The Ameliorative Role of 5-Aminolevulinic Acid (ALA) Under Cr Stress in Two Maize Cultivars Showing Differential Sensitivity to Cr Stress Tolerance

Heavy metal (HM) contamination of the environment is a serious threat to sustainable crop production. Among the HMs, chromium (Cr) is one of the most toxic HMs that is known to negatively affect growth and metabolic activities of diverse crop plants. The present study was designed to investigate the ameliorative role of 5-aminolevulinic acid (ALA) under Cr stress in two maize (Zea mays L.) cultivars showing differential sensitivity to Cr tolerance. ALA is a biosynthesis precursor and it has a dominant regulatory effect related to physiological, respiratory, and photosynthesis processes in various plant species. Three concentrations of Cr (0, 5, and 10 mg kg⁻¹) were tested under the graded levels of ALA application (0, 12.5, and 25 mg L⁻¹). The results indicated that Cr stress differentially reduced plant growth attributes, gas exchange characteristics, photosynthetic pigments, and biomass in both the cultivars. Oxidative stress increased as evidenced in the form of electrolyte leakage, malondialdehyde, and hydrogen peroxide (H₂O₂) accumulation in plants. The anti-oxidative enzyme activities, that is, catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) both in the leaves and roots of maize cultivars decreased due to Cr stress. The concentration of Cr increased in roots and shoots of maize under Cr levels without ALA. Under Cr stress, ALA exogenous application markedly enhanced plant growth, photosynthetic pigments, gas exchange capacity, and biomass. Furthermore, ALA application decreased the Cr-induced oxidative stress in maize cultivars by improving the activities of CAT, POD, and SOD in plants. After ALA application, the Cr concentrations and total Cr uptake by plants differently decreased in both cultivars. The 6103 cultivar of maize was found to be a tolerant cultivar against Cr stress due to its strong defensive system with a higher rate of antioxidant enzyme activities. On the other hand, the other maize cultivar (9108) was found to be a sensitive cultivar against Cr stress due to its weak defense system with higher contents of reactive oxygen species. These findings suggest that ALA can play a regulatory role in maintaining optimum plant growth and efficient photosynthetic processes under Cr-challenged habitats in maize. Thus, ALA application may be used as a sustainable remedial strategy to alleviate Cr-induced stress in maize cultivars.