As a non-essential element of plants, Cd is one of the most toxic substances in heavy metal toxicity. Cd stress can induce plants to produce reactive oxygen species (ROS), leading to cell membrane peroxidation, causing disturbance of plant physiological processes and inhibiting plant growth (Xu et al. 2006; An et al. 2019). Bioaccumulation comprehensively reflects the degree of physiological damage to plants, and is a direct manifestation of plants' response to stress (Aihemaiti et al. 2020). Studies have shown that a certain concentration of Cd2+ inhibits the growth of wheat seedlings and reduces their biomass (Liu et al. 2015; Chang et al. 2017; Zhang et al. 2021; Wu et al. 2022). The results of this study showed that 50mg·L− 1 Cd2+ treatment inhibited the growth of two wheat seedlings, and the plant height, shoot and root dry weight of the two wheat seedlings decreased (Fig. 1D, 1E and 2E, 2F), which is consistent with the results of Chang et al. (2017). Foliar application of a certain concentration of EBR can promote the growth of wheat seedlings, increase the plant heights and biomass of wheat seedlings, and the effective concentration of EBR is 0.1 mg·L− 1 (Fig. 1A, 1B and 2A, 2B), our results are consistent with the results of Yin et al. (2016) as regards concerning a certain concentration of EBR can relieve cadmium on soybean seedling growth, and the different concentrations of EBR required may be due to different experimental materials. It has been reported that Cd can induce changes in root morphology and number (Qiu and Huang 2008; Rizvi and Khan 2018), in this investigation, treatment with Cd2+ alone increased the number of roots of the two varieties of wheat (Fig. 1F), which may be due to the ability of wheat to increase its absorption capacity to accommodate Cd2+ stress, while EBR treatment showed little effects on root number reduction (Fig. 1C), indicating that EBR was ineffective in this biological process. Our previous study (Shen et al. 2017) has confirmed that exogenous ABA can promote Cd accumulation in roots and reduce Cd accumulation in leaves, this study found that the effect of EBR on Cd accumulation was similar to that of ABA (Fig. 2C, 2D).
Photosynthesis is an important basic physiological activity of plants. Photosynthetic pigments are important pigment molecules for photosynthesis, and chlorophyll is the most important one (Limón-Pacheco and Gonsebatt 2009; Shang and Shen 2018). Studies have shown that excessive cadmium stress leads to plant chloroplast and pigment disintegration (Hou et al. 2007; Mano et al. 2009). The results of this study showed that there was no significant change in chlorophyll content of the two wheat cultivars after treatment with 50 mg·L− 1 Cd2+, indicating that the leaves of two varieties of wheat seedlings chlorophyll synthesis did not change much, probably because 50 mg·L− 1 Cd2+ could not inhibit or promote the chlorophyll synthesis of these two varieties of wheat (Fig. 3E). Foliar application of EBR of 0.001 or 0.01 mg·L− 1 had little effect on the chlorophyll content of LX77 and JM22 seedlings (Fig. 3A), but the EBR of 0.1 or 1.0 mg·L− 1 was sprayed on the foliage, the chlorophyll content of LX77 seedlings increased significantly. While the concentration of EBR is 1.0 mg·L− 1 which significantly reduced the chlorophyll contents of JM22 seedlings, indicating that different wheat varieties responded differently to high concentration EBR. However, the leaf chlorophyll contents of JM22 are always higher than LX77, indicating that JM22 may be have an efficient mechanism of chlorophyll synthesis.
The vitality level and growth of roots directly affect the nutrient status and yield of above-ground parts (Li et al. 2016b), and root vigor is one of the key indicators of roots to characterize the roots of plants (Noctor et al. 2002). The results showed that the root vigor of different varieties of wheat seedlings had different responses to 50mg·L− 1 Cd2+ treatment (Fig. 3F). The root vigor of LX77 seedlings decreased significantly, and the root vigor of JM22 seedlings did not change significantly. It indicated that the roots of JM22 wheat seedlings were more resistant to Cd2+ stress. After spraying a certain concentration of EBR, the root vigor of LX77 seedlings decreased first and then increased, and increased significantly when the EBR concentrations were 0.1 or 1 mg·L− 1, the root vigor of JM22 with the similar effects (Fig. 3B), indicating that the foliar application of EBR can improve the root vigor, similarly biological functions of GA and SADH in grape shoot vigor ( Ibrahim et al. 2007).
Osmotic regulation is a physiological response of plants to adapt to stress, and soluble proteins and sugars are important osmotic adjustment substances (Liu et al. 2015). This study found that 50mg·L− 1 Cd2+ treatment alone inhibited the synthesis of soluble protein in LX77 seedlings, and promoted the synthesis of soluble protein in JM22 seedlings (Fig. 3G), indicating that the sensitivity of different wheat varieties to Cd2+ stress are different. After spraying EBR with different concentrations, the soluble protein content of the two wheat cultivars increased first and then decreased (Fig. 3C), and reached the maximum when the EBR concentration was 0.1 mg·L− 1, indicating that a certain concentration of EBR could promote soluble protein synthesis of wheat seedlings under Cd stress. This study showed that 50mg·L− 1 Cd2+ treatment promoted the synthesis of soluble sugars in two varieties of wheat seedlings (Fig. 3H), which increased the soluble sugar content of both varieties. Foliar application of different concentrations of EBR could not increase the soluble sugar contents of LX77 seedlings, but promoted the increase of soluble sugar content of JM22. It is suggested that the soluble sugar synthesis in different varieties of wheat seedlings has different sensitivity to EBR.
AsA and GSH are the main antioxidants in plants, which can eliminate excess ROS produced during cell metabolism and reduce the damage caused by membrane lipid peroxidation (Sayed and Gadallah 2019). It also plays an important role in free radical scavenging reactions in plants (Noctor et al. 2002; Huang et al. 2020). The results of this study showed that the content of AsA and GSH in the two wheat cultivars decreased significantly after treatment with 50mg·L− 1Cd2+ alone (Fig. 4F, 4G), and the decrease level of JM22 was smaller than LX77, indicating that the resistance of JM22 to Cd2+ stress was stronger than that of LX77. After spraying EBR at different concentrations, the contents of AsA and GSH in the two wheat cultivars increased first and then decreased (Fig. 4A, 4B), indicating that the EBR could increase the AsA and GSH contents in a range of concentrations of EBR to the two wheat cultivars under Cd2+ stress.
Excessive Cd2+ will directly destroy the cell membrane, causing the loss of intracellular substances and the peroxidation of cell membrane lipids, leading to the accumulation of ROS (Shen et al. 2017). H2O2 is the main form of ROS, and MDA is the product of membrane lipid peroxidation. Therefore, H2O2 and The level of MDA reflects to some extent the degree of damage to plant cells (Sun and Song 2015; Li et al. 2016a). Li et al. (2015) shown that cadmium stress leads to an increase in MDA contents in leaves of Solanum nigrum L., and a certain concentration of EBR can reduce the MDA contents of Solanum nigrum leaves under cadmium stress. The results of this study showed that the H2O2 and MDA contents of the seedlings of LX77 seedlings were increased significantly after treatment with 50mg·L− 1 Cd2+ alone (Fig. 4H, 4I), indicating that Cd2+ caused the cell membrane lipid peroxidation of LX77 seedlings. The MDA contents of JM22 seedlings did not change significantly, and the H2O2 contents increased significantly but the increasing rate was less than that of LX77 (Fig. 4H, 4I). The relative conductivity of roots did not change much in both LX77 and JM22 (Fig. J), indicating that 50mg·L− 1 Cd2 + affected the relative conductivity of roots in two varieties are limited. Compared with Cd2+ treatment alone, the MDA and H2O2 contents and relative conductivity of the two wheat cultivars decreased to some extent after foliar application of different concentrations of EBR. The EBR concentrations were 0.01 or 0.1 mg·L− 1 are the best.
Enzymes and non-enzymatic systems in plants resist oxidative stress. Studies have found that the synergistic effect of enzymatic antioxidant system and non-enzymatic antioxidant system can effectively alleviate the toxicity of ROS accumulation induced by stress in wheat (Sui et al. 2011; Shen et al. 2017). Antioxidant enzymes SOD, POD and CAT are important protective enzymes for scavenging ROS in plants (Shen et al. 2017). They can synergistically and effectively eliminate ROS and reduce the damage of plant cells (Limón-Pacheco and Gonsebatt 2009). Study has shown that foliar application of 0.5 mg·L− 1 EBR can increase the SOD, POD and CAT activities of soybean seedlings under cadmium stress (Liu et al. 2015). This study found that the SOD activities of LX77 seedlings were decreased significantly after treatment with 50mg·L− 1Cd2+ alone (Fig. 5D),while the SOD activities of wheat seedlings of JM22 were increased a little (Fig. 5D), indicating that Cd2+ stress inhibited the LX77 seedlings heavily. Compared with Cd2+ treatment alone, the SOD activities of both wheat cultivars increased first and then decreased after foliar application of different concentrations of EBR (Fig. 5A). indicating that foliar application of a certain concentration of EBR could increase the SOD activity of wheat seedlings under Cd stress. After treatment with 50mg·L− 1 Cd2+ alone, the POD activities of both wheat cultivars increased (Fig. 5E), and the leaves were sprayed with different concentrations of EBR, the POD activities of wheat cultivars did not increase significantly (Fig. 5B), indicating that EBR could not stimulate the POD activities of wheat seedlings under Cd2 + stress. After treatment with 50 mg·L− 1 Cd2+ alone, the CAT activities of LX77 seedlings were increased while the CAT activities of JM22 seedlings were decreased (Fig. 5F), indicating that the CAT activities of different varieties of wheat varied. Compared with the treatment of Cd2+ alone, foliar application of a certain concentration of EBR can not promote CAT activities in both wheat seedlings. The role of antioxidant enzymes, or indirect regulation of plant response to oxidative stress, requires further investigation.