Amelioration of cadmium stress by supplementation of melatonin and ZnO-nanoparticles through physiochemical adjustments in Brassica oleracea var. capitata

The Brassicaceae family, including the globally consumed Brassica oleracea var. capitata, faces multiple stressors like heavy metal toxicity, impacting their growth and quality. These stressors significantly impact plant growth, productivity, and overall quality. Particular nanoparticles and phytohormones play a pivotal role in regulating plant responses to such abiotic stressors. In the current study, the utilization of zinc oxide nanoparticles (ZnONPs) and melatonin (MT) was explored to enhance the resilience of Brassica oleracea var. capitata to cadmium (Cd) stress. The nanoparticles were synthesized in an environmentally friendly manner using Clinopodium vulgare L. leaf extract. Microscopic and spectroscopic techniques were employed to characterize the ZnO-NPs. Treatments were chosen as different concentrations of nanoparticles i.e., NP1 = 25 mg L-1 ZnO, NP2 = 50 mg L-1 ZnO, NP3 = 100 mg L-1 ZnO, MT = 200 mu M, and NP0 (distilled water). The results revealed that ZnO-NPs showed a concentration-dependent effect on B. oleracea growth parameters and chlorophyll content, with the foliar application of MT further augmenting the impact. ZnO-NP treatments effectively reduced Cd concentrations while concurrently elevating zinc (Zn) levels in shoots and roots. The addition of exogenous MT further potentiated these effects. Notably, the treatments with 50 and 100 mg L-1 ZnO-NPs individually resulted in a growth increase of 38 % and 40 %, respectively, along with corresponding Zn content elevations of 20.1 % and 24 % compared to the control. The 100 mg L-1 ZnO-NPs treatment extended shoot length by 63.5 % and enhanced Zn content by 51 % compared to the control. ZnO-NPs and MT elevated total Cd absorption in shoots and roots, reducing post-harvest bioavailable Cd concentration in soil compared to the control. In conclusion, the synergy of ZnO-NPs and melatonin holds promise as a viable strategy to both counteract Cd toxicity and promote Zn bio-fortification in crops, presenting implications for healthier produce in contaminated soils.