Low pH alleviated salinity stress in ginger seedlings by enhancing light eciency and equilibrium of mineral elements

Abstract

medicine. Ginger is increasingly favored by people due to its avor and health bene ts, and the amount of ginger produced and grown is increasing (Food and Agriculture Organization of the United Nations, FAO). However, soil salinization is getting worse and worse, resulting in a decrease in ginger yield.
Therefore, there is an urgent need to explore the effects of salt stress on the growth of ginger, in order to improve the growth of ginger under salt stress in combination with other ways.
Many studies indicated that the photosynthesis and chlorophyll content studies on ginger are mainly in antibiotics and drought [18][19][20][21][22]. However, little study was reported on photosynthesis and chlorophyll content under salt stress with low pH in ginger. Therefore, the objectives of this study were to investigate the alleviating effect of low pH in salinity stress tolerance in ginger seedlings, and the growth and biomass, chlorophyll content, photosynthesis-related enzymes, sugar metabolism and mineral content of ginger under different treatments were investigated.

Growth and biomass
Under salt-free conditions, there was no signi cant difference in the height of ginger seedlings at pH 6 (T1) and pH 4 (T2) (p < 0.05) (Supplementary Table 1), which indicated that low pH wolud not affect the growth of ginger. However, salt stress signi cantly inhibited the growth of ginger seedlings, but the effect of low pH (T4) combined with salt stress on the growth of ginger seedlings was lower than that of high pH (T3). The trends of stem thickness, leaf, stem, rhizome and root fresh weight of ginger seedlings under different treatment conditions were similar to those of ginger seedling height. For example, T3 treatment reduced the fresh weight of ginger leaves and roots by 56.52% and 40.06% compared to the control group, while T4 treatment reduced the fresh weight of ginger shoots and roots by 51.40% and 34.87% (Supplementary Table 1).

Photosynthetic parameters
In order to investigate the photosynthetic capacity of ginger seedlings under salt stress, Pn, Gs, Ci, Gs, Tr, WUE and Ls were measured. Fig.2 showed the results of Pn, Gs, Ci, Tr, WUE and Ls of ginger leaves under different H + treatments with or without salt stress. Salt stress signi cantly decreased Pn, Gs, Tr, and WUE under T3 treatment were decreased remarkably by 47.21%, 42.67%, 17.19% and 36.26%, respectively, while Ci was increased by 18.92%, compared to control (T1). Meanwhile, under T4 treatment, Pn, Gs, Tr, and WUE were decreased by 25.42%, 31.90%, 12.50% and 14.76%, respectively, while Ci was increased by 8.62%, compared to control (T1). The Ls of ginger seedlings treated with T3 and T4 decreased by 41.75% and 20.26%, respectively, compared to control (T1). And compared to T3, T4 treatment signi cantly improved the Pn, Gs, Tr, WUE and Ls of ginger seedlings. However, pH treatment only had a certain effect on Pn and WUE in ginger seedlings leaves, but they did not reach signi cant differences at the level of P<0.05 (T1).

Pigment contents
Salt stress (T3 and T4 treatment) signi cantly reduced the content of Chl a, Chl b, Car and Chl a+b, which reduced by 9.33%, 28.28%, 7.59% and 13.01%, respectively, compared with T1 (Table 1). Meanwhile, the content of Chl a, Chl b, Car and Chl a+b slightly increased under T4 treatment compared to T3 treatment. There was no considerable difference caused by different pH in the content of Chl a, Chl b, Car and Chl a+b in ginger leaves under salt stress. For root activity, as shown in Table 1, the effects of different treatments on the root activity of ginger seedlings were different, and the changes of root activity after salt stress treatment were greater than those without stress. Salt stress alone decreased the root activity . by 26.94% (T3), however, in low pH with salt stress (T4) only decreased the root activity by 19.57% (Table 1).

Chlorophyll uorescence
To analyze the changes of different ginger photosystems in response to salt stress, chlorophyll uorescence parameters were measured. Salt stress had signi cant negative effects on chlorophyll uorescence parameters of ginger seedlings, which was manifested by decreased Fv/Fm, qP, ΦPSII and increased NPQ compared to control (T1) (Fig. 3). T3 treatment reduced Fv/Fm, qP, ΦPSII by 9.62%, 12.90% and 28.96%, respectively; T4 treatment decreased by 6.85%, 7.87% and 14.35%, respectively, compared to normal conditions (T1). In contrast, the value of NPQ was upregulated by 23.27% under T3 treatment and 14.35% under T4 treatment, compared to T1 treatment. However, under the same salt stress conditions, T4 treatment had less effect on chlorophyll uorescence compared with T3 treatment.

Reducing sugar, Sucrose and Starch content
The effects of different treatments on reducing sugar, sucrose and starch content of ginger seedlings are shown in Table 3. Both low pH and salt stress would decrease the content on reducing sugar, sucrose and starch, but the effect of salt stress was more obvious than low pH. Salt stress alone (T3) reduced the reducing sugar, sucrose and starch content by 51.75%, 63.42% and 54.33%, respectively, compared to control (T1). However, low pH combined with salt stress (T4) only decreased reducing sugar, sucrose and starch by 35.06, 48.62 and 31.48%, respectively, compared to T1. Table 3 shows that the effect of low pH (T2) on reducing sugar and sucrose was signi cant, but the effect on starch content is light, and there was no signi cant difference at the level of P <0.05.
The activity of SS and SPS SS and SPS are key enzymes of carbon metabolism, and their activities affect the content of carbohydrates in plants. Different treatment had signi cant effects on the activity of SS and SPS in the of ginger leaves, as shown in Fig. 4. For SS, there is no signi cantly difference on its activity between T1 and T2 treatment. For SPS, the ginger seedlings treated with low pH (T2) had higher SPS activity than the control group (T1). Salt stress alone reduced the activity of SS and SPS by 41.57 and 30.34%, respectively, compared to control (T1). The activity of SS and SPS were reduced by 30.3 and 6.15%, respectively, under salt with low pH (T4) treatment compared to control (T1).

Content of phosphorus and Nitrogen
Fig . 5 showed the results on the content of P and N of ginger leaves under salt stress with or low pH. Only lowering the pH (T2) has no signi cant (P<0.05) effect on the content of P and N in ginger seedlings. Compared to control (T1), salt stress alone (T3 treatment) reduced the content of P and N by 57.90 and 16.35%, respectively; and low pH with salt stress (T4 treatment) decreased content of P and N by 47.48 and 11.08%, respectively. T4 treatment contained higher P and N content than T3 treatment ginger seedlings.

Mineral composition content
As shown in Fig. 6, salt stress signi cantly increased the Na content in leaves of the plants and signi cantly decreased the content of K, Mg, Ca, Fe and Zn. The content of Na of ginger seedling leaves was enhanced by 101.55% under salt stress alone and increased by 40.62% under low pH with salt stress. Comparative to control (T1), salt stress alone signi cantly reduced the content of K, Mg, Ca, Fe and Zn by 27.27%, 32.52%, 28.08%, 47.01% and 41.73%, respectively; low pH with salt stress (T4) decreased the content of K, Mg, Ca, Fe and Zn by 17.36%, 29.38%, 13.78%, 40.64% and 31.75%, respectively.

Ultrastructure Morphometric
Transmission electron microscopy was used to observe the changes of whole mesophyll cells and chloroplasts in ginger seedling leaves. As shown in Fig.7, ginger seedlings leaves grown under pH 6 and pH 4 without salinity treatment possess typical mesophyll cells and chlorophyll shapes; simultaneously, large number of starch grains packed well in the cell, and the chloroplast contains a great quantity of orderly grana lamellae (Fig. 7 T1 and T2). However, the size of the cells and chloroplasts of ginger grown under 100 mmol·L -1 salinity changed differently. Compared with the leaves without treated with salinity, the cells of the salt treated ginger leaves were disordered and irregular, the cytoplasm was dissolved and the chloroplast shape was severely swollen. Additionally, in the salinity treatment group, the starch grains decreased and the permeability granules increased signi cantly in the leaves, the grana lamellae were also loosed ( Fig. 7 T3). However, this situation will be improved by lowering the pH value. In the absence of salt treatment, the mesophyll cells and chloroplasts of ginger seedlings were very similar under pH 4 and pH 6. But, in the case of salinity treatment, the cell morphology of the low pH (pH 4) treated seedlings is slightly improved compared with the high pH treatment (pH 6), such as an increase in starch grains.

Discussion
Salt stress resulted in reduced plant growth and biomass, and even plant death [23]. This study showed that salt induction had a negative impact on the growth and biomass of ginger seedlings, but under salt treatment conditions, ginger seedlings at lower pH have better growth state and higher biomass than high pH treatment. The decrease in plant growth under salt stress was mainly due to the imbalance of nutrients. In addition, salt stress could inhibit cell division and cell elongation, and also reduce the extracellular water potential and water bioavailability of the root area of the plant, causing the plants had a low absorption of water and nutrients, which led to damage to the metabolic activity in the system [24]. Other studies had also found that tomato and wheat growth and biomass also decline under salt stress [2,25]. Low pH could improve the growth and biomass of ginger seedlings under salt stress may be attributed to photochemical physiology and biochemical reactions. The effects of low pH on photosynthesis of forced ginger seedlings under salt stress may be mainly the effect on light and dark responses, as shown in Fig. 1. Low pH combined with salt stress on ginger seedlings' chlorophyll content, photosynthetic enzyme activity, sugar metabolism-related factors, and the changes in chlorophyll content, photosynthetic enzyme activity, sugar metabolism-related factors, and essential elements of ginger seedlings under low pH combined with salt stress could appropriately characterize the corresponding photosynthesis process.
Salinity often adversely affects plant growth and photosynthesis. The damage caused by salt to plants is mainly due to the inhibition and destruction of photosynthesis, and the decrease of photosynthetic e ciency is one of the most important reasons for the reduction of plant biomass under salt stress. The inhibition of photosynthesis undre salt stress was mainly affected by the stomatal and non-stomatal restrictions [7]. In the present study, salt stress decreased Pn, Gs, Tr and Ls in ginger seedlings, but increased Ci, there is a positive correlation between Pn, Gs and Ci, which indicated that the changes of photosynthetic rate of ginger seedlings caused by salt stress were mainly caused by the unrestricted stomata caused by the decrease of electron transfer rate [26], and the reduction in Ls also indicated that stomatal limitation is non-dominant. Although salt stress decreased Pn, Gs, Tr, and Ls in ginger seedlings, increased Ci, the changes of Pn, Gs, Tr, Ls and Ci in ginger seedlings under low pH with salt stress (T4) were signi cantly lower than the salt stress alone group (T3). These results suggested that low pH could improve photosynthesis of ginger seedlings under salt stress by improving non-stomatal related factors. In addition, the increase of Pn is also positively correlated with the content of chlorophyll, so low pH may reduce the loss of chlorophyll content The water use e ciency (WUE) is usually used to evaluate the adaptability of gas exchange, which is of great signi cance to the plant's ability to adapt to adversity stresses [27]. Therefore, WUE was closely related to the CO 2 concentration in plants, additionally, it was also affected by leaf Tr, which was in turn caused by the decreased Cs [28]. In this research, salt stress decreased WUE, Tr and Cs, so salt stress may reduce Tr by lowering Cs and causing stomatal closure, and eventually reduce WUE. Stomatal closure reduces carbon dioxide xation as photosynthetic rate [25]. Nevertheless, low pH increased the WUE of ginger seedlings under stress condition. This indicated that H + could alleviate the harmful effects of salt stress on ginger seedlings by enhancing Cs and water use in photosynthesis. This was consistent with the mechanism of applying exogenous ALA to increase WUE in cucumber leaves under salt stress [29].
Chlorophyll is essential for photosynthesis, and its content has an obvious correlation with leaf photosynthetic capacity [30]. And chlorophyll content and root activity can be used as indicators to indicate the health status of plants. Salt stress negatively affects chlorophyll content and root activity, leading to decreased photosynthesis and plant growth. Consistently, in this study, there is no doubt that salt stress reduced chlorophyll concentration (total Chl, Chl a, Chl b, Car) and root activity in ginger seedlings. Chl and Car are the main photosynthetic pigments in higher plants, and their content is inevitably affected by salt stress. Salt-induced alterations in chlorophyll content may be due to salt stress caused Mg precipitation to impair biosynthesis, or to change the bond between chlorophyll and thylakoid membrane protein, increased Chl-degrading enzyme chlorophyllase activity and accelerated chlorophyll degradation, this effect is attributed to increased Na + content of toxic cations [31][32][33]. Salt stress also caused reduction in chlorophyll contents in mungbean leaves, which might be caused by membrane swelling in chloroplasts and/orexcess Na + ions in the leaves [24]. In this research, as expected, in the saltstressed plants, Na accumulated excessively in leaves (Fig. 6). And different H + concentration gradients on the root cell membrane of many plant species have different exclusion effects on Na + [34]. Low pH combined with salt stress could increase the H + concentration and separate Na + into the vacuole to exclude Na in the roots of ginger seedlings, thus increasing the chlorophyll content and root activity. The chlorophyll content of ginger seedlings was higher than that of salt stress alone under low pH with salt stress, which showed that H + was bene cial to maintain the normal photosynthesis capacity of ginger seedlings and improve their adaptability to salt stress ( Table 2). This can also be corroborated by the results of chlorophyll uorescence (Fig. 3). Chlorophyll uorescence is commonly used to measure the tolerance and adaptability of plants to extreme environments, which is a method to re ect the effectiveness of Photosystem II (PS II). In our study, salt stress reduced Fv/Fm ΦPSII and qP, but increased NPQ in ginger seedlings, which was also reported in salt-stressed tomato seedings, eggplant (Solanum melongena) and wheat [25,26,35]. These results suggested that PSII electron transport in ginger seedlings was inhibited by salinity, and excessive excitation is dissipated by thermal forms, the decrease in qP also indicates that the proportion of plastid quinones in PSII is increased, resulting in a decrease in Fv/Fm ratio [20]. The Fv/Fm, ΦPSII, and qP values increased signi cantly, and NPQ decreased signi cantly under low pH with salt stress, which suggested that H + may protect PSII from overexcitation and maintain the structural integrity of thylakoid membranes, so that the electron transfer and photosynthesis of ginger seedlings proceed normally, thereby improving the light energy conversion e ciency of PS II.
In addition, some studies suggested that the decrease in PSII activity caused by salt stress may be attributed to changes in CO 2 xation e ciency. Rubisco activity in plants is mainly regulated by Rubisco activating enzymes, glycolytic enzymes and Calvin cycling enzymes [36]. FBPase can hydrolyze and remove phosphate from fructose 1,6-bisphosphate to form chlorophyll 6-phosphate, participate in the carbon dioxide (Calvin) cycle, and sucrose and starch biosynthesis in plants [37]. FBA is also a key enzyme involved in the Calvin cycle, and is involved in glycolysis, gluconeogenesis [38]. In this study, the activities of Rubisco, FBA, and FBPase were reduced under T3 and T4 treatments, which indicates that salt threatens to reduce the e ciency of carbon dioxide xation, inhibit the Calvin cycle, and biosynthesis of sucrose and starch, this is also consistent with the results of reducing sugar, sucrose, and starch contents in Table 3. The above results further validated that the effect of salt stress on photosynthesis of ginger seedlings is non-stomatal limitation. Similar results were also found in salt-stressed cucumber seedlings [7]. However, low pH with salt stress increased Pn, Gs and the activities of Rubisco, FBA and FBPase, and reduced Ci. The results showed that low pH could increase the photosynthetic rate of ginger seedlings by reducing non-stomatal restriction, enhancing the activities of photosynthetic related enzymes, promoting Calvin cycle, sucrose and starch biosynthesis.
Sugar is the nal product of photosynthesis and acts as an osmotic regulator or nutrition and metabolic signal to participate in plant defense against abiotic stress such as salt stress [39]. In order to study the effect of salt stress on the sugar metabolism of ginger seedlings, sugar changes and sugar metabolismrelated enzyme activities in differently treated ginger seedlings were analyzed. The content of reducing sugar, sucrose and starch in ginger was decreased after salt treatment, this probably because the sugar was consumed in order to meet the energy demand for ginger growth [14]. In addition, the decrease in sugar content of ginger seedlings may also be attributed to the reduction of sugar metabolism-related enzyme activities by salt stress. Sucrose is the main product of photosynthesis, sucrose phosphate synthase (SPS) and sucrose synthase (SS) are considered to be key enzymes in sucrose metabolism [40].
In ginger seedlings, salt stress decreased SS and SPS activities. And the results of photosynthetic related enzyme activity ( Table 3) also indicated that inhibition of Calvin cycle under salt stress may also reduce sugar synthesis. Therefore, the changes in the above factors led to reduced sugar content in ginger seedlings. Nevertheless, low pH with salt stress treatment improved the reducing sugar, sucrose and starch content in ginger, and the activities of SS and SPS were also improved. H + may increase the activity of sugar metabolism-related enzymes to accumulate sugar in ginger seedlings, and then play the role of an osmotic protector, thereby promoting osmotic steady state [41]. Furthermore, the enhancement of the Calvin cycle is another important factor to increase the sugar content of ginger seedlings. Nitrogen (N) is a component of plant cell components, including amino acids, proteins and nucleic acids, which is an important element in plant growth and development [42]. Phosphorus (P) is another important nutrient in plants and can be used as a constituent element of essential biomolecules such as phospholipids and ATP [43]. The results showed that salt treatment induced decrease in N and P concentration in ginger plants, which may hinder the synthesis of related substances in plants, leading to reduced plant growth. The decrease of N concentration caused by salt stress may be due to the interference of N absorption and utilization by salt. The reduction of phosphorus content in ginger seedlings caused by salt stress is consistent with the results of Zhang et al. [43], which reported that phosphorus concentration was decreased in maize under salt stress. However, plants under low pH with salt stress have consistently higher N and P concentration than the plants under salt stress, the results showed that low pH can increase the content of N and P in salt-stressed plants by providing su cient H + to Exclude Na + [32]. Wang et al. [16] also found that lowering the pH of cotton roots reduced salt stress and improved P content and plant growth.
Potassium (K), calcium (Ca), zinc (Zn), magnesium (Mg), and zinc (Zn) are important for chlorophyll synthesis and photosynthesis, as well as for enzyme systems and plant carbon cycling, for example, Ca can act as a second messenger that acts as a cytokinin in promoting chlorophyll synthesis; Mg 2+ is the core element of the chlorophyll molecule [25], so these minerals play a vital role in salt tolerance. In this study, as expected, in the salt-stressed ginger seedlings, Na accumulated excessively in leaves, while K, Mg, Ca, Zn and Fe contents decreased signi cantly. This may be because salt stress caused excessive Na + accumulation in ginger seedlings, resulting in a lack of other mineral ions, which would lead to a decrease in chlorophyll content and affect photosynthesis in ginger seedlings [44], which was consistent with the results in Table 1. In addition, Na + and essential mineral ions (such as K + and Ca 2+ ) have an antagonistic effect between their absorption sites, and the increase in Na content would also affect other mineral content [1]. Na + /K + ratio in the cytoplasm is essential for maintaining normal physiological functions of cells [45], the increase of Na + /K + ratio in ginger seedlings under salt stress would inhibit the related metabolic activities of ginger seedling cells. Moreover, the lower Mg, Zn, Ca and Fe concentrations induced by salt may cause the destruction of chlorophyll structure. The results in Fig. 7 also indicated that salt stress would destroy the cell structure of ginger seedlings. However, low pH with salt stress decreased Na content and increased other mineral content, this may be attributed to the low pH decreased the Na content and thus enhance the absorption of trace micronutrients. These results indicated that ginger seedlings under low pH with salt stress have higher absorption and transport capacity of K + , Mg 2+ , Zn 2+ , etc., to ensure that the concentration of ions involved in key metabolic activities in the leaves is su cient. And, low pH also maintained the integrity of chloroplast structure in cells, promoted the production of chlorophyll, improved photosynthesis and promoted ginger growth by increasing the content of essential metal elements.
Salinity stress would change the ultrastructure of plant cell membrane and cell wall, destroy the structure of mitochondria and chloroplast, and change the degree of aggregation of rough endoplasmic reticulum [46]. In this study, the changes of whole mesophyll cells and chloroplasts of ginger seedlings in salinity was observed. Compared with T3 treatment, T4 treatment ginger seedlings maintained the structural integrity of the cells without chloroplast swelling, starch grains disorder and thylakoid changes, indicating that cellular homeostasis was maintained by low pH. The accumulation of starch grains in the chloroplast under T4 treatment was higher than T3 treatment, which indicates that the carbon assimilation of plant photosynthesis was activated and more energy was generated (the results in Table 3 also showed that the ginger seedlings under low pH treatment (T4) contain higher starch content than T3 treatment. This observation is consistent with the higher energy requirements to maintain normal cell morphology under salt stress [46]. In addition, osmotic adjustment is another important physiological mechanism to improve plant adaptation to salinity stress higher levels of starch have higher osmotic adjustment ability [26]. Therefore, low pH may be able to alleviate the destruction of the intact cells of ginger seedlings under salt stress, reduce the swelling of chloroplasts, and provide a normal necessary place for the normal operation of photosynthesis.

Conclusion
Salt stress is one of the major abiotic stresses that inhibit plant growth. This study demonstrated that salt stress signi cantly inhibited the growth and decreased photosynthesis, the pigment contents, sugar content and enzyme activities, and mineral content in ginger leaves. However, low pH with salt stress not only increased the growth and height of chlorophyll content and chlorophyll-related index, but also increased the photosynthesis-related enzyme activities and sugar metabolism capacity, and mineral content of ginger seedlings under salt stress. Moreover, it is worthwhile noting that low pH simultaneously increased the accumulation of K, Mg, Ca, Fe and Zn. In addition, ultrastructure pictures demonstrate that low pH can increase photosynthesis by reducing damage to the chloroplast of ginger seedlings under salt stress. In conclusion, low pH may reduce the damage to ginger seedlings under salt stress by enhancing photosynthesis. The molecular mechanism of how low pH can improve salt stress of ginger seedlings needs further research. Plant materials (ginger) used in this study were from our lab (College of Horticulture Science and Engineering, Shandong Agricultural University, Tai'an, China; Professor Kun Xu is the chairmen of the laboratory, Email: xukun@sdau.edu.cn). A pot culture experiment was performed from April to October 2018 in Tai'an, Shandong Province, China. On May 14, the ginger variety "Shandong No. 1" was sown in 40 pots (diameter: 25 cm, height: 30 cm) containing clean quartz sand. To minimize variables, soilless cultivation was used in this study. In June and July, a 50% shading screen was used to reduce sunlight transmission. Pots were irrigated daily with Hoagland nutrient solution.

Experimental treatments
NaCl and Na 2 SO 4 were used to simulate neutral salt stress, and pure water with different pH values (HCl/H 2 SO 4 =1/1) was used to simulate hydrochloric acid stress treatment. A two-way random block design was used: the primary treatment includes two pH values, normal and low pH (6.0 and 4.0, respectively), and the secondary treatment includes two salinity treatments, 0 and 100 mmol L −1 Na + . Therefore, this study includes four treatments: T1 (pH 6, salinity 0), T2 (pH 4, salinity 0), T3 (pH 6, salinity 100 mmol L -1 ) and T4 (pH 4 salinity 100 mmol L -1 ). Each treatment was repeated 3 times with 6 plants each. All treatments were performed on a ginger tri-chain fork and measurements were taken 20 days after treatment.

Photosynthetic parameter
Select the functional leaves of the ginger and measure the photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO 2 concentration (Ci) by portable photosynthesis system (Ciras-3, PP SYSTEMS, USA) using the method of Li et al. [18] with slight modi cation. When Pn reached a steady state at each light intensity level, the data was obtained 5 times for each treatment, and the average value was calculated for its photosynthetic parameter.

Pigment concentration
The chlorophyll content of the leaves was determined by 80% acetone extraction. A fresh sample of 0. where V is the total volume of acetone extract (ml), and W is the fresh weight (g) of the sample.

Chlorophyll uorescence
The photochemical quenching coe cient (qP), non-photochemical quenching coe cient (NPQ), quantum e ciency of PS II (ΦPS II), and variable uorescence/ uorescence maximum (Fv/Fm) were measured according to Liu et al [20]. At the time of measurement, 5 plants were averaged for each treatment.
Reducing sugars and Sucrose were calculated according to the method by Handel [49] using the standard graph of glucose.
Starch content was calculated according to the method by Hannachi and Van Labeke [45]. Statistical analysis LSD (least signi cant difference) was used to analyze differences between the measurements in each treatment by using DPS.

Availability of data and materials
The data that support the ndings of this study are available from the corresponding author upon reasonable request.
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.

Competing interests
The authors have no con icts of interest to declare    Table 2 Effects of low pH on the activities of Rubisco, FBA and FBPase in ginger leaves under salt stress T1 (pH 6, salinity 0), T2 (pH 4, salinity 0), T3 (pH 6, salinity 100 mmol L -1 ) and T4 (pH 4 salinity 100 mmol L -1 ) Different small letters in a column of the same treatment days indicate signi cance at the 5% level.  Table 3 Effects of low pH on Reducing sugar, Sucrose and Starch content in ginger leaves under salt stress. DW stands for dry weight. T1 (pH 6, salinity 0), T2 (pH 4, salinity 0), T3 (pH 6, salinity 100 mmol L -1 ) and T4 (pH 4 salinity 100 mmol L -1 ) Different small letters in a column of the same treatment days indicate signi cance at the 5% level. Figure 1 Impacts of low pH on photosynthesis processes of ginger under salt stress. "↓" indicates a decrease, "↑"

Figures
indicates an increase Page 21/25

Figure 4
Effects of low pH on the activity of SS (A) and SPS (B) in ginger leaves under salt stress. T1 (pH 6, salinity 0), T2 (pH 4, salinity 0), T3 (pH 6, salinity 100 mmol l-1) and T4 (pH 4 salinity 100 mmol l-1) Different small letters in a column of the same treatment days indicate signi cance at the 5% level.

Figure 5
Page 24/25 Effects of low pH on the content of P (A) and N (B) in ginger leaves under salt stress. T1 (pH 6, salinity 0), T2 (pH 4, salinity 0), T3 (pH 6, salinity 100 mmol l-1) and T4 (pH 4 salinity 100 mmol l-1) Different small letters in a column of the same treatment days indicate signi cance at the 5% level.

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