Combating Mercury Stress in Spinach (Spinacia oleracea L.) with Zinc Oxide and Providencia Vermicola, Assessing Changes in Defense Mechanism and Organic Acid Exudation Patterns

Soil contamination with toxic heavy metals [such as mercury (Hg)] is becoming a serious global problem because of the rapid development of the social economy. Although plant growth-promoting bacteria (PGPB) and nanoparticles (NPs) are the major protectants to alleviate metal toxicity, the study of these chemicals in combination to ameliorate the toxic effects of Hg is limited. A pot experiment was conducted over 60 days using spinach (Spinacia oleracea L.) to investigate the effects of varying Hg levels (0 mM, 20 mM, and 40 mM) combined with different concentrations of zinc oxide nanoparticles [ZnO - NPs (50 mg/L and 100 mg/L)] and Providencia vermicola (5 ppm and 10 ppm) on various morpho-physio-biochemical responses. The experiment followed a completely randomized design (CRD) with four replicates per treatment. Data were collected on plant growth parameters, photosynthesis, oxidative stress markers, antioxidant enzyme activities, organic acid levels, and Hg uptake. Statistical analysis was performed using two-way ANOVA to determine the significance of the treatments, and Tukey's HSD test (P < 0.05) was used for post hoc comparisons of means. Results from the present study showed that the increasing levels of Hg in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Hg in the soil significantly (P < 0.05) increased oxidative stress indicators in term of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patter in the roots of S. oleracea. The negative impact of Hg toxicity can overcome the application of P. vermicola and ZnO - NPs, which ultimately increased plant growth and biomass by capturing the reactive oxygen species, and decreased oxidative stress in S. oleracea by decreasing the Hg contents in the roots and shoots of the plants. Research findings, therefore, suggest that the combined application of P. vermicola and ZnO - NPs can ameliorate Hg toxicity in S. oleracea, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids. [GRAPHICS]