Compared to antioxidants and polyamines, the role of maize grain-derived organic biostimulants in improving cadmium tolerance in wheat plants

Recently, the strategy of seed soaking has been successfully applied using extracts from different plant parts for healthy growth of plant under different environmental stresses. Compared to antioxidants like ascorbic acid (AsA) and glutathione (GSH) or polyamines (PM) like spermine (SPM), spermidine (SPD), and putrescine (PUT), the effects of seed soaking using maize grain extract (MGE) on the biomass, productivity, phytohormones, and antioxidant defense system and its different components were examined with Cd2+-stressed wheat plants. In a preliminary study, seed soaking using AsA + GSH or PUT + SPD + SPM was more effective in increasing shoot fresh and dry weights, SPAD chlorophyll, and grain yield, and reducing malondialdehyde (MDA) content than individuals. In addition, MGE at 2% was more efficient than other concentrations. Therefore, they were selected for the main study. In the main study, compared to the control, seed soaking in AsA + GSH, PUT + SPD + SPM or MGE had positive effects on plant growth, yield, photosynthetic efficiency, contents and redox states of MA and GSH, contents of PAs and plant hormones to varying degrees. Proline content and its metabolism enzymes activity, contents of soluble protein, N-compounds, soluble sugars, and alpha-tocopherol (alpha-TOC), and activities of antioxidant enzymes were not affected. However, contents of MDA and hydrogen peroxide (H2O2) were significantly reduced under normal conditions. Under Cd2+ stress (1.2 mM), along with the detrimental increases in the contents of MDA, H2O2 and Cd2+, contents of N-compounds, soluble sugars, proline content and its metabolism enzymes activities, MA and GSH and their redox states, and polyamines, and activities of antioxidant enzymes were increased. In contrast, plant growth and yield, photosynthetic efficiency, soluble protein, and plant hormones were significantly reduced compared to the control. However, all of these attributes were significantly improved to varying degrees along with reduced contents of Cd2+, MDA, and H2O2 by seed soaking in AsA + GSH, PUT + SPD + SPM or MGE compared to the Cd2+-stressed control. Compared to MA + GSH or PUT + SPD + SPM, seed soaking in MGE at 2% conferred the best results. Therefore, it is recommended to soak wheat seeds using MGE to improve plant growth and productivity by restricting the inhibitory influences of oxidative stress induced by Cd2+ stress.