Application of Thiourea Alleviates Adverse Effects of Heavy Metals Stress in Wheat

This research examined the role of thiourea (TU) in alleviating the adverse effects of heavy metal stress on the growth and physiological traits of wheat, cv. Azar 2 under greenhouse conditions. The pots were contaminated with metal plant smelting waste. Thiourea was also applied by two methods: seed priming and foliar spraying. Thiourea treatments were applied at seven levels, which included no application of TU, seed priming at concentrations of 500, 1000, and 1500 mg L−1, and foliar spraying at tillering stage at the same concentrations. Soil contamination with heavy metals has detrimental effects on plant functions. Under contamination conditions, the first day of seedling emergence was delayed (157%), and emergence percent (42.4%), survived seedling (24.4%), seedling’s leaf area (53.6%) and number (43.8%) and its dry weight (36.2%) decreased significantly compared to non-contaminated soil. In contrast, seed priming with TU, especially at concentrations of 1000 and 1500 mg L−1, increased seedling function and performance. Heavy metal stress also caused damage to the plant at the pre-stem elongation stage. Under stress conditions, total soluble sugars (28.6%), proline content (174.7%), electrolyte leakage(169.6%), leaf temperature (205.9%), root density (53.1%), lead (Pb), cadmium (Cd), and zinc (Zn) (229.5%) concentration were increased; while the relative water content (23.9%), leaf area (47.1%), plant tissues dry weight, root volume (14.3%), photosynthetic pigments and chlorophyll stability index (52.0%), nitrogen (N) (37.9%), phosphorus (P) (53.0%) and potassium (K) (56.4%) content decreased. Similar to seedling stage, the higher concentrations of TU had more effects on plant functions than 500 mg L−1. In most of the measured traits, foliar spraying showed better effects than to priming method. Surprisingly, root dry weight and volume had higher values with 500 mg L−1 treatment compared to higher concentrations of TU. Also, seed priming was more effective than foliar spraying for these two traits. It seems that treatments that caused more aerial development had fewer effects on the development of the underground part. Therefore, different concentrations of TU changed the photoassimilates allocation. The application of TU enhanced mineral nutrition both under stressed and non-stressed conditions, while decreasing heavy metal content in treated plants. This effect was noticeable for Zn content. The Zn content was increased by TU application with both methods under non-stress conditions, while decreased under stress conditions compared to non- application of TU treatment. Overall, thiourea (TU) enhanced wheat resistance to stress by regulating nutrient uptake and improving physiological characteristics. Our data showed that the higher concentrations of TU, especially 1000 mg L−1 and by foliar spraying method, are more effective than other concentrations and seed priming method. In addition, the results show that thiourea is a compound that, at different concentrations, has multiple effects on plant physiological performance and allometric relationships. These effects can be observed by changing the partitioning of photoassimilates into root and aerial parts, regulating the accumulation of essential elements and heavy metals in leaves, and changing the density and volume of roots. Ultimately, this research concluded that the application of TU as a growth regulator can be an effective strategy for improving wheat's resistance to heavy metal stress and increasing crop quality in contaminated soils. These findings will help farmers and researchers find better ways to address the challenges posed by heavy metal contamination in wheat and other field crops.