24-Epibrassinolide and Spermine Combined Treatment Sustains Maize (Zea mays L.) Drought Tolerance by Improving Photosynthetic Efficiency and Altering Phytohormones Profile

24-Epibrassinolide (EBL) and/or spermine (Spm) applications regulate photosynthetic process and hormonal balance in drought-stressed plants. A pot experiment was conducted to investigate the potential effects of 25 mg l−1 EBL and/or 0.1 mg l−1 Spm applied to maize (Zea mays L.; hybrid Giza 129) exposed to water deficiency (50 and 75% field capacity). Plastic pots were planted with maize plants and designed in a complete randomized design with four replications. Drought significantly impaired photosynthetic pigments content, photochemical reactions of photosynthesis, net photosynthetic rate, transpiration rate, stomatal conductance, maximum quantum efficiency of PSII photochemistry, electron transport rate, actual photochemical efficiency of PSII, photochemical quenching coefficient, effective quantum yield of PSII photochemistry, activities of Rubisco, Rubisco activase, and carbonic anhydrase, seeds carbohydrate content as well as concentrations of auxins, cytokinins, and gibberellins. These changes were significantly modulated in drought-affected plants after EBL and Spm combined application. Moreover, this combined treatment under water shortage conditions inhibited the increased concentrations of intercellular CO2, non-photochemical quenching coefficients, and abscisic acid as well as diminished the enhanced activity of glycolate oxidase. These results reinforce the utility of this combined treatment not only in improving the photosynthetic capability but also in regulating the hormonal homeostasis as a powerful strategy to enhance the plant drought tolerance. Indeed, exogenous application of 25 mg l−1 Spm + 0.1 mg l−1 EBL can preserve the photosynthetic apparatus activity under water deficiency.