Rapeseed seed germination and seedling growth under NaCl stress
NaCl stress hindered the growth and development of rapeseed seedlings (Fig. 1). Increased level of NaCl consistently showed significant influence on germination rate, germination index, root length of seedlings, fresh and dry weight of rapeseeds. When the NaCl rate was increased from 150 to 200 mM, there were corresponding percent decrease of 18.03%, and 25.42% in germination rate, 41.65% and 48.13% in germination index and 56.16% and 78.99% in vigor index, respectively as compared to CK (Table 1). These results showed that NaCl at the rate 150 mM is sensitive to rapeseeds gemination and seedlings growth.
Table 1
Effect of NaCl stress on the germination index of rapeseed
NaCl concentration(mM) | Germinaton/% | Germinaton index | vitality index | plant height/cm | root length/cm | fresh weight/g | dry weight/g |
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0(CK) | 98.33 ± 1.33 ab | 45.71 ± 1.08 a | 2.19 ± 0.05 b | 1.82 ± 0.07 a | 5.64 ± 0.36 a | 0.15 ± 0.01 bc | 0.018 ± 0.00 ab |
50 | 99.33 ± 0.67 a | 43.57 ± 0.72 a | 2.96 ± 0.05 a | 1.96 ± 0.14 a | 3.98 ± 0.18 b | 0.20 ± 0.01 a | 0.020 ± 0.00 a |
100 | 88.00 ± 0.00 bc | 32.24 ± 0.55 b | 1.86 ± 0.03 c | 1.84 ± 0.10 a | 3.66 ± 0.18 bc | 0.18 ± 0.01 ab | 0.017 ± 0.00 ab |
150 | 80.67 ± 2.91 cd | 26.67 ± 2.97 bc | 0.96 ± 0.11 d | 1.48 ± 0.12 b | 3.18 ± 0.24 c | 0.14 ± 0.01 c | 0.015 ± 0.00 bc |
200 | 73.33 ± 6.67 d | 23.71 ± 3.49 c | 0.46 ± 0.07 e | 1.24 ± 0.07 b | 1.92 ± 0.15 d | 0.09 ± 0.01 d | 0.011 ± 0.00 c |
Effects of GABA priming on seed germination and seedling growth of rapeseed under NaCl stress
Under 150 mM NaCl stress, treated by GABA priming relieves the inhibitory effect of salt stress on seedling growth (Fig. 2) Accordingly, CK2 treatment showed reduction in germination rate, germination index, vigor index, seedling height, root length, fresh weight and dry weight of rapeseed by 60.4%, 79.27%, 93.09%, 25%, 85.21%, 66.67% and 46.48%, as compared with CK1, respectively (Table 2). GABA priming of the rapeseed seeds accelerated the germination rate, germination index, vigor index, seedlings root length, fresh and dry weight as compared with CK2 seedlings. GABA at the rate 1.5 mM had showed consistently significant effect for all indexes except for seedlings height. The current findings indicated that GABA (1.5 mM) seed priming could successfully mitigate the adverse effect of salt stress on the germination and seedlings growth of rapeseed.
Table 2
Effects of GABA priming on rapeseed germination and seedling growth under NaCl stress
GABA concentration(mM) | Germinaton/% | Germinaton index | vitality index | hight/cm | root length/cm | fresh weight/g | dry weight/g |
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0(CK1) | 100.00 ± 0.00 a | 47.14 ± 0.44 a | 3.76 ± 0.03 a | 0.80 ± 0.03 a | 5.68 ± 0.03 a | 0.21 ± 0.02 a | 0.023 ± 0.00 a |
0(CK2) | 39.60 ± 5.91 d | 9.77 ± 1.82 f | 0.26 ± 0.05 e | 0.60 ± 0.03 b | 0.84 ± 0.03 c | 0.07 ± 0.01 c | 0.013 ± 0.00 d |
0.5 | 55.60 ± 4.35 c | 16.6 ± 0.72 cd | 0.75 ± 0.03 d | 0.72 ± 0.06 a | 1.36 ± 0.06 c | 0.13 ± 0.01 b | 0.017 ± 0.00 c |
1.0 | 73.60 ± 5.27 b | 21.09 ± 1.98 cd | 1.15 ± 0.11 c | 0.54 ± 0.02 b | 3.80 ± 0.02 b | 0.15 ± 0.01 b | 0.018 ± 0.00 c |
1.5 | 92.80 ± 3.72 a | 33.54 ± 2.16 b | 2.00 ± 0.13 b | 0.60 ± 0.03 b | 5.20 ± 0.03 a | 0.20 ± 0.02 a | 0.022 ± 0.00 a |
2.0 | 54.40 ± 5.00 c | 15.57 ± 2.22 de | 0.79 ± 0.11 d | 0.74 ± 0.05 a | 1.32 ± 0.05 c | 0.13 ± 0.01 b | 0.021 ± 0.00 ab |
2.5 | 35.20 ± 3.88 d | 10.66 ± 1.66 ef | 0.42 ± 0.06 e | 0.56 ± 0.02 b | 1.08 ± 0.02 c | 0.11 ± 0.01 b | 0.018 ± 0.00 bc |
CK1: priming with deionized water and treated with deionized water ; CK2: priming with deionized water and treated with 150mM NaCl stress. |
Effects of GABA priming on root activity and chlorophyll content of rapeseed seedlings under NaCl stress
Accordingly, the root activity and chlorophyll content of CK2 were reduced by 82.14% and 31.70%, as compared with CK1, respectively (Fig. 3). Compared with CK2, GABA priming can effectively alleviate the effect of salt stress on root system and leaf photosynthesis of rapeseed seedlings. Among these treatments, 1.5 mM GABA priming significantly improved root vitality and increased chlorophyll content by 77.02% and 38.64%, respectively. The results indicated that GABA (1.5 mM) seed priming could successfully enhance root activity in rapeseed seedlings and promote the synthesis of photosynthetic pigments in rapeseed leaves.
The effect of GABA triggering on MDA and H2O2 of rapeseed seedlings under NaCl stress
MDA and H2O2 content of rapeseed seedlings were significantly affected by GABA priming under NaCl stress (Fig. 4). The results indicate a significant increase in the contents of MDA and H2O2 of CK2 seedlings compared to CK1, with an increase of 72.19% and 176.57%, respectively (Fig. 4). However, GABA priming treatments significantly reduced the MDA and H2O2 content in rapeseed seedlings ,as compared to CK2. Specifically, the MDA content of 1.5 mM priming treatment decreased by 41.68% (Fig. 4A), while the MDA content reduced by 54.30% (Fig. 4B). This suggests that priming treatment of rape seeds with 1.5 mM GABA under NaCl stress can significantly reduce the content of MDA and H2O2 in rapeseed seedlings, thereby mitigating oxidative damage caused by NaCl stress.
Effect of GABA triggering on the activity of antioxidant system in rapeseed seedlings under NaCl stress
Antioxidant enzymes and antioxidants in rapeseed seedlings were significantly affected by GABA priming under NaCl stress (Fig. 5, Fig. 6).The result indicated that exposure to NaCl stress resulted in a significant increase in the activities of SOD, POD, CAT, and APX n rapeseed seedlings (Fig. 5). In comparison to CK2, GABA priming significantly increased the activity of SOD, POD, CAT, and APX (Fig. 5). Notely, 1.5 mM GABA priming significantly increased SOD, POD, CAT, and APX activity by34.81%, 239.46%, 66.20%, and 57.84% respectively. These results suggest that seed priming treatment with GABA (1.5 mM) improved the antioxidant system of rapeseed seedlings.
Except antioxidant enzymes, antioxidants play a crucial role in mitigating oxidative damage in plants induced by salt stress(Yang et al. 2018). The study shows that the AsA content of rapeseed seedlings was significantly reduced when treated with NaCl, while the GSH content increased. Compared with CK2, rapeseed seedlings treated with GABA priming showed a significant increase in AsA content, with the 1.5 mM GABA treatment showing a remarkable increase of 89.77% (Fig. 6A). The content of GSH increased by 8.90% when treated with 1.5 mM GABA compared to CK2(Fig. 6B). However, when seedlings were treated with other concentrations under NaCl stress, the GSH content decreased, but the change was not significant. These results indicate that GABA (1.5 mM) can induce oxidative stress response in plants under NaCl stress, leading to an improvement in the efficiency of ROS scavenging. Discussion
Role of GABA in germination of rapeseed under salt stress.
Seed germination is a complex physiological process that is easily affected by abiotic stresses such as high temperature and salt damage(Wang,2015). Generally, low-concentration NaCl stress does not have a significant effect on plant seed germination. However, higher-concentration NaCl treatment can inhibit both plant seed germination and seedling growth(Cao et al.,2019). Some studies have found that when the seeds of sweet sorghum (Sorghum dochna Snowden.) were cultivated with 0.4% NaCl, the germination rate and germination potential did not change significantly, and when the NaCl concentration was higher than 0.8%, the germination of sorghum seeds began to be inhibited(Wang et al., 2019). The results of this study is similar. Accordingly, at a NaCl rate of 50 mM, the germination rate and germination index of rapeseed seeds did not significantly differ from CK (Table 1). However, as the NaCl concentration increased, seed germination and seedling growth were impeded. The inhibition was particularly severe when NaCl concentrations exceeded 150 mM, leading to a significant reduction in both seed germination and seedling formation.
GABA, a non-protein amino acid, is synthesized by glutamic acid decarboxylase and degraded by polyamines(Li et al., 2021). Numerous studies have demonstrated that the application of GABA can improve seed germination rates, accelerate germination, increase the conversion value of storage materials, promote germ root and elongation, and increase the fresh and dry weight of early plant seedlings (Wang, 2020; Zhao et al., 2021). Additionally, it has a positive regulatory role in various adversity stresses (Zhang et al., 2020). According to research, soaking wheat (Triticum aestivum L.) seeds with GABA can enhance the growth of seedlings. Treatment with 0.5 mM GABA has shown a significant increase in the germination rate and fresh weight of wheat. Moreover, GABA treatment has also reduced the damage caused by salt stress to wheat seedlings (Zhang et al., 2015). According to this study, seeds primed with 1.5 mM GABA under NaCl stress conditions showed a significant increase in the germination rate and germination index of rape seeds, as well as promoted the growth of seedlings. These results demonstrate that seed priming with 1.5 mM GABA can effectively alleviate the damage caused by salt stress on seed germination and seedling formation.
GABA priming increased root activity and chlorophyll content of rapeseed seedlings under salt stress
The plant root system plays a vital role in the absorption of water and mineral elements. When subjected to NaCl stress, the cells respond through ion balance and osmotic regulation (Liu et al., 2017). The root activity is a reflection of its ability to oxidize, reduce, absorb, and synthesize substances. Furthermore, the size of the root system is crucial in maintaining the growth of the aboveground parts of plants(Yi et al., 2018). In a study conducted on maize (Zea mays L.) under NaCl stress conditions, it was observed that treatment with 0.5 mM GABA led to an increase in root length, root surface area, and root volume, thereby promoting root development (Wang, 2016). Similarly, in rapeseed seedlings subjected to NaCl stress, priming with 1.5 mM GABA resulted in significantly higher root length and root activity as compared to CK2 (Fig. 3). This increase in root activity aids in the absorption of water and minerals, consequently enhancing the salt tolerance of rapeseed seedlings.
Under NaCl stress, plant root activity weakens and there is insufficient nutrient and water supply to the aboveground parts, leading to plant malnutrition and a decrease in chlorophyll content in leaves(Ling et al., 2020). Chlorophyll is closely linked to plant photosynthetic efficiency. Maintaining stable chlorophyll content helps plants maintain normal photosynthesis progress under adverse conditions. Reduced chlorophyll content in plants can lead to a decrease in the photosynthetic rate and accumulation of net photosynthetic products, ultimately affecting plant growth (Katarzyna et al., 2015). Recent studies(Liu, 2019)have demonstrated that GABA can stabilize and protect chloroplast thylakoids by reducing hydroxyl radical content, thus preventing cell damage in low temperature environments. The research of Chen(Chen, 2017)supports the use of GABA to reduce the decline rate of chlorophyll content under stress conditions. This can help alleviate damage to photosynthetic tissue caused by adverse conditions. In this study, it was found that the chlorophyll content in cotyledons of rapeseed seedlings treated with GABA was significantly higher than that of CK2. The most significant effect was observed with a 1.5 mM GABA priming treatment. This suggests that priming rapeseed with GABA can effectively alleviate the negative impact of NaCl stress on seedling photosynthetic pigments. Moreover, it is beneficial for the normal photosynthesis of seedlings..
GABA priming regulated the antioxidant enzyme system of rapeseed seedlings under salt stress
Under NaCl stress, excessive production of ROS can accumulate in plant cells, leading to severe oxidative damage to plant cell membranes (Yin. et al,2020). MDA, a byproduct of membrane lipid peroxidation, is frequently used to assess the extent of oxidative damage in plant cells (M. et al.,2016). To balance the production and removal of ROS, plants have developed an antioxidant enzyme system through long-term evolution (Liu et al., 2017). Research (Liu et al., 2017) has indicated that the application of GABA from an external source can lower the content of O2.−, H2O2, and MDA in tomato seedling leaves. Carillo (2018) suggested that the application of GABA can aid in the removal of ROS and improve the plants' stress resistance. The study revealed that treating rapeseed seedlings with GABA priming can decrease the levels of H2O2 and MDA when exposed to NaCl. The most significant effect was observed with 1.5 mM GABA treatment. The antioxidant enzyme system in plant cells, including SOD, POD, CAT, and APX, can be destroyed when exposed to NaCl stress. However, when activated, these enzymes can effectively remove excess H2O2, O2.−, and other ROS, thereby reducing oxidative damage to plants (Ibrahim et al., 2020). Tian et al. (2005) demonstrated that the use of GABA increased the activities of SOD, POD, and CAT in maize seedlings, reducing damage caused by NaCl stress. In this study, rapeseed seedlings primed with 1.5 mM GABA displayed significantly higher activities of SOD, POD, CAT, and APX under NaCl stress compared to CK2. The results demonstrate that seed priming with an optimal concentration of GABA effectively regulates the antioxidant enzyme system of plants under NaCl stress. This process mitigates excess ROS, reduces oxidative damage, and ultimately enhances the salt tolerance of rapeseed.
The AsA-GSH cycle is one of the most important antioxidant protection systems for scavenging ROS (Ian et al.,2007), which mainly composed of antioxidant substances AsA and GSH and key enzymes of the AsA-GSH cycle. Studies have shown that the rapid operation of AsA-GSH cycle in plants is beneficial to reduce the level of ROS in plants and alleviate the oxidative stress of plants(Sun et al.,2015). AsA is the main scavenging enzyme in the AsA-GSH cycle (Gausiya et al.,2016), while GSH mainly promotes the stabilization of membrane protein structures (Yang et al.,2018). The study concluded that NaCl stress treatment caused a significant decrease in the AsA content of rapeseed seedlings. However, the GSH content increased, indicating that the seedlings tried to maintain cell membrane stability by increasing GSH content under NaCl stress. Compared to CK2, the AsA and GSH content in seedlings treated with GABA significantly increased. The most significant effect was observed with 1.5 mM GABA priming. It suggest that GABA priming (1.5 mM) can induce oxidative stress in plants in response to NaCl stress, thereby improving the efficiency of scavenging ROS in the AsA-GSH cycle.