Vegetative buds conditions under which the individual steps of development take place. This can be illustrated by the example of germination in contrast to growth and development. Seeds which germinate in the autumn or winter (such as those of winter cereals) do so at temperatures that are considerably lower than those relevant for germination in the spring or summer. Independently of the temperature range in which germination is possible, the rate of germination itself shows the typical temperature dependence of biological processes (Q10 between 1.5 and 2.5; see Chap. 1.3.2). Such germination behaviour can be modified by inhibitory mechanisms (e.g. the requirement for a prolonged exposure of imbibed seeds to cold - stratification - or the fire-dependent opening of seeds in pyrophytes), which prevent premature germination at unfavourable times.
Frost represents a long-term stress situation in contrast to, for example, fire, and can last for hours, days and even months. Frost acts not only as a temperature stress, but primarily as a drought stress due to the freezing of tissue water. Plants undergo a process of hardening during the advent of the colder season or in the presence of permanent frost during their juve nile development. This enables them to withstand even the sometimes extremely severe frosts which occur in nature (air temperatures as low as -70 °C have been recorded in Siberia).
Temperature stress does not occur only in extreme locations, and plants can be subjected to large ranges of temperature even during the course of a "normal" day (see Chap. 2.1, section on radiation climate). Temperatures of above +50 °C occur frequently at the immediate surface of the soil during extended periods of solar irradiation, particularly if there is no plant growth and the soil is dark and thus reflects little of the incident sunlight. However, air temperature decreases rapidly with the distance from the soil surface, and the temperature gradient within the soil itself is even steeper (see Figs. 4.3.2 and 2.1.6). Whereas an air temperature of +40°C was measured about 10 cm above soil exhibiting a surface temperature of +50°C, the soil temperature at a depth of 10 cm was below 25 °C (see Fig. 2.1.6). Temperature gradients within the soil and within the surface-near air layer also occur on clear nights, whereby their direction is reversed and their amplitude is of a lesser magnitude. Cooling of surfaces to tempera-
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