Negative impacts of elevated ozone on dominant species of semi-natural grassland vegetation in Indo-Gangetic plain.

Increasing tropospheric ozone (O3) concentrations in most regions of the world have led to significant phytotoxicity to all types of vegetation. Indo-Gangetic Plains of India is one of the hot spot areas with high O3 concentrations throughout the year although O3 phytotoxicity on grassland species in this region is not explored. Therefore the present study was conducted to assess the responses of a dominant species, Ischaemum rugosum Salisb, a C4 grass and a co-dominant species Malvastrum coromandelianum (L.) Garcke, a C3 forb under future elevated O3 (non filtered ambient + 20 nl l-1; NFA+) concentration compared to non filtered ambient (NFA; 48.7 nl l-1, 8 h mean) for 9 weeks from 15th May to 15th July 2016 in mix-culture using open-top chambers (OTCs). Plants were assessed for physiological, biochemical and growth parameters including biomass accumulation during vegetative and reproductive stages to assess the O3 induced responses. Under NFA+, higher reductions were observed in physiological parameters, growth and total biomass accumulation in M. coromandelianum compared to I. rugosum while both the species suffered membrane damage. Enhancement in contents of ascorbic acid and tannin in I. rugosum while proline and total phenolics in M. coromandelianum led to more protection of former species compared to later from oxidative damage. No significant change in stomatal conductance in I. rugosum while significant increase in M. coromandelianum might have led to more accumulation of O3 inside the plant, thus more negatively affecting the performance of later species. The present study concludes that M. coromandelianum (C3 photosynthetic pathway) will be relatively more negatively affected compared to I. rugosum (C4 photosynthetic pathway) under future O3 concentrations.

Ozone phytotoxicity to Panicum maximum and Cenchrus ciliaris at Indo-Gangetic plains: an assessment of antioxidative defense and growth responses.

Two common tropical grassland species, Panicum maximum Jacq. (Guinea grass) and Cenchrus ciliaris (Buffel grass) of Indo-Gangetic plains were assessed for their responses under future level of O3 (ambient +30 ppb) using open top chambers. Plants were assessed for foliar injuries, pigments, growth, biomass accumulation, histochemical localization of reactive oxygen species (ROS), antioxidant defense system and ROS scavenging activities at two stages. Foliar injuries were noticed at an early stage in P. maximum compared to C. ciliaris. Significant reductions were observed in total chlorophyll, growth and total biomass in both species. Significant increases in contents of melondialdehyde and ascorbic acid in P. maximum while total phenolics and thiols in C. ciliaris were found. Histochemical analysis showed more production of superoxide radicals and hydrogen peroxide in leaf tissues of P. maximum compared to C. ciliaris. It can be concluded that higher level of primary antioxidants (total phenolics and thiols) along with superoxide dismutase and ascorbate peroxidase scavenged O3 effectively in C. ciliaris causing less reduction of biomass which is used as a feed for cattles. In P. maximum, more photosynthates were allocated for defense, leading to higher reduction in total biomass compared to C. ciliaris. The leaf area ratio was higher in P. maximum compared to C. ciliaris under elevated O3. The study further suggests higher susceptibility of P. maximum compared to C. ciliaris under future level of O3 exposure.

Responses of a semi-natural grassland community of tropical region to elevated ozone: An assessment of soil dynamics and biomass accumulation.

Despite knowing the phytotoxic effects of tropospheric ozone (O3), which is of global concern, there is no study so far reported about its impacts on grassland community of tropical regions. Therefore, we assessed the responses of a semi-natural grassland community of Indo-Gangetic plains to elevated O3 exposure (Ambient + 20 ppb) compared to ambient after three years of exposure using open-top chambers. Percent decreases were found in above (26%; p ≤ 0.002) and belowground (30%; p ≤ 0.003) biomass under elevated compared to ambient O3 exposure. Percent decrements in total organic carbon (TOC; 24%; p ≤ 0.001), total nitrogen (29%; p ≤ 0.001) and available phosphorus (11%; p ≤ 0.002) in the soil were also observed under elevated O3 exposure. Exposure at elevated O3 reduced soil microbial biomass and activities of ß-glucosidase, amylase, urease and phosphatase, while polyphenol oxidase and peroxidase showed enhancement in their activities, showing negative effects on belowground soil health. Percent reduction in root shoot ratio (10%; p ≤ 0.05) depicts that less C-allocation towards root system led to a reduction in TOC in the soil, which could affect C-sequestration under elevated O3 condition in the semi-natural grasslands. Elevated O3 also affected enzymes participating in N and P-cycles, causing reductions in total nitrogen and phosphorus. The study concludes that projected O3 concentrations have serious implications for aboveground biomass as well as belowground soil health in tropical areas, identified as hotspots of O3 in the world.

Assessment of Ozone Sensitivity in Three Wheat Cultivars Using Ethylenediurea.

Three wheat (Triticum aestivum L.) cultivars [HD 2987 (ozone (O3) sensitive), PBW 502 (intermediately sensitive) and Kharchiya 65 (O3 tolerant)] with known sensitivity to O3 were re-evaluated using ethylenediurea (EDU; 400 ppm) to ascertain the use of EDU in determiningO3 sensitivity under highly O3-polluted tropical environments. EDU treatment helped in improving the growth, biomass, photosynthetic pigments and the antioxidative defense system of all the wheat cultivars. Under EDU treatment, PBW 502 retained more biomass, while HD 2987 showed better performance and ultimately the greatest increment in yield. Cultivar Kharchiya 65 also showed a positive response to EDU as manifested with an increase in pigment contents, total biomass and enzymatic antioxidants; however, this increment was comparatively lower compared to the other two cultivars. The results indicated that EDU did not have many physiological effects on cultivars but helped in counteracting O3 primarily by scavenging reactive oxygen species and enhancing the antioxidative defense system where superoxide dismutase emerged as the major responsive biochemical parameter against ambient O3. The observed results clearly indicated that differential O3 sensitivity in three wheat cultivars established by the previous study is in accordance with the present study using EDU as a sensitivity tool, which is an easy and efficient technology in comparison to chamber and Free-Air Carbon dioxide Enrichment (FACE) experiments although its mechanistic understanding needs to be further validated.