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.