40. Natural Herbicides
Questions and problems:
2. What is a terpene? What is a monoterpene? How are they different? Find examples of each of these classes of compounds.
3. What practical use might be made of allelopathics?
Allelophathy and Terpenes
Allelopathy refers to the suppression of growth of one plant species by another due to the release of toxic substances.1 The toxic substances are usually released in liquids which contain terpenes. Plants release these toxic substances into the environment by volatilization from leaves, leaching from leaves by rain water, fog or dew, leaching from plant litter, decay of sloughed tissue from roots, and exudation from roots.2
Terpenes are “a group of hydrocarbons present in turpentine, liquid resin, or essential oils” which are particularly produced by conifers. Terpenes are divided into six types according to the number of isoprene units they contain. An isoprene unit has a chemical formula of C5H8, which can undergo self-combinations or even polymerization into the following:
a) Monoterpenes with 2 isoprene units. (C10H16) Examples are pinene, nerol, citral, camphor, menthol, limonene. 3
b) Sesquiterpenes(C15H24) with 3 isoprene units. Examples are nerolidol, farnesol. 4
c) Diterpenes(C20H32) with 4 isoprene units. Examples are phytol, vitamin A1. 5
d) Triterpenes(C30H48) with 6 isoprene units. One of the examples is Squalene. 6
e) Tetraterpenes(C40H64) with 8 isoprene units. One of the examples is carotene (provitamin A1).7
f) Polyterpenes with a large number of isoprene units.( (C5H8)n ). One of the examples is rubber.8
During the early investigations on allelopathy, scientists did not consider the application of it in agriculture. In present study, possible applications were discovered.9 Essential oils, which contain terpenes, are found to have power to inhibit seed growth of another plant. For example, the walnut tree (Juglans regia, Juglandaceae) perform the most effective allelopathy by the release of juglone, which is “an extremely toxic naturally occurring quinone with high potency against both plants and animals”.10 Another exapmple of allelopathy are the oak trees in the Quercus genus (Fagaceae) which “employ biosynthesis based on the phenylalanine ammonia-lyase pathway to replease large amounts of allelopathic phenolics in their defence”.11 Result from a study shows that seeds that germinated in presence of essential oils usually did not develop normally. Radicle and coleoptile growth were inhibited. Fifty percent inhibition of growth occurred at a lower oil concentration than for germination, and radicle growth was somewhat more sensitive than coleoptile growth.12 It is probably the inhibit of growth mainly inhibit the growth of roots into the soil. Af ter leaf fall in summer, in dense populations of Origanum syriacum and Micromeria fruticosa, almost no other plants develop near the aromatic plants. In pot experiments, in which leaves of these species were mixed into the top layer of the soil, there was marked inhibition of growth and germination.13 These observations implied the occurrence of allelopathic interactions. These data suggest that essential oils could be used to inhibit emergence of weeds. Further studies especially in formaulation of essetial oils for application, are still required to apply this technique to agriculture.14
References
1. “Allelopathy.” Encuclopedia Britannica Online. 2005. Encyclopedia Britannica.
27 September. 2005 <
http://www.britannica.com/dictionary?book=Dictionary&va=allelopathy&query=allelopathy>
2. Birkett, Michael A., Chamberlain, Keith, Hooper, Anthony M. & Pickett, John A.. “Does allelopathy offer real promise for practical weed management and for explaining rhizosphere interactions involving higher plants.” Plant and Soil.
Netherlands: Kluwer Academic Publishers, 2001.
3. Kirst, Burkhard “Terpenes.” Department of Biology, Chemistry, Pharmacy. Institute of Chemistry. 1998-02-12. <
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html>
4. Kirst, Burkhard “Terpenes.” Department of Biology, Chemistry, Pharmacy. Institute of Chemistry. 1998-02-12. <
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html>
5. Kirst, Burkhard “Terpenes.” Department of Biology, Chemistry, Pharmacy. Institute of Chemistry. 1998-02-12. <
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html>
6. Kirst, Burkhard “Terpenes.” Department of Biology, Chemistry, Pharmacy. Institute of Chemistry. 1998-02-12. <
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html>
7. Kirst, Burkhard “Terpenes.” Department of Biology, Chemistry, Pharmacy. Institute of Chemistry. 1998-02-12. <
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html>
8. “Terpene.” Wikipedia. 26 September 2005. Wikipedia. 27 September. 2005 <
http://en.wikipedia.org/wiki/Terpene>
9. Dudai, N., Poljakoff-Mayber, A., Mayer, A. M., Putievsky, E., Lerner, H. R.. “Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides.” Journal of Chemical Ecology, Vol. 25, No.5, 1999. 7 January, 1999. Division of Aromatic Plants, Dept. of Botany, The Hebrew University of Jerusalem. 27 September, 2005.
10. Dudai, N., Poljakoff-Mayber, A., Mayer, A. M., Putievsky, E., Lerner, H. R.. “Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides.” Journal of Chemical Ecology, Vol. 25, No.5, 1999. 7 January, 1999. Division of Aromatic Plants, Dept. of Botany, The Hebrew University of Jerusalem. 27 September, 2005.
11. Birkett, Michael A., Chamberlain, Keith, Hooper, Anthony M. & Pickett, John A.. “Does allelopathy offer real promise for practical weed management and for explaining rhizosphere interactions involving higher plants.” Plant and Soil.
Netherlands: Kluwer Academic Publishers, 2001.
12. Dudai, N., Poljakoff-Mayber, A., Mayer, A. M., Putievsky, E., Lerner, H. R.. “Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides.” Journal of Chemical Ecology, Vol. 25, No.5, 1999. 7 January, 1999. Division of Aromatic Plants, Dept. of Botany, The Hebrew University of Jerusalem. 27 September, 2005.
13. Dudai, N., Poljakoff-Mayber, A., Mayer, A. M., Putievsky, E., Lerner, H. R.. “Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides.” Journal of Chemical Ecology, Vol. 25, No.5, 1999. 7 January, 1999. Division of Aromatic Plants, Dept. of Botany, The Hebrew University of Jerusalem. 27 September, 2005.
14. Dudai, N., Poljakoff-Mayber, A., Mayer, A. M., Putievsky, E., Lerner, H. R.. “Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides.” Journal of Chemical Ecology, Vol. 25, No.5,
1999. 7 January, 1999. Division of Aromatic Plants, Dept. of Botany, The Hebrew University of Jerusalem. 27 September, 2005.
