عنوان مقاله [English]
Throughout their life cycle, plants are subjected to various types of environmental stresses which include salinity, water deficit, temperature extremes, toxic metal ion concentration and UV radiation. These environmental factors limit the growth and productivity of plants to various degrees, depending upon severity of stress. Heavy metal toxicity is one of the major current environmental health problems and potential dangerous due to bioaccumulation through the food chain and in plant products for human consumption. Cadmium (Cd) is a high toxic trace element that enters the environment mainly from industrial processes and phosphate fertilizers. It can reach high levels in agricultural soil and is easily accumulated in plants. Cd ions are taken up readily by the plant roots and translocated to the above-ground vegetative parts. The presence of Cd at higher concentrations in the soil damages root tips, reduces nutrient and water uptake, impairs photosynthesis and inhibits growth of the plants. Furthermore, Cd directly or indirectly induces reactive oxygen species (ROS), which affect the redox status of the cell and cause oxidative damage to proteins, lipids, and other biomolecules. Cd damages the nucleoli in cells of the root tip, alters the synthesis of RNA, inhibits ribonuclease activity and inhibits the DNA repairing mechanism. Understanding the biochemical and molecular responses to Cd stress is essential for a holistic opinion of plant resistance mechanisms of heavy metal stress.
Materials and Methods
In this study, the effect of different concentrations of cadmium on proline and phenolic contents, activity of some antioxidative enzymes such as superoxide dismutase, catalase and ascorbate peroxidase, phenylalanine ammonialyase in Lense culinaris Medik. was studied. Therefore, two-week-old plantlets were treated with different concentrations of cadmium chloride (0 as control, 0.125, 0.25, 0.5, 1, 2.5 and 5 mM) of cadmium chloride for 10 days and then the above mentioned factors were investigated. The experiments were carried out by using a complete randomized block design with three replications. The statistical analyses were carried out using the SAS version 9. Changes in biochemical parameters were tested statistically by performing one-way analysis of variance (ANOVA). The treatment means separated using Duncan's multiple-range test (DMRT) taking P < 0.05 as significant.
Results and Discussion
Surveying the results indicated that the amount of non-enzymatic antioxidant compounds (phenol and proline) were increased in treated seedlings compared with the control. Review of the literature indicates that a stressful environment results in an overproduction of proline in plants which in turn imparts stress tolerance by maintaining cell turgor or osmotic balance; stabilizing membranes thereby preventing electrolyte leakage; and bringing concentrations of ROS within normal ranges, thus preventing oxidative burst in plants. The increase in phenolic content may be due to protective function of these compounds against heavy metal stress by metal chelation and ROS scavenging. The results obtained from measuring the activity of antioxidant enzymes showed that with increasing concentrations of cadmium, the activity of antioxidant enzymes superoxide dismutase, catalase and ascorbate peroxidase were increased in lentil seedlings. However, this increase was more evident at concentrations greater than 0.5 mM. These results suggest that lentil seedlings tend to cope with free radicals generated by Cd through coordinated, enhanced activities of the antioxidative enzymes involved in detoxification. With increasing levels of cadmium in the medium, the activity of the phenylalanine ammonialyase also increased. The trend of phenylalanine ammonialyase activity induction in response to different concentrations of cadmium, was similar to those of antioxidant enzymes. Although the induction of activity of this enzyme was higher than the antioxidant enzymes. PAL is a key enzyme in the phenolic metabolism that has been reported to protect plants against stress conditions via synthesizing various phenylpropanoid products such as simple phenols, anthocyanin, flavonoid, and lignin.
The present study demonstrated that antioxidative system in L. culinaris underwent biochemical changes to survive under high concentrations of cadmium. Increase in metal chelate components (free phenols and proline) in all treatment levels proves this fact. Proline and phenolic compounds produced by the Phenylpropanoid Pathway in lentil played a major role in its response to Cd stress. Also that prolonged stress induced by Cd concentrations, can result into the activation of antioxidant enzymes such as superoxide dismutase, catalase and ascorbate peroxidase.
Key words: Antioxidant enzymes, Cadmium, Lentil, Phenylalanine ammonialyase, Total phenolics