O3 with aquas solution is superoxide radicles

O3from the atmosphere interact with the leaf surface, are partitioning into somedegraded on the leaf surface and remaining passes through the stomata of theleaf. Depending on different leaf surface (Altime et al., 2006), moisture (Zanget al., 2006), and temperature (Fowler et al.

, 2001), distruction level varywith these property. The remaining O3 reaches to intercellular spaceswhich react in different manners with water phase of cellular medium resultingin decomposition of O3, with olefins, antioxidants and is absorbedby the cell wall of the mesophyll cells. Many physical factors affects the O3uptake were PAR, temperature, vapour pressure deficit , ambient CO2,soil water potential and stomatal conductance (Fuhrrer, 2008).Internally O3first interact to form free radicles and peroxides  and the process is very fast in the presenceof high pH. O3 may brings changes in the pH of the solution and  the value changes from 6 to 8 pH doesnotcauses any cellular damages as this reaction carried out in apoplast away fromcellular metabolic process. The first free radicles after interaction withaquas solution is superoxide radicles (O2.

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-) ‘ followed by hydroxyl radicles(OH), singlet oxygen (O21), and hydrogen peroxide (H2O2)(Azada 1980).Olefins ( Ethylene and isoprene) releases by plants presence inair cavities and cell wall  react with O3. Apoplastic ascorbic acid react with the O3, limits the O3 damages  ( Eller and Sparks, 2006) and the plants whichis less sensitive to O3 may contains high content of it.

Both reduced andoxidized ascorbate are present, reduced ascorbate undergo oxidation in thepresence of O3 to formed oxdised ascorbate in the apoplastic region. Thisoxidized ascorbate is transfer to cytosol for reduction and back to apoplast. Inlong term fumigation, an injury symptom is appearing because antioxidant levelsare fall and O3 reachs to plasma membrane. In addition to ascorbic acid, phenoland amino acid having sulfylhydryl groups are antioxidant present in apoplasticregion. Plant is protected in the presence of enzymes like superoxide dismutaseand peroxide which act against the free radicles and peroxides formed fromreaction between O3 and unsaturated hydrocarbons.

If somehow O3 escape fromapoplastic region and reaches to plasma membrane, it either react with plasmamembrane or diffuse to cytosol in the case high O3 concentrations. Twodifferent fate, when O3 is reaches at plasma membrame , it either react withantioxidant system or/and  sense byproteinous receptor acting as chemosignalling, thus signaling pathway isgenerated with the help of intracellularly located secondary messengermolecules. Reactive oxygen species (ROS) produces by decomposition of O3molecules in intercellular space, cell wall and plasma membrane are also actingas signaling molecules. On the other hand, O3 may directly penetrate to plasmamembrane, reaches to the cytosol , is either decompose to form free radiclesand peroxides or react with cellular organelles. More frequent study related tomembrane permeability as O3 causes increases in conductivity resulting inpassive moment of H2O, ions (K, Ca, Mg) and some organic molecules. This leadsto loss in cell turgidity (Guderian 1985) and releases of ions is not becauseof membrane breakage, rather it react with sulfhydryl groups of membrane ATPase(Chevrier et al.

, 1990).Incase of community level study, here comes the concept of canopy architectureand species composition. In respose to O3, species composition may changeresulting in canopy change of the community (fuhrrer, 2008).

Canopy conductance(gc) at different levels are vary in the community with the fact that low gc atlower conopy layers compared to upper canopy layer. This factor is not becauseof low O3 at low layer of canopy rather it because of low PAR enrichment(Davison et al., 2003).

Therefore canopy uptake is again affect by many factorslike temperature, PAR value and vapour pressure deficit. Grassland are moredamaged after harvested with the fact of no more canopy layers and species aredirectly exposed to O3 concentration results in changes in communityproductivity (Ainsworth, 1995)


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