Allelopathic Effects of Aqueous Extracts from Sorghum (Sorghum bicolor L.) and Russian Knapweed (Acroptilon repens L.) on Seedling Growth and Enzymes Activity of Wheat, Sugar beet, Common Lambsquarters and Redroot Pigweed

The continued use of synthetic herbicides has resulted in herbicide-resistant weeds as well as in negative impacts upon human health and the environment. Nowadays, agricultural techniques are focused on sustainable agricultural production. Allelopathy is defined as any direct or indirect positive or negative effect of one plant on the other plant species through the release of chemicals into the environment. It plays a significant role in agroecosystems, and affects the growth, quality and quantity of the produce. A number of plant species have been reported to have an allelopathic effect on other plant species. Allelochemicals produced by one crop species can influence the growth, productivity, and yield of other crops. Sorghum (Sorghum bicolor L.) is reported as one of the most allelopathic crops used extensively as cover and smother crops and is also incorporated into the soil for weed suppression. Russian knapweed (Acroptilon repens L.) has also been shown to produce phytotoxic compounds, which may contribute to its competitive behavior. Studies of the extract from Russian knapweed has suggested the presence of plant growth inhibitors.
Material and In order to evaluate the response of wheat (Triticum aestivum L.), sugar beet (Beta vulgaris L.), common lambsquarters (Chenopodium album L.) and redroot pigweed (Amaranthus retroflexus L.) to allelopathic effects of shoot aqueous extract of sorghum (Sorghum bicolor L.) and Russian knapweed (Acroptilon repens L.), a greenhouse experiment as factorial experiment based on randomized completely design with three replications was conducted in Faculty of Agriculture, University of Maragheh in 2016. The factors were included shoot aqueous extract of sorghum and Russian Knapweed and extract concentrations at 0 (distilled water as control), 5, 10 and 20% (m/v). In order to make the required aqueous extract, the maceration method was used according to the previously described method with some modifications. Briefly, the aerial parts of sorghum and Russian knapweed were dried under shade and powdered mechanically. For making the stock extract, 50, 100 and 200 g of the powdered plants were added to 1 l of distilled water and was placed in a closed container for 48 h with frequent agitation until the soluble matter was dissolved. The extract was filtered through Whatman filter paper. The concentration of the resulting extract was 5, 10 and 20% (v/v).
Seedlings growth was measured in terms of shoot fresh and dry weight and root fresh and dry weight. The lowest shoot dry and fresh weight were observed in 20% extract concentration of Russian knapweed and sorghum. Growth indices loss in redroot pigweed was greater with application of 20% extract of sorghum. However, growth loss in wheat, sugar beet and common lambsquarters was greater with application by extract of Russian knapweed. The results revealed that the sorghum and Russian knapweed aqueous extract had stimulatory effect on the proline content of the seedlings. Enhanced proline along with increase in water extract concentration suggests an allelochemical induced stress. Accumulation of proline indicates cellular damage in the target tissue caused by the ROS generated by peppermint allelochemicals. The present study concludes that peppermint allelochemicals induce oxidative stress in tomato through generation of ROS and upregulation of the activities of some scavenging enzymes. This result is in agreement with the findings of Batish et al. (8). They noted that proline enhances tolerance and provides protection against abiotic stress by avoiding ROS-induced damage to photosystems, membranes and proteins. Different concentrations of water extract had significant effect on the sugar contents of crops seedling. In comparison with the control, the sugar contents was generally increased in all treatments. Increase in sugar content is an indication of reduction in the activity of some respiratory enzymes and reduced consumption of sugar in low-growing plants. These results are in line with the findings of Abdulghader et al. (2008) where the level of soluble sugar in radish leaves was increased by heliotrope allelochemicals. In contrast, the soluble sugar content of seeds was reduced in the presence of the leaf litter leachates of some selected tree species. Our results showed that, by increasing concentration of the extract, there was an increase in the activity of antioxidant enzymes in the aerial parts of crops. This indicates that oxidative stress could play a role in phytotoxic phenomenon. Any increase in the activity of antioxidant enzymes demonstrates that sorghum and Russian knapweed water extract exposure causes excessive generation of O2-, resulting in oxidative stress. Increased activity of these scavenging enzymes can be due to the induction of secondary defensive mechanism against oxidative stress caused by sorghum and Russian knapweed allelochemicals. Allelochemicals absorbed by plant cells should be detoxified. The detoxification and the response of plant cells to it result in increased activity of antioxidant enzymes.
Application of aqueous extract of sorghum and Russian knapweed at 20% concentration increased the activity of catalase, peroxidase, polyphenol oxidase enzymes and proline concentration by 100.85, 62.02, 24.94 and 143.61%, respectively over control. On basis of the decrease in seedling growth and increase in activity of antioxidant enzymes, it could therefore, be concluded that sorghum and Russian Knapweed shoot aqueous extract as bioherbicide may help control growth of weeds.