The Effect of Milk Thistle (Silybum marianum L.) Extract on Growth and Enzyme Activities of Common Cocklebur (Xanthium strumarium L.) Seedling

Abstract

Introduction Allelopathy means any process involving the secondary metabolites (allelochemicals) produced by plants, algae, bacteria, and fungi (excluding animals) that influences the growth and development of agricultural and biological systems, with positive or negative effects. Plants produce secondary metabolites and in certain circumstances, these can act as phytotoxins, inhibiting or promoting some biochemical or physiological processes in the other plants or organisms. Toxicity of allelochemical compounds include effects on growth, chlorophyll content, photosynthesis, transpiration, stomatal conductance, relative growth rate, and oxygen uptake in many crops. Furthermore, the influence of allelochemicals on alpha amylase, oxidase, sucrose synthase activity and malon dialdehyde concentration were analysed. The role of numerous plant secondary metabolites is still unclear, and this raises curiosity for both plant physiologist and ecologists. Inhibitory effects on germination and establishments of crops caused by residues of either crops or weeds have lead to investigation of the release of toxic compounds from such residues. This present study was designed to evaluate the allelopathic potential of milk thistle for controlling common cocklebur in vegetables. Materials and Methods In order to investigate the effect of milk thistle aqueous extract on growth and enzyme activities of common cocklebur seedlings, two separate experiments were carried out in randomized complete design and randomized complete block designs with five and four replications, respectively, in spring 2012 in seed technology laboratory and greenhouse of Azad University, Shoushtar branch, Shoushtar, Iran. Milk thistle aqueous extract concentrations were involved (0, 5, 10 and 15% (v/v)) in petridish and (0, 10, 20 and 30% (v/v)) as spraying on common cocklebur seedlings under greenhouse conditions. Whole plants of milk thistle were gathered from medicine plant farm of Azad University. Then, their flowers were cut and their shoots were dried in 60 °C and grinded. For preparing aqueous extract 100 g powder of milk thistle dissolved in 1000 ml distilled water and maintained at 20 °C. The solution was filtered and cleared. The aqueous extract was considered as stock and other aqueous extracts were prepared from it. After application extract concentration in petridish and pot, seedling fresh weight, activity of sucrose synthase, alpha amylase, catalase enzymes and malon dealdehyde concentration of common cocklebur were measured. Statistical calculations were accomplished through the SPSS software and graphs were drawn by the Excel software. Results and Discussion Results showed that fresh weight of common cocklebur seedlings were affected by milk thistle extract. Milk thistle extract reduced alpha amylase enzyme activity and seedling growth of common cocklebur. In addition, increase in aqueous extract of milk thistle resulted in reduction and increase in catalase and malon dialdehyde concentration in cocklebur seedling; respectively. The minimum alpha amylase enzyme activity (2.1 nM/g seed/min) and seedling fresh weight (0.32 g) were observed in 15% aqueous extract. Malon dialdehyde concentration of cocklebur seedlings was 0.4 (nM/g FW) in 15% aqueous extract of milk thistle. Catalase enzyme activity was the lowest (1.4 nM H2O2/mg Protein/min) and the greatest (27 protein absorption/60s) in 15% aqueous extract of milk thistle. In greenhouse experiment, also, with increasing aqueous extract of milk thistle was reduced seedling fresh weight, catalase and sucrose synthase enzyme activities. Catalase and sucrose enzyme activities were 1.8 (nM H2O2/mg Protein/min) and 2 (nM/ mg protein/min) at 30% aqueous extract of milk thistle. Futhermore, increase in aqueous extract of milk thistle resulting in increase cell memberane destruction and malon dialdehyde concentration in cocklebur seedling tissue. The lowest (0.0029 nM/g FW) and greatest (0.93 nM/g FW) malon dialdehyde concentration was at 30% aqueous extract. Conclusions In this study, with increasing the amount of milk thistle extract, values of measured traits including fresh weight, sucrose synthase and catalase enzymes of cocklebur seedlings compared to control treatment (distilled water) have a decreasing trend, but malon dialdehyde concentration of cocklebur seedlings had increased compared to the control. The destruction of cell membranes under the influence of cocklebur allelopathic compounds can be one reason for the reduced growth of weed seedlings. Reduced sucrose synthase activity, led to reduced production of sucrose that was associated with reduced seedling growth. The present study confirmed that milk thistle can be used as an allelopathic plant for weed suppression in agroecosystems through its release of allelochemicals. Knowledge about the challenges related to the demonstration of allelopathy, as an ecological significant mechanism, is important in the assessment of ecological effects of allelopathic plants.

Keywords

• Antioxidant
• Alpha amylase
• Catalase
• Malon dialdehyde
• Sucrose synthase