Allelopathic effects of Xanthium strumarium L. on germination and seedling growth of Mung bean (Vigna radiata L. Wilczek)

Xanthium strumarium (Cocklebur) is a genus of flowering plants in the family Asteraceae, native to the America, Eastern Asia, South Africa and Australia. It is coarse, herbaceous annual plant growing to 50-120 cm tall, which invades agricultural lands. Its seedlings and seeds contain the glycoside carboxyatractyloside and can be poisonous to animals, including cattle, horses and pigs. Its successful spread in so many parts of world has mainly attributed to its allelopathic properties, which enable it to complete effectively with native crops and/or pasture species. Vigna radiata is an important food legume cultivated in rainfed areas in the west and northwest of Iran. One of the main reasons of its lower yield is competition from weeds, because it does not compete well with weeds. Thus, weeds reduce yield and quality of mungbean through competition for light, moisture and nutrient. This study was aimed at determining the possible effects of the widespread weed X. strumarium L. whole plant A.E. on germination and seedling growth of mungbean under laboratory and greenhouse conditions.

Naturally growing common cocklebur plants around our university campus were randomly uprooted and collected during flowering stage in August 2018. The plants were immediately brought to laboratory and the whole plant cut into 2 to 3 cm pieces, air-dried in greenhouse and ground in a mill of 2 mm sieve size. Then 3, 6 and 9 g of ground materials were mixed in 100 ml distilled water and soaked for 24 h at room temperature (21 to 22 oC) and filtered through four layers of cheesecloth. The aqueous extracts were stored in conical flasks in dark until use. Both in laboratory bioassays or greenhouse studies were done from August to December, 2018 and the rates of 90, 180 and 270 g powdered material or 3, 6 and 9% extracts were based on the previous studies.  

Results showed that mean rate of germination in control treatment was higher than that of other treatments, but there was not any significant difference between control and 3% treatment. The lowest value for this trait belonged to aqueous extract i.e. 9%. The increase in X. strumarium L. A.E. concentration was concomitant with decrease of the mean rate of mungbean seed germination. In this case, there was a negative linear regression between A.E. concentration and plumule length as well. It is noteworthy that decrease in germination rate led to a delayed emergence and poor establishment of mungbean seedlings. X. strumarium A.E. did not adversely influence plumule dry weight of mungbean in the 3% A.E. treatment, compared to the control. The A.E. concentrations of 6 and 9% resulted in significant reduction of 20.87 and 30.97% in plumule length compared with those of the control, respectively. The soil incorporation of dry mix residues decreased both germination and seedling growth of mungbean. There was an increase in inhibitory effect with increase in amount of dry residues incorporated. However, no significant difference was observed between control and 90 g for both germination (%) and rate, and root and shoot lengths. Compared to soil incorporated dry mix residues, the soil surface applied dry whole plant residues drastically inhibited the mungbean germination and seedling growth. This is congruent with previous report. All rates of soil surface placed X. strumarium dry mix residues significantly inhibited the mungbean seedling emergence (%) and rate and root and shoot lengths over the control. 

High concentrations of X. strumarium L. A.E. negatively influenced seed germination rate, plumule length and seedling dry weight of mungbean. However, their effects were not considerable at low concentrations. In conclusion, we can claim that mismanagement of weed population in field condition especially high densities of X. strumarium L. weed and presence of high concentrations of water soluble chemicals produced by this weed adversely influence early growth (delayed emergence and dwarf and weak seedlings) of mungbean plants. Eventually, this trend negatively influences homogenous emergence of plants and the result would be plants susceptible to biotic and abiotic stresses, leading to very low marketable yields.