Response of endogenous ABA and GA to cold resistance of Brassica rapa L. and Brassica napus L.

Cold resistance is critical in winter rapeseed production in northern China. The abscisic acid (ABA) plant hormone, also known as “stress hormone” is an important signal molecule for the regulation of plant cold resistance. Study of possible regulatory mechanisms of endogenous ABA and GA is needed to develop a valuable insight into the mechanism of cold resistance both in winter rapeseed Brassica rapa L. and Brassica napus L. Thus we investigated the regulation of endogenous hormones in cold-resistance using eight winter rapeseed cultivars with different gradients of cold tolerance in a pot experiment. Different winter rapeseed cultivars were cultivated in a growth chamber with temperature conditions of 25 ℃, 10 ℃, 2 ℃ and 5 ℃, respectively. The endogenous ABA, GA and chlorophyll contents were measured. Regression analysis showed a significantly negative linear correlation between temperature and endogenous ABA content, with a regression equation of y = ax + b. With decreasing of temperature, the endogenous ABA content increased slightly at 10 ℃ and then drastically at both 2 ℃ and 5 ℃. There were remarkable differences in ABA content among the temperature treatments and interaction of temperature and crop cultivar. The trend in endogenous GA content was the exact contrast of that of ABA, highlighting the antagonism between the both phytohormones. Temperature had no significant effect on endogenous ABA content for temperatures > 0 ℃ in different winter rapeseed cultivars. However, temperature effect on endogenous ABA content of B. rapa was more significant than that of B. napus, and the cold tolerant cultivar more sensitive at a temperature of 5 ℃. The increase in ABA content contributed to the change in chlorophyll, which initially increased and then decreasing with decreasing temperature, and with an overall declining trend. The response mechanisms between B. rapa and B. napus were different, which resulted in lower chlorophyll content in B. rapa than in B. napus. The leaf wilting date of B. rapa delayed well into wintering stage, with leaf wilting period of strongly cold resistant varieties occurring early to halt root nutrient uptake. Winter rapeseed root stored enough organic matter to increase its cold resistance and defend against low temperature injury. Thus with decreasing temperature, endogenous ABA content of winter rapeseed leaf increased. However, as chlorophyll degraded, B. rapa winter rapeseed leaf withered early, occurring in advance along with high ABA content after low temperature stress. This was the main reason and the stronger characteristic biological performance of B. rapa over B. napus in terms of cold resistance.