REDUCTION IN THE PHOTOSYNTHETIC RATE OF ACER, QUERCUS AND FRAXINUS SPECIES CAUSED BY SULFUR-DIOXIDE AND OZONE

The relative sensitivity of leaf photosynthesis to fumigation with sulphur dioxide (SO2) and ozone (O3) was determined over a range of light intensity and relative humidity for sugar maple Acer saccharum Marsh, black oak Quercus velutina Lamb and white ash Fraxinus americana L. Treatments included fumigation at low (0.2-0.4), intermediate (0.5-0.6) and high (1.3-1.5 cal cm-2 min-1) light intensity and low (22-43%) and high (55-92%) relative humidity. After one week of fumigation at low humidity and low light intensity the rate of photosynthesis was 52, 46 and 80% for treatment with 50 pphm (1.4 mg m-3) SO2, 52, 73 and 100% for treatment with 50 pphm (1.1 mg m-3) O3 and 56,59 and 62% for treatment with 50 pphm SO2 plus 50 pphm O3, respectively for black oak, sugar maple and white ash. After three weeks of fumigation SO2 reduced photosynthesis to 26, 57 and 93% while O3 reduced photosynthesis to 57, 45 and 94%, respectively in black oak, sugar maple and white ash. Visible symptoms did not appear during fumigation for any of these experiments except that the onset of autumnal coloration for some fumigated leaves occurred up to two weeks earlier than for non-fumigated leaves. Fumigation for one week with 50 pphm SO2 plus 50 pphm O3 at high humidity and high light intensity reduced photosynthesis to 15% in sugar maple and to 30% in white ash. Visible symptoms of red or purple discoloration for white ash and reddish flecking in sugar maple occurred for all leaves fumigated at these conditions. The photosynthetic rate of leaves exposed to SO2 plus O3 at high relative humidity and either intermediate or low light intensity was reduced to 44 to 65% with visible symptoms occurring on some, but not all, replicates. © 1979.