The initiation of elongation growth during long-term low-temperature stay of spring-type oilseed rape may trigger loss of frost resistance and changes in photosynthetic apparatus
Introduction
Recently published results show that spring-type oilseed rape is able to cold acclimate to a level comparable with winter type, but only after cold prehardening [1], which results in both the increase of photosynthetic activity and growth cessation during cold acclimation [2], [3], [4], [5]. This observation is consistent with results obtained for field grown spring oilseed rape when some spring cultivars were characterised with frost resistance similar to the less resistant winter cultivars [6]. However, the winter survival of spring-type plants is usually very low [6], [7]. It is suggested that spring rape cannot survive winter because of its limited ability to prevent generative shoot elongation during winter even when temperature rises only slightly above 0°C [8]. Thus, one of the possible reasons of limited cold-acclimation abilities and low frost resistance often observed in spring-type plants [9], [10], [11] can be the limited capability to cease the elongation growth during cold acclimation. In consequence, less energy is accessible for processes associated with acclimation.
Other results pointed at another possible reason of low acclimation abilities observed in spring-type plants. They cannot recover high photosynthetic activity after shift from warmth to cold-acclimating temperatures (0–5°C) [12], [13], [14]. High photosynthetic rate during cold acclimation is a requirement for the expression of freezing tolerance in cold-tolerant plants because it provides energy for cold acclimation. Our results showed that spring-type oilseed rape can adjust photosynthesis to subsequent cold during prehardening, that is during the first phase of cold acclimation that occurs at moderate low temperatures (10–15°C) [1].
It was also found in field-grown oilseed rape cultivars that some properties of photosynthetic apparatus observed in winter correlated well with growth rate observed during winter [8]. Cultivars with higher photosynthetic electron transport rate retained the cessation of elongation growth for a longer period [8]. It may suggest that maintenance of sufficient photosynthetic activity and slow elongation growth rate during cold acclimation is controlled by the same mechanism. This observation is in agreement with the results concerning acclimation of photosynthetic apparatus observed after long-term exposure to low temperatures [12], [13], [14]. Although prehardening occurred in higher temperatures than photosyntetic low temperature acclimation, and the latter was observed only in winter-type plants [12], [13], [14], the mechanism of both these processes is probably the same. Prehardening was observed only when moderate low temperature occurred in the light [2]. Such conditions may be described as conditions of elevated PSII excitation pressure. Plants showed elevated PSII excitation pressure reflecting imbalances between energy supply and consumption when exposed to either high light intensity or low temperature [15], [16], [17]. Both prehardening [2] and growth under high PSII excitation pressure [17] resulted in acclimation of photosynthetic apparatus and the decrease in elongation growth rate, but it was not involved directly in the development of frost resistance.
The aim of the present investigation was to compare the adjustments of prehardened spring- and winter-types oilseed rape to prolonged (up to 10 weeks) exposure to low non-freezing temperatures. The present paper tried to determine whether the loss of frost resistance observed in spring-type oilseed rape may be the effect of a tendency to start the elongation growth during the low-temperature stay. Additionally, the experiment was devised to study interactions between elongation growth rate, properties of photosynthetic apparatus and frost resistance under these conditions.
Section snippets
Plant material and growth conditions
Seedlings of spring oilseed rape (Brassica napus L. var. oleifera f. annua cv. Star; DLF Trifolium, Denmark) and winter oilseed rape (Brassica napus L. var. oleifera f. bienae cv. Górczanski; MHBPiNR, Poland) were grown eight specimens per 20-cm pot containing loam soil: sand: peat (1:1:1; v:v:v). Seeds were germinated under controlled-environment conditions: 300 μmol m−2 s−1 PPFD (sodium light, ‘Agro’, Philips), 20°C, photoperiod 12 h. When about 50% of plants emerged from soil the temperature
Frost resistance
Changes in frost resistance, observed during 10 weeks of low-temperature stay, differ fundamentally between spring and winter cultivars of oilseed rape (Fig. 1). When resistance of leaf discs is taken into consideration, both cultivars reached the maximal level of resistance on the 42nd day of acclimation. LT50 was −20.3 and −20.7°C for winter and spring rape, respectively. In the case of whole plants, the maximal level of resistance was reached in different terms — on the 28th day for spring
Frost resistance of winter and spring-type oilseed rape during long-term low-temperature treatment
According to Levitt [25], in winter annuals, the growth rate must fall in the autumn to enable the use of excess photosynthetic products in the cold-acclimation process, and consequently promotion of elongation growth must lead to consumption of accumulated photosynthetic products, e.g. sugars and loss of freezing resistance. Our results indicate that the decrease in frost resistance observed in spring-type plants was associated with the beginning of elongation growth of petioles and epicotyle
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