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Effects of fluctuating levels of ozone or nitrogen dioxide alone and in sequence on plant growth and photosynthesis

University of British Columbia

Radish, Raphanus sativus L., plants were exposed daily to ozone, O₃ , or nitrogen dioxide, NO₂ , or sequences of the two pollutants. The exposure profiles for both gases approximated sine waves with peak concentrations of 120 ppb (parts per billion by volume; n1 l ⁻1). In the case of O₃ , this is close to the reported threshold for adverse effects while with NO₂ it is below the reported threshold. The sequences involved different combinations of exposures to NO₂ from 06:00 to 10:00h and/or 18:00 to 22:00h and O₃ from 10:00 to 18:00h. O₃ alone caused no significant effects on growth after 21 days but N0₂ alone early in the day caused a small growth increase. Increases in the quantum yield of chlorophyll fluorescence and in photochemical quenching observed in both treatments after 6 days of exposure indicated increased photosynthetic CO₂-assimilation that, in the case of O₃ , was presumably offset by increased maintenance/repair respiration since no increase in dry matter occurred. Exposures to NO₃ in sequence with O₃ had increasingly negative effects on growth. Since O₃ tended to prevent stomatal closure early and late in the day, the estimated NO₂ fluxes in the sequential treatments were greater than those with NO₂ alone. However, since growth reductions were observed with the sequences, the mechanisms of stimulation of assimilation were counteractive. No visible necrotic injury was observed in any treatment and O₃ alone resulted in a significant effect on ethylene release, a decrease. This suggests that O₃-induced increases in ethylene release reported in the literature may have reflected the occurrence of incipient or actual foliar injury. The lack of growth effects due to O₃ alone but their occurrence with NO₂-O₃ sequences indicates that exposures to other pollutants in ambient air must be considered in establishing objectives or standards of air quality to protect vegetation. Studies with a C-3 grass, Poapratensis L., Kentucky bluegrass, and a C-4 grass, Setaria viridis Beauv., green foxtail, treated with similar sine-wave exposures to O₃ within the modified cavity of an electron paramagnetic resonance (EPR) spectrometer, permitted observation of the effects of exposure on both the free radical signals in photosystems I and II and on chlorophyll fluorescence attributable to photosystem II. Both Signal I (from P700⁺ in PSI) and Signal II (from Tyr-160 in the D2 protein of PSII) were stimulated by O₃₋ However, the fact that Signal I observed in white light in bluegrass during exposure to O₃ rose to the level of Signal I in far-red light indicates reduced electron flow through PSI. These effects and concomitant effects on chlorophyll fluorescence confirm a major effect of O₃ on the water-splitting side of PSII. In Kentucky bluegrass, the level of Signal I in white light was approximately one half of that in far-red light, while in foxtail the situation was reversed presumably because of a contribution by PSII light-harvesting to PSI in the bundle-sheath chloroplasts. Measurements of the relative levels of Signal I in white or far-red light may therefore provide a means of assessing the extent of such contributions in other C-4 species.

Thesis/Dissertation

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2009-04-03

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http://hdl.handle.net/2429/6789

Plant Science

University of British Columbia

UBCV

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