| Summary: | In this paper, the effect of isosteviol on the physiological metabolism of <i>Brassica napus</i> seedlings under salt stress is explored. <i>Brassica napus</i> seeds (Qinyou 2) were used as materials, and the seeds were soaked in different concentrations of isosteviol under salt stress. The fresh weight, dry weight, osmotic substance, absorption and distribution of Na<sup>+</sup>, K<sup>+</sup>, Cl<sup>−</sup>, and the content of reactive oxygen species (ROS) were measured, and these results were combined with the changes shown by Fourier transform infrared spectroscopy (FTIR). The results showed that isosteviol at an appropriate concentration could effectively increase the biomass and soluble protein content of <i>Brassica napus</i> seedlings and reduce the contents of proline, glycine betaine, and ROS in the seedlings. Isosteviol reduces the oxidative damage to <i>Brassica napus</i> seedlings caused by salt stress by regulating the production of osmotic substances and ROS. In addition, after seed soaking in isosteviol, the Na<sup>+</sup> content in the shoots of the <i>Brassica napus</i> seedlings was always lower than that in the roots, while the opposite was true for the K<sup>+</sup> content. This indicated that under salt stress the Na<sup>+</sup> absorbed by the <i>Brassica napus</i> seedlings was mainly accumulated in the roots and that less Na<sup>+</sup> was transported to the shoots, while more of the K<sup>+</sup> absorbed by the <i>Brassica napus</i> seedlings was retained in the leaves. It is speculated that this may be an important mechanism for <i>Brassica napus</i> seedlings to relieve Na<sup>+</sup> toxicity. The spectroscopy analysis showed that, compared with the control group (T1), salt stress increased the absorbance values of carbohydrates, proteins, lipids, nucleic acids, etc., indicating structural damage to the plasma membrane and cell wall. The spectra of the isosteviol seed soaking treatment group were nearly the same as those of the control group (T1). The correlation analysis shows that under salt stress the <i>Brassica napus</i> seedling tissues could absorb large amounts of Na<sup>+</sup> and Cl<sup>−</sup> to induce oxidative stress and inhibit the growth of the plants. After the seed soaking treatment, isosteviol could significantly reduce the absorption of Na<sup>+</sup> by the seedling tissues, increase the K<sup>+</sup> content, and reduce the salt stress damage to the plant seedlings. Therefore, under salt stress, seed soaking with isosteviol at an appropriate concentration (10<sup>−9</sup>~10<sup>−8</sup> M) can increase the salt resistance of <i>Brassica napus</i> seedlings by regulating their physiological and metabolic functions.
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