Physiological basis of the disturbances in frost tolerance of winter oilseed rape as a result of deaclimation processes – role of brassinosteroids
Oilseed rape (Brassica napus L.) is one of the main oil plants that are cultivated in the world. Oilseed rape gives a high-quality fat and is a source of valuable feed. The yield of winter oilseed rape is higher than the yield of spring oilseed rape, but part of its growing season is during the winter months, which is connected with the exposure of the plants to frost. Well cold acclimated (during autumn) oilseed rape plants are able to easily survive frost, especially under snow cover. However, the problem of recent years is the occurrence during autumn (or early spring) short periods with abnormally elevated temperatures. It results from climate changes and disturbs the natural process of preparing plant metabolism to survive frost (reducing plant frost tolerance). The phenomenon is called deacclimation of plants. As a result of the decreased frost tolerance farmers may be forced to plow entire winter oilseed rape plantations injured during the winter. The problem also applies (although to a lesser extent) to spring oilseed rape, and the phenomenon of deacclimation is dangerous for this form in early spring, when sudden frost may occur after a period of increased temperature.
As a result of the experiments carried out as part of the project, it was established, among others that oilseed rape cultivars (including those currently in cultivation in Poland) differ to some extent in their tolerance to deacclimation. Spring cultivar (Felix) is rather susceptible to deacclimation in comparioson to winter cultivars. Among the winter cultivars, Rokas tolerates deacclimation relatively well and maintains a high level of frost resistance (based on plant regrowth after frost – parameter RT50). Studies devoted to the characteristics of the physiological and biochemical basis of the deacclimation process prove that periods of elevated temperature lasting several days (e.g. 7 days with a temperature of 16oC (day)/9oC (night)) fully or partly reverse the metabolic changes induced by cold acclimation at +4oC. Shift of the hormonal balance in direction of the increase in the level of growth/development hormones and the decrease in the content of protective ABA hormone as a result of deacclimation is one of the possible causes of reduced frost tolerance of plants after deacclimation, which constitutes a basis for the resumption of growth. During deacclimation, the photosynthesis is intensified. The structure of chloroplast membranes and the ratio of building them fatty acids are changed in direction characteristic for plants before exposure to cold. Additionally, reduced accumulation of protective proteins from the HSP family (Heat Shock Proteins) under the influence of deacclimation was also confirmed in most of the examined cultivars. It is difficult to visually assess whether plants are deacclimated or not (this requires taking leaf samples and laboratory analysis). However, as project research has shown, the condition of the crop can be monitored using non-invasive methods. Methods for measuring the spectral properties of leaves (so-called leaf reflection) have proven to be promising. Some parameters, e.g. ARI 1 and 2 (reflecting the level of anthocyanin pigments), clearly illustrate the changes caused by deacclimation. These type of measurements can be made using drones or satellites to monitor crop fields and may allow to capture the moment when plants become deacclimated in late autumn/winter/early spring and become more sensitive to a sudden frost. This gives an opportunity to use preventive measures (i.e. spraying of some hormones/regulators). The project studies demonstrated the activity of some hormones from the brassinosteroid group, as well as the additional commercial preparation Asahi SL towards improving the frost tolerance of deacclimated plants. The effect of regulators was dependent on the intensity of frost.