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BIOCHEMICAL EXPLANATIONS OF MORE THAN ADDITIVE INHIBITORY EFFECTS OF LOW ATMOSPHERIC LEVELS OF SULPHUR DIOXIDE PLUS NITROGEN DIOXIDE UPON PLANTS

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SUMMARY

Three clones of Lolium perenne L. known to show different sensitivities to SO2 pollution, together with other grasses (Dactylis glomerata L., Phleum pratense L. and Poa pratensis L.), were fumigated with low levels of SO2, NO2 and SO2+ NO2. Ratios of glutamate dehydrogenase activity (GDH) to that of glutamine synthetase (GS) were significantly raised by SO2 (6·8 parts 10-8 for 11 weeks) or SO2+ NO2 in the SO2-sensitive cloned cultivar S23 of L. perenne and by SO2+ NO2 in cloned Lolium perenne material which has high resistance to SO2 (S23 Bell resistant). In Lolium derived from Pennine upland (Helmshore clone) GDH/GS ratios were unchanged in the presence of SO2 or SO2+ NO2 but all three clones showed slight elevations in ratios in response to NO2 fumigation alone.

Within a week after treatment with NO2, all grasses had significantly higher nitrite reductase activities than similar plants given clean air, SO2 or SO2+ NO2 treatments. The presence of SO2 appeared to destroy the ability of the plant to respond to NO2, and this inhibition of a potential detoxification mechanism of nitrite is believed to be one of the main reasons why the SO2+ NO2 combination exhibits more than additive effects upon the growth of grasses.

All three clones of Lolium perenne and also Phleum pratense showed enhanced ATP formation in the presence of low (6·8 parts 10-8 for 20 weeks) levels of NO2, but reduced cyclic photophos-phorylation in the presence of either SO2 or SO2+ NO2. At higher levels of shorter duration (25 parts 10-8 for 11 days), rates of ATP formation, ATP content and energy charge ratios were higher in NO2 fumigated tissue than in controls, but lower in SO2+ NO2 treated tissue, although basic electron transport systems were unaffected. It is believed that the principal effect of the pollutant combination is upon proton gradients within the photosynthetic membranes leading to general deficiency in ATP which is necessary both for growth and the repair of secondary pollutant damage elsewhere.

Parts 10-8 (parts per hundred million) = quantity in μ m-3× (1/mol. wt × 106) × 0·0224 × 108 at s.t.p. * for 11 weeks) or SO2+ NO2 in the SO2-sensitive cloned cultivar S23 of L. perenne and by SO2+ NO2 in cloned Lolium perenne material which has high resistance to SO2 (S23 Bell resistant). In Lolium derived from Pennine upland (Helmshore clone) GDH/GS ratios were unchanged in the presence of SO2 or SO2+ NO2 but all three clones showed slight elevations in ratios in response to NO2 fumigation alone.

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