Azolla pinnata as a phytoremediator: improves germination, growth and yield of maize irrigated with Ni-polluted water.

The removal of pollutants from the environment has become a global demand. The current study aimed to relieve the Ni toxicity effect on the germination, growth, and grain yield of maize by using Azolla pinnata as a phytoremediator. Azolla-treated and untreated nickel solutions [0 (control), 24, 70, 140 and 190 ppm] were applied for germination and pot experiments. Electron microscope examination cleared the Ni accumulation in Azolla's cell vacuole and its adsorption on the cell wall. The inhibition of the hydrolytic enzyme activity reduces maize germination; maximal inhibition was 57.1% at 190 ppm of Ni compared to the control (100%). During vegetative growth, Ni stimulated the generation of HO (0.387 mM g F Wt at 190 ppm of Ni), which induced maximal lipid peroxidation (3.913 µMDA g F Wt) and ion leakage (74.456%) compared to control. Chlorophyll content and carbon fixation also showed significant reductions at all Ni concentrations; at 190 ppm, they showed maximum reductions of 56.2 and 63%, respectively. However, detoxification enzymes' activity such as catalase and antioxidant substances (phenolics) increased. The highest concentration of Ni (190 ppm) had the most effect on constraining yield, reaching zero for the weight of 100 grains at 190 ppm of Ni. Azolla-treated Ni solutions amended all determinant parameters, indicating a high percentage of changes in hydrolytic enzyme activity (125.2%) during germination, chlorophyll content (77.6%) and photosynthetic rate (120.1%). Growth measurements, carbon fixation, and yield components showed a positive association. Thus, we recommended using Azolla as a cost-effective and eco-friendly strategy to recover Ni-polluted water.