Effect of sequences of ozone and nitrogen dioxide on chlorophyll fluorescence in radish
AbstractOzone (O3) is the most important of the phytotoxic gaseous air pollutants. It causes substantial decreases in crop yields worldwide and bears adverse effects on vegetation in general. On the other hand, nitrogen dioxide (NO2) is the air pollutant most likely to be associated with O3 because it is a precursor of O3. The objective of this study was to determine the effects of sequential exposures to nitrogen dioxide (NO2) and ozone (O3) on chlorophyll fluorescence in radish, Raphanus sativus L. Radish plants were exposed daily to O3 or NO2, or sequences of the two gases. The exposure profiles for both gases approximated sine waves with peak concentrations of 120 ppb (parts per billion by volume, nl l-1). In the case of O3, this is close to the reported threshold for adverse effects; while for 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 22:00hr and O3 from 10:00 to 18:00hr. Relative to the control, early and early + late NO2 resulted in stimulations of quantum yield (Y) and photochemical quenching (qP), with late NO2 resulting in little or no change. In contrast, early, late and early + late NO2 in combination with O3 resulted in progressive reductions in these variables. The overall effect of O3 treatment was to stimulate quantum yield and qP, both of which are indicative of increased photochemistry. Late NO2 exposures caused no significant effects relative to the control. However, late NO2 failed to result in a significant stimulation of photochemistry in the chloroplast, but caused significant residual increases in non-photochemical quenching (qN) during the middle of the day, responses which imply increased photo-protection capacity. Stimulation of quantum yield and photochemical quenching by O3, O3 + late NO2, O3 + early NO2, early NO2 and late + early NO2 implies that
CO2-fixation was limited by processes at PSII. However, apart from the early NO2 related stimulation of qN, all exposures involving O3 led to a decrease in qN implying inability to regulate photosnynthesis resulting from changes in the thylakoid membrane. In the case of NPQ, all exposures including O3, decreased this parameter suggesting impaired proper functioning of the xanthophyll cycle associated with the light-harvesting complex of photosystem II.