Green-synthesized bimetallic zinc ferrite (franklinite) nanoparticles prevent drought-induced growth reduction and oxidative stress in sorghum bicolor through improved nutrient content and the antioxidant defense

Sorghum bicolor (L.) Moench is a cereal crop renowned for its ability to moderately tolerate drought stress, however prolonged and severe droughts can affect its growth and yield. This study reports on the dual role of green-synthesized bimetallic Zinc-Iron Oxide (ZnFe2O4 NPs) nanoparticles (NPs), which are to enhance sorghum growth and mitigate oxidative damage under drought stress. The 34 nm ZnFe2O4 NPs synthesized from rooibos, were confirmed by an absorbance peak at 296 nm, a Fourier-Transform-Infrared Spectroscopy peak at 547 cm−1, and characterized by a crystallized face-centered-franklinite nature. Sorghum seedlings germinated from seeds primed with different ZnFe2O4 NPs concentrations of 5 mg/L, 10 mg/L, and 15 mg/L, were established for 14 days, followed by withholding water for 7 days to induce drought stress. Drought led to a decrease in shoot length (SL), fresh weight (FW), dry weight (DW), and total chlorophyll (Chlt) content by 43.6 %, 58.0 %, 63.9 % and 45.0 % respectively. These decreases correlated with the alteration of vascular bundle tissues causing nutrient imbalances, overaccumulation of reactive oxygen species, and lipid peroxidation thus inducing oxidative damage in sorghum seedlings. However, priming with NPs particularly, 15 mg/L ZnFe2O4 NPs, increased SL (138 %), FW (251.23 %), DW (251 %), and Chlt (104 %), and effectively reduced oxidative stress, through the improved antioxidant activity. The results suggest that ZnFe2O4 NPs can be employed as a strategy to prevent the harmful effects of drought stress, while improving crop growth.