Selenium Improves Yield and Quality in Prunella vulgaris by Regulating Antioxidant Defense, Photosynthesis, Growth, Secondary Metabolites, and Gene Expression Under Acid Stress

Abstract

Prunella vulgaris, an essential component of traditional Chinese medicine, is suitable for growing in soil with a pH value ranging from 6.5 to 7.5. However, it is primarily cultivated in acidic soil regions of China, where its growth is frequently compromised by acidic stress. Selenium (Se) has been recognized for its potential to enhance stress tolerance in plants. However, its role in acid-stress-induced oxidative stress is not clear. In this study, the effects of varying Se concentrations on the growth and quality of P. vulgaris under acidic stress were investigated. The results showed that acid stress enhanced antioxidant enzyme activities, non-enzymatic antioxidant substances, and osmolyte content, accompanied by an increase in oxidant production and membrane damage. Furthermore, it decreased the photosynthetic capacity, inhibited root and shoot growth, and diminished the yield of P. vulgaris. In contrast, exogenous application of Se, particularly at 5 mg L^-1, markedly ameliorated these adverse effects. Compared to acid-stressed plants, 5 mg L^-1 Se treatment enhanced superoxide dismutase, peroxidase, ascorbate peroxidase, and glutathione peroxidase activities by 150.19%, 54.94%, 43.43%, and 45.55%, respectively. Additionally, soluble protein, soluble sugar, and proline contents increased by 11.75%, 23.32%, and 40.39%, respectively. Se application also improved root architecture and alleviated membrane damage by reducing hydrogen peroxide, superoxide anion, malondialdehyde, and electrolyte leakage levels. Furthermore, it significantly enhanced the photosynthetic capacity by elevating pigment levels, the performance of PSI and PSII, electron transfer, and the coordination of PSI and PSII. Consequently, plant growth and spica weight were significantly promoted, with a 12.50% increase in yield. Moreover, Se application upregulated key genes involved in flavonoid and phenolic acid metabolic pathways, leading to elevated levels of total flavonoids, caffeic acid, ferulic acid, rosmarinic acid, and hyperoside by 31.03%, 22.37%, 40.78%, 15.11%, and 20.84%, respectively, compared to acid-stressed plants. In conclusion, exogenous Se effectively alleviated the adverse effects of acid stress by improving the antioxidant system, growth, and photosynthetic capacity under acid stress, thus enhancing the yield and quality of P. vulgaris.

Keywords: Prunella vulgaris L.; Selenium; acid stress; photosystem; root morphology; secondary metabolites.

Figure 1

Effects of selenium on activities of SOD (A), POD (B), APX (C) and GPX (D) in P. vulgaris under acid stress. FW: fresh weight, CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 2

Effects of exogenous selenium on the contents of soluble protein (A), soluble sugar (B), and proline (C) in P. vulgaris under acid stress. FW: fresh weight. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 3

Effects of selenium treatment on the contents of H₂O₂ (A), O₂⁻ (B), MDA (C), and EL (D) in P. vulgaris under acid stress. FW: fresh weight. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 4

Effects of exogenous selenium on the contents of Chl a (A), Chl b (B), Car (C) and Chl a + b (D) in P. vulgaris under acid stress. FW: fresh weight. Cha co CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 5

Effects of exogenous selenium on P_n (A), G_s (B), C_i (C), and T_r (D) in P. vulgaris under acid stress. FW: fresh weight. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 6

Effects of selenium on morphology and biomass characteristics of P. vulgaris under acid stress (A) Branch number per plant; (B) Spica number per plant; (C) Spica length; (D) Spica width; (E) Spica weight; (F) Whole plant weight. DW: dry weight. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L−1 Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 7

The V_t curves (A), △V_t curves (B), and 820 nm modulated reflection curve (C) of P. vulgaris under different Se treatments. V_t = (F_t − F₀)/(F_m − F₀): the relative variable fluorescence (Vt) at any time; △V_t = V_{t (Se)} − V_{t (CK)}; O, K, J, I, and P represent different phases in the OJIP curve. MR/MR₀: the 820 nm modulated reflection curve; MR is the modulated reflection at different time points; MR₀ is the MR value of far-red light irradiated at 0.7 ms. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively.

Figure 8

Effects of selenium on W_k (A), V_J (B), M₀ (C), and φE₀ (D) in P. vulgaris under acid stress. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 9

The function and coordination of PSII and PSI in P. vulgaris treated with Se under acid stress. F_v/F_m (A); PI_abs (B); ΔI/I₀ (C); and Φ_PSI/PSII (D). CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).

Figure 10

The transcript abundance of rosmarinic acid biosynthesis genes of plants treated with Se. (A) C4H coumarate 4-hydroxylase; (B) 4CL 4-coumaroyl CoA ligase; (C) PAL phenylalanine ammonia-lyase; (D) TAT tyrosine aminotransferase. CK: control. AS: acid stress. AS+S1, AS+S5, and AS+S10: treated with acid stress and 1, 5, and 10 mg L⁻¹ Se solution, respectively. After treatment with the selenium spray, the seedlings underwent 15 days of acid stress and were subsequently tested. The data are represented as the mean ± standard deviation. Significant differences are indicated by different lowercase letters above the bars according to a Duncan’s multiple range test (p < 0.05).