Medicinal Plant Research 2015, Vol.5, No. 6, 1-9
6
mitochondria. These features are typical of secretory
tissues with high metabolic activity. Secretion is an
active process in which energy is used for metabolic
compartmentation, ion extrusion, or biosynthesis of
products (Fahn, 1988). Hence, presence of a large
number of mitochondria in glandular trichomes is
indicative of active secretion. Presence of large vacuoles
is related to the storage of metabolites and ions in the
secretory apparatus (Figueiredo and Pais, 1994). At 50
mg/kg As, trichomes of
O. basilicum
showed extensive
RER and large mitochondria, indicating that the
activity was higher in glandular cells that stimulated
the production of EO. The existence of more
prominent plasmodesmata connecting the cytoplasm
of the secretory cells indicates enhanced intercellular
transport of compounds within the trichome.
Mitochondrial aberration in the form of inflated
cristae and less electron dense cytoplasm was seen at
150 mg/kg As. Underdeveloped organelles in the
trichomes at 150 mg/kg As could be the possible
reason for low yield in EO. This clearly demonstrates
that higher doses of As affect the cells and disrupt
their metabolic machinery. The variation observed in
yield are manifestations of changes observed at
cellular and subcellular levels.
Characteristic basil aroma appears due to 1,8-cineol,
methyl cinnamate and linalool (Lee et al., 2005).
Camphor, 1,8-cineol and linalool are also known to be
biologically active components (Morris et al., 1979).
These compounds possess antimicrobial and
antioxidative properties with high therapeutic value
(Raseetha et al., 2009). GC analysis revealed that
linalool, the major constituent in
O. basilicum
was not
affected by exposure to As, but relative concentration
of methyl eugenol, methyl cinnamate, 1,8-cineol and
camphor diminished when plants were subjected to As
stress (Table 2). Such a change in constituents of EO
can be considered detrimental to oil quality. Low
concentration of 1,8-cineol and absence of camphor at
higher As levels depict that there is a compromise in
defense potential as these two constituents are
involved in allelopathic reactions (Rice, 1979).
Compounds responsible for typical aroma of basil are
affected due to As toxicity, which also degrades the
antioxidative property of basil. Such changes in the
EO composition in response to heavy metal stress
have also been reported in
Rosmarinus officinalis
(Deef, 2007),
Mentha arvensis
(Prasad et al., 2010)
and
Salvia officinalis
(Stancheva et al., 2009).
Compromise on quality of therapeutically important
compounds reflect inactivation of enzymes or
redirection of metabolic functions to maintain growth
(Murch et al., 2003).
3.
Material and Methods
3.1.
Plant material, growth conditions and treatments
Seeds of
Ocimum basilicum
were procured from
Germplasm Conservation Division, National Bureau
of Plant Genetic Resources (NBPGR), New Delhi
(India). Pot culture experiments were conducted in
Botanical Garden, University of Delhi, Delhi (India).
Seeds were sown in October in pots (38 cm diameter)
filled with 4 kg air dried soil and compost in equal
ratio. Soil was clay loam, pH 7.2, NO
3
N 125 mg/kg,
available P 0.5 mg/kg and K 120 mg/kg. Air-dried
soil
was amended with disodium hydrogen arsenate
[Na
2
HAsO
4
.7H
2
O] at concentrations of 10, 50 and 150
As mg/kg soil, each with three replicas. Pots without
As served as control. Twenty seeds were sown in each
pot and five uniformly growing seedlings per pot were
retained for further studies. Plants were grown under
natural conditions of light, temperature and humidity.
Plants were harvested after four months of growth
period. Whole plants were scooped out. Fresh weight
of the shoot system was taken. Plant tissue was oven
dried at 72
o
C and weight was recorded.
3.2.
EO extraction
Fifty grams of dried shoots (leaves and stems) were
subjected to steam distillation-extraction for 4 h
according to the protocol given by Rao et al. (2005)
with slight modifications. The condensate obtained
from distillation was collected and divided into
sub-samples. Each sub-sample was mixed with 100 ml
hexane to trap the dissolved EO. EO obtained from
the samples was dried over anhydrous sodium
sulphate to make it moisture free. Quantity of EO was
measured by calibrated burette. Percentage EO yield
of tissue samples was calculated by the formulae
given by Rao et al. (2005):
EO yield (%) = Amount of EO recovered (g) / Amount
of crop biomass distilled (g) x 100
3.3.
EO analysis
EO analysis was carried out by gas chromatography
using Shimadzu GC 2014 equipped with flame
ionization detector (FID) and an electronic pressure
