An influence of cold stress on temperature of maize shoots --Kolesnichenko, AV, Pobezhimova, TP, Grabelnych, OI, Tourchaninova, VV, Voinikov, VK It was earlier considered that, because of the particularities of the organism, plants are not able to adjust their temperature. However, in the 60's it was found that during the blossoming of Aroide, strong activation of alternative cyanide-resistant respiration causes thermogenesis to occur (Wilson, Smith, Z. Pflanzenphysiol. 65:124-129, 1971). This fact allowed some researchers to suggest that cyanide-resistant alternative oxidase can also participate in processes of plant thermoregulation during low-temperature stress (Vanlerberghe, McIntosh, Plant Physiol., 100:115-119, 1992). Recently it was found that uncoupling proteins, which are homologues of mammalian mitochondrial uncoupling proteins (UCPs), exist in plants (Vercesi et al., Nature, 375:24, 1995). Researchers who found these proteins supposed that they participate in plant protection from low-temperature stress (Laloi et al., Nature, 389:135-136, 1997). Some years ago cytoplasmatic protein CSP 310 was discovered, that also uncouple oxidation and phosphorylation in winter cereals' mitochondria during low-temperature stress (Kolesnichenko et al., Russ. J. Plant Physiol., 43:771-776, 1996). The mechanism of CSP 310 uncoupling action is still unknown but there are some data that show that CSP 310 is present in maize mitochondria (Kolesnichenko et al., J. Therm. Biol., 25:203-209, 2000). Previously it was shown that under cold shock (-4 C, 1 h), living winter wheat shoots can generate heat and their temperature was above 0 C for the initial 25-30 min (Vojnikov et al., Biochem. Physiol. Pflanzen., 179:327-330, 1984). We supposed that other cereals also could produce heat during cold stress. So, the present work was aimed at the investigation of an influence of cold stress on temperature of maize seedling shoots.

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.

Figure 1. Temperature difference between alive and killed shoots of maize at 0 C (1) and —4 C (2).

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