Soil Chemical Analysis

Human health effects of air pollution

The aim of this review is to assess the mode of action and role of antioxidants as protection from heavy metal stress in roots, mycorrhizal fungi and mycorrhizae. Based on their chemical and physical properties three different molecular mechanisms of heavy metal toxicity can be distinguished: (a) production of reactive oxygen species by autoxidation and Fenton reaction; this reaction is typical for transition metals such as iron or copper, (b) blocking of essential functional groups in biomolecules, this reaction has mainly been reported for non-redox-reactive heavy metals such as cadmium and mercury, (c) displacement of essential metal ions from biomolecules; the latter reaction occurs with different kinds of heavy metals. Transition metals cause oxidative injury in plant tissue, but a literature survey did not provide evidence that this stress could be alleviated by increased levels of antioxidative systems. The reason may be that transition metals initiate hydroxyl radical production, which can not be controlled by antioxidants. Exposure of plants to non-redox reactive metals also resulted in oxidative stress as indicated by lipid peroxidation, H(2)O(2) accumulation, and an oxidative burst. Cadmium and some other metals caused a transient depletion of GSH and an inhibition of antioxidative enzymes, especially of glutathione reductase. Assessment of antioxidative capacities by metabolic modelling suggested that the reported diminution of antioxidants was sufficient to cause H(2)O(2) accumulation. The depletion of GSH is apparently a critical step in cadmium sensitivity since plants with improved capacities for GSH synthesis displayed higher Cd tolerance. Available data suggest that cadmium, when not detoxified rapidly enough, may trigger, via the disturbance of the redox control of the cell, a sequence of reactions leading to growth inhibition, stimulation of secondary metabolism, lignification, and finally cell death. This view is in contrast to the idea that cadmium results in unspecific necrosis. Plants in certain mycorrhizal associations are less sensitive to cadmium stress than non-mycorrhizal plants. Data about antioxidative systems in mycorrhizal fungi in pure culture and in symbiosis are scarce. The present results indicate that mycorrhization stimulated the phenolic defence system in the Paxillus-Pinus mycorrhizal symbiosis. Cadmium-induced changes in mycorrhizal roots were absent or smaller than those in non-mycorrhizal roots. These observations suggest that although changes in rhizospheric conditions were perceived by the root part of the symbiosis, the typical Cd-induced stress responses of phenolics were buffered. It is not known whether mycorrhization protected roots from Cd-induced injury by preventing access of cadmium to sensitive extra- or intracellular sites, or by excreted or intrinsic metal-chelators, or by other defence systems. It is possible that mycorrhizal fungi provide protection via GSH since higher concentrations of this thiol were found in pure cultures of the fungi than in bare roots. The development of stress-tolerant plant-mycorrhizal associations may be a promising new strategy for phytoremediation and soil amelioration measures.

Plant responses to abiotic stresses: heavy metal‐induced oxidative stress and protection by mycorrhization

Reactive oxygen species (ROS) were initially recognized as toxic by-products of aerobic metabolism. In recent years, it has become apparent that ROS plays an important signaling role in plants, controlling processes such as growth, development and especially response to biotic and abiotic environmental stimuli. The major members of the ROS family include free radicals like O2● −, OH● and non-radicals like H2O2 and 1O2. The ROS production in plants is mainly localized in the chloroplast, mitochondria and peroxisomes. There are secondary sites as well like the endoplasmic reticulum, cell membrane, cell wall and the apoplast. The role of the ROS family is that of a double edged sword; while they act as secondary messengers in various key physiological phenomena, they also induce oxidative damages under several environmental stress conditions like salinity, drought, cold, heavy metals, UV irradiation etc., when the delicate balance between ROS production and elimination, necessary for normal cellular homeostasis, is disturbed. The cellular damages are manifested in the form of degradation of biomolecules like pigments, proteins, lipids, carbohydrates and DNA, which ultimately amalgamate in plant cellular death. To ensure survival, plants have developed efficient antioxidant machinery having two arms, (i) enzymatic components like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR); (ii) non-enzymatic antioxidants like ascorbic acid (AA), reduced glutathione (GSH), α-tocopherol, carotenoids, flavonoids and the osmolyte proline. These two components work hand in hand to scavenge ROS. In this review, we emphasize on the different types of ROS, their cellular production sites, their targets, and their scavenging mechanism mediated by both the branches of the antioxidant systems, highlighting the potential role of antioxidant

Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants

Health risks of heavy metals in contaminated soils and food crops irrigated with wastewater in Beijing, China

Acidic soils are widespread covering nearly 40% of the world’s total arable land area. However, soils of certain regions are naturally acidic but an increase in soil acidification as a result of accelerating anthropogenic activities is becoming a global issue. High emissions of acid precursors (nitrogen, sulfur, and carbon dioxide) in the atmosphere are chiefly responsible for acid precipitation, which in turn is a pre-eminent factor for soil acidification. Long-term application of nitrogen fertilizers is a major contributor in acidification of agricultural soil. Soil acidification is an important edaphic stress, which leads to cation leaching, instability in the soil aggregate structure, increases metal toxicity, lowers the soil nutrient availability, and consequently affects the soil biological properties and plant performances. The present chapter aimed to assess the consequent effects of soil acidification on plants and the plant community structure. It includes causes, processes, plants’ responses, and remedial measures to combat soil acidification due to increasing pollution. Different plants may show different sensitivity to acidity and have diverse an optimal pH range for nutrient uptake. Besides, depletion of basic cations (Na, Ca, Mg, and K) due to leaching and increased solubility of toxic metals (Al and Mn) in soil restrict the plant’s access to water and nutrients, thereby causing severe injury to roots, a reduction in crop yield, and an increase in plant susceptibility to pathogens. Plant diversity, species richness, and occurrence of species are significantly influenced by acidification of soil. Alteration in the plant community structure, in turn, may affect the ecosystem structure and functions. Acidification of soil could plausibly be ameliorated by nutrient management practices and by addition of acid-neutralizing substances.

Soil Acidification and its Impact on Plants

Because tropospheric O3 is a secondary pollutant, its concentration in the troposphere largely depends upon different variables that play major roles in its in-situ production. Formation of O3 in the troposphere largely depends upon the emission of it’s precursors like nitrogen oxides (NOx) and volatile organic compounds (VOCs) along with a set of favourable meteorological conditions such as high temperature, intense solar radiations, long sunshine hours, wind speed/direction, etc. In addition, tropospheric O3 concentration also depends upon the stratospheric intrusion which shows significant seasonal and zonal variations. Apart from O3 formation, the O3 budget in the troposphere is also determined by O3 destruction/depletion, which is more prominent in marine boundary layer, characterized by the production of halogen species like chlorine (Cl), bromine (Br) and iodine (I) and their respective oxides. This chapter emphasizes the role of different factors determining O3 formation/depletion and the conditions which significantly affect the tropospheric O3 budget. The effect of climate change variables on tropospheric O3 budget is also discussed.

Tropospheric Ozone Budget: Formation, Depletion and Climate Change

Restoration of mine spoil is a prime need for coal industry. The study of ground cover vegetation provides essential information about the species diversity and their successional trends during the restoration. The present study was conducted to analyze the structure and biomass accumulation of ground vegetation developing in different plantation stands of an opencast coal mine spoil in a dry tropical region. Different plantation stands showed variations in species diversities. Exotic herbs were more dominant in comparison to native herbs. Pennisetum pedicillatum, an exotic herb showed maximum Importance Value Index in most of the plantation stands. Total number of species varied between 12-18 in different plantation stands. Speces richness and evenness increased with increasing age of the plantations. Variations in total biomass accumulation of ground vegetation were also significant among different plantations. These results suggest that reforestation programme with exotic species on coal mine spoil has been successful in colonization of ground vegetation under different plantations. Gravellia pteridifolia plantations showed most successful ground cover among different plantation stands.

Development of ground vegetation under exotic tree plantations on restored coal mine spoil land in a dry tropical region of India.

Tropospheric ozone (O3) phytotoxicity is a major threat to agricultural production leading to global yield loss of about 3.9–15% for wheat, 8.5–14% for soybean and 2.2–5.5% for maize. At a global scale, the United States displays a decline in O3 level of about 17% from 2000–2015. O3 concentration in the United States is about two times higher than that of Europe. Despite the declining trend in these two continents, the prevailing concentrations are still high enough to affect the plant growth and productivity. Beside this, a significant rise in O3 pollution is observed in various Asian countries. China, the fastest developing nation of Asia, has shown an increase of surface O3 concentration by 0.58 ppbv/yr. from 1994 to 2007. A continuous increase in the emission of O3 precursors in southern and central Asia has been reported due to weaker legislative policies in the developing countries, contributing to more O3 prevalence in such regions. A simulation study projected an increase of global tropospheric O3 by 4.3 ± 2.2 ppb with maximum increment displayed in South Asia, South-East Asia and Middle Eastern region.

Madhoolika Agrawal

Papers

Soil Acidification and its Impact on Plants

Tropospheric Ozone Budget: Formation, Depletion and Climate Change

Development of ground vegetation under exotic tree plantations on restored coal mine spoil land in a dry tropical region of India.

Ozone Toxicity and Remediation in Crop Plants

Ozone flux-effect relationship for early and late sown Indian wheat cultivars: Growth, biomass, and yield