24-Epibrassinolide-Succinic Acid Conjugate Is Involved in the Acclimation of Rape Plants to Salt Stress

The influence of the conjugate of 24-epibrassinolide with succinic acid (tetrahydrosuccinate of 24-epibrassinolide, EBL THS) and 24-epibrassinolide (EBL) on the acclimation of rapeseed plants (<i>Brassica napus</i> L.) to chloride salinity (150 mM NaCl) was investigated. After two weeks of growth in Hoagland-Snyder medium, the rapeseed seedlings were transferred to the same medium supplemented with EBL or EBL THS (10 nM) for 4 h, after which NaCl (150 mM) was added; parameters were taken on the 1st, 3rd, 5th and 7th days. It was established that salt stress inhibited growth processes (by 19-45%), reduced the chlorophyll and carotenoid contents (by 19-50%), photosystem II efficiency (by 13-19%), tissue hydration (by 3.54%), and osmotic potential (by three times), increased lipid peroxidation (LPO) (by 1.5-2 times), and proline accumulation (by 1.4-18 times), and altered ion status, increasing the concentrations of Na⁺ and Cl^- ions while decreasing the levels of K⁺, Ca²⁺, Mg²⁺, S²⁺, Fe²⁺, Al³⁺, and P³⁺. The short-term pretreatment of plants with EBL THS, similar to EBL, reduced the inhibitory effects of NaCl on growth processes, pigment content (to a greater extent with EBL THS), the efficiency of photochemical processes in photosystem II, the accumulation of Na⁺ ions, and in the case of EBL THS, the accumulation of Cl^- ions. Both regulators (especially EBL THS) reduced LPO, and stimulated the accumulation of NaCl-induced proline, which was organ-specific and dependent on the duration of stress. EBL THS stimulated the activity of superoxide dismutase and peroxidase, whereas EBL primarily stimulated peroxidase. Thus, it was demonstrated for the first time that EBL THS, like EBL, increased the salt tolerance of rapeseed plants, but had a more pronounced stress-protective effect, primarily at the level of antioxidant system components.