Assessment of the Allelopathic Potential of Extracts of Lavendula officinalis

Human has always introduced various methods to control weeds. One of the best and the most modern methods is to use synthetic herbicides. The synthetic herbicides offer the same advantages compared to the other methods. They are selective, easy to apply, act quickly, relatively inexpensive, and can be used where other methods don’t work well. Therefore, their application have increased dramatically in the agricultural world and generated a series of additional problems such as herbicide resistance in weeds, soil and water pollution and the toxicity effects on human health and non-target species. With increasing global awareness about these disadvantages, the herbicidal potential of natural compound are focused because they have a short half-life and novel sites of action. Many natural compounds taken from many plant species, especially they are aromatic ones, have been studied for their herbicidal potential. For example, sorgoleone, isolated from Sorghum sp.; ailanthone, isolated from Ailanthus sp.; artemisinin, isolated from Artemisia sp.; coumarin, isolated from Ruta sp. and so on (5). The great interests in this field caused that the molecule of leptospermone isolated from Callistemon citrinus was optimized and then commercialized as mesotrione and sulcotrione herbicides (group F2/27) that inhibit p-hydroxyphenyl pyruvate dioxygenase (HPPD), a key enzyme in the biosynthesis of carotenoid and plastoquinone (9). Lavender (Lavandula sp.) is a medicinal plant with allelopathic properties which belongs to the Lamiacae family. The substantial components of lavender are made up of coumarin. Despite this, 18 structural analogues of coumarin extracted from lavender, coumarin itself had the most phytotoxic on Lolium sp. (9). The antibacterial, antifungal, insecticidal and herbicidal activities were demonstrated by the lavender essence. The current research aimed to determine the allelopathic potential of lavender essence on the germination indices of six plant species.
This study was separately and simultaneously carried out in 6 experiments based on a completely randomized design with four replications, which each one involved with one plant species. In each experiment, the influence of eight concentrations of coumarin including zero (control), 4, 20, 40, 100, 200, 300, and 400 ppm extract were assessed on germination and seedling growth of one plant species. There was 245 ppm coumarin from 100 g leaf of lavender. Weed species included redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), barnyard grass (Echinochloa cruss-gali (L.) Moench), johnson grass (Sorghum halepense (L.) Pers.), common purslane (Portulaca oleracea L.); and corn crop (Zea mays L. cv. Sc 704). The seeds of all weeds were collected from weeds which were around fields located near Mashhad, Iran. The seeds of maize were obtained from the Agricultural and Natural Resources Research Center of Mashhad, Iran. Before the start of experiment, the seed dormancy-breaking treatments were conducted to increase the seed coat permeability of weed species. Hence, the seeds of A. retroflexus, C. album, E. cruss-gali, S. halepnse, and P. oleracea were acid-scarified in concentrated sulfuric acid (98%) for 5, 1, 3, 4 and 1 min, respectively; and then rinsed with distilled water. Twenty seeds of each plant species were placed in Petri dish on two layer of filter paper. Petri dishes were transferred into incubator after 10 ml of treatment solutions were added into them. Daily observations were made on the number of seeds germinated in each dish. After 12 days, root and shoot length was measured. Then, they were harvested and oven-dried at 75ºC for a period of 48 h and the dry weight was determined. The data were changed to individual plant and subjected to analyze by both the variance analysis using PROC GLM in SAS software.
Despite the differences between studied plants, the results indicated a high toxicity of lavender extract. All components of germination in all plants were significantly affected by lavender extract. Totally, allelopathic effects of extract from lavender increased with increasing its concentration and caused the germination percentage, the germination rate, the root and shoot length, and the seedling dry weight decreased. Low concentrations of lavender extract led to a stimulatory effect on seed germination common purslane, barnyard grass, johnson grass and corn. The results of this study showed that coumarin has the phytotoxic effect against Z. mays, S. halepnse, E. cruss-gali, A. retroflexus, C. album, and P. oleracea. Generally, Z. mays and P. oleracea were most tolerant and sensitive to coumarin.
The present study concludes that corn and common purslane had the highest and the lowest sensitivity to lavender extract, respectively. However, further studies are required to identify a selectivity index, using dose-response experiments to develop a selective or non-selective natural herbicide.