Several cyanobacteria have been used as effective natural biostimulants under different stresses, but the utilization of Spirulina platensis has not been extensively investigated. The effects of living S. platensis (25 mg/L on dry weight basis) added twice as soil addition on growth, photosynthetic pigmentation, and antioxidant defenses of Triticum aestivum plants grown under normal and two drought stresses (80 and 60% Field capacity) were evaluated. Under drought stress conditions, growth parameters (shoot height, fresh, and dry weights, photosynthetic pigments) and antioxidant defenses were significantly inhibited by recording relatively lower values of the measured characters than the control. The treatment of wheat plants with S. platensis was successful in improving all growth metrics. Under 80 and 60% FWC, the fresh and dry weight of the shoot increased by approximately 25%, while the height of the shoot improved by approximately 33%. The contents of photosynthetic pigment (chl. a, b, and carotenoids) of T. aestivum were significantly induced by 38%, 31%, and 34%, respectively, under 60% FWC. Antioxidant capacity percentages as scavengers of different free radicals (OH¯⋅, H2O2, NO, and metal chelating), and inhibition of lipid peroxidation, in addition to enzymes (CAT and APX), non-enzymatic antioxidants (phenolics, reducing power, total antioxidants) were significantly improved in Spirulina-enriched soil. The activity of antioxidant enzymes (SOD, CAT, POD, and APX) increased significantly in the shoot of wheat seedling by 150%, 83%, 193% and 11%, respectively, under 60% FWC. Components of soluble metabolites (carbohydrates, free amino acids, and soluble proteins) of T. aestivum were significantly enhanced in Spirulina-treated soil. The roots of wheat seedlings treated with S. platensis showed the highest increase in soluble proteins, free amino acids, and soluble carbohydrates (21.67, 4.68, and 41.67 mg g−1 FW, respectively). The application of Spirulina-based biostimulators is aligned with the sustainable agriculture concept by significant improving the content of photosynthetic pigments, confirming the correlation between growth and the measured antioxidants parameters of T. aestivum. The PCA biplot indicated a great contradiction between chlorophyll pigments, soluble metabolites (proteins, amino acids, and carbohydrates), phenolics, reducing power and flavonoids, and all growth indicators and antioxidant abilities. Therefore, the results of this study support the use of the Spirulina platensis treatment, which was mainly effective in improving the growth of wheat plants by reducing the detrimental effects of drought stress in dry conditions.
