Responses of Bloom-Forming Heterosigma akashiwo to Allelochemical Linoleic Acid: Growth Inhibition, Oxidative Stress and Apoptosis

Algal blooms have been occurring in many regions worldwide, and allelochemicals are important algaecides used to control harmful algal blooms (HABs). The allelopathic effects of linoleic acid (LA) on the harmful raphidophyte Heterosigma akashiwo were studied, and the possible mechanisms were investigated through analyses of population growth dynamics, cellular ultrastructure and the physiological levels of H. akashiwo. The results showed that the inhibitory effect of LA on H. akashiwo cells increased with an increasing LA concentration. The levels of ROS and MDA were significantly elevated, indicating oxidative stress and lipid peroxidation due to LA exposure. At the same time, LA also activated the antioxidant system, including superoxide dismutase (SOD), catalase (CAT), and POD, and non-enzymatic antioxidants such as reduced AsA and glutathione (GSH). Transmission electron microscopy (TEM) revealed that the morphology of the algal cells was impaired in an LA-dependent manner. Annexin V-FITC/PI double staining and flow cytometric analysis revealed that LA exposure decreased the cellular mitochondrial membrane potential (MMP), increased the rate of apoptosis. LA modulated bcl-2/bax homeostasis and increased the expressions of cytochrome c and caspases-3 and -9, proving that LA induced cell death via the mitochondria-mediated apoptotic pathway. It was suggested that LA had allelopathic effects on H. akashiwo, inducing physiological and morphological changes and finally triggering the apoptosis of H. akashiwo. All of these results showed that LA might have the potential as an algaecide to control harmful algae.

Allelopathic effects of Androstachys johnsonii Prain on germination and growth of Zea mays L. and Vigna subterranea (L.) Verdc.

Allelopathic effects of the donor plants are mostly regarded as harmful to target plants such as traditional crops. Traditional crops play pivotal role as staple food for rural communities and hence ensuring food security. Study of the allelopathic effects of Androstachys johnsonii Prain on the germination and growth of Zea mays L. and Vigna subterranea (L.) Verdc was conducted. Different plant leachates concentrations of 0.675%, 1.25%, 2.5% and 5% were prepared through serial dilutions. Germination bioassays were arranged in a complete randomized setup of three replicates for each treatment for seven days. Germination rates were calculated, radicle and hypocotyl lengths were also measured. Germination rates of Z. mays L. and V. subterranea (L.) Verdc. ranged from 50 - 100% and 18 - 76%, respectively. Significant differences of P< 0.005 were noticed from the germination rates of V. subterranea (L.) Verdc subjected to high concentrations of different extracts used. Statistically there was no significant differences between germination rates of Z. mays L. treated with all concentrations of leaf leachates against germination rates of the seeds treated with distilled water. Outstandingly, the germination rate and early growth of Z. mays L. were promoted by less concentrated extracts of leaf, root, stem bark and soil which then suggest future use of allelochemicals from Androstachys johnsonii Prain for promotion of maize plant productions. Based on the beneficial attributes of allelochemicals from Androstachys johnsonii on the germination and growth of Z. mays L., planting of Androstachys johnsonii Prain in maize fields is highly recommended.

Allelopathic Activity and Characterization of Allelopathic Substances from Elaeocarpus floribundus Blume Leaves for the Development of Bioherbicides

To help protect the environment as well as increase agricultural production, the use of synthetic herbicides must be reduced and replaced with plant-based bioherbicides. Elaeocarpus floribundus is a perennial, evergreen, and medium-sized plant grown in different areas of the world. The pharmaceutical properties and various uses of Elaeocarpus floribundus have been reported, but its allelopathic potential has not yet been explored. Thus, we carried out the present study to identify allelopathic compounds from Elaeocarpus floribundus. Aqueous MeOH extracts of Elaeocarpus floribundus significantly suppressed the growth of the tested species (cress and barnyard grass) in a dose- and species-dependent way. The three most active allelopathic substances were isolated via chromatographic steps and characterized as (3R)-3-hydroxy-β-ionone, cis-3-hydroxy-α-ionone, and loliolide. All three substances significantly limited the seedling growth of cress, and the compound (3R)-3-hydroxy-β-ionone had stronger allelopathic effects than cis-3-hydroxy-α-ionone and loliolide. The concentrations of the compounds required for 50% growth inhibition (I50 value) of the cress seedlings were in the range of 0.0001–0.0005 M. The findings of this study indicate that all three phytotoxic substances contribute to the phytotoxicity of Elaeocarpus floribundus.

Allelopathic Effects of Essential Oils on Seed Germination of Barley and Wheat

In this study, we evaluated the allelopathic effects of essential oils (EOs) from six different plant species, namely, lavender (Lavandula angustifolia), hyssop (Hyssopus officinalis), English thyme (Thymus vulgaris), lovage (Levisticum officinale), costmary (Chrysanthemum balsamita), and cumin (Cuminum cyminum), on seed germination and seedling growth of barley (Hordeum vulgare) and wheat (Triticum aestivum). The main constituents of the EOs of L. angustifolia were 47.0% linalool acetate and 28.4% linalool; H. officinalis’ main constituents were 39.8% cis-pinocamphone, 9.8% trans-pinocamphone, 11.4% β-pinene, and 7.5% β-phellandrene; T. vulgaris’ were 38.2% para-cymene, 25.6% thymol, and 13.6% γ-terpinene; L. officinale’s were 64.8% α-terpinyl acetate and 14.7% β-phellandrene; C. balsamita’s were 43.7% camphor, 32.4% trans-thujone, and 11.6% camphene; C. cyminum’s were 49.6% cumin aldehyde, 10.4% para-cymene, 11.6% α-terpinen-7-al, and 9.1% β-pinene. All six EOs exhibited an allelopathic effect and suppressed the seed germination and seedling development of wheat and barley; however, the concentrations that exhibited a suppressing effect were different among the plants. C. cyminum EO completely suppressed both barley and wheat germination at 10-, 30-, and 90-µL application rates, making it the most effective treatment among the tested EOs. C. balsamita’s and H. officinalis’ EOs at 30 and 90 µL application rates completely suppressed barley and wheat radicles per seed, radicle length (mm), seedling height (mm), and germination (%). L. angustifolia’s EOs at 30- and 90-µL and T. vulgaris’ EO at 90 µL application rates also completely suppressed barley and wheat radicles per seed, radicle length (mm), seedling height (mm), and germination (%). C. balsamita’s, H. officinalis’, L. angustifolia’s, and T. vulgaris’ EOs at a 10 µL application rate reduced barley radicle length, seedling height, and % germination relative to the control. Wheat seed germination % was completely suppressed by the application of L. angustifolia’s and T. vulgaris’ EOs at 30 and 90 µL, while T. vulgaris’ EO at 10 µL rate reduced the germination relative to the control. Interestingly, C. balsamita and H. officinalis at 10 µL did not reduce wheat germination; however, they did reduce the number of radicles per seed, radicle length (mm), seedling height (mm), germination (%), and vigor index. Furthermore, L. officinale’s EO reduced the measured indices (radicles per seed, radicle length, seedling height, and vigor index) at the 10, 30, and 90 µL application rates relative to the non-treated control; however, none of the application rates of L. officinale’s EO had a suppression effect on wheat germination. This study demonstrated the allelopathic effects of the EOs of six different herbal plant species on seed germination of barley and winter wheat. The results can be utilized in the development of commercial products for controlling pre-harvest sprouting of wheat and barley. Further research is needed to verify the results under field conditions.

Allelopathic Effects of Chrysanthemoides monilifera subsp. monilifera on Lolium rigidum in Wheat Field: Implications on the Reduction of Chemical Loads in Agro-Ecosystems

Weed control through allelopathic plants is a promising approach that may minimize many of negative consequences of synthetic herbicides. We have studied potential of Chrysanthemoides monilifera subsp. monilifera (boneseed) leaf extract for controlling growth of Lolium rigidum (annual ryegrass) in wheat (Triticum aestivum) fields. Both pre-and post-emergent ryegrass-control experiments were conducted in greenhouse using field soil. Treatments such as boneseed leaf extracts (5 and 10% for pre-emergent and 10 and 20% for post-emergent experiments) alone or as a mixture combined with different strength (¼ and ½ strength) of pre-emergent (boxer gold) and post-emergent (hussar OD) herbicides were applied on pre- and post-emergent ryegrass and wheat. The findings revealed that none of the boneseed leaf extracts alone or as mixture had significant inhibitory impact on pre-emergent ryegrass compared with herbicide alone. Although we observed significant inhibitory impacts on post-emergent ryegrass with boneseed leaf extracts alone (10 and 20%) compared with control, they were negligible compared to full strength herbicides. Mixtures had significant inhibitory impact on post-emergent ryegrass compared with herbicide alone with same doses and impact increased with herbicide concentration. Despite the greater impacts by higher herbicides concentration alone, findings suggest the use of mixture of ¼-strength herbicide and 10% boneseed leaf extract was able to control ryegrass successfully than the herbicide alone without adverse impacts on wheat. This study suggests that use of boneseed leaf extract mixed with lower doses of post-emergent herbicides may be effective in controlling ryegrass with concomitant reductions in expenses and ecological health risks linked with the practice of synthetic herbicides.

Allelopathic Properties of Hemp

Summary Allelopathic effects of different plant allelochemicals have become the focus of studies that aim to determine new pesticidal compounds. Plant extracts with different chemical compositions can be obtained using different methods of extraction, whereas studies dealing with allelopathic properties use biotests to obtain fast and precise results. Such studies were undertaken after noticing the allelopathic effects of industrial hemp (Cannabis sativa L.) on the surrounding crops. Trials that involve the application of hemp extracts obtained by different methods resulted in determining the hemp allelopathic effects on Stellaria media (L.)Scop., Beta vulgaris ssp. vulgaris var. vulgaris, Lupinus albus L., corn (Zea mays L.), Cyperus rotundus L., Matricaria recutita L., Lepidium sativum L., lettuce (Lactuca sativa L.), rapeseed (Brassica napus L.), wheat (Triticum aestivum L.), rye (Secale cereal M. Bieb.), Ambrosia artemisiifolia L., Chenopodium album L., and Sorghum halepense L..

Study of the allelopathic effects of Medicago sativa on the germination characteristics of Secale montanum

This study investigated the allelopathic effects of Medicago sativa L. on the germination characteristics of weed rye (Secale montanum L.) Guss. In order to study the allelopathic effects of different concentrations of aqueous extract (0, 10, 20 and 30%) of different M. sativa parts (root, leaves, stem and aerial parts of the plant including (stems, leaves and flowers) on initial growth of rye (S. montanum) seedlings a factorial experiment arranged in RCB design was carried out with three replications in the greenhouse of Agricultural College, Mahabad Islamic Azad University in 2017. The greenhouse data showed that the plant height, root and biomass dry weight of the weed decreased so that the declining trend had a significant effect (p≤0.05) on all traits of rye studied in the present experiment. Among the aqueous extracts of M. sativa that of the root had the greatest allelopathic potential.

Allelopathic effects of Epichloë fungal endophytes: experiment and meta-analysis

Host-specific Epichloë spp. endophytic fungal symbionts of pooid grasses that produce herbivore-deterring alkaloids and alter the grass host's metabolite and protein profiles. Early observations suggested that Epichloë may have negative allelopathic effects on neighbouring plant species, particularly Trifolium spp. clovers, but subsequent allelopathy tests produced variable results. We examined two hypotheses: (1) Epichloë strains differ in allelopathic effect, and (2) Epichloë allelopathy negatively affects other plant species. We performed a greenhouse experiment using root exudates from Lolium perenne L. hosting different E. festucae var. lolii (Latch, M.J. Chr. & Samuels) C.W. Bacon & Schardl strains to compare their allelopathic effects on native legumes and forbs. We then used meta-analysis to examine the evidence to date for allelopathic effects of Epichloë endophytes. We found little difference in effect among E. festucae var. lolii strains and very little evidence for negative allelopathic effects of Epichloë in cool-season grasses across a range of methodologies, target plant species, and response measures. Negative allelopathic effects were detected only for root hair measures, which were from a single study. Positive effects on biomass were found for some experimental subgroups, including legumes. However, the majority of response variables showed no evidence for Epichloë allelopathy. Although there is currently little evidence for negative Epichloë allelopathic effects, our meta-analysis identified several research gaps. Experiments testing the functional belowground effects of Epichloë presence may help to determine its effects on non-host plant performance via plant-soil feedbacks.