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.

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Summary Data are presented which fully corroborate previous provisional findings that two populations of Peltigera canina differ quite markedly, when air‐dry, in their response to heat stress. In addition, observations upon a number of other species are described, and it was shown that thallus temperatures as low as 25 °C can severely stress Peltigera scabrosa. The design of the stress treatment paralleled naturally occurring stress periods and temperature levels found under summer field conditions. Stress effects were assessed by monitoring nitrogenase activity, net photosynthetic rates and respiration rates. The mycobiont usually has a much greater tolerance of thermal stress than the algal component, so that net photosynthetic rates of the lichens are seriously impaired whilst concurrent respiration rates can be fully maintained. This probably accounts for the extreme temperatures required by previous workers to induce a stress effect when this was defined solely by a reduction of respiration rates. The significance of these findings is discussed in relation to the ecology and geographical distribution of lichens.

Despite the contrasting thermal and light environment of sun and shade ecotypes of Cladonia stellaris, analysis of the pattern of net photosynthesis throughout the year as well as enzyme polymorphism in each morphotype has found little genotypic difference. The large disparity between photosynthetic capacity in the sun and shade replicates is shown to be due to a difference in chlorophyll content. There is a significant decline in photosynthetic capacity in November and this is shown to be reversible uncoupling of energy transduction in some photosynthetic units during the winter period. The control of coupling and uncoupling of photosynthetic units is found to be mediated by both day length and thallus temperature and a double environmental signal in June and July of short days coupled to low temperature will initiate and finalize the event within 12 h. This response pattern is discussed in relation to identical capacity changes in other lichen species. Apart from a single isoenzyme difference in esterase polymorphism, no differences in the zymograms of leucine aminopeptidase, phosphoglucoisomerase, or acid phosphatase were detected and this overall genetic homogeneity was equally evident in a general protein gel. This unexpected degree of homogeneity is discussed in terms of the difficulties in assessing enzyme polymorphisms in different plant populations, without maximum refinement of protein preparation methods and electrophoretic techniques. It is concluded that the two quite distinct sun and shade morphotypes only reflect phenotypic differences. In contrast, however, significant differences are reported in the distribution of specific enzymes between the tops and bases of the podetia.

SUMMARYThe wetting burst, resaturation respiration and basal respiration in Peltigera polydactyla, P. praetextata and Nostoc commune has been examined using both infrared gas analysis and oxygen electrodes. The physical nature of the wetting burst is further confirmed by drying replicates in COj-free air, after which the burst is eliminated. The data also shows that resaturation respiration is very dependent on prior events in the field. A prior fast drying rate produces higher resaturation respiration rates than does slow drying. However the length of time a lichen remains fiiily saturated is largely responsible for the amplitude of resaturation respiration and induces a very evident seasonal component in P. polydactyla. The extent of carbon loss and its potential ecological impact is questioned. INTBO'DUCTIONReid (1960) demonstrated a pronounced increase in the respiration rate of a number of lichen species, immediately following thallus rehydration. He also showed an apparent correlation between the size of this ' respiratory burst' and the corresponding habitat characteristics of contrasting lichen species. The 'respiratory burst' was apparently more pronounced in lichens from mesic habitats and this relationship has since been reported by Smith and Molesworth (1973) and Earrar and Smith (1976). Dilks and Proctor (1974) and Krochko, Winner and Bewley (1979) have also used the magnitude of the 'respiration burst' as a measure of drought stress. There have been a number of suggestions as to the mechanism of the 'respiratory burst', and ali relate it to some aspect or other of uncontrolled metabolism immediately following thallus rehydration. Krochko et al. (1979) suggest an uncoupling of mitochondrial respiration, whilst Farrar and Smith (1976) point to a breakdown in cell compartmentalization as a result of the period of dessication. Conversely Dhinsda and Matowe (1981) indicate an increase in lipid peroxidation during the re-establishment of membrane integrity to be a more likely cause. However, no hard evidence is available to support any one of these explanations more than the others. Despite this absence of a causal explanation it is very evident that a simple increase in respiration is not the sole source of the nnodified gas exchange which occurs in the first few hours following rewetting.Smith and Molesworth (1973) define three distinct stages that characterize the complete sequence of events in Peltigera polydactyla: the first minute is characterized by a large and rapid, non-metabolic release of COj. This is followed