© Benaki Phytopathological Institute
Herbicidal effect of essential oils
51
and 64μl. Taking into account that the vol-
ume of the Petri dishes was approximately
100 cm
3
, the above quantities were equal to
concentrations (v/v) of 0, 0.08, 0.16, 0.32 and
0.64μl of essential oil per ml of Petri dish, re-
spectively. Each Petri dish was sealed by film
so as to prevent escape of the volatile com-
pounds and all the Petri dishes were placed
in a growth chamber in the dark at 22°C. The
Petri dishes were arranged in a completely
randomized design. After seven days of in-
cubation, the germinated seeds were count-
ed and their radical length was recorded.
Each treatment was replicated three times
and the bioassay trial was repeated twice
and data were averaged. Bioassay data were
analyzed over repetition time by using a fac-
torial approach (essential oil x essential oil
concentration). Data was expressed in % of
control and was arcsine square root trans-
formed before the ANOVA to reduce the
heterogeneity, but means presented are
back-transformed. LSD (5%) values were em-
ployed for the mean comparison. The ANO-
VA was performed separately for each weed
species and oil.
Results and Discussion
Essential oil composition
The main essential oils’ constituents
and their percentage yield are present-
ed in Table 1. In
S. hortnesis
oil, carvacrol
(46.94%) and
γ
-terpinene (29.14%) were the
main components, followed by
a
-terpinene
(5.16%) and
p
-cymene (4.62%). In
M. officina-
lis
oil the dominant components were gera-
nial (47.16%) and neral (36.10%).
Inhibitory effects
Both germination and root length of each
species were significantly affected by essen-
tial oil concentration (P<0.001), however not
by the interaction of essential oil x oil con-
centration. Statistically significant difference
in the effect of the two essential oils was re-
corded. In particular, increased inhibition of
L. rigidum
germination and
Ph. brachystachys
root length were recorded when treated with
M. officinalis
essential oil (Table 2). Since ger-
mination and root length was significantly af-
fected by oil concentration, ANOVA was per-
formed separately for each weed species and
oil. More specifically, the germination of
L.
rigidum
ranged from 70 to 99% and 57 to 91%
of control after the application of
S. horten-
sis
and
M. officinalis
essential oils, respective-
ly, while, the root length of this weed spe-
cies ranged from 28 to 54% and 25 to 66%
of control, respectively (Table 3). Inhibition
of
Ph. brachystachys
germination was more
pronounced and ranged from 28 to 75% and
from 21 to 85% of the control when treated
with
S. hortensis
and
M. officinalis
oil, respec-
tively (Table 4). The root length of this weed
species ranged from 24 to 71% and from 8 to
74% of control, respectively (Table 4).
The main components of the essen-
tial oil of
M. officinalis
are citral (neral and
geranial), citronellal, linalool, geraniol and
β
-caryophyllene-oxide (Adzet
et al.,
1992).
Citral is a mixture of two monoterpenes:
geranial (citral A) and neral (citral B). It is a
volatile essential oil component of lemon-
grass [
Cymbopogon citrate
(Poaceae)] and
other aromatic plants and shows allelo-
pathic traits (Dudai
et al.,
1999). In a previ-
ous study, citral exhibited low phytotoxicity
to soybeans and high phytotoxicity to vel-
vetleaf [
Abutilon theophrasti
, Medik. (Malva-
ceae)], large crabgrass [
Digitaria sanguinalis
,
L. Scop. (Poaceae)], redroot pigweed [
Ama-
ranthus retroflexus
, L. (Amaranthaceae)] and
italian ryegrass [
Lollium multiflorum
, Lam.
(Poaceae)] (Vaughn & Spencer, 1993). Citral
was found to be a strong inhibitor of wheat,
Amaranthus palmeri
S. Wats. (Amaranthace-
ae) and
Brassica nigra
L.
(Brassicaceae) seed
germination (Dudai
et al.,
1999). Dudai (2007)
reported that citral is absorbed by wheat
seed through the abscission layer, and that
it reaches highest concentration in the em-
bryo, where it accumulates in the aleurone,
scutellum and parts of the endosperm. As
reported by Chaimovitsh
et al
. (2012) citral,
as an allelochemical or a potential herbicide
interferes with cell division cytokinesis and
cell elongation. More specifically, they re-
ported that at lower concentrations citral in-
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