Copper (Cu) holds a significant importance in plant metabolism as it serves as an essential micronutrient but becomes toxic at higher concentrations. Nitric oxide (NO), a key signaling molecule, and nitrogen (N) play essential roles in combating toxicity of some metals. This study explores the potential of interactive effects of NO as 100 mu M SNP (sodium nitroprusside, NO source) and N (80 mg N kg-1 soil) in mitigating Cu (100 mg Cu kg-1 soil) stress in mustard (Brassica juncea L.) plants. The impaired physio-biochemical changes, photosynthetic efficiency, and the expression level of genes associated with photosynthesis, and N assimilation under Cu stress were ameliorated with the exogenous application of NO and N. The combined treatment of NO and N conspicuously lowered reactive oxygen species (ROS) and its related impacts. It also enhanced the activity and relative expression of antioxidant enzymes, including ascorbate peroxidase (APX), glutathione reductase (GR), and superoxide dismutase (SOD) as well as N assimilation enzymes, such as nitrate reductase (NR) and nitrite reductase (NiR). The supplementation of NO and N also triggered the expression of rbcL (large subunit of Rubisco), photosystem (photosystem II D1 protein; psbA and photosystem II protein B; psbB) and markedly improved photosynthetic capacity under Cu stress. The study highlights the significance of NO and N as a potential strategy to counteract Cu-induced stress in crops. It suggests a synergistic or interactive effect between the two substances as a phytoremediation strategy for enhancing crop growth and productivity in Cu-contaminated soils. Understanding the mechanisms behind NO and N mediated stress alleviation could facilitate the development of targeted approaches to enhance plant resilience against heavy metal stress.

Urban air pollution has been a global challenge world-wide. While urban vegetation or forest modelling can be useful in reducing the toxicities of the atmospheric gases by their absorption, the surge in gaseous pollutants negatively affects plant growth, thereby altering photosynthetic efficiency and harvest index. The present review analyses our current understanding of the toxic and beneficial effects of atmospheric nitrogen oxides (NOx), hydrogen sulphide (H2S) and carbon monoxide (CO) on plant growth and metabolism. The atmospheric levels of these gases vary considerably due to urbanization, automobile emission, volcanic eruptions, agricultural practices and other anthropological activities. These gaseous pollutants prevalent in the atmosphere are known for their dual action (toxic or beneficiary) on plant growth, development and metabolism. NO seems to exert a specialized impact by upregulating nitrogen metabolism and reducing tropospheric ozone. High H2S emission in specific areas of geothermal plants, fumarolic soils and wetlands can be a limitation to air quality control. Certain shortcomings associated with the designing of field experiments, sensitivity of detection methods and simulation development are yet to be overcome to analyze the precise levels of NO, H2S and CO in the rhizosphere of diverse agro-climatic regions. Several laboratory-based investigations have been undertaken to assess the roles of atmospheric gases, namely NOx, CO, H2S, and particulate matter (PM). However, in order to enable natural and sustainable mitigation, it is essential to increase the number of field experiments in order to identify the pollutant-tolerant plants and study their interactive impact on plant growth and agriculture.

Since the advent of industrialization, there has been a significant increase in the accumulation of heavy metals (HMs) in the soil, which further pose a major threat to plant growth and productivity. To address this issue and meet the global demand for food and energy, it is crucial to develop strategies that mitigate the toxicity induced by increased levels of HMs. One viable option is the supplementation of various phytohormones and gasotransmitters. Gasotransmitters are well-known for their ability to counteract a plethora of stresses in plants, with multifunctional hydrogen sulfide (H2S) being a promising candidate for reducing HM- induced oxidative stress. Furthermore, H2S has also been reported to regulate various physiological processes, such as seed germination, senescence and ageing in plants both under normal and stressed conditions. This review concurrently underscores the significance of H2S in plants subjected to HM stress. It also elucidates H2S's role as both a standalone stress mitigator as well as a synergistic component when combined with other stress-alleviating agents, resulting in the mitigation of HM toxicity, enhanced plant growth, stabilization of physiological processes and the upregulation of antioxidative metabolic activities. Overall, this review accentuates the necessity for sustainable and eco-friendly approaches to manage HM stress and emphasizes the potential of H2S as a promising solution to alleviate HM-related damage to plants.

Fertilizer-intensive agriculture leads to emissions of reactive nitrogen (Nr), posing threats to climate via nitrous oxide (N2O) and to air quality and human health via nitric oxide (NO) and ammonia (NH3) that form ozone and particulate matter (PM) downwind. Adding nitrification inhibitors (NIs) to fertilizers can mitigate N2O and NO emissions but may stimulate NH3 emissions. Quantifying the net effects of these trade-offs requires spatially resolving changes in emissions and associated impacts. We introduce an assessment framework to quantify such trade-off effects. It deploys an agroecosystem model with enhanced capabilities to predict emissions of Nr with or without the use of NIs, and a social cost of greenhouse gas to monetize the impacts of N2O on climate. The framework also incorporates reduced-complexity air quality and health models to monetize associated impacts of NO and NH3 emissions on human health downwind via ozone and PM. Evaluation of our model against available field measurements showed that it captured the direction of emission changes but underestimated reductions in N2O and overestimated increases in NH3 emissions. The model estimated that, averaged over applicable U.S. agricultural soils, NIs could reduce N2O and NO emissions by an average of 11% and 16%, respectively, while stimulating NH3 emissions by 87%. Impacts are largest in regions with moderate soil temperatures and occur mostly within two to three months of N fertilizer and NI application. An alternative estimate of NI-induced emission changes was obtained by multiplying the baseline emissions from the agroecosystem model by the reported relative changes in Nr emissions suggested from a global meta-analysis: -44% for N2O, -24% for NO and +20% for NH3. Monetized assessments indicate that on an annual scale, NI-induced harms from increased NH3 emissions outweigh (8.5-33.8 times) the benefits of reducing NO and N2O emissions in all agricultural regions, according to model-based estimates. Even under meta-analysis-based estimates, NI-induced damages exceed benefits by a factor of 1.1-4. Our study highlights the importance of considering multiple pollutants when assessing NIs, and underscores the need to mitigate NH3 emissions. Further field studies are needed to evaluate the robustness of multi-pollutant assessments.