High Temperature

Heat stress occurs when a rise in temperature has negative consequences for a plant. It is a complex function of intensity (temperature in degrees), duration and the rate of temperature increase. For plants inhabiting very cold climates such as the Arctic, temperatures in the region of 15°C can already be a source of heat stress. In a temperate climate, heat stress takes place in the temperature range of 35-40°C. In scientific literature, heat stress denotes temperatures that exceed the optimum values by around 10-15°C (Larkindale et al. 2005). Plants can be divided into three groups, subject to their sensitivity to high temperature (Fig. 5.8) . In geographic zones with a hot climate, in habitats marked by high fluctuations in daily temperature (soil surface, littoral zone, shallow waters) or seasonal fluctuations and in volcanic areas, temperature levels can be lethal for vascular land plants. High absorption of solar energy during windless weather can increase the temperature inside plant tissues in excess of the ambient temperature. Creeping grass shoots, the runners and tillers of young plants can also be subjected to heat stress. The lethal temperature range (thermal death point) is determined by the duration of tissue exposure to high temperature (Table 5.2). Only single-celled organisms can complete their life cycle during continued exposure to temperatures higher than 50°C, while only prokaryotic organisms can survive in temperatures higher than 60°C.

3.1 Consequences of Heat Exposure

At very high temperatures, severe cellular injury and even death may occur within minutes or even seconds (due to denaturation and/or aggregation of proteins), while at moderately high temperatures,

Fig. 5.8 Classification of the plants, subject to their sensitivity to high temperature (adapted from Stushnoff et al. 1984)
Table 5.2 The lethal temperature range (thermal death point) characteristic for varied types of plants

Type of plants

Thermal death point

Aquatic plants and plants growing in shaded habitats

38-42°C following several hours of exposure

Temperate plants with hydrated and metabolically

45-55°C following several hours of exposure

active organs

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