The Temperature Challenge. Temperature Dependence of Life

Temperature is the second dominant factor (after precipitation) that determines the global structure of vegetation. On more than 60% of the continental surface of the Earth, frost can occur at least temporarily. Because of the fundamental importance of water for life, frost (i.e. temperatures below the freezing point of water) is a major threat for any type of organism. This applies in particular to poikilothermic organisms such as plants, which equilibrate with the temperature of their environment. Therefore, frost tolerance is a major factor for the distribution of the terrestrial vegetation on Earth.

On the other side, even in the hottest regions (with the exception of volcanoes)—Al’Aziziyah in Libya and Death Valley in California, USA, with the highest ever recorded temperatures of 57.8 °C and 56.7 °C, respectively—higher plant life is possible, provided that moisture is available. Extreme temperatures are commonly accompanied by other stresses— in particular, a shortage of water, which is either frozen at sub-zero temperatures or simply not available in hot places.

Temperature Dependence of Life. For most organisms, temperature ranges of active life can be determined, as well as temperature limits beyond which life is not possible. Since metabolism requires water, active life is confined to conditions under which water is in the liquid state, and only under high pressure (e.g. in the deep sea) is life possible at temperatures around and even above 100 °C.

On the other side, freezing of cellular liquids can be avoided—for example, by high concentrations of compatible solutes or antifreeze compounds, such as specialised proteins—and therefore life processes can continue even in a small temperature range below zero, albeit at low intensity. A dose-response diagram for temperature usually shows an asymmetrical curve with an optimum range of about 10 K, a steep drop towards higher temperatures and a smooth decline towards lower temperatures (Fig. 4.1). Imbalances of metabolism increase with deviation from the optimal range, thereby hampering cellular and developmental processes such as growth and reproduction.

Fig. 4.1 Life processes of an organism, described as a function of temperature. The relative growth rate (R) or percentage of survival, respectively, may be used as a measure of life:

On the high-temperature side, heat damage to proteins contributes the most to such imbalances, while on the low-temperature side, membrane functions and different rates of deceleration of enzymatically catalysed reactions are the main reasons for the occurrence of imbalances. For each organism a temperature range of hardly detectable life functions can be derived. Extreme temperatures beyond this range result in lethal damage.

Within the tolerable temperature range, the extent of damage does not exceed the capacity of a plant for repair at more favourable temperatures. This status of “latent life” is known as one of the five cardinal points of temperature tolerance of an organism (cold death, cold latent life, temperature optimum for active life, heat latent life and heat death) (Fig. 4.1).