Sensitivity of agricultural crops to tropospheric ozone: a review of Indian researches.

Tropospheric ozone (O3) is a long-range transboundary secondary air pollutant, causing significant damage to agricultural crops worldwide. There are substantial spatial variations in O3 concentration in different areas of India due to seasonal and geographical variations. The Indo-Gangetic Plain (IGP) region is one of the most crop productive and air-polluted regions in India. The concentration of tropospheric O3 over the IGP is increasing by 6-7.2% per decade. The annual trend of increase is 0.4 ± 0.25% year-1 over the Northeastern IGP. High O3 concentrations were reported during the summer, while they were at their minimum during the monsoon months. To explore future potential impacts of O3 on major crop plants, the responses of different crops grown under ambient and elevated O3 concentrations were compared. The studies clearly showed that O3 is an important stress factor, negatively affecting the yield of crops. In this review, we have discussed yield losses in agricultural crops due to rising O3 pollution and variations in O3 sensitivity among cultivars and species. The use of ethylene diurea (EDU) as a research tool in assessing the losses in yield under ambient and elevated O3 levels also discussed. Besides, an overview of interactive effects of O3 and nitrogen on crop productivity has been included. Several recommendations are made for future research and policy development on rising concentration of O3 in India.

Salinity alleviates the toxicity level of ozone in a halophyte Mesembryanthemum crystallinum L.

Mesembryanthemum crystallinum (Ice plant) is an annual halophytic plant species spread in the coastal areas of the Mediterranean Sea, Egypt. Information about the behaviour of halophytes under the future concentration of ozone (O3) is scanty. Therefore, we have assessed the effects of elevated O3 (ambient + 20 ppb), moderate salinity (200 mM NaCl), and their combined treatment (salinity + elevated O3) on various morphological, growth, physiological, biochemical and anatomical parameters of Egyptian ice plant. Under salinity stress, plant growth, percentage of pigmented leaf and its thickness, ROS levels, antioxidative enzymes, and ROS scavenging activities were increased, while photosynthetic pigments and efficiency were decreased compared to the control. Elevated O3 exposure led to reductions in most of the growth parameters and pigments, while ROS levels, histochemical localization of H2O2 and ·O2-, antioxidative enzymes and non-enzymatic antioxidants (betacyanin, phenolics, thiols and ascorbic acid) showed increases. Surprisingly, salinity alleviated the oxidative stress of elevated O3 due to the rise of SOD activity, antioxidant compounds, and a decrease of ·O2- production rate with concomitant increases of most of the growth parameters. Thick lower collenchyma and enhancement of xylem parenchyma under O3 and combined treatment suggested that anatomical acclimation also operated under O3 stress and salinity played a vital role in the growth of this plant under combined stress. Results showed that salt is essential for the optimum development of this species and its role is extended to alleviate the oxidative damage caused by elevated O3. The results further recommend the use of Egyptian M. crystallinum as a O3 tolerant crop for saline areas along the Mediterranean Sea coast.

Responses of an old and a modern Indian wheat cultivar to future O3 level: Physiological, yield and grain quality parameters.

A field study was conducted to understand the physiological responses, yield and grain quality of an old (HUW234) and a modern (HD3118) wheat cultivar exposed to elevated ozone (O3). The cultivars were grown under ambient O3 (NF) and ambient +20 ppb O3 (NF+) conditions using open-top chambers (OTCs). The comparative study of an old and a modern cultivar showed variable physiological responses under elevated O3 exposure. Elevated O3 in old cultivar caused high reductions in Rubisco activity (Vcmax) and electron transport rate (J) compared to modern cultivar with simultaneous reductions in the rate of photosynthesis and chlorophyll fluorescence. In modern cultivar, high stomatal density and conductance caused higher O3 uptake thereby triggering more damage to the adjacent stomatal cells and photosynthetic pigments coupled with reductions in photosynthetic rate and photosynthetic nitrogen use efficiency (PNUE). Modern cultivar also showed relatively high reduction in grain yield compared to old one under NF + treatment. Furthermore, grain quality traits (such as starch, protein and amino acids) of modern cultivar were better than old cultivar under ambient O3, but showed more deterioration under NF + treatment. Results thus indicated that modern cultivar is relatively more susceptible to O3 and showed more negative impacts on plant performance, yield and quality of grains compared to old cultivar.

ROS production and its detoxification in early and late sown cultivars of wheat under future O3 concentration.

The present field study was planned with an objective to unravel the mechanisms behind the differential responses of early and late sown wheat cultivars with respect to their defense capacity to scavenge ROS induced under elevated O3 (EO3). Experiments were performed under ambient and elevated levels of O3 (ambient + 20 ppb) to plants inside open-top chambers (OTCs). Ozone concentrations, stomatal flux of O3 and meteorological parameters were measured throughout the experiment. Contents of superoxide radicals (O2-) and hydrogen peroxide (H2O2) and their localization, lipid peroxidation, antioxidative enzyme activities, ascorbic acid and total phenolic contents were measured at vegetative and reproductive developmental stages. EO3 exposure induced higher stomatal flux of O3 in early sown cultivars. Higher contents of O2-, H2O2 and lipid peroxidation were noticed under EO3 in all the cultivars but the magnitude of increases was higher in late sown cultivars at the reproductive stage. Activities of glutathione reductase (GR) and ascorbate peroxidase (APX) were higher in late sown cultivars under EO3. Ascorbic acid and total phenolic contents were significantly higher in early sown than late sown cultivars under EO3 treatment. The present study concludes that early sown cultivars are more efficient in their defense response due to higher induction of enzymatic and non-enzymatic antioxidants, while the induction of enzymatic antioxidants was more distinct in late sown cultivars. Non-enzymatic linked defense mechanism requires additional metabolic cost than enzymatic defense, making early sown cultivars more susceptible to EO3. Differential response of early and late sown cultivars with respect to antioxidative defense against O3 stress suggests that yield responses are governed by the time of sowing and intrinsic defense responses of the cultivars. In future with rising trend of O3, early sown cultivars are expected to be more vulnerable to oxidative stress compared to late sown cultivars.