Geoscience Reference
In-Depth Information
(Said-Al Ahl and Hussein, 2010). Similar results of an inhibi-
tory effect of high level of salinity were also found on lemon
balm (Ozturk et al., 2004),
Majorana hortensis
(Shalan et al.,
2006),
Matricaria chamomile
(Razmjoo et al., 2008),
Salvia
officinalis
(Ben Taarit et al., 2010) and basil (Said-Al Ahl et al.,
2010). On the contrary, an increase of essential oil yield due to
lower levels of salinity has been reported in other plant species,
for example,
Satureja hortensis
(Baher et al., 2002) and
Salvia
officinalis
(Hendawy and Khalid, 2005). It was also shown that
essential oil yield of coriander leaves was stimulated only under
low and moderate stress, while it decreased at the high salinity
level. At low stress, (
E
)-2-decenal, (
E
)-2-dodecenal and dode-
canal contents increased (Neffati and Marzouk, 2008).
12.5
Biotic factors
Plants produce diversity of natural products or secondary
metabolites with a prominent function of protection against
predators and microbial pathogens and are important for the
communication of the plants with other organisms. They also
serve as defence against abiotic stress (e.g. UV-B exposure)
(Schafer and Wink 2009), and are insignificant for growth and
developmental processes (Rosenthal et al., 1991).
A vast majority of the different structures of terpenes pro-
duced by plants as secondary metabolites are presumed to be
involved in defence as toxins and feeding deterrents to a large
number of plant-feeding insects and mammals (Gershenzon
and Croteau, 1991).
For example, the pyrethroids (monoterpenes esters) that
occur in the leaves and flowers of
Chrysanthemum
species
show strong insecticidal responses (neurotoxin) to insects
such as beetles, wasps, moths, bees, and so on, and is also a
popular ingredient in commercial insecticides because of low
persistence in the environment and low mammalian toxicity
(Turlings et al., 1995).
Abietic acid is a diterpene found in pines and leguminous
trees. It is present in or along with resins in resin canals of
the tree trunk. When these canals are pierced by feeding
insects, the outflow of resin may physically block feeding and
serve as a chemical deterrent to continued predation (Bradley
et al., 1992). The milkweeds produce several better-tasting
glucosides (sterols) that protect them against herbivory by
most insects and even cattle (Lewis and Elvin-Lewis, 1977).
Halogenated coumarin derivatives work very effectively
in
