Medicinal Plant Research 2015, Vol.5, No. 6, 1-9
2
yield of these plants (Zheljakov et al., 2008). Similarly,
EO yield of
Mentha piperita
and
Ocimum basilicum
is
also not affected by the treatments of Cd, Pb and Cu in
soil. However, application of these heavy metals can
alter the composition of EO (Zheljakov et al., 2006).
Ocimum basilicum
L. (Lamiaceae), commonly known
as sweet basil is an annual aromatic medicinal herb
native to India and other regions of Asia. In India,
basil is mainly cultivated in Assam, Bihar, Uttar
Pradesh and West Bengal where soil is severely
polluted with As (Heikens, 2006; Rao et al., 2007). It
grows luxuriantly in variety of soil types and
agroclimatic conditions (Begum et al., 2002). EO
derived from basil is widely utilized in high-grade
perfumes, aromatherapy, flavoring liquors and as
herbal spice (Bahl et al., 2000; Kumar et al., 2004).
EO contains biologically active constituents that
possess antimicrobial (Elgayyar et al., 2001),
fungistatic (Reuveni et al., 1984), insecticidal (Bowers
and Nishida, 1980) and allelopathic properties (Rice,
1979). Traditionally, basil has been used for the
treatment of headache, cough, diarrhoea, constipation,
warts and kidney malfunction (Politeo et al., 2007).
The yield and composition of EO is dependent on
many environmental factors, agrochemical practices
and the type of cultivar (Jirovetz et al., 2003).
Leaves of
O. basilicum
bear non-glandular and
glandular trichomes on both the abaxial and adaxial
surfaces. Non-glandular trichomes are uniseriate,
pointed, straight or hook-like and their function is to
confer defence to plants. Glandular trichomes are
responsible for biosynthesis, secretion and accumulation
of EO (Fischer et al., 2011). According to Wolff et al.
(2012) high concentration of heavy metal in soil
affects the structural integrity of the trichomes.
However, there is no detailed experimental evidence
to address how trichomes respond to high As levels in
soil, and how the alteration of trichome structure
influences the EO yield and quality. Studies on
how ultrastructure of glandular trichomes is affected
in treated plants and how cell organelles of functional
importance such as endoplasmic reticulum (ER),
mitochondria and plastids respond to high As
concentrations has not been carried out in relation to
As toxicity.
The present study was designed to investigate: (i)
effect of different As concentrations in soil on EO
yield and composition of basil plants; (ii) impact of As
in the soil on morphology and ultrastructure of EO
secreting glandular trichomes. The study has been
undertaken to understand the EO secretion and
glandular trichome function in uncontaminated
(control) and As- contaminated soil.
2. Results
1.1.
Effect of As on growth
Table 1 shows that
O. basilicum
plants exposed to
increasing As concentrations showed progressive
decrease in the vegetative growth i.e. shoot length and
shoot biomass in terms fresh and dry weight. Highest
reduction was observed at 150 mg/kg As. Shoot length
was significantly reduced by 34.65%, compared to
control, while difference between other treatments
was not significant. Shoot fresh and dry mass
accumulation was inhibited by 31.78 and 27%
respectively at 150 mg/kg As. However, an increase in
shoot length, fresh and dry weight was observed at 10
mg/kg As.
1.2.
Effect of As on EO yield and composition
Hydrodistillation of leaf extract from the three month
old basil plants was carried out. EO yield of the
control plants was 0.20% (w/w). At 10 and 50 mg/kg
As, EO yield significantly increased 3.5-4 times
to reach 0.77 and 0.8% respectively, whereas at 150
mg/kg As the yield significantly reduced up to 0.08%
(Figure. 1). EO components estimated in control
plants were linalool (0.0013 mg/g DW), methyl
cinnamate (0.013 mg/g DW), camphor (0.0072 mg/g
DW), 1,8-cineol (0.009 mg/g DW) and methyl
eugenol (0.028 mg/g DW). Linalool concentration
increased 3-4 times under 10-150 mg/kg As as
compared to the control. Methyl cinnamate
concentration at 10 mg/kg As showed down regulation
up to 72.30%, whereas at 50 and 150 mg/kg As it
was not detected. Concentration of 1,8-cineol
and methyl eugenol decreased by 67.7 and 96%
respectively at 150 mg/kg As in comparison to EO
extracted from leaves of control plants. Camphor was
severely affected by As toxicity and not detected at all
in any of the As-treated plants (Table 2).
1.3.
Effect ofAs on trichome density and development
1.3.1.
Trichome density
On the adaxial surface, trichome density increased 2-3
