Effects of Exogenous Isosteviol on the Physiological Characteristics of Brassica napus Seedlings under Salt Stress

Abstract

In this paper, the effect of isosteviol on the physiological metabolism of Brassica napus seedlings under salt stress is explored. Brassica napus 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⁺, K⁺, Cl^-, 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 Brassica napus seedlings and reduce the contents of proline, glycine betaine, and ROS in the seedlings. Isosteviol reduces the oxidative damage to Brassica napus seedlings caused by salt stress by regulating the production of osmotic substances and ROS. In addition, after seed soaking in isosteviol, the Na⁺ content in the shoots of the Brassica napus seedlings was always lower than that in the roots, while the opposite was true for the K⁺ content. This indicated that under salt stress the Na⁺ absorbed by the Brassica napus seedlings was mainly accumulated in the roots and that less Na⁺ was transported to the shoots, while more of the K⁺ absorbed by the Brassica napus seedlings was retained in the leaves. It is speculated that this may be an important mechanism for Brassica napus seedlings to relieve Na⁺ 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 Brassica napus seedling tissues could absorb large amounts of Na⁺ and Cl^- to induce oxidative stress and inhibit the growth of the plants. After the seed soaking treatment, isosteviol could significantly reduce the absorption of Na⁺ by the seedling tissues, increase the K⁺ content, and reduce the salt stress damage to the plant seedlings. Therefore, under salt stress, seed soaking with isosteviol at an appropriate concentration (10^-9~10^-8 M) can increase the salt resistance of Brassica napus seedlings by regulating their physiological and metabolic functions.

Keywords: Brassica napus seedlings; isosteviol; physiological characteristics; salt stress.

Figure 1

Effect of isosteviol on the biomass of Brassica napus seedlings under salt stress. T1: control group; T2: NaCl; T3: NaCl + isosteviol (10⁻¹⁰); T4: NaCl + isosteviol (10⁻⁹); T5: NaCl + isosteviol (10⁻⁸); T6: NaCl + isosteviol (10⁻⁷); T7: NaCl + isosteviol (10⁻⁶). Data are presented as the mean ± S.D. (n = 5 in each group). Different lowercase letters indicate significant differences between different treatments (p ≤ 0.05). (A) Fresh weight of shoot; (B) fresh weight of root; (C) dry weight of shoot; (D) dry weight of root.

Figure 2

Effects of isosteviol on the osmotic substance contents in the tissues of Brassica napus seedlings under salt stress. T1: control group; T2: NaCl; T3: NaCl + isosteviol (10⁻¹⁰); T4: NaCl + isosteviol (10⁻⁹); T5: NaCl + isosteviol (10⁻⁸); T6: NaCl + isosteviol (10⁻⁷); T7: NaCl + isosteviol (10⁻⁶); FW: fresh weight. Data are presented as the mean ± S.D. (n = 5 in each group). Different lowercase letters indicate significant differences between different treatments (p ≤ 0.05). (A) Glycine betaine content of shoot; (B) glycine betaine content of root; (C) proline content of shoot; (D) proline content of root; (E) soluble protein content of shoot; (F) soluble protein content of root.

Figure 3

Effects of isosteviol on Na⁺, K⁺, and Cl⁻ contents in different tissues of Brassica napus seedlings under salt stress. T1: control group; T2: NaCl; T5: NaCl + isosteviol (10⁻⁸); DW: dry weight. Data are presented as the mean ± S.D. (n = 5 in each group). Different lowercase letters represent significant differences in the total amount of the three ions within shoots and roots in the same treatment group (p ≤ 0.05), and different capital letters represent significant differences in the total amounts of the three ions among the different treatment groups (p ≤ 0.05).

Figure 4

Effect of exogenous isosteviol on the reactive oxygen species (ROS) content of Brassica napus seedlings under salt stress. T1: control group; T2: NaCl; T3: NaCl + isosteviol (10⁻¹⁰); T4: NaCl + isosteviol (10⁻⁹); T5: NaCl + isosteviol (10⁻⁸); T6: NaCl + isosteviol (10⁻⁷); T7: NaCl + isosteviol (10⁻⁶); FW: fresh weight. Data are presented as the mean ± S.D. (n = 5 in each group). Different lowercase letters indicate significant differences between different treatments (p ≤ 0.05). (A) H₂O₂ content of shoot; (B) H₂O₂ content of root; (C) O₂^·− content of shoot; (D) O₂^·− content of root.

Figure 5

Effect of isosteviol on Fourier transform infrared spectroscopy (FTIR) of Brassica napus seedling tissues under salt stress. T1: control group; T2: NaCl; T3: NaCl + isosteviol (10⁻¹⁰); T4: NaCl + isosteviol (10⁻⁹); T5: NaCl + isosteviol (10⁻⁸); T6: NaCl + isosteviol (10⁻⁷); T7: NaCl + isosteviol (10⁻⁶).

Figure 6

Correlation analysis (T2–T7 treatment groups) of the effects of exogenous isosteviol on the physiological indicators of Brassica napus seedlings under salt stress. *: Indicates a significant correlation at the 0.05 level (p ≤ 0.05).

Figure 7

Principal component analysis of physiological indicators of Brassica napus seedlings treated with isosteviol under salt stress. X1: shoot fresh weight; X2: root fresh weight; X3: shoot dry weight; X4: root dry weight; X5: shoot glycine betaine; X6: shoot proline; X7: shoot soluble protein; X8: root glycine betaine; X9: root proline; X10: root soluble protein; X11: shoot Na⁺; X12: shoot K⁺; X13: shoot Cl⁻; X14: root Na⁺; X15: root K⁺; X16: root Cl⁻; X17: shoot H₂O₂; X18: shoot O₂^·−; X19: root H₂O₂; X20: root O₂^·−.