The effect of salicylic acid foliar application on improving shoot and root growth of tomato (Lycopersicon esculentum L.) under cadmium stress conditions

Introduction Heavy metals are non-biodegradable and persistent in nature thereby disrupting the environment and causing huge health threats to humans. Cadmium (Cd) is a toxic heavy metal that enters the environment through various anthropogenic sources, and inhibits plant growth and development. Cadmium toxicity may result from disturbance in plant metabolism as a consequence of disturbance in the uptake and translocation of mineral nutrients. The use of plant hormones has been introduced as a simple and suitable strategy to reduce the effect of heavy metals in plants. A new method for reducing the effect of cadmium on plants is the use of growth regulators such as salicylic acid (SA). Tomato (Lycopersicon esculentum) is an important vegetable that is rich in minerals, various vitamins and antioxidant compounds. Since the cultivation of tomatoe, as an important and highly productive crop, is very common in many parts of Iran, there is a possibility of contamination of the soil under cultivation with cadmium. Therefore, the present study was conducted to evaluate the the effects of foliar spraying of salicylic acid on reducing Cd-induced stress in tomato (Lycopersicon esculentum L.). Materials and methods A factorial experiment based on a complete randomized design with three replications was conducted in a greenhouse of Razi University. Factors included Cd at three levels (0, 15 and 30 mg kg-1 as Cd(NO3)2) and SA at three levels (0, 250, and 500 μM). The solutions containing Cd-(NO3)2 were sprayed uniformly on the sub-samples separated from the original soil sample according to the desired concentrations. The samples were kept moist for 30 days at moisture conditions close to field capacity. At the four-leaf stage, SA solutions were sprayed three times on the foliage of the plants, until the beginning of flowering. After harvesting, some characteristics including soluble sugars and proline contents, plant height, dry weights of shoots and roots, and root volume and length were determined. All plant parameters were then averaged for each pot. Also, Cd concentrations in extracts obtained from the digestion of leaf tissues, were measured by Varian AA220 atomic absorption spectrophotometer. The analysis of variance (ANOVA) and comparison of means (Duncan's multiple range test) were performed using SPSS-16 software. Results and discussion The results revealed that Cd stress reduced all plant characteristics, such as plant height, root volume and root length, as well as, dry weights of shoots and roots, and elevated leaf Cd concentration, proline content and soluble sugars in tomato. However, the SA application resulted in improvements in growth parameters. Also, the results showed that the interaction effects of Cd and SA on the most growth characteristics such as plant length, shoot and root dry weights, stem diameter, leaf area, and proline content, soluble sugars and Cd concentration were significant (P < 0.01). The highest amount of soluble sugars (0.48 mg kg-1), proline content (26.3 mmol g-1) and Cd concentration (0.685 mg) were obtained in 30 mg kg-1 of Cd and 0 μM SA. Also, the highest amount of soluble sugars (0.53 mg kg-1), plant length (44.6 cm), root length (19.6 cm), shoot dry weight (7.51 g), and leaf area (268.2 cm2) were found in the treatment of 500 μM SA and 0 mg kg-1 Cd. Conclusion The application of salicylic acid effectively increased the all measured growth parameters, including plant biomass, and total root volume. Cd stress reduced growth indices and, increased proline content and soluble sugars in tomato. Also, it seems that under Cd-induced stress, SA is an effective approach for improving crop growth by increasing plant resistance. In general, the application of appropriate concentration of salicylic acid (500 μM), as a plant hormone, is an effective, simple and low-cost strategy to reduce the adverse effects of Cd (30 mg kg-1 soil) stress in tomato.