Geoscience Reference
In-Depth Information
deficit, that is, drought or high soil salinity. In cases of high
soil salinity and also in other conditions such as flooding and
low soil temperature, water exists in the soil solution, but plants
cannot take it up—a situation commonly known as 'physi-
ological drought'. Drought occurs in many parts of the world
every year, frequently experienced in the field-grown plants
under arid and semi-arid climates. Regions with adequate
but non-uniform precipitation also experience water-limiting
environments. Since the dawn of agriculture, mild to severe
drought has been one of the major production-limiting factors.
Consequently, the ability of plants to withstand such stress is of
immense economic importance. The general effects of drought
on plant growth are fairly well known. However, the primary
effect of water deficit at the biochemical and molecular levels
are not considerably understood yet and such understanding is
crucial. All plants have tolerance to water stress, but the extent
varies from species to species. Knowledge of the biochemical
and molecular responses to drought is essential for a holistic
perception of plant resistance mechanisms to water-limited
conditions in higher plants.
Water stress is one of the most important environmen-
tal stresses that can depress growth and alter the biochemi-
cal properties of plants (Zobayed, 2005). According to Franz
(1983), Palevitch (1987) and Marchese and Figueira (2005), one
of the most important factors affecting secondary metabolism
is soil water capacity. Usually, limited availability of water has
a negative effect on plant growth and development. However,
a non-severe water deficit has sometimes proved beneficial for
the accumulation of biologically active compounds in medici-
nal and aromatic plants (Palevitch, 1987). Ghershenzon (1984)
demonstrated that in herbaceous plants and shrubs, terpenes
tend to increase under stress, mainly under severe water deficit
conditions. This type of stress is known to increase the amount
of secondary metabolites in a variety of medicinal plants, for
example, artemisinin in
Artemisia annua
L. (Charles et al.,
1993), ajmalicine in
Catharanthus roseus
(Jaleel et al., 2008)
and hyperforin in
Hypericum perforatum
(Zobayed et al.,
2005). The response of essential oil yield and composition to
water stress varies with the duration and severity of stress.
Putievesky et al. (1990) also reported that as irrigation intervals
became more extended, herbage yield and essential oil yield
were reduced in
Pelargonium graveolens
. Similarly, Rajeswara
Rao et al. (1996) reported that a wet season encouraged vegeta-
tive growth of rose-scented geranium and resulted in a higher
essential oil yield.
