Drought is a major abiotic stress that confronts plant growth and productivity, thus compromising food security. Plants use physiological and biochemical mechanisms to cope with drought stress, but at the expense of growth. Green-synthesized nanoparticles (NPs) have gained great attention in agriculture due to their environmental friendliness and affordability while serving as potential biofertilizers. This study investigates the role of hematite (αFe2O3) NPs, synthesized from Aspalathus linearis (rooibos), to improve Sorghum bicolor growth under drought stress. About 18 nm, spherical, and highly agglomerated hematite (αFe2O3) NPs were obtained. Sorghum seeds were primed with 5, 10, and 15 mg/L αFe2O3 NPs, and, after seven days of germination, the seedlings were transferred into potting soil, cultivated for fourteen days, and were subsequently water deprived (WD) for a further seven days. A reduction in plant height (78%), fresh (FW; 35%) and dry (DW; 36%) weights, and chlorophyll (chl) content ((total chl (81%), chla (135%), and chlb (1827%)) was observed in WD plants, and this correlated with low nutrients (Mg, Si, P, and K) and alteration in the anatomic structure (epidermis and vascular bundle tissues). Oxidative damage was observed as deep blue (O2●−) and brown (H2O2) spots on the leaves of WD plants, in addition to a 25% and 40% increase in oxidative stress markers (H2O2 and MDA) and osmolytes (proline and total soluble sugars), respectively. Seed priming with 10 mg/L αFe2O3 NPs improved plant height (70%), FW (56%), DW (34%), total Chl (104%), chla (160%) and chlb (1936%), anatomic structure, and nutrient distribution. Priming with 10 mg/L αFe2O3 NPs also protected sorghum plants from drought-induced oxidative damage by reducing ROS formation and osmolytes accumulation and prevented biomolecule degradation. The study concludes that green synthesized hematite NPs positively influenced sorghum growth and prevented oxidative damage of biomolecules by improving nutrient uptake and osmoregulation under drought stress.

Salinity stress is one of the major environmental constraints responsible for the reductions in agricultural productivity. Salinity affects crop growth, by causing osmotic and ionic stresses, which induce oxidative damage due to increased reactive oxygen species (ROS). Exogenous application of natural compounds can reduce the negative impacts of salinity stress on plants. This study evaluated the antioxidant capacity of chitosan, a biopolymer to reduce the salt-induced oxidative damage on sorghum plants. Morpho-physiological and biochemical attributes of sorghum plants stressed with 300 mM NaCl, in combination with chitosan (0.25 and 0.5 mg/mL), were assayed. Salt stress decreased growth, fresh (66.92%) and dry (48.26%) weights, affected the shape and size of the stomata, caused deformation of the xylem and phloem layers, and increased the Na+/K+ (1.3) and Na+/Si+ (5.4) ratios. However, chitosan effectively reversed these negative effects, as supported by decreased Na+/Si+ ratio (~0.9) and formed silica phytoliths. Oxidative stress was exerted as observed by increased H2O2 (44%) and malondialdehyde (125%) contents under salt stress, followed by their reduction in chitosan-treated sorghum plants. Salt increased proline (318.67%), total soluble sugars (44.69%), and activities of SOD (36.04%) and APX (131.58%), indicating sorghum’s ROS scavenging capacity. The antioxidant capacity of chitosan was measured by determining its ability to reduce oxidative damage and minimizing the induction of the antioxidant defense system. Chitosan reduced oxidative stress markers, proline, total soluble sugars, and the antioxidant enzyme activities by more than 50%. Fourier Transform Infrared Spectra of chitosan-treated samples confirmed a reduction in the degradation of biomolecules, and this correlated with reduced oxidative stress. The results suggest that chitosan’s antioxidant capacity to alleviate the effects of salt stress is related to its role in improving silicon accumulation in sorghum plants.

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Salinity is one of the major detrimental abiotic stresses at the forefront of deterring crop productivity globally. Although the exogenous application of phytohormones has formerly proven efficacious to plants, their effect on the moderately stress-tolerant crop “Sorghum bicolor” remains elusive. To investigate this, S. bicolor seeds primed with methyl jasmonate (0; 10 and 15 μM MeJa) were exposed to salt (200 mM NaCl) stress, and their morpho-physiological, biochemical, and molecular attributes were measured. Salt stress significantly decreased shoot length and fresh weight by 50%, whereas dry weight and chlorophyll content were decreased by more than 40%. Furthermore, salt-stress-induced oxidative damage was evident by the formation of brown formazan spots (indicative of H2O2 production) on sorghum leaves and a more than 30% increase in MDA content. However, priming with MeJa improved growth, increased chlorophyll content, and prevented oxidative damage under salt stress. While 15 µM MeJa maintained proline content to the same level as the salt-stressed samples, total soluble sugars were maintained under 10 µM MeJa, indicating a high degree of osmotic adjustment. Shriveling and thinning of the epidermis and xylem tissues due to salt stress was prevented by MeJa, followed by a more than 70% decrease in the Na+/K+ ratio. MeJa also reversed the FTIR spectral shifts observed for salt-stressed plants. Furthermore, salt stress induced the expression of the jasmonic acid biosynthesis genes; linoleate 92-lipoxygenase 3, allene oxide synthase 1, allene oxide cyclase, and 12-oxophytodienoate reductase 1. In MeJa-primed plants, their expression was reduced, except for the 12-oxophytodienoate reductase 1 transcript, which further increased by 67%. These findings suggest that MeJa conferred salt-stress tolerance to S. bicolor through osmoregulation and synthesis of JA-related metabolites.

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