical compistion, toxicity and mosquito repellency of Ocimum selloi oil, Journal of Ethnopharmacology, 88: 253-260
http://dx.doi.org/10.1016/S0378-8741(03)00233-2

Peterson C., and Coats J., 2001, Insect repellents- past, present and future, Pesticide Outlook, 12: 154-158
http://dx.doi.org/10.1039/b106296b

Ravindran J., Samuel T., Alex E., and William J., 2012, Adulticidal activity of Ageratum houstonianum Mill. (Asteraceae) leaf extracts against three vector mosquito species (Diptera: Culicidae), Asian Pacific Journal of Tropical Biomedicine, 2(3): 177-179

Roemantyo S.H., 1990, Ethnobotany of Japanese incense, Economic Botany, 44: 413-416

Sakthivadivel M., and Daniel T., 2008, Evaluation of certain insecticidal plants for the control of vector mosquitoes viz., Culex quinquefasciatus, Anopheles stephensi and Aedes aegypti, Applied Entomology and Zoology, 43(1): 57-63
http://dx.doi.org/10.1303/aez.2008.57

Samuel T., Ravindran K.J., and Arivoli S., 2011, Screening of plant extracts for ovicidal activity against Culex quinquefasciatus Say (Diptera: Culicidae), Applied Botany, 40: 5456-5460

Samuel T., Ravindran K.J., Eapen A., and William S.J., 2012a, Repellent activity of Ageratum houstonianum Mill. (Asteraceae) leaf extracts against Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae), Asian Pacific Journal of Tropical Disease, 2(6): 478-480
http://dx.doi.org/10.1016/S2222-1808(12)60104-2

Samuel T., Ravindran K.J., Eapen A., and William S.J., 2012b, Effect of Ageratum houstonianum Mill. (Asteraceae) leaf extracts on the oviposition activity of Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae), Parasitology Research, 111(6): 2295-2299
http://dx.doi.org/10.1007/s00436-012-3083-7
PMid:22903449

Samuel T., Ravindran K.J., and Arivoli S., 2012c, Bioefficacy of botanical insecticides against the dengue and chikungunya vector Aedes aegypti (L.) (Diptera: Culicidae), Asian Pacific Journal of Tropical Biomedicine, 2: S1842-S1844
http://dx.doi.org/10.1016/S2221-1691(12)60505-X

Samuel T., Ravindran K.J., and Arivoli S., 2012d, Screening of twenty five plant extracts for larvicidal activity against Culex quinquefasciatus Say (Diptera: Culicidae), Asian Pacific Journal of Tropical Biomedicine, 2: S1130-S1134
http://dx.doi.org/10.1016/S2221-1691(12)60372-4

Sears R., 1996, An ethnobotanical survey of insect repellents in Brazil, in Tropical Resources Institute Working Pages, New Have

Shaalan E.A.S., Canyonb D., Younesc M.W.F., Wahaba H.A., and Mansoura A.H., 2005, A review of botanical phytochemicals with mosquitocidal potential, Environmental International, 31: 1149-1166
http://dx.doi.org/10.1016/j.envint.2005.03.003
PMid:15964629

Sharma V.P., Nagpal B.N., and Srivastava A., 1993, Effectiveness of neem oil mats in repelling mosquitoes, Transactions of the Royal Society of Tropical Medicine and Hygiene, 87(6): 626
http://dx.doi.org/10.1016/0035-9203(93)90263-P

Sukumar K., Perich M.J., and Boober L.R., 1991, Botanical derivatives in mosquito control: A review, Journal of the American Mosquito Control Association, 30: 122-124

Tawatsin A., Wratten S.D., Scott R.R., Thavara U., and Techadamrongsin Y., 2001, Repellency of volatile oils from plants against three mosquito vectors, Journal of Vector Ecology, 26(1): 76-82 PMid:11469188

Thangam T.S., and Kathiresan K., 1992, Smoke repellency and killing effects of marine plants against Culex quinquefasciatus, Tropical Biomedicine, 9: 35-38

Trigg J.K., 1996, Evaluation of a Eucalyptus – based repellent against Anopheles species in Tanzania, Journal of the American Mosquito Control Association, 12: 243-246
PMid:8827599

Trigg J.K., and Hill N., 1996, Laboratory evaluation of Eucalyptus based repellent action against four biting arthropods, Phytotherapy Research, 10: 43-46http://dx.doi.org/10.1002/(SICI)1099-1573(199606)10:4<313::AID-PTR854>3.0.CO;2-O
http://dx.doi.org/10.1002/(SICI)1099-1573(199606)10:4<313::AID-PTR854>3.3.CO;2-F

Tyagi B.K., Ramnath T., and Shahi A.K., 1997, Evaluation of repellency of Tagetes minuta (Family: Compositae) against the vector mosquitoes Anopheles stephensi Liston, Culex quinquefasciatus Say and Aedes aegypti (L.), Insect Pest Control, 39: 184-185

Vernede R., and Marnix M.M., 1994, Smoke as a form of personal protection against mosquitoes, a field study in Papue, New Guinea, Southeast Asian Journal of Tropical Medicine and Public Health, 25(4): 771-775
PMid:7667730

Wink M., 1993, Protection and application of phytochemicals from an agricultural perspective, Phytochemistry and Agriculture, 34: 171