Thermal time

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The thermal time is a parameter used in biology to describe the temperature dependence of development processes in cold-blooded organisms . It corresponds to the summed up product of temperature (often the daily mean temperatures) and duration.

Basics

All cold-blooded organisms are dependent on a certain range of ambient temperatures for development and growth. Development is only possible above a certain minimum temperature. If the temperature continues to rise, it is accelerated until an optimum temperature is finally reached. If the temperature rises further, it is inhibited again until finally no growth is possible again above a maximum temperature. It is often observed that the growth rate in the range between the minimum and optimal temperature increases approximately linearly in proportion to the temperature itself. Since the final size of animals and plants is often more or less constant within a certain range of fluctuation, the growth rate automatically determines the duration of the organism's development . This means: within the relevant temperature range, the temperature sum and development time are equivalent to one another. The product of the two is called the "thermal constant", it is usually given in "degree days". A constant value can thus be used instead of the temperature-dependent development time itself. This development time normalized according to the temperature (measured in degree days instead of days) is the thermal time. On the same basis, if the change in growth rate has been measured over a certain temperature range in the laboratory or greenhouse (under controlled temperature), the minimum temperature for the respective species can be determined by extrapolation.

The linear increase and thus the equivalence of temperature and time is not a matter of course. Actually, based on the RGT rule , one could assume an exponential relationship. A linear relationship has been confirmed empirically. It may be based on an increasing inactivation of enzymes above their optimal temperature, which cancels out the (thermodynamically based) increase when the temperature rises. The linear relationship is particularly evident in plants and insects, while in cold-blooded vertebrates the relationships are more complex. Due to the linear increase it is u. a. explain why z. B. tropical plants, which (at higher temperatures) have a faster development time, at the same time must have a higher minimum temperature for development.

application

The concept of thermal time is mainly used within biology in the application-oriented disciplines. On its basis, for example, development or germination times of widespread crops or pests in certain climatic zones are determined. Sowing and harvesting dates can be optimized. The concept can also be used to predict changes in the event of climate change. The concept is less commonly applied to natural ecosystems.

literature