Environmental stress and Umbilicaria lichens: the effect of high temperature pretreatments.

The heat sensitivity of net CO2 exchange in four Umbilicaria species has been examined. Plants were exposed to pulses of heat from 2 to 24 h, were exposed either air-dry or saturated with water, and then were examined repeatedly under stress-free conditions to see if the responses were stable over many days or weeks. The initial responses found were maintained in all cases up to one month implying that the effects were not reversible. Hydrated plants were much more sensitive than dry plants, but in both cases the sensitivity increased with exposure time. All species showed basically the same critical temperatures thus suggesting that selection in the field was not operating on the basis of differential heat sensitivity. This idea was supported both by observations of thallus temperatures in the field during periods of hot weather, and by observations of thallus temperatures exposed to controlled high temperatures. Both approaches suggested that temperatures high enough to induce heat stress in the field, do not occur.