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

• Main Author: Mazarura, Upenyu
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6789

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.