Dragon spurge (Euphorbia dracunculoides L.) is a leading chickpea weed, causing enormous crop losses. Chemical herbicides, such as as pendimethalin and s-metalochlor, though capable of providing fairly good control of dragon spurge, are phytotoxic to chickpea, causing chlorosis – a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green colour of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plants have little or no ability to manufacture carbohydrates through photosynthesis and may die, unless the cause of the chlorophyll insufficiency is treated. In chickpea, chlorosis is also manifested in reduced plant height and reduced root and shoot biomass.
There is growing consensus that synthetic herbicides cause declining species diversity in agro-ecosystem, and can lead to both shifts in weed population and herbicide resistance among previously susceptible weed species. Selective but equally or more effective environmentally-safe herbicides for weed control can come from plant derived materials.
A team of researchers, from the Department of Agronomy, University of Agriculture, Faisalabad, Pakistan (Abdul Khaliq, Amar Matloob, Asif Tanveer, RN Abbas and M. B.Khan), have recently published research indicating that sorghum can potentially provide an effective bio-herbicide for the control of dragon spurge. If effective in field conditions, this would be a boon for chickpea farmers and the environment in general - a possibility even more signoficant given that both sorghum and chickpea are important dryland crops.
Chickpea is the world’s second-largest cultivated food legume. Initially, it grows very slowly and is a poor weed competitor. It is currently grown on over 11 million hectares in the Mediterranean, western Asia, South Asia, Australia and Sub-Saharan Africa, especially in Eastern and Southern Africa. World chickpea production is well over 9 million tonnes. 96% cultivation is in developing countries. India is the world’s largest producer and consumer of chickpea, accounting for over 66% of world production. Other major producers are Pakistan, Turkey, Iran, Myanmar, and Canada. Ethiopia and Kenya are the leading producers in Sub-Saharan Africa.
Allelopathy is the biological process by which an organism produces one or more biochemicals, known as known as allelochemicals, which influence the growth, survival, and reproduction of other organisms. These biochemicals can have beneficial or detrimental effects on the target organisms. Allelopathy can be exploited for natural weed management in agro-ecosystems. Allelopathic interactions are species (donor and receiver) specific and concentration dependent. Sorghum (Sorghum bicolor) produces phytotoxic allelochemicals that can inhibit weeds. The allelopathic activity of sorghum is selective and concentration-dependent.
In undertaking the research, field-grown sorghum plants were harvested at maturity, dried under shade, chopped, and re-dried in an oven at about 70 °C for 48 hrs. The oven-dried plant samples were then ground and passed through a mesh, soaked in distilled water for 24 hours at a ratio of 1g per 10ml. The solution was then filtered and diluted at 25, 50 and 75% for testing on samples of dragon spurge. Seeds of the weed were collected from field-grown mature plants and cleaned manually to ensure their purity. They were surface-sterilized with a 10:1 water:bleach solution and rinsed with distilled water. Some seeds were then set to germinate in petri dishes on filter paper treated with 25, 50, 75 and 100% concentrations of the sorghum extract, with control samples similarly treated with distilled water.
The sorghum aqueous extract inhibited the germination and seedling growth of the treated weed seeds. The inhibitory activity varied with extract concentration. Concentrations above 25% delayed the weed seed germination more than twofold and at 100% extract concentration, the proportion of seeds that germinated was 12% - i.e. final seed germination was inhibited by 88%. The seedling growth was also inhibited by the sorghum extract at concentrations above 25%, with a progressive reduction in seedling shoot-length with increasing concentrations of the extract. The root length was also inhibited by 15-42%. The inhibitory effect at all concentrations was more evident on root length than on shoot length. Morphological disorders such as root twisting and distorting were also observed.
The activity of different allelochemicals is often modified by their joint action, and allelochemicals present in different plant extracts can exhibit synergism – different extracts can hence be combined to further enhance their efficacy. As part of the same research, the team also tested an aqueous extract of sunflower (Helianthus annuus L.). They established that the sunflower extract had similar inhibitory effects on dragon spurge as the sorghum extract, but was more effective at lower concentrations. The extracts were however more effective in combination at all concentrations than when tested individually. The variable influence of the individual sorghum and sunflower extracts on dragon spurge seed germination and seedling growth may be explained by differences in the types of the allelochemicals the contain and their concentration. The higher effectiveness of the combined extracts shows synergism between these chemicals. The combination can be used as bio-herbicide for management of this weed, provided that these results are reproducible under field conditions – which can only be established field testing.
The study is published in the Pakistan Journal of Weed Science Research under the title Bio-herbicidal properties of sorghum and sunflower aqueous extracts against germination and seedling growth of dragon spurge.