Low Temperature

Periodic temperature drops below zero degrees are reported on around 64% of the Earth's surface. The lowest temperatures are noted in Antarctica, reaching around -50°C in coastal areas and up to -90°C in the interior. The minimum temperature at which a given species can survive is one of the main criteria determining plant distribution on our planet. In a temperate climate, low-temperature stress eliminates or inhibits the growth and yield of valuable plants and crops (Xin and Browse

Fig. 5.4 Schematic illustration of the influence on growth and morphology of roots and above-ground organs of potato seedlings (adapted from Marschner 1995)

2000; Jan et al. 2009). Plants indigenous to colder regions are usually well adapted to chilling temperatures and are, therefore, not significantly impaired by cold periods, apart from a general slowing down of the metabolic rate and growth. In a temperate climate, plants respond differently to freezing temperatures and the winter environment than other factors that occur irregularly. In the winter, chilling temperatures do not come as a surprise for plants that have adapted to the periodic, adverse vegetation factors in the course of evolution. Low temperatures are accompanied by short daytime and low radiation intensity. The adaptation to growth inhibiting factors is characteristic of the dormant state (Jan et al. 2009) .

There are two types of injuries a plant can sustain through exposure to low temperatures (Fig. 5.5). On the other hand, many plants that are native to cold climates can survive extremely low temperatures without injury (Levitt 1980).

An analysis of freezing winter temperatures as an environmental stressor should also account for the impact of other adverse factors such as low light intensity and short daytime. The above conditions arrest the growth and development of vegetation (Hopkins 2006).

The plants' ability to survive freezing and other adverse temperature changes differs from the remaining stressors. Levitt's stress avoidance theory (1980) does not apply in this case. Plants are unable to avoid freezing temperatures, and they can only protect themselves from the negative consequences of cold by increasing their tolerance to chilling. Many plants enter the dormant state to survive harsh winter weather. This is a typical feature of adaptation to freezing which is a genetically inherited trait.

Plants can be classified into three categories based on the range of lethal temperatures and the characteristics of mechanisms conditioning their resistance to low temperatures (Fig. 5.6).

2.1 Consequences of Chilling and Freezing Stress

There are two theories explaining the plants' primary response to temperature stress. The first concept, formulated by Lyons (1973), states that low temperatures induce the phase transition of cell membranes where a liquid-crystal structure is transformed into a crystal (gel) phase.

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