Effect of exogenous 2,4-epibrassinolide on trace element absorption and chlorophyll fluorescence of Medicago sativa L. seedlings under NaCl stress

Due to the serious secondary soil salinization, salt injury has become a limiting factor of the development of high quality alfalfa industry in Northwest and North China. Brassinosteroid (BR) has been globally recognized as a new plant growth hormone with high and broad spectrum activity, which could regulate plant growth and development and mitigate a series of abiotic stresses due to high salinity, heavy metal contamination, high temperatures, low temperatures, drought and hypoxia. It also plays an important role in regulating the photosynthesis and absorption of trace element of plants. Given the importance of the study of the effects of BR on trace element absorption and fluorescence kinetics parameters in Medicago sativa L. seedlings under salt stress, we investigated salt resistance and possible physiological regulation mechanism of M. sativa seedlings induced by exogenous 2,4-Epibrassinolide (EBR). The effects of EBR on the absorption, transportation and allocation of trace elements, leaf PSⅡ function, electron transport rate and light allocation in seedlings of M. sativa cv. Zhongmu No.3 and M. sativa cv. Longzhong under NaCl stress were determined using the hydroponics method. Four treatments were conducted in the experiment ― CK (distilled water), 150 mmol·L^-1 NaCl, 0.1 μmol·L^-1 EBR and 150 mmol·L^-1 NaCl + 0.1 μmol·L^-1 EBR. The results showed that Cu²⁺ content increased significantly in different organs (leave, stem and root) of the seedlings, also Fe²⁺, Mn²⁺ and Zn²⁺ contents significantly decreased and then Fe²⁺/Na⁺, Mn²⁺/Na⁺, Cu²⁺/Na⁺ and Zn²⁺/Na⁺ declined markedly under 150 mmol·L^-1 NaCl stress. The metabolic process of the uptake, transportation and distribution of inorganic ions was disordered and PSⅡ reaction center damaged. Concurrently, there was a drop in the transportation ratio of photosynthetic electrons (ETR) and photochemical reaction energy. NaCl stress facilitated antenna dissipated energy and reaction center dissipated energy, which resulted in a drop in photosynthetic capacity. This condition was reversed by the addition of 0.1 μmol·L^-1 EBR under NaCl stress. Addition of EBR significantly decreased Cu²⁺ content in different plant organs (leave, stem and root), while significantly increased contents of Fe²⁺, Mn²⁺ and Zn²⁺, and ratios of Fe²⁺/Na⁺, Mn²⁺/Na⁺, Cu²⁺/Na⁺ and Zn²⁺/Na⁺. The uptake and transportation of inorganic ions were effectively regulated, accompanied by a decrease in antagonism among positive ions like Na⁺, Fe²⁺, Mn²⁺, Cu²⁺, Zn²⁺, etc. Leaf F₀ and NPQ significantly decreased and F_m, F_v/F₀, F_v/F_m, ФPSⅡ, F_v′/F_m′, qP and ETR significantly increased. Absorbed light energy was allocated more to photochemical reaction energy, less light was allocated to antenna pigment dissipation and excess light energy to reaction center. This illustrated that EBR improved the selective absorption, transportation and distribution of inorganic ions and maintained ion metabolic balance under NaCl stress. Furthermore, EBR effectively alleviated any harm to M. sativa seedlings caused by NaCl stress by raising ETR, reducing antenna heat dissipation and excess light energy of reaction center and maintaining a higher level PSⅡ photochemistry activity. This further stimulated even energy allocation between PSⅠ and PSⅡ and decreased damage to PSⅡ reaction center.