Chitosan Nano-particles Application on Growth, Physiology and Mineral Element Contents of Milk thistle (Silybum marianum) under Salinity Stress Condition

Salinity stress is one of the most limiting factors in plant growth and physiological activities. To extend crop production in saline regions, application of plant promoting growth substances like Chitosan is necessary to modify the adverse effects of salinity. Milk thistle is an agro-medical plant that is familiar as a weed. However, Silymarin (which is produced by Milk thistle as a secondary metabolite) is one the most important material for treating such disease as; liver diseases, blood fat, diabetes, hepatitis, and cancer. Application of bio stimulants is one of the solutions for decreasing adverse effects of abiotic and biotic stresses on plants and increasing quality and quantity of plant yield production, so chitosan is a compound that induces defense mechanisms against stresses.
This experiment was conducted to evaluate the soil application of chitosan on vegetative, generative and physiological activities of Milk thistle in 2016. The experiment was conducted in Research Greenhouse of College of Agriculture the University of Tehran, Karaj, Iran. The experimental treatments were arranged as factorial based on a randomized complete block design (RCBD) with three replications. The first factor comprised of four saline irrigation water levels of control (Urban water with 1.2 dS m-1), 4, 8 and 12 dS m-1 and the second factor was application of different levels of Chitosan consisting of control (without chitosan application), 0.01, 0.05, and 0.1 percent of chitosan based on 1-kilogram dry soil weight in Rhizobags. Chitosan with high molecular weight was purchased from Sigma Aldrich. 12 seeds of Milk thistle were sown in each plastic pot (23×24 cm diameter and height) containing 8.0 kg of soil, at the depth of 2 cm. After 72 days planting, plants with a uniform growth were selected for experiments. At least three plants were randomly selected from each treatment. The plant root and shoot were cut at the base and weighed to determine the dry root and shoot weight. Samples were dried at 60οC for 48 hours, and their mean root and shoot dry weight were recorded for each treatment at each replicate. Also, Statistical calculations were done by SAS software, 9.1 version, and means were compared by Duncan multiple test with 5% probability.
The result showed salinity stress had significant effect on all of the growth, physiologic and ionic trait (p≤0.01). Mean comparisons in chitosan levels showed the maximum total biomass, leaf area meter, relative water content and membrane stability index were achieved with application of 0.05% nano chitosan. In 4 dS m-1 saline water, the highest total dry mass was obtained at 0.01% of chitosan by increasing of 21.6% compared to control (p≤0.01). In all of the salinity levels, chitosan application increased the leaf area index in compared to control (without chitosan application). The maximum leaf area index (4.264) was achieved in 0.05% chitosan concentration under non saline condition. Under 12 dS m-1 salinity, application 0.05% chitosan concentration decreased by 10.4% and 16.5% in sodium content and sodium potassium ration in shoot, respectively. The mechanism of action of chitosan on growth is not clear. It was also found that chitosan may induce a signal to synthesize plant hormones such as gibberellins and enhance growth and development by some signaling pathway related to auxin biosynthesis. Chitosan stimulates vital processes of plants on every level of biological organization, from single cells and tissues, through physiological and biochemical processes, to changes on the molecular level related to the expression of genes. The result of the experiment demonstrated the adverse effect of salinity on growth, physiological activity, and nutrient content in Milk thistle. Application of chitosan at 0.05% (v/v) in soil could modify the adverse effect of salinity on total biomass, leaf area index, relative water content, and membrane stability index; however, 0.01% chitosan concentration was more effective on photosystem II efficiency.
Based on the result of this experiment, salinity stress has negative effects on growth, physiology and mineral element contents of Milk thistle. Application of 0.01% of chitosan could modify the adverse effects of irrigation water salinity on Milk thistle. Generally, application of chitosan could decrease Na effects in the Milk thistle, so the rate of Na in the shoot and root was decreased under high levels of salinity. The present study demonstrated that chitosan can be used as an ecofriendly compound to protect milk thistle plants as well as to enhance growth and biochemical parameters under saline water.