چكيده لاتين :
Introduction: Nowadays, the global effort in modern agriculture is to reduce the use of harmful herbicides
and introduce new methods of weed control, one of which is the use of allelopathic properties. Among
allelopathic species, aromatic plants have the ability to transmit allelochemical constituents as essential oils
through diffusion and thus affect their surrounding organisms. Since the use of essential oils and volatile
compounds will not have residues on the crop, these allelopathic compounds can be very effective in preventing
weed growth before and after crop cultivation. Studies have shown that some essential oils or their components
effectively reduce plant growth. This study was aimed to identify novel allelopathic species and their inhibitory
compounds.
Materials and Methods: Plant samples consisting of 112 different plant organs including root, rhizome,
corm, stem, leaf, flower, fruit, fruit peel, aerial part, and plant exudate such as oleogum belonging to 97 aromatic
species from 16 different plant families were collected from different areas of Iran. The essential oils were
extracted by water distillation using Clevenger. The oils were then dehydrated in glass containers with
anhydrous sodium sulfate and stored at 4 °C during the experiment. The study was conducted in two separate
experiments on germination and seedling growth, and lettuce was used as the test plant. The effect of 112
essential oils at two concentrations of 1 and 3 μL was investigated. The factorial experiments were conducted in
completely randomized design with four replicates. Allelopathic effects of volatile compounds were evaluated
by cotton swab method. The traits related to germination including Germination percentage (G%), Mean
Germination Time (MGT), percentage of seed Dormancy induction (D%), percentage of embryo death as Lethal
effect (L%), and seedling growth traits consist of Hypocotyl (H), Radicle (R), and Vigor Index (VI) were
studied. The components of the strongest allelopathic essential oils were analyzed by headspace gas
chromatography and mass spectrometry.
Result and Discussion: The results showed significant inhibitory effects of essential oils even on 1 µl
concentration on the studied traits. The essential oils of Pelargonium graveolens, Pimpinella anisum and Thymus
daenensis had the greatest inhibitory effect on G% (100% inhibitory); and the essential oils of Amomum
subulatum, Atremisia sieberi, Dracocephalum moldavica and Thymus transcaspicus had the highest effect on
MGT (more than 224%). The essential oils of Ziziphora tenuior, Pimpinella anisum, and Pelargonium
graveolens caused the highest seed dormancy (more than 22%). The essential oil of Pimpinella anisum and
Thymus daenensis caused the highest percentage of embryo death (100% lethality). According to seedling
growth, the essential oils of Pimpinella anisum, Origanum vulgare, Perovskia abrotanoides and Thymus
daenensis resulted in the highest inhibition of hypocotyl (more than 92%). Thymus daenensis and Origanum
vulgare essential oil exhibited the greatest effect on decreasing radicle growth (more than 97%). Overall, the
essential oils of Anise, Oregano, Russian Sage and “Denaee” Thyme were the most inhibitors of seedling
growth. Finally, with respect to germination and seedling growth percentages, 25 essential oils mainly from the
Lamiaceae family (12 essential oils), and some other families e.g. Asteraceae (4 essential oils), Apiaceae (3
essential oils), Rutaceae and Geraniaceae (2 essential oil), Liliaceae and Zingiberaceae (1 essential oil) had the
greatest inhibitory effect on the Vigor index. According to the results of the headspace gas chromatography,
compounds such as borneol, eucalyptol, limonene, alpha and beta-pinene, carvacrol, and camphene were the
dominant compounds in essential oils with severe inhibitory effects. Other studies have also reported phytotoxic,
antibacterial, antifungal, and pesticidal effects of some of these compounds, scant literature is, however,
available on the allelopathic effects of some other ones.
Conclusion: Our results showed that the essential oils of medicinal and aromatic species were highly toxic to
lettuce. In addition to inhibition of germination percentage and seedling growth, traits such as seed dormancy
induction, embryo death and delayed germination were also affected by essential oils. In allelopathic researches, such traits are often overlooked, while they can play an important role in growth inhibition and are valuable traits
for weed control. Further research is needed to identify the compounds responsible for allelopathic activity in
these plants essential oils and to understand the biological roles of these compounds in natural ecosystems. Such
information could provide a broader perspective for researchers on the production of new bioactive chemicals
from natural products. Identification of allelopathic plants and compounds, and their biological functions are
important for the biological control of weeds in organic agriculture as well as species that can adversely affect
forestry projects.
