| Tropospheric ozone (O3) is a naturally occurring greenhouse gas formed as a product of photochemical reactions with precursors:NOx and volatile organic compounds (VOCs). Over the industrial period, anthropogenic precursor emissions from fissil fule and biomass buring have led to elevated ozone concentrations. Ozone is known to damage plants and ozone levels can lead to stomatal closure, which reduces the uptake of ozone concentration, and in turn limits the damaging effects of ozone. Ozone coused cellular damage inside leaves that adversely reduced photosynthetic rates and stomatal conductance, dry matter accumulation and partitioning, and accelerate leaf senescence.Different species have different ozone sensitivities, and species exhibit significant interspecies variation in the impacts of elevated ozone, and the responses of stomatal conductance to ozone have interspecies variation. Ozone uptake at the level was predominantly controlled by stomatal aperture. Factors that influence the stomatal conductance can alter the flux of ozone into intercellular spance of leaf. Water stress was believed to be one of the most important modifying factors of ozone sensitivity. As water stress was accompanied by decreased stomatal conductance, decreasing ozone damage to the leaves. Increased nitrogen may result in below-ground competition for resources shifts to above-ground competition. Some experiments suggested that nitrogen supply could reduce ozone sensitivity or protect plants against to ozone damage or accelerate leaf senescence.In this experiment, three wheat species were selected as the test materials to explore effects of different water stress and nitrogen on the ozone sensitivity. The main results were as follows:1. Effects of ozone on winter wheat and its relativesO3significantly (p<0.05) decreased PNsatå’Œgs.Overall, there were considerable interspecific variations (p<0.05). T.aestivum (-10.15%) displayed higher PNsat values than T.monococcum (-8.03%) and T.turgidum (-4.79%). The decline of dry matter accumulation was caused by the decrease in P>Nsat.O3decreased shoot dry matter (-16.67%), root dry matter (-32.67%) but increased root/shoot ratio (34.17%). The species showed03-induced reduction in Fm, Fv/Fm and Yield, but increase in qP and NPQ of24.70%and32.61%, respectively. Effect of03-induced on Fv/Fm of T.monococcum (-5.38%) was higher than T.aestivum (-2.08%). T.turgidum and T.monococcum showed03-induced increase in NPQ of42.21%and48.56%, repectively, compared with20.93%in T.aestivum.Winter wheat exhibited significant interspecies variation in the impacts of elevated ozone on photosynthesis, chlorophyll a fluorescence and growth. T.aestivum was more sensitivity to ozone followed by T.monococcum, but T.turgidum appeared to be the most ozone-tolerant.2. Effects of drought on the ozone sensitivity of winter wheat and its relativesCompares with the normal water supply, drought significantly decreased PNsat, gs, shoot dry matter, root dry matter and total matter. Drought caused significantly reduction in ozone flux by partial stomatal closure in species, but these three species showed significant remarkly different responses to elevated ozone and drought. Drought resulted in interspecies differences in winter wheat and its relatives in the response to ozone. Combined drought and ozone exposure significantly reduced PNsat of T.turgidum by13.41%, but non-significant reduced in T.monococcum and T.aestivum by5.88%and2.17%, repectively. Effect of03-induced on chlorophyll a fluorescence paremeters of T.aestivum and T.monococcum were higher than T.turgidum. Combined drought and ozone significantly (p<0.05) increased qP and NPQ of T.aestivum and T.monococcum, compared with no significant difference in T.turgidum. Plant resposens to ozone stress treatments were assessed by determining in vivo biochemical parameters, gas exchange, chlorophyll a fluorescence and biomass matter. T.turgidum and T.monococcum demonstrated higher O3tolerance than T.aestivum, with the latter exhibiting higher drought tolerance than the former. T.turgidum and T.monococcum lost O3tolerance, while T.aestivum showed improved tolerance to O3under combined drought and O3exposure. These indicated that the existence of differential bebaviour of the three wheat species between O3stress and the combination of drought and O3stress.3. Effects of nitrogen adding on the ozone sensitivity of winter wheat and its relativesNitrogen adding caused significantly (P=0.000) interspecies differences in PNsat, gsand dry matter accumulation of winter wheat and its relatives in the response to ozone. Nitrogen adding significantly increased PNsat, gs, biomass matter of T.turgidum followed by T.monococcum and T.aestivum. Nitrogen made a big difference to shoot dry matter than root dry matter. Nitrogen significantly affected the biomass of T.monococcum combined nitrogen and ozone made higher effects on the T.turgidum followed by T.aestivum; increased the dry matter accumulation of T.monococcum. Nitrogen adding increased Fm, Fv/Fm and Yield, but no significant difference. Nitrogen reduced qp and NPQ of T.aestivum and T.monococcum, compared with increase in T.turgidum.Combined nitrogen and ozone exposure reduced PNsat and gs of T.aestivum, followed by T.turgidum and T.monococcum. The interaction between ozone and nitrogen stresss reduced shoot dry matter, root dry matter and total dry matter, and indicated more decrease in root dry matter than shoot dry matter. Compared with a single O3stress, the combination of nitrogen and O3stress reduced Fm, Fv/Fm and Yield but increased qP and NPQ. Significantly (P<0.01) reduced Fm, Fv/Fm and Yield of T.aestivum, followed by T.turgidum and T.monococcum. Overall, nitrogen adding may mitigate the effects of elevted O3stress on photosynthesis and biomass in wheat species.4. Effects of nitrogen and drought on the ozone sensitivity of winter wheat and its relativesThe combination of drought and nitrogen adding reduced PNsat and gs, and wheat species exhibited significant interspecies variation. T.turgidum and T.monococcum demonstrated higher decrease in PNsat than T.aestivum. The decrease in shoot dry matter, root dry matter and total dry matter caused by the combined of drought and nitrogen adding, which inhibited higher decrease in shoot biomass than root biomass. Under the ozone stress, the combination of drought and nitrogen adding reduced PNsat and gs, and wheat species exhibited significant interspecies variation. The combination stress significantly reduced the T.aestivum and T.monococcum, but no significant difference in T.tuigidum. Under the combination of ozone, drought and nitrogen adding, compared with drought, nitrogen adding may increase the interaction of combination on PNsat, gs, shoot dry matter, root dry matter and total dry matter, exhibiting significant interspecies variations. Compared with nitrogen adding, drought may increase the interaction of combination on PNsat, gs, shoot dry matter, root dry matter and total dry matter of wheat species, exhibiting significant interspecies variations. And the combination stress caused significantly effects on T.monococcum, and higher decrease in root dry matter than shoot dry matter. Overall, under the high ozone concentration conditions, drought and nitrogen adding may increase the effects of combination stress on wheat species, in which the effect of drough stress on wheat was more significant than nitrogen stress. |
