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SUMMARY Mature apple (Malus domestica Borkh.) trees were exposed to episodes of 100 and 150 nl I1 ozone in 1990 and 1991 respectively, in open-top chambers at the University of Nottingham, Sutton Bonington Campus, UK. The episodes were for 8 h daily and for not more than 3 d in succession, with a seasonal total of 37 d in each year. Leaf fall was monitored by counting leaves on marked shoots at frequent intervals. A significant interaction between ozone exposure and leaf age was observed: exposure to ozone accelerated autumn leaf fall in old leaves. The relationships between experimental conditions, leaf age and plant material are discussed, and the implications for UK flora are considered.

The half‐lives of leaves of 202 taxa of vascular plants were estimated mostly from the literature and ranged from 1 to 350 wk. The longevity of leaves in the plant groups studied is in the order Pinophyta > Pteridophyta > Liliatae > Magnoliatae. Substantial differences are evident between the superorders of flowering plants proposed by Dahlgren and the subclasses proposed by Cronquist. The mean leaf half‐life for families of Magnoliatae was shown to be inversely related to their Advancement Index. Length of life of leaves, therefore, appears to have taxonomic significance, with more primitive vascular plants tending to have longer‐lived leaves than the more advanced. The ecological significance of leaf half‐life is less clear, except in that annual leaf fall is not necessarily associated with deciduousness. The capacity to shed individual leaves may give the angiosperms a flexibility in environmental response which allows them a competitive advantage over the gymnosperms except in the most stressful habitats.

SUMMARY The sensitivity of five Pinus and three Picea populations to SO2 was compared in various types of fumigation experiment (i) > 9 months at 200 μg m−3, (ii) 6 h at 8000 μ m−3, (iii) 9 weeks at 800 μg m−3, (iv) 12 h (overnight) at 17 000 μg m−3 preceded by either 42 days or 6 h at 800 μg m−3. 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 μg m−3(measured as relative growth rate) and the degree of acute injury at 8000 μg m−3. In general the populations of Picea were more sensitive to low concentrations of SO2 but more resistant to acute injury than those of Pinus. At 800 μg m−3 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 SO2 cannot be used to predict the responses of conifers to low concentrations of SO2 in the field.

Summary Little attention has been paid to the potential importance of light in the ecology of lichens. Data is presented which shows that Peltigera canina var. praetextata from a woodland floor can rapidly acclimate to low or high light levels and thus maintain a given level of photosynthesis irrespective of the leaf canopy status. This photosynthetic adjustment takes place naturally in the field during leaf fall in October. P. scabrosa shows a similar response to low light and furthermore, acclimation to low levels of illumination can readily take place in air-dry material. In contrast populations of Peltigera aphthosa collected from the dense shade of spruce in northern Ontario are extremely sensitive to quite modest levels of ‘high’ light, with pronounced damage occurring under 350 μE m-3 8-1 illumination. Contrasting populations of P. aphthosa collected from open habitats are not affected by this level of illumination. The maintenance of the correct day length during air-dry storage is also very important since a rapid alteration of daylength produces a marked although temporary photosynthetic decline. On the other hand continuous daylight induces a rapid and continuous decline of photosynthesis probably leading eventually to the death of the phycobiont.