Effects of NaCl and NaHCO3 Stress

Ying BAO Jiaxin WANG Chao CHEN Xinmiao YU

Abstract With Hemerocallis fulva ‘Golden Doll as an experimental material, the effects of different concentrations of neutral salt (NaCl) and alkaline salt (NaHCO3) stresses on photosynthesis and chlorophyll fluorescence characteristics in H. fulva seedlings were studied. The results showed that salt stress treatment significantly reduced the photosynthetic capacity of H. fulva . Under the NaCl and NaHCO3 stresses, the photosynthetic characteristics and chlorophyll fluorescence parameters of H. fulva seedlings were basically the same, but the photosynthetic characteristics and chlorophyll fluorescence parameter values of H. fulva seedlings were significantly different under different salt types and salt concentrations. With the extension of days of salt stress and the increase of salt concentration, the initial fluorescence yield ( Fo ) and non-photochemical quenching coefficient ( NPQ ) increased; the maximum fluorescence yield ( Fm ), maximum photochemical efficiency ( Fv/Fm ), PSII actual photosynthetic quantum yield ( Y ) and apparent quantum efficiency ( AQY ) all showed a downward trend; and moreover, with the extension of days of salt stress and the increase of salt concentration, the net photosynthetic rate ( Pn ) decreased and the intercellular CO2 concentration ( Ci ) increased. It was speculated that under salt stress, the photosynthetic characteristics of H. fulva leaves were inhibited. On the one hand, the non-stomatal limiting factor, i.e. , the chlorophyll content decreased, which led to the inhibition of photosynthetic characteristics. On the other hand, the decrease in the photosynthetic performance of mesophyll cells led to a decrease in the net photosynthetic rate of H. fulva . The changes of photosynthetic characteristics and chlorophyll fluorescence parameters in H. fulva caused by salt stress were closely related to the types of salts and salt concentration. High salt stress significantly inhibited the photosynthetic capacity of H. fulva ‘Golden Doll, and the effect of NaHCO3 on H. fulva seedlings was significantly greater than that of NaCl.

Key words Hemerocallis fulva ‘Golden Doll; Salt stress; Photosynthetic characteristics; Chlorophyll fluorescence

Received: February 9, 2020Accepted: April 6, 2020

Supported by Science and Technology Research Project of Institutions of Higher Education in Hebei Province (BJ2017102); Tangshan Normal University Doctoral Fund (2014A06).

Ying BAO (1983-), female, P. R. China, lecturer, PhD, mainly devoted to the research about stress physiology and resistance breeding of ornamental plants.

*Corresponding author. E-mail: baoying090924@126.com.

Among various environmental stresses, salt stress is one of the most serious environmental factors that affect plant growth and development. The research on the adaptability of plants to salt stress has become a hot topic in the world. The large area and wide distribution of saline land in China, coupled with industrial pollution, the development of irrigated agriculture and the improper use of agricultural fertilizers, have led to the continuous expansion of the area of secondary saline soil. Soil salinization has become a continuing problem that restricts the sustainable development of agriculture in China. In addition, most of the large and medium-sized cities in China are located in coastal areas, and with the economic development, the greening and beautification of saline-alkali land have attracted more and more attention. However, the higher soil salt content limits the application of landscaping plants［1-2］. Therefore, evaluating and screening garden plants suitable for growth in saline-alkali areas is of great significance for urban greening and ecological environment improvement.

Studies have shown that salt stress can affect plant growth in at least two ways. On the one hand, salt stress can directly affect the growth of plants; and on the other hand, salt stress can indirectly affect the growth and development of plants by inhibiting the photosynthetic characteristics of plants, and the higher the salt concentration, the longer the stress time and the more significant the degree of impact on plants［3］. However, there is no definitive conclusion as to which main factors cause the net photosynthetic rate of plants to decrease. Plants convert absorbed light energy into chemical energy, and mainly achieve energy conversion through two light systems (PSI and PSII). PSII is very sensitive to salt stress, and it plays an important role in the response of plants to salt stress, while the fluorescence parameters can reflect various indicators of PSII. Photosynthesis is a very important life activity during plant growth, so it is of great significance to study its photosynthetic characteristics and chlorophyll fluorescence parameters.

Hemerocallis fulva ‘Golden Doll is a perennial herb. It has beautiful flowers, rich colors, strong adaptability and strong cold resistance. It can overwinter in the north of China in open field. It is widely used for the improvement of ground cover, flower landscape and saline land. It is a very good landscaping plant［4］. At present, the research on the salt tolerance of H. fulva is mainly focused on the response to neutral salt (such as NaCl) stress, and mainly on the screening of salt-tolerant Hemerocallis varieties, changes in resistance physiological indicators, etc., while the studies on alkaline salt stress are relatively few, and there is a lack of systematic research. There are few reports on the response mechanism of H. fulva 餾 photosynthesis to salt stress.

This study started with the photosynthetic rate and chlorophyll fluorescence characteristics of H. fulva , and discussed the changes in photosynthetic characteristics and chlorophyll fluorescence parameters of H. fulva under different salt concentrations and salt types (NaCl and NaHCO3), aiming to reveal the effect of salt stress on H. fulva ‘Golden Doll. It will provide a theoretical basis for a series of problems such as artificial control of salt stress and cultivation in saline-alkali land.

Materials and Methods

Experimental materials

The material used in this experiment was seedlings of H. fulva ‘Golden Doll with the same growth condition, free of diseases and pests. The day/night temperature of the artificial climate room was 25 ℃/18 ℃; the photoperiod was 12 h/12 h; the illuminance was 600 μmol/(m2·s); and the relative humidity was 50%.

Material culture and treatment

Before the experiment, NaCl and NaHCO3 solids were dissolved in Hoagland nutrient solution, respectively, and they were prepared into neutral and alkaline salt solutions with concentrations of 50, 150 and 250 mmol/L for use. H. fulva was planted in plastic flower pots containing a nutrient soil which was prepared from peat and perlite mixed according to the volume ratio of 3∶1 . The plants were regularly watered and placed in an artificial climate room for 2 weeks to keep the material growth status basically the same. H. fulva ‘Golden Doll plants with the same growth state were selected, and corresponding prepared salt solutions were added to the substrate at about 16:00 to perform 7 kinds of treatments: control (CK), 50 mmol/L NaCl, 150 mmol/L NaCl, 250 mmol/L NaCl, 50 mmol/L NaHCO3, 150 mmol/L NaHCO3, and 250 mmol/L NaHCO3. Three repetitions were set for each treatment. Data was measured after 1, 3, 5, 7 and 9 d of treatment, respectively.

Experimental methods

Determination of pH value of cultivation substrate during treatment

After deionized water was boiled, it was cooled to 50 ℃ for use. Each of 7 kinds of treated soil samples (10 g) was taken and dissolved in 50 ml of deionized water, followed by stirring with a glass rod for 3 to 5 min to mix well while avoiding contamination. The mixtures were stood for 10 min. Finally, a pH meter electrode was inserted into the upper suspension, and the reading was recorded.

Determination of photosynthetic parameters

In the morning of a sunny day, the portable pulse modulation chlorophyll fluorometer (PAM-2500, WalZ) was used to measure the slow fluorescence kinetics and fast light curve of the second to third functional leaves under the growth point of a plant. The net photosynthetic rate ( Pn ) and intercellular CO2 concentration ( Ci ) were measured with a GFS-3000 portable photosynthesis system. During the measurement, the illuminance was (1 000±50) μmol/(m2·s) and the temperature was about 25 ℃. Each measurement was repeated 3 times.

Determination of fluorescence induction curve parameters

A portable chlorophyll fluorometer was used to determine the plant fluorescence induction curve. During the measurement, the leaves were dark-adapted for 20 min in advance, then the measurement light was turned on to determine the initial fluorescence ( Fo ), followed by the execution of a saturated pulse light to determine the maximum fluorescence ( Fm ), and finally the actinic light was turned on［an illumination intensity of 270 μmol/(m2·s)］and lasted 5 min with the execution of a saturation pulse every 20 s. Fluorescence parameters such as the maximum photosynthetic quantum yield ( Fv/Fm ) of PSII, the actual photosynthetic quantum yield ( Y ) of PSII, the electron transfer rate ( ETR ) through PSII, and the non-photochemical quenching coefficient ( NPQ ) were acquired through the PAN-WIN software (WalZ).

Determination of light response curve

The light response curve parameters were measured referring to Chen et al. ［5］. The photosynthetically active radiation ( PAR ) gradient was set to 0, 1, 5, 30, 63, 140, 270, 618, 980, and 1 385 μmol/(m2·s), each of which lasted for 10 s. After measuring the light induction curve, the fluorometer was adjusted to the light response curve measurement mode to fit the light response curve by the EilersandPeeters formula to obtain the parameters of apparent quantum efficiency ( AQY ) and maximum electron transfer rate ( ETR max ).

Determination of chlorophyll content

The chlorophyll content was determined according to the method of Shen et al. ［6］. After 9 d of salt stress treatment, a number of fresh plant leaves were wiped to remove the surface dirt, and shredded after the removal of the veins. A 0.5 g of the freshly cut sample was added into a mortar, and added with a small amount of quartz sand and calcium carbonate. Into the mortar, 2 to 3 ml of 95% ethanol was added, and the mixture was ground into a homogenate, which was then added with 95 ml of 10% ethanol, and further ground until the tissue turned white. After standing for 3 to 5 min, the mixture was filtered into a 25 ml brown volumetric flask, and diluted to constant weight with 95% ethanol. About 3 ml of chloroplast pigment extract was added into a cuvette with a light path of 1 cm, and determined for D 645 nm and D 663 nm , with 95% ethanol as a blank control. The total chlorophyll content was calculated according to the Arnon formula. The determination was repeated 3 times.

ρa ＝(12.7 D 663 nm -2.69 D 645 nm )× V/m ; ρb ＝(22.9 D 645 nm -4.68 D 663 nm )× V/m ; total chlorophyll content＝ ρa＋ρb .

Where D 663 nm is the absorbance at a wavelength of 663 nm; D 645 nm is the absorbance at a wavelength of 645 nm; V is the final volume of the extract (L); m is the fresh leaf mass (g); and ρa and ρb are the chlorophyll a content and chlorophyll b content, respectively.

Data statistics and analysis

PAM-WIN software, Microsoft Excel and SPSS software were used to perform relevant data processing and analysis.

Results and Analysis

Determination of pH value of the substrate under different concentrations of NaCl and NaHCO3 stresses

It can be seen from Table 1 that the pH value of the substrate after the treatment with the neutral salt and alkaline salt generally increased with the salt concentration increasing. However, the trend of pH increase after the alkaline salt treatment was more significant.

Effects of different concentrations of NaCl and NaHCO3 stresses on Fo and Fm of H. fulva leaves

Effects of different concentrations of NaCl and NaHCO3 stresses on the initial fluorescence yield ( Fo ) of H. fulva leaves

Fo can reflect the degree of permanent damage to plant leaf PSII caused by adversity stress. As can be seen from Fig. 1, no matter under the stress of the neutral salt or the alkaline salt, as the salt concentration increased, Fo showed an upward trend, and the longer the stress treatment time, the greater the increase, indicating greater permanent damage to PSII in the leaves of H. fulva . After 9 d of salt stress treatment, with the salt concentration increasing, the Fo of H. fulva plants in the neutral salt treatments increased by 9.62%, 13.28% and 16.93%, respectively, and the Fo of the plants in alkaline salt treatments increased by 9.62%, 16.93% and 27.89%, respectively. The results of two kinds of salt stresses indicated that the alkaline salt stress had a greater permanent damage to PSII of H. fulva leaves than that of the neutral salt stress.

Effects of different concentrations of NaCl and NaHCO3 stresses on the maximum fluorescence yield ( Fm ) of H. fulva leaves

Fm reflects the electron transfer status and maximum electron transfer potential through PSII. As can be seen from Fig. 2, compared with the CK, whether under the stress of the neutral salt or the alkaline salt, as the salt concentration increased, Fm showed a decreasing trend, and the longer the treatment time, the greater the decrease, which indicated that the electron transfer ability of PSII in H. fulva was obviously inhibited. Compared with the CK, Fm of the neutral salt treatments showed a significant decrease after 5 d of treatment; and Fm of the alkaline salt treatments began to show a significant decrease after 7 d of treatment. After 9 d of salt treatment, with the increase of the salt concentration, the Fm of the H. fulva plants treated with the neutral salt decreased by 17.65%, 31.10% and 34.50%, respectively, and the Fm of the H. fulva plants treated with the alkaline salt decreased by 27.20% , 29.88% and 35.06%, respectively. The results of the two kinds of salt stresses indicated that the alkaline salt stress inhibited maximum electron transport potential of PSII significantly greater than the neutral salt stress.

Effects of different concentrations of NaCl and NaHCO3 stresses on the Fv/Fm and Y of H. fulva leaves

Effects of different concentrations of NaCl and NaHCO3 stresses on the maximum photochemical efficiency ( Fv/Fm ) of H. fulva  leaves

It can be seen from Fig. 3 that under the stress treatment with the neutral salt or the alkaline salt, with the increase of the salt concentration, Fv/Fm showed a downward trend, and the longer the stress treatment time, the greater the decrease. After 9 d of salt stress treatment, the Fv/Fm of the H. fulva plants treated with the neutral salt decreased by 8.89%, 16.81% and 20.72%, respectively, and the Fv/Fm of H. fulva plants treated with the alkaline salt decreased by 15.22% , 19.02% and 25.34% , respectively. When comparing the two kinds of salt stresses, the damage of the alkaline salt to H. fulva was significantly greater than that of the neutral salt.

Effects of different concentrations of NaCl and NaHCO3 stresses on the actual photosynthetic quantum yield ( Y ) of PSII in H. fulva leaves

It can be seen from Fig. 4 that regardless of the neutral salt or the alkaline salt, Y decreased compared with the CK, and the decrease rate gradually increased with the extension of the treatment time; and the higher the salt concentration, the more significant the stress effect. After 9 d of treatment, with the increase of the salt concentration, Y of the plants treated with the neutral salt decreased by 6.55% , 19.71% and 28.54% , respectively, and Y of the plants treated with the alkaline salt decreased by 5.68%, 21.75% and 33.01%, respectively. The results of the two kinds of salt stresses showed that the damage of the alkaline salt to the PSII reaction center of H. fulva was more obvious.

Effects of different concentrations of NaCl and NaHCO3 stress on non-photochemical quenching ( NPQ ) of H. fulva leaves

As can be seen from Fig. 5, compared with the CK, after the neutral salt stress treatment, NPQ showed a significant upward trend after 7 d of treatment, indicating that the plants gradually converted light energy into heat energy and lost it. After 9 d of treatment, NPQ of the plants treated with the neutral salt increased by 11.92% , 29.02% and 32.12%, respectively, as the salt concentration increased; and compared with the CK, NPQ of the plants treated with 50 mmol/L alkaline salt decreased by 38.46% , and NPQ of the plants treated with 150 mmol/L alkaline salt increased by 23.07%. The results of two kinds of salt stresses showed that the alkaline salt stress was more harmful to the PSII system of H. fulva , and the plants under the alkaline salt stress could not convert light energy into heat energy to dissipate it out of the body.

Effects of different concentrations of NaCl and NaHCO3 stresses on light response curves of H. fulva leaves

Effects of NaCl and NaHCO3 stresses on apparent quantum efficiency ( AQY ) of H. fulva leaves

AQY represents the initial slope of the light response curve, and reflects the efficiency of the leaves use of light energy, especially weak light. It can be seen from Fig. 6, whether under the stress treatment of the neutral salt or the alkaline salt, as the salt concentration increased, the apparent quantum efficiency showed a downward trend, and the longer the treatment time, the greater the decrease. After 9 d of salt treatment, as the salt concentration increased, the apparent quantum efficiency of the plants treated with the neutral salt decreased by 48.70%, 59.75% and 64.84%, respectively, and the apparent quantum efficiency of the plants treated with the alkaline salt decreased by 60.04%, 65.03% and 68.49%, respectively. The results of the two kinds of salt stresses showed that the alkaline salt treatment significantly weakened the plants use efficiency of weak light compared with the neutral salt.

Effects of different concentrations of NaCl and NaHCO3 stresses on the maximum electron transfer rate ( ETR max ) of H. fulva  leaves

ETR﹎ax  is the maximum photosynthetic rate when there is no photoinhibition, that is, the potential maximum relative electron transfer rate, which represents the maximum ability that the photosynthetic characteristic of leaves can achieve. As can be seen from Fig. 7, no matter under the stress treatment of the neutral salt or the alkaline salt, as the salt concentration increased, the maximum electron transfer rate showed a downward trend, and the longer the salt treatment time, the greater the decrease. After 9 d of salt stress treatment, as the salt concentration increased, the maximum electron transfer rate of the plants treated with the neutral salt decreased by 37.51%, 47.91% and 52.91%, respectively, and the maximum electron transfer rate of the plants treated with the alkaline salt decreased by 41.39 %, 58.02% and 69.48%, respectively. When comparing the two kinds of salt, the alkaline salt treatment had more obvious inhibitory effect on the maximum photosynthetic efficiency of plants.

Effects of different concentrations of NaCl and NaHCO3 stresses on photosynthetic characteristics of H. fulva leaves

Effects of different concentrations of NaCl and NaHCO3 stresses on the net photosynthetic rate ( Pn ) of H. fulva leaves

The net photosynthetic rate can directly reflect the function of plants photosynthetic system, and is also a measure of whether the photosynthetic system of plant is functioning normally. It can be seen from Fig. 8 that compared with the CK, the net photosynthetic rate of the plants treated with the alkaline salt decreased with the increase of the concentration of salt treatment and the extension of the treatment time; and the net photosynthetic rate of the treatment with the neutral salt at 50 mmol/L was slightly higher than that of the CK, by 2.72%, and that of the plants treated with the neutral salt at 150 and 250 mmol/L was significantly lower than the CK. After 9 d of salt treatment, as the salt concentration increased, the net photosynthetic rate of the plants treated with the neutral salt at 150 and 250 mmol/L decreased by 42.91% and 72.38%, respectively, and the net photosynthetic rate of the plants treated with the alkaline salt at 50, 150 and 250 mmol/L decreased by 29.99%, 61.84% and 73.35%, respectively. The results of the two kinds of salt stresses showed that the net photosynthetic rate of the H. fulva leaves in the alkaline salt stress treatments was significantly more inhibited than that in the neutral salt stress treatments.

Effects of different concentrations of NaCl and NaHCO3 stresses on intercellular CO2 concentration ( Ci ) in H. fulva leaves

The intercellular CO2 concentration of plant leaves is another important factor that affects the photosynthetic characteristics of plants. It provides a direct source for the carbon synthesis of photosynthetic characteristics. It can be seen from Fig. 9 that with the extension of the treatment time, the intercellular CO2 concentration in leaf mesophyll cells generally showed an upward trend. There was a significant difference in Ci between the CK and the 250 mmol/L neutral salt treatment; and there were significant differences between the plants treated with the alkaline salt at 150 and 250 mmol/L and the plants of the CK. After 9 d of salt treatment, as the salt concentration increased, the intercellular CO2 concentration in the leaves of the plants treated with the neutral salt increased by 16.59%, 54.38% and 101.54%, respectively, and the intercellular CO2 concentration in the leaves of plants treated with the alkaline salt increased 16.16%, 88.47% and 130.40%, respectively. When comparing the two kinds of salts, the increase in the intercellular CO2 concentration in the leaves of the plants in the alkaline salt treatments was significantly greater than that in the neutral salt treatments.

Effects of different concentrations of NaCl and NaHCO3 stresses on the total chlorophyll content in H. fulva leaves

The reduction in the total chlorophyll content reflects the degree of damage to chloroplasts. The lower the chlorophyll content, the more severe the stress damage to the plant. It can be seen from Fig. 10 that, whether it was the neutral salt or the alkaline salt, the total chlorophyll content of H. fulva leaves showed a downward trend. Compared with the CK, the total chlorophyll content of H. fulva leaves decreased by 23.39% under the neutral salt stress at 250 mmol/L, and decreased by 37.66% under the alkaline salt stress at 250 mmol/L. From the results of two kinds of salt stresses, it could be seen that the alkaline salt had more obvious damage to H. fulva  .

Discussion

Salt stress severely affects the growth and development of plants［7］. Plant leaf photosynthesis is an important foundation for plant growth and development［8］, while chlorophyll is the material basis for photosynthetic light energy capture in plants, including chlorophyll a and chlorophyll b. The chlorophyll content of plant leaves can be used as a physiological indicator to measure the resistance of plants to stress. Under salt stress treatment, the chlorophyll content of most plants decreases. On the one hand, it may be because that salt stress leads to a decrease in the activity of chloroplast pigment synthase and chlorophyll synthesis is blocked. On the other hand, it may be because that salt stress triggers the disorder of plant chloroplast function, and the chloroplast morphological structure is damaged, which leads to the decrease of chlorophyll content in the leaves［9］. In this study, as the concentration of salt treatment increased, the total chlorophyll content gradually decreased, indicating that salt stress caused a change in the pigment composition of H. fulva leaves. Whether it was the neutral salt or the alkaline salt stress, with the increase of salt concentration, the chlorophyll content of H. fulva leaves decreased significantly, which might be caused by the damage to chloroplast structure of H. fulva  leaves caused by high salt stress. This result is consistent with previous studies on sorghum［10］ and cotton［11］. The differences in the decreases in chlorophyll content indicated that there were differences in the degree of photosynthesis affected by different types of salt stresses. Compared with the neutral salt, the alkaline salt stress led to a more obvious decrease in the total chlorophyll content, indicating that the alkaline salt stress caused the chloroplast structure of H. fulva leaves to be more seriously damaged, and the reduction of chlorophyll content might directly affect the normal photosynthesis of H. fulva .

High salt stress treatment can significantly reduce the photosynthesis of plants［12］. Previous studies have shown that the initial site of plant photosynthesis damage is closely related to PSII, and salt stress will lead to the damage of the plant chloroplast photosynthetic structure, thereby reducing the original light conversion efficiency of PSII and inhibiting the potential activity of PSII［13］. The increase in primary fluorescence yield ( Fo ) is the result of the reversible or irreversible inactivationity of the PSII reaction center［14］, which can reflect the degree of permanent damage of PSII in plant leaves under stress. Fv/Fm can reflect the potential maximum light energy conversion efficiency of intact plant leaves, and is used to measure the conversion efficiency of original light energy by PSII［15-16］. Many research results show that salt stress can cause the Fm and Fv/Fm of plant leaves to decrease, and simultaneously cause Fo to rise［17-19］. This study also obtained the same research results. Salt stress had a significant inhibitory effect on the photosynthetic characteristics of H. fulva leaves photosynthesis, especially the activity of photosystem II, and this inhibition was closely related to the salt concentration and salt type. With the increase of salt concentration, Fm and Fv/Fm both decreased, indicating that photoinhibition occurred in H. fulva leaves. And Fo increased at the same time, indicating that PSII was destroyed, which is also confirmed in previous studies［20-21］. Meanwhile, compared with the neutral salt, the alkaline salt stress made the degree of photo-inhibition of H. fulva leaves more intense.

The non-photochemical quenching coefficient ( NPQ ) represents the portion of the light energy absorbed by the PSII antenna pigment that cannot be used for the transfer of photosynthetic electrons and is dissipated in the form of heat［22］. In this study, the NPQ of H. fulva leaves gradually increased with the increase of the salt stress concentration and the extension of salt stress time, indicating that salt stress weakened the utilization of light energy, and the energy diffused by heat dissipation after the leaves suffered from photoinhibition also gradually increased［23］; and the increase rate under the alkaline salt stress was significantly higher than that under the neutral salt stress, indicating that salt stress caused the PSII excitation energy distribution of H. fulva leaves to change, and it adapted to the salt stress environment by increasing the heat dissipation to consume too much excitation energy, which is consistent with the research conclusion of Qi et al. ［24］.

Many important photosynthetic physiological parameters can be estimated from the plant photosynthesis light response curve. Apparent quantum efficiency ( AQY ) reflects the ability of plant leaves to absorb, convert and utilize light energy under low light［25-26］. A significant feature of photoinhibition is the decline in apparent quantum efficiency［14］. In this study, after 9 d of salt treatment, as the salt concentration increased, the apparent quantum efficiency of plants treated with the neutral salt and alkaline salt stress decreased significantly, and compared with the neutral salt, the alkaline salt treatment was more obviously weakened the use efficiency of weak light in H. fulva leaves.

The net photosynthetic rate ( Pn ) can directly reflect the function of the plant photosynthesis system, and is also a measure of whether the plant photosynthesis system is functioning normally［27］. The Ci concentration of plant leaves is another important factor that affects the photosynthetic characteristics of plants. It provides a direct source of carbon synthesis for photosynthetic characteristics. In this study, whether it was the neutral salt or the alkaline salt stress, with the increase of the salt stress concentration and the extension of the stress time, the net photosynthetic rate of H. fulva leaves decreased, and Ci increased and was significantly higher than the CK, which might be because that high salt stress caused the accumulation of a large amount of salt ions in the cells of H. fulva , which destroyed the structure of chloroplasts, resulting in damage to the photosynthetic organs of the leaves, decreased chlorophyll content, and decreased photosynthetic activities of the mesophyll cells, indicating that non-stomata limitation had become the main reason for the reduction of photosynthetic rate of plant leaves［28-29］.

In this study, after high salt stress, Fm , Fv/Fm , Y , ETR and AQY of H. fulva leaves decreased significantly, while Fo and NPQ increased sharply, indicating that high salt stress led to a decline in photosynthetic function of H. fulva leaves, and inactivation of PSII reaction center. Although some of the excess excitation energy was also dissipated by non-photochemical quenching ( NPQ significantly increased), due to high salt concentration or prolonged stress time, the defense system of H. fulva leaves was destroyed, resulting in serious damage to photosynthetic mechanism, which accords with the result of previous studies.

Under salt stress, the growth of H. fulva ‘Golden Doll was obviously inhibited, and the effect of NaHCO3 on H. fulva seedlings was significantly greater than that of NaCl, which is consistent with previous studies［32-33］. The common feature of the neutral salt and alkaline salt damage to plants is that the accumulation of Na+ changes the normal ion balance in plant leaf cells. The difference is that in addition to the destruction of ion balance in alkaline salt stress, high pH will also destroy the acid-base balance in the microenvironment of plant leaf cells, that is, alkaline salt stress has a stronger damage effect on plant mesophyll cells［34］.

The results of this study showed that under salt stress, the photosynthetic characteristics of H. fulva leaves were inhibited. On the one hand, the non-stomatal limiting factor, i.e. , the chlorophyll content decreased, which led to the inhibition of photosynthetic characteristics. On the other hand, the decrease in the photosynthetic performance of mesophyll cells led to a decrease in the net photosynthetic rate of H. fulva . The changes of the photosynthetic characteristics and chlorophyll fluorescence parameters in H. fulva caused by salt stress were closely related to the types of salts and salt concentration. High salt stress significantly inhibited the photosynthetic capacity of H. fulva ‘Golden Doll, and the effect of NaHCO3 on H. fulva seedlings was significantly greater than that of NaCl.

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