Allelopathic Effects of Sorghum (Sorghum bicolor L.) and Russian Knapweed (Acroptilon repens L.) Aqueous Extract on Seed Germination Indices and Enzyme Activity of Some Field Crops and Weeds

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Introduction: The loss of crop yield due to weeds is enormous. Potential yield reductions caused by uncontrolled weed growth throughout a growing season have an estimated range of 45-95%, depending on ecological and climatic conditions. Overusing synthetic herbicides may affect the environmental, human health and food. Furthermore, increasing use of herbicides has resulted in a dramatic increment in the herbicide resistance among weeds, and over 307 weed resistant biotype belonging to 183 species (110 dicots and 73 monocots) have been identified worldwide. Cultivating crops with allelopathic potential can reduce the dependency on synthetic herbicides and increase crop yields. Sorghum (Sorghum bicolor L.) has been reported as one of the most allelopathic crops extensively used as cover and smother crops and also incorporated in the soil for weed suppression. Moreover, Russian knapweed (Acroptilon repens L.) has been shown to produce phytotoxic compounds and plant growth inhibitors, which may contribute to its competitive behavior. The phenomenon of allelopathy has been suggested to be one of the possible alternatives for achieving sustainable weed management. Materials and Methods: In order to evaluate the allelopathic effects of sorghum (Sorghum bicolor L.) and Russian knapweed (Acroptilon repens L.) extract on seed germination indices and enzymatic activity of wheat (Triticum aestivum L.), sugar beet (Beta vulgaris L.), common lambsquarters (Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.), an experiment was conducted as factorial experiment based on randomized completely design with three replications in Faculty of Agriculture, University of Maragheh in 2016. The investigated factors were four crops (wheat, sugar beet, common lambsquarters and redroot pigweed), shoot aqueous extract (sorghum and Russian Knapweed) and aqueous extract concentrations at 0 (control), 5, 10 and 20% (m/v). Sorghum and Russian knapweed crops were collected during July 2016. The plants were taken to the laboratory and kept fresh in a refrigerator. In addition, Kato-Noguchi et al., (17) method was followed for extraction. Samples of the above-ground tissues (stems and leaves) of sorghum and Russian knapweed were washed thoroughly with tap water and rinsed with distilled water. They were clipped and then passed through a 1 mm screen before storage in a refrigerator at 2 ℃. Each 50, 100 and 200 g samples was extracted by soaking it in 1 L of distilled water at 24 ℃ during 24 h in a shaker. All extracts were filtered through two layers of cheese cloth to remove fibers. Distilled water was also considered as the control treatment. Results and Discussion: Results indicated that percentage and rate of germination, radicle and plumule length, radicle dry weight, plumule dry weight, seedling dry weight and vigor length index of all crops decreased significantly by increasing aqueous extract concentration of sorghum and knapweed. These results are in agreement with those previously reported that the degree of inhibition increased by increasing extract concentration. Using Russian knapweed aqueous extract, germination percentage was 12.62 percent greater than sorghum aqueous extract. Moreover, inhibitory effect of sorghum aqueous extract on germination rate loss was higher than that of aqueous extract of Russian knapweed. As to field crops, inhibitory effect of sorghum and Russian knapweed extract on sugar beet was higher than wheat. Moreover, the effect of sorghum and knapweed extract on radicle, plumule and seedling fresh weight loss of redroot pigweed and common lambsquarters (weeds) was greater as compared with that on field crops (wheat and sugar beet). Increasing concentration of sorghum extract decreased seed vigor indices of common lambsquarters and redroot pigweed by 83.36 and 87.15% relative to control, respectively. Activity of peroxidase and polyphenoloxidase enzymes was reduced and increased, respectively, as a result of an increment in extract concentration. Batish et al. (7) have demonstrated that parthenin (Parthenium hysterophorus) impairs mung bean growth by affecting respiration, protein content and activities of protease and peroxidase enzymes. The minimum value of plumule and seedling length was observed for redroot pigweed and common lambsquarters weeds with application of 20% aqueous extract of sorghum and Russian knapweed. The values of weed tolerance index (WTI) revealed that redroot pigweed and common lambsquarters were more sensitive to aqueous extract of sorghum and Russian knapweed. The WTI combines several individual measured parameters and ranges between 0 and 1. In this case, a low WTI indicates a high susceptibility to biotic or abiotic stress originated from crop extracts. Conclusions: Our results showed that aqueous extracts of sorghum and Russian knapweed had an inhibitory effect on seed germination indices of wheat, sugar beet, common lambsquarters and redroot pigweed. It can be also concluded that sorghum and Russian knapweed possess weed-suppressing ability and seem to be useful for developing natural herbicides.