Response of winter oilseed rape to imitated temperature fluctuations in autumn-winter period

Low temperature is a limiting factor in plant cultivation in many areas in the world. Thermal fluctuations and harshening cold events frequently occur in temperate climates during autumn-winter period and can trigger cold stress in plants, leading to a critical decrease in crop yield. We aimed to explore responses of winter oilseed rape (Brassica napus L. ssp. oleifera biennis Metzg) to imitated fluctuating and harshening conditions. Cold acclimation, continuous increasing cold stress, de-acclimation and re-acclimation treatments were applied in order to imitate autumn-winter conditions and simulate winter oilseed rape development through this period. Pot experiments were conducted in controlled climate conditions using two cultivars of differing cold tolerance. Leaf blade cell plasma membrane H+-ATPase activity, ethylene emission level, free proline content, survival of plants and phenotypical symptoms were evaluated. The suppression of H+-ATPase activity, a rapid stimulation of the ethylene emission at the beginning of cold stress treatment, and decrease of it at lower temperatures, also rise in free proline content were observed in non-acclimated plants under increasing cold stress conditions. These responses coincided with a decrease in survival. However, cold acclimation alleviated negative consequences of harshening thermal conditions and resulted in a modified pattern of tested traits. So that acclimated plants, especially those of the tolerant genotype, maintained higher rates of survival and displayed higher content of free proline, H+-ATPase activity and stable ethylene emission levels. Intensive adjustments in free proline levels were observed under fluctuating temperatures. Hardening conditions (acclimation and re-acclimation) induced the accumulation of free proline and de-acclimation resulted in a significant reduction. Surprisingly, de-acclimated plants exposed to re-acclimation managed to increase proline content to levels higher than in acclimated plants.