Regulation Of Brassinolide On Ion Homeostasis During Salt Stress In Canola Seedlings

Soil salinization posed a severe threat to crop growth, reducing the yield of crop, while brassinosteroids (BRs) could promote plant salt resistance, relieving the effects of salt stress. 24-epibrassinolide (EBL) was used as the delegate to study the effects of BRs on salt resistant regulation, especially the regulation in the ion homeostasis of the dicotyledons of canola(Brassica napus L. cv. Nanyanyou 1). The effects of different concentrations NaCl stress on photosynthesis, osmotic adjustment ability and ion homeostasis of canola seedlings were studied when spayed with different concentrations of EBL. Then, the effects of 10"10 mol·L-1 EBL application on ion distribution at different levels, such as different organs, leaf symplast, leaf apoplast and relative ion content (percentage of total ions) in root, petiole, leaf blade cells of canola seedlings under 150 mmol·L-1 NaCl stress conditions were focused to study. The results were showed as follows:(1) Dry weight (DW) of canola seedlings was decreased significantly under 100 and 200 mmol-L’1 NaCl, however,10"12,10"10,10"8,10"6 mol·L-1 EBL foliar spraying application all increased the DW of stressed plants. Also,10-10 mol·L-1 EBL (EBL2) treatment made the seedlings DW peaked in both salt-stressed canola plants. EBL2 application promoted relative water content (RWC) and osmotic adjustment ability (OAA) of salt-stressed canola, furthermore, it partly reversed net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci) and transpiration rate (Tr) of canola leaves and limitation values (Ls) under salt stress at varying degree. EBL could improve K+/Na+ homeostasis of the different organs of canola seedlings under salt stress, increase the SK,Na of leaf and root and decrease the SK,Na of petiole, which means that EBL can regulate the selective absorbtion and transportion of K+ and Na+ to keep ion homeostasis and improve salt resistance of canola seedlings.(2) Under 150 mmol·L-1 NaCl stress, EBL2 application could increase K+ content in petiole, but did not affected K+ content in leaf main vein and leaf blade significantly, while for Na+ and Cl-, EBL2 application could reduce the contents of them in petiole, leaf main vein and leaf blade, which causing the increase of K+/Na+ in petiole, leaf main vein and leaf blade, implying that improvement of ion homeostasis in leaves caused by EBL was as the result of exclusion of Na+ and Cl-.(3) Compared with control, the contents of Na+ and Cl- in leaf symplast and apoplast of canola seedlings under 150 mmol·L-1 NaCl stress were increased significantly, and EBL2 application reduced their contents in salt-stressed plants. However, EBL did not affect K+ content in leaf symplast and leaf apoplast significantly. Application of EBL2 increased K+/Na+ in leaf symplast and leaf apoplast and increase the steepness of ion content between leaf symplast and apoplast. Our results indicated that EBL could decrease the accumulation of Na+ and Cl-, which tends to accumulate these ions in the vacuole, and improve K+/Na+in leaf symplast and leaf apoplast to contribute to canola salt resistance.(4) Under 150 mmol·L-1 NaCl stress, EBL2 application could decrease the relative Na+ and Cl" content and increase the relative K+ content in root, petiole and leaf cells of canola seedlings, suggesting that EBL can affect ion distribution of different organs cells, such as increasing K+/Na+ to improve salt resistance.From the above-mentioned results, we found that foliar spraying EBL can increase ion homeostasis of canola seedlings to improve photosynthesis and osmotic adjustment under NaCl stress. In the ion balance regulation:EBL reduced Na+translocation from the roots to the shoots, and reduced the contents of Na+ and Cl- in leaf symplast and leaf apoplast. It also reduced the relative Na+ and Cl" content and increased the relative K+ content in root, petiole and leaf cells of canola seedlings. The results also suggested that improvement of ion homeostasis in leaves caused by EBL was as the result of exclusion of Na+ and Cl-, and EBL had much smaller impact on K+ in leaves. In addition, EBL can promote ions accumulate in the vacuole to reduce their distribution in leaf apoplast of canola.