SUMMARYGenotypes of Lolium perenne, Festuca rubra, Dactylis glomerata and Holcus lanatus which appeared to be differentially sensitive to acute or chronic SOj injury were compared in various morphological and physiological studies to examine some possible mechanisms of tolerance. Stomatal number and size showed no relationship with tolerance, but stomatal resistance in clean air showed a relationship with intra-and interspecific differences in tolerance to acute injury. Also, acute doses of SOj increased stomatal resistance in tolerant, but not in sensitive species.Membrane permeability was increased by acute doses of SO2, but there were no differential effects which could be related to tolerance. Uptake of ^^COg by leaf segments in buffered solution was reduced in the presence of sulphite to a greater extent in genotypes of Lolium perenne sensitive to chronic injury. Prefumigation for 13 months with a low concentration of SOj increased the sensitivity to sulphite, especially in the sensitive genotypes.The species tolerant of acute SO2 injury, Dactylis glomerata and Holcus lanatus, had lower rates of '^COg fixation than sensitive species, and were affected less by sulphite. There were no relationships between prefumigation or sensitivity to chronic injury and the uptake or oxidation of sulphite in three Lolium perenne genotypes. Species sensitive to acute injury absorbed morê^S Oj than tolerant species. Oxidation of the absorbed radioactivity was fastest in the most tolerant and slowest in the least tolerant species.It is concluded that many of the differential responses to acute injury can be explained by avoidance mechanisms. Some potential mechanisms of tolerance to chronic injury are discussed.

SUMMARY First‐trifoliate leaves of four‐leaved soybean plants were exposed to 35SO2 in the light or dark, and the uptake and translocation of radioactivity measured. Nearly four times more 35SO2 was taken up in the light (stomata open) than in the dark (stomata closed) but approximately 50% of the radioactivity was leached from the leaf surface in the dark compared with 5% in the light. A greater proportion of the source‐leaf radioactivity was translocated in the dark but, in absolute terms, the quantity of radioactivity moved was less. Radioactive products were found in all parts of the plant after exposure in both light and dark. The expanding trifoliate leaf contained the greatest concentration of translocated radioactivity in both cases. Light and dark pre‐ and post‐treatments had little effect on uptake of 35SO2 or translocation of the products. 35SO42− was detected in the nutrient solution within 1 h of exposure of leaves to 35SO2.

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SUMMARYThe sensitivity of five Pinus and three Picea populations to SOg was compared in various types of fumigation experiment (i) > 9 months at 200 /ig m-^, (ii) 6 h at 8000 /ig m-^, (iii) 9 weeks at 800 fig m », (iv) 12 h (overnight) at 17 000 /xg vcT^ preceded by either 42 days or 6 h at 800 fig m ^ To complement the fumigation experiments the growth of four of the populations was compared at a polluted (London) and a control site (Silwood Park).There was no relationship between the relative sensitivity of the populations to 200 /ig nC( measured as relative growth rate) and the degree of acute injury at 8000 fig mT^. In general the populations oi Picea were more sensitive to low concentrations of SOg but more resistant to acute injury than those of Pinus. At 800 fig m~^ the initial symptoms were of acute injury, but later whole needles died and were shed. Populations prone to the initial injury were not necessarily sensitive to the later type, and vice versa.Leaf fall was accelerated and dry wt gain depressed at the polluted site compared with the control and these effects were much greater in Picea than in Pinus populations.It is concluded that the order of sensitivity obtained in screening experiments depends largely on the conditions used, and that short-term experiments with high concentrations of SOg cannot be used to predict the responses of conifers to low concentrations of SOg in the field.