Plants activate physiological responses against salinity stress through hormone signaling pathways such as melatonin (M) and methyl jasmonate (MeJ). These hormones trigger defense responses, but comparing their individual and combined effects under salt stress has not been studied. This research investigates defense responses in tomato plants induced by 100 mu M of M and MeJ, along with their combined application (MeJ+M, 100+100 mu M) under non-stress, threshold (0.9 g NaCl kg-1 soil) and severe (1.8 g NaCl kg-1 soil) salinity conditions. Compared to melatonin, MeJ application caused adverse effects, including chlorophyll degradation (34.2 %), root inhibition (17.2 %), and elevated H2O2 (28.9 %), O2-center dot (33.7 %), and malondialdehyde (14.3 %) in the plant under non-stress conditions. Evaluation of the MeJ+M treatment in non-stress conditions indicated that M prevented MeJ-induced damage. Besides, the optimal potassium uptake and plant growth were obtained in the MeJ+M treatment under non-stress and threshold salinity levels. Phytohormones application increased enzymatic antioxidant activity (superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase), modified the activity of phenylalanine ammonia-lyase and polyphenol oxidase, and consequently boosted non-enzymatic antioxidants (phenolic, flavonoid, and anthocyanin content), resulting in a significant reduction of damage from severe salinity stress. However, due to their almost similar physiological changes induced by MeJ, M, and MeJ+M, these treatments were not superior compared to each other in severe stress. Thus, owing to the disruption of the normal morpho-physiological processes in non-stress conditions by MeJ, M can be considered a safer treatment for practical usage. Additionally, the MeJ+M application can not only optimize antioxidant protection under stress conditions but also stimulate plant growth under non-stress conditions.

Background This study aimed to investigate the alterations in biochemical and physiological responses of oat plants exposed to antimony (Sb) contamination in soil. Specifically, we evaluated the effectiveness of an arbuscular mycorrhizal fungus (AMF) and olive mill waste (OMW) in mitigating the effects of Sb contamination. The soil was treated with a commercial strain of AMF (Rhizophagus irregularis) and OMW (4% w/w) under two different levels of Sb (0 and 1500 mg kg-1 soil).Results The combined treatment (OMW + AMF) enhanced the photosynthetic rate (+ 40%) and chlorophyll a (+ 91%) and chlorophyll b (+ 50%) content under Sb condition, which in turn induced more biomass production (+ 67-78%) compared to the contaminated control plants. More photosynthesis in OMW + AMF-treated plants gives a route for phenylalanine amino acid synthesis (+ 69%), which is used as a precursor for the biosynthesis of secondary metabolites, including flavonoids (+ 110%), polyphenols (+ 26%), and anthocyanins (+ 63%) compared to control plants. More activation of phenylalanine ammonia-lyase (+ 38%) and chalcone synthase (+ 26%) enzymes in OMW + AMF-treated plants under Sb stress indicated the activation of phenylpropanoid pathways in antioxidant metabolites biosynthesis. There was also improved shifting of antioxidant enzyme activities in the ASC/GSH and catalytic pathways in plants in response to OMW + AMF and Sb contamination, remarkably reducing oxidative damage markers.Conclusions While individual applications of OMW and AMF also demonstrated some degree of plant tolerance induction, the combined presence of AMF with OMW supplementation significantly enhanced plant biomass production and adaptability to oxidative stress induced by soil Sb contamination.