Effects of Exogenous Spermidine on Seed Germination and Physiological Metabolism of Rice Under NaCl Stress

Abstract

Salt stress is one of the principal abiotic stresses limiting agricultural production and seriously inhibiting seed germination rates. This study selected the salt-tolerant rice variety HD961 and the salt-sensitive rice variety 9311 as experimental materials to investigate the physiological and metabolic effects of exogenous Spd seed priming on rice seeds and seedlings under NaCl stress. The experiment involved treating rice seeds with 0.1 mmol·L^-1 Spd and then subjecting them to 100 mmol·L^-1 NaCl stress for 24 h, with sampling for analysis at the 24 h and the four-leaf-one-heart stage. The results indicated that under NaCl stress, the rice's germination and vigor indices significantly decreased. However, exogenous Spd seed priming reduced the accumulation of malondialdehyde, enhanced the capacity for osmotic adjustment, and increased the amylase and antioxidant activity by 50.07% and 26.26%, respectively. Under NaCl stress, the morphological development of rice seedlings was markedly inhibited, whereas exogenous Spd seed priming improved the aboveground and belowground biomass of the rice under stress conditions, as well as the content of photosynthetic pigments. It also reduced the damage to seedlings from electrical conductivity, helped maintain ionic balance, and promoted the excretion of Na⁺ and Cl^- and the absorption of K⁺ and Ca²⁺. In the salt-sensitive rice variety 9311, the soluble protein content increased by 15.12% compared to the salt-tolerant rice variety HD961, especially under 100 mmol·L^-1 NaCl stress, when the effect of exogenous Spd seed priming was more pronounced. In summary, these findings might provide new research perspectives and strategies for improving the salt tolerance of rice under NaCl stress.

Keywords: NaCl stress; rice; seed germination; seedling growth; spermidine.

Figure 1

The effect of exogenous spermidine seed soaking on the germination rate of two rice varieties, HD961 (A) and 9311 (B), under salt stress.

Figure 2

Response mechanism of germination potential (A), germination index (B), vigor index (C), and average germination time (D) of 9311 rice seeds to exogenous spermidine under salt stress. Here, S is spermidine seed treatment, N is NaCl treatment, NS is a combination of NaCl and spermidine seed treatments, and CK is no NaCl. In the following figures and tables, S, N, NS, and CK represent the same meanings. Different letters indicate statistically significant differences (p < 0.05).

Figure 3

Response mechanism of root length (A), shoot length (B), and fresh weight (C) of 9311 rice variety to exogenous spermidine seed soaking under salt stress for 1, 3, 5, and 7 d. Different letters indicate statistically significant differences (p < 0.05).

Figure 4

Response mechanism of ATP content (A), soluble sugar (B), and soluble starch (C) in 9311 rice variety to exogenous spermidine seed soaking under NaCl stress. Different letters indicate statistically significant differences (p < 0.05).

Figure 5

Response mechanisms of total amylase activity (A), α-amylase activity (B), and β-amylase activity (C) in the 9311 rice variety to exogenous spermidine soaking under NaCl stress. Different letters indicate statistically significant differences (p < 0.05).

Figure 6

Influence of exogenous spermidine seed soaking on MDA content (A), electrolyte leakage in leaves (B) and roots (C), and H₂O₂ distribution in leaves (D) of 9311 rice variety under NaCl stress. Different letters indicate statistically significant differences (p < 0.05).

Figure 7

Response mechanisms of superoxide dismutase (A), peroxidase (B), catalase (C), ascorbate peroxidase (D), glutathione (E), and ascorbic acid (F) contents in 9311 rice seeds to exogenous spermidine soaking under NaCl stress. Different letters indicate statistically significant differences (p < 0.05).

Figure 8

Response mechanism of polyamine content (A), spermidine content (B), zeatin content (C), gibberellin (D), auxin (E), abscisic acid (F), and ethylene content (G) in rice seeds to exogenous spermidine soaking under NaCl stress. Different letters indicate statistically significant differences (p < 0.05).

Figure 9

Response mechanism to exogenous spermidine soaking under NaCl stress of total chlorophyll (A), chlorophyll a (B), chlorophyll b (C), and carotenoid (D) contents of 9311 rice seedlings. Different letters indicate statistically significant differences (p < 0.05).

Figure 10

Response mechanism to exogenous spermidine soaking under NaCl stress of net photosynthetic rate (A), transpiration rate (B), stomatal conductance (C), and intercellular carbon dioxide concentration (D) of 9311 rice seedlings. Different letters indicate statistically significant differences (p < 0.05).

Figure 11

Response mechanism of rice seed germination and seedling growth to exogenous spermidine on HD961 and 9311 under salt stress. Red indicates a positive correlation between the two parameters, and blue indicates a negative correlation. * p ≤ 0.05.

Figure 12

Response mechanism of rice seed germination and seedling growth to exogenous spermidine under salt stress.