The temperature of chilled seedlings was recorded by a copper-constantan thermocouple with sensitivity of about 0.025 C (wire diameter 0.1 mm) connected to the input of a high-sensitive microvoltmeter. For the measurement, seedling shoots (3 g) were tightly packed in a small container at 20 C and then transferred to a thermostat with an experimental temperature (0 or -4 C). Temperature changes were recorded for 1 h. The shoot sample then was placed in hot water (95 C) to stop all metabolic processes, and then the temperature changes were recorded in killed samples cooled from 20 C to the experimental temperature. Thus, we obtained temperature curves following chilling with one tissue sample for living and for dead tissue and calculated the temperature difference (DT0) between "killed" and "alive" seedling shoot tissue.
The study of an influence of cold shock on temperature of maize shoots showed that maize seedlings, like winter wheat seedlings, are able to generate heat during cold stress (Fig. 1). When maize seedlings were exposed to cold shock at 0 C, the temperature difference between "alive" and "killed" seedling shoots was up to 1.25 — 1.5 C during the 20 min of cold shock. Subsequent chilling of maize shoots caused the reduction of temperature difference between "alive" and "killed" seedling shoots to 0.5 C. At the same time, the results show that increasing the cold stress intensity caused an increase of heat production by maize shoots at the first moment of cold shock: if the maximum temperature difference between "alive" and "killed’ shoots at 0 C was about 1-1.5 C, then at —4 C it was about 3-3.5 C (Fig. 1). At the same time, at —4 C after 35 min of cold shock temperature difference between "alive" and "killed’ shoots was not detected — seedlings were killed by low temperature. Therefore, based on the data obtained we can conclude that in maize a low-temperature stress defense mechanism exists that involves heat generation by seedling shoots.
1. Temperature difference between alive and killed shoots of maize
at 0 C (1) and —4 C (2).
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