The effects of cadmium stress on the growth and physiological characteristics of Sassafras tzumu Hemsl. were studied in pot experiments. Five Cd levels were tested (CT: 0 mg/kg, Cd5: 5 mg/kg, Cd20: 20 mg/kg, Cd50: 50 mg/kg, and Cd100: 100 mg/kg). The growth and physiological characteristics of the sassafras seedlings in each level were measured. The results showed that soil Cd had negative influences on sassafras growth and reduced the net growth of plant height and the biomass of leaf, branch and root. The contents of hydrogen peroxide and malondialdehyde increased with increasing cadmium concentration; this indicated that cadmium stress had a certain effect on the peroxidation of the inner cell membranes in the seedlings that resulted in damage to the cell membrane structure. Superoxide dismutase activity increased in treatments Cd50 and Cd100 compared to that in the other treatments, while peroxidase activity increased steadily with increasing cadmium concentration; these results suggest that peroxidase is likely the main protective enzyme involved in the reactive oxygen removal system in sassafras seedlings. The increase in proline content with increasing cadmium concentration indicated that cadmium stress induced proline synthesis to resist osmotic stress in the seedlings. Compared to that in CT, the soluble sugar content declined under the different treatments. The soluble protein content first increased and then decreased with increasing cadmium stress. At different levels of cadmium stress, the chlorophyll content in the seedlings first increased and then decreased, and it was higher in the Cd5 and Cd20 treatments than that in the CT treatment. These results reflect that cadmium has photosynthesis-promoting effects at low concentrations and photosynthesis-suppressing effects at high concentrations. The photosynthetic gas exchange parameters and photosynthetic light-response parameters showed downward trends with increasing cadmium concentration compared with those in CT; these results reflect the negative effects of cadmium stress on photosynthesis in sassafras seedlings.
Research Article
Growth and Physiological Characteristics of Sassafras tzumu Hemsl. Seedlings Under Cadmium (Cd) Stress
https://doi.org/10.21203/rs.3.rs-120872/v1
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Heavy metal stress
Antioxidant enzyme activities
Tree growth
Osmotic regulating substance
Photosynthetic characteristics
In recent years, environmental pollution and ecological damage have become increasingly serious due to the rapid development of industrialization. Among these issues, soil heavy metal pollution has become one of the most prominent environmental problems in the world. Cadmium (Cd) is a toxic, silver-white metal element. Because of its water solubility, fluidity, and toxicity, Cd can be easily absorbed by plant roots, and it can alter plant structural and functional properties, inhibit seed germination and root elongation [1]. Cd directly or indirectly inhibits physiological processes such as respiration, photosynthesis, water movement and gas exchange [2]. Additionally, Cd can affect the metabolism, chlorophyll synthesis [3], and antioxidant enzyme activity [4] of plants.
The absorption and accumulation of Cd can affect the normal growth of plants. Cd stress leads to a decline in biomass and photosynthetic rate of plants, also causes oxidative damage and imbalance of nutrient uptake [5, 6]. And apart from inhibiting the formation of photosynthetic pigments, reducing the efficiency of photosynthesis, Cd stress can also increase the accumulation of active oxygen and enhance peroxidation [7, 8]. Through the generation and synthesis of various enzymatic antioxidants, non-enzymatic antioxidants, osmolytes and chelating agents, can plants reduce the stress caused by heavy metals [9]. Because that Cd can accumulate in plants and enter human body through the food chain, causing chronic poisoning and endangering human health [10], it is of great importance to find an effective method to remediate Cd-contaminated soils.
Woody plants have proved an effective means for removing or stabilizing toxic metals from contaminated soils, with their high accumulation of heavy metals, perennial trait, high biomass production, and fast growth [6, 11–13]. Sassafras tzumu Hemsl. belongs to the genus Sassafras and the family Lauraceae. It is often used for shipbuilding and making high-quality furniture. Sassafras trees are beautiful and have red leaves in autumn, which makes them excellent ornamental trees for gardens [14, 15]. Sassafras has good prospects in terms of timber production and garden use. Therefore, most studies on sassafras in China have focused on genetic breeding and reproduction technology, cultivation and afforestation technology, disease and pest control, etc. [16–18]. However, few studies have been performed on the physiological conditions of sassafras under heavy metal stress. In view of this, this study aimed to investigate the effects of Cd stress on the growth, photosynthetic and physiological characteristics of sassafras and to analyze the physiological response of sassafras to Cd stress in order to provide a theoretical basis for further research on the resistance of sassafras to Cd stress.
2.1. Experimental design
The plants used in this experiment were 1-year sassafras seedlings, which are common native tree species in Sichuan. The seedlings were obtained from the same source and were pests and diseases free. The tested soil was a yellow soil taken from a forest farm in Ya’an city, Sichuan Province. The pH of the test soil was 5.75, and the total nitrogen, phosphorus and potassium contents were 9.10 g/kg, 0.64 g/kg, and 12.59 g/kg, respectively.
The experiment was conducted at the teaching and research station of Sichuan Agricultural University. Its geographical coordinates are east longitude 103°51¢29² and north latitude 30°42¢18². Approximately 12 kg of mixed soil was weighed and placed into each pot. The content of Cd2+ (mg/kg) in the soil of each pot was calculated according to the dry weight of the soil, and Cd2+ was added to the pots in the form of a CdCl2 aqueous solution. A single-factor test design was used to establish 5 cadmium treatment levels: CT (0 mg/kg), Cd5 (5 mg/kg), Cd20 (20 mg/kg), Cd50 (50 mg/kg), and Cd100 (100 mg/kg). Five replicates were established for each treatment group, and all treated plants were placed in a greenhouse.
The experiment began in early May 2019. CdCl2 was applied 5 times, with an interval of 15 d; the same amount of pure water was added to CT. At the time of application, the prepared CdCl2 solution was poured evenly onto the soil surface in the basin. Any CdCl2 solution that exuded from the pot was collected at the bottom of the basin with a tray pad and poured back into the soil. The CdCl2 applications ended in mid-July 2019. After 30 d of plant growth, the plant photosynthetic and physiological characteristics were measured. The plants were harvested at the end of December 2019, and the plant roots, branches and leaves were harvested separately.
2.2. Determination of the plant growth index
The height and ground diameter of each plant were measured before and after the experiment, and the subtraction method was used to calculate the results. The height of the seedlings was measured with a ruler (precision: 0.1 cm). An electronic Vernier caliper (precision: 0.1 mm) was used to measure the ground diameter from two perpendicular directions at the root neck, and the average value was calculated. The plant samples were washed with deionized water, and then the plant organs, i.e., roots, branches and leaves, were harvested separately. In the lab, the plant organs were put in an oven at 105 °C for 30 min, after which they were dried at 70 °C to constant weight and weighed. The biomass of each dried organ was then calculated.
2.3. Determination of the physiological characteristics
In the 5 replicates of each treatment, five mature fresh functional leaves were randomly selected from the middle to upper part of the tree canopy and placed separately into an ice box for the measurement of each index.
The content of free proline (Pro) was extracted with sulfonyl salicylic acid and determined with acidic ninhydrin colorimetry [19]. The content of soluble protein (SP) was determined by the Coomassie brilliant blue method [20]. The malondialdehyde (MDA) and soluble sugar contents (SS) were determined by the thiobarbituric acid heating colorimetric method [20]. The activity of superoxide dismutase (SOD) was determined by the hydroxylamine method [21]. Catalase (CAT) activity was determined by the ammonium molybdate method [22]. Peroxidase (POD) activity was determined by guaiacol spectrophotometry [20]. The H2O2 content was determined with a method based on H2O2 and titanium ions forming a colored [TiO(H2O2)]2+ coordination compound (the specific absorption peak is 410 nm) [23]. All enzyme activities were determined with kits produced by Nanjing Jiancheng Biological Research Company.
2.4. Determination of the chlorophyll content
Two to three mature functional leaves of the plants in each treatment were randomly selected for the determination of chlorophyll content. During the determination, after removing the veins of the leaf, cut the remaining parts into pieces, and store the samples in refrigerator for preservation. An amount of 0.1 g leaves was accurately weighed and placed into a 10 mL centrifuge tube. An amount of 9 ml chlorophyll extraction solution (80% acetone and anhydrous ethanol 1:1 mix) was added, and the mixture was placed in the dark and left for more than 24 h until the leaves were completely white. Spectral measurements were performed at wavelengths of 663 nm and 646 nm.
2.5. Determination of the photosynthetic parameters
The net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intracellular CO2 concentration (Ci) were determined with an LI-6800 portable photosynthesis system (Li-Cor Inc, USA). Three plants were selected from each treatment, three leaves were selected from each plant, and ten data points were recorded for each leaf.
2.6. Determination of the photosynthetic light-response curve
The LI-6800 portable photosynthesis system (Li-Cor Inc, USA) was used to measure the photosynthetic light-response curves of the leaves selected for the previous determination. The photosynthetically active radiation (PAR) gradient values were 1800, 1600, 1000, 800, 600, 400, 200, 100, 75, 50, 25, and 0 mmol/m2/s, the CO2 concentration was set to 400 ppm, and the room temperature was set to 30°C. Each leaf was photoinduced for 20 min before the determination, and in this test, the photoinduction strength was 800 mmol/m2/s.
The photosynthetic light-response curves of sassafras leaves treated with different concentrations of cadmium were fitted by using Ye’s [25] modified linear hyperbolic model. The fitting equation of the modified linear hyperbolic model is as follows:
When Pn=0, the light compensation point (LCP) can be obtained:
With dPn/dI=0, the light saturation point (LSP) can be obtained:
When I=Is, Pnmax can be obtained:
2.7. Statistical analysis
All data were collected in Excel 2016 and statistically analyzed with SPSS 24.0. The significance of the differences among means was assessed by one-way analysis of variance (ANOVA). Multiple comparisons were performed mostly with Duncan’s test to compare the mean values between treatments (p<0.05). The figures were drawn in Origin 8.5.
3.1. The effects of Cd stress on plant growth
The net growth in plant height decreased with increasing Cd concentration, but there were no significant differences among treatments (P<0.05). The net growth in ground diameter showed an increasing trend, but there were no significant differences among treatments (P<0.05). The biomass of leaves, branches and roots decreased with the aggravation of cadmium stress, and there were significant differences between treatments and CT for both roots and branches (except in Cd5); however, no significant differences were found for leaf biomass (P<0.05). (Table 1)
Table 1. The growth characteristics of sassafras seedlings under different levels of cadmium treatment.
|
Treat-ment |
Net growth in plant height/cm |
Net growth in ground diameter/cm |
Leaf biomass/g |
Branch biomass/g |
Root biomass/g |
Total biomass/g |
|
CT |
36.19±3.58a |
4.12±0.83a |
25.94±0.77a |
45.46±1.23a |
48.45±1.15a |
119.85±2.73a |
|
Cd5 |
24.89±0.51a |
3.38±0.92a |
22.79±1.50a |
44.22±0.96a |
39.64±1.25b |
106.65±1.42ab |
|
Cd20 |
33.03±6.49a |
7.11±1.73a |
21.01±9.20a |
40.96±1.28b |
35.20±2.80b |
97.17±7.76ab |
|
Cd50 |
29.2±3.15a |
6.58±1.17a |
20.58±2.03a |
38.84±0.09b |
35.09±1.44b |
94.51±3.26bc |
|
Cd100 |
23.65±3.13a |
6.71±1.04a |
19.39±1.54a |
32.82±1.00c |
34.42±1.44b |
86.63±1.32c |
Different lowercase letters in the same column indicate significant differences between different cadmium concentration treatments (P<0.05).
3.2. The effect of Cd stress on physiological characteristics
3.2.1. The contents of H2O2 and MDA in sassafras leaves under Cd stress
The H2O2 content decreased first and then increased with increasing cadmium concentration, but there were no significant differences among treatments. The MDA content showed the opposite trend to the H2O2 content as the cadmium concentration increased; however, there were still no significant differences among treatments. (Figure 1)
3.2.2. Response of antioxidant enzyme activity to Cd stress
SOD activity fluctuated with the increasing cadmium concentration, while the CT treatment showed the highest SOD activity. CAT activity showed a fluctuation trend. POD activity showed a slowly increasing trend, and the highest activity was observed in the Cd100 treatment. (Figure 2)
3.2.3. Changes in the contents of osmotic regulation substances in response to Cd stress
The Pro content showed a fluctuating upward trend; the Cd100 treatment had the maximum Pro content, but there were no significant differences among treatments. The content of SS was the highest in the CT treatment, while the SS content in the other treatments first increased and then decreased. The SP content first increased and then decreased with increasing cadmium concentration and reached a maximum in the Cd50 treatment; however, the differences between treatments were not significant. (Figure 3)
3.3. The effects of Cd stress on photosynthetic characteristics
3.3.1. Chlorophyll content in sassafras leaves under cadmium stress
With the increase in the cadmium concentration, the chlorophyll a and b contents and the total chlorophyll content in sassafras leaves first increased and then decreased, reaching a maximum in Cd20. There were no significant differences in the chlorophyll a content or the total chlorophyll content under the Cd5, Cd20 and Cd50 treatments, while the Cd100 treatment was significantly different from CT (P<0.05). The chlorophyll b content showed a similar change trend as the chlorophyll a content, but a significant difference between the chlorophyll a and b contents appeared in Cd50 (P<0.05). (Figure 4).
3.3.2. Changes in photosynthetic gas exchange parameters in sassafras under cadmium stress
The leaf Pn was significantly affected by the different Cd treatments. With the increase in Cd concentration, the Pn in the sassafras leaves showed a downward trend. The Pn values in Cd20, Cd50, and Cd100 (but not Cd5) were significantly different from that in CT. The different concentrations of Cd did not significantly affect Gs, Ci or Tr (Figure 5).
3.3.3. The effect of Cd stress on the photosynthetic light-response curve
When the PAR was weak (PAR£200 mmol/m2/s), the Pn of leaves showed a nearly linear increasing trend, the increase in the Pn in CT was significantly higher than that in the cadmium treatments, and the increase in the Pn of Cd5, Cd20, Cd50 and Cd100 decreased with the increase in cadmium concentration. The increasing trend of the Pn of leaves slowed under different Cd concentrations when the PAR exceeded 200 mmol/m2/s. When the PAR was between 800~1800 mmol/m2/s, the Pn of leaves tended to be stable and eventually reached the light saturation point when leaf photosynthesis exhibited photoinhibition (Figure 6).
The modified orthogonal hyperbolic model from Ye [15] was used to fit the photosynthetic response curves of the sassafras leaves at the different cadmium concentrations, and the characteristic parameters of the light response were calculated (Table 2).
The maximum net photosynthetic rate (Pnmax) of the leaves decreased significantly with increasing Cd concentration. The highest Pnmax appeared in CT, at 9.19 mmol/m2/s; then, Pnmax decreased in the different treatments, by 37.11% (Cd5), 50.38% (Cd20), 80.20% (Cd50) and 84.22% (Cd100). The dark respiration rates (Rds) of the sassafras leaves under the different Cd concentrations were not significantly different from that of CT (Table 2).
The light compensation point (LCP) of the leaves was the highest in the Cd50 treatment and the lowest in the Cd100 treatment; however, there were no significant differences among the treatments. The light saturation point (LSP) of the leaves was the highest at Cd5 and the lowest at Cd100, and there were also no significant differences among the treatments (Table 2).
Table 2. Photosynthetic light-response parameters.
|
Treatment |
Pnmax (µmol/m²/s) |
𝛼 |
Rd (µmol/m²/s) |
LCP (µmol/m²/s) |
LSP (µmol/m²/s) |
|
CT |
9.19±0.007a |
0.06±0.003b |
1.63±0.029abc |
29.47±1.015a |
1113.86±11.919a |
|
Cd5 |
5.78±0.139b |
0.13±0.006a |
1.81±0.112ab |
17.92±0.968a |
2326.13±1157.22a |
|
Cd20 |
4.56±0.043c |
0.05±0.003b |
1.32±0.161bc |
30.37±3.252a |
1182.54±100.105a |
|
Cd50 |
1.82±0.027d |
0.06±0.008b |
1.21±0.122c |
105.59±70.914a |
868.16±3.473a |
|
Cd100 |
1.45±0.014e |
0.06±0.003b |
1.83±0.065a |
−42.29±112.027a |
723.38±2.234a |
Pnmax: Maximum net photosynthetic rate; 𝛼: Initial quantum efficiency; Rd: Dark respiration rate; LCP: Light compensation point; LSP: Light saturation point. R2: 0.94-0.99. Different lowercase letters in the same column indicate significant differences between treatments with different cadmium concentrations.
Cadmium (Cd) is highly mobile in plant tissues and affects plant physiological growth [26]. This study demonstrated that Cd stress in soil had a negative influence on sassafras growth and reduced the net growth in plant height and leaf, branch and root biomass. The decrease of the net growth in plant height and biomass may have been due to the toxicity of Cd to photosynthetic organs [27] and plant structure [28]. Under Cd stress, the biomass allocation pattern of sassafras also changed; the degree of the decline in aboveground biomass was higher than that of the decline in underground biomass, probably because cadmium limited the effectiveness and availability of soil nutrients [19]. These effects intensified the competition for nutrients among different plant organs; therefore, the plants retained their underground biomass at the expense of aboveground biomass in order to obtain more soil nutrients and maintain normal growth. In addition, cadmium stress can limit and inhibit plant photosynthesis, which weakens their photosynthetic production capacity and ultimately reduces their biomass; these impacts were more evident on aboveground biomass than on underground biomass [29].
When plants are subjected to abiotic stress, H2O2 plays a key regulatory role in their organs and protects the plants from the harm caused by the abiotic stress [30]. The MDA content, which is an indicator of lipid peroxidation in cellular organelles, is usually upregulated in response to various external stimuli [31]. In this study, the H2O2 and MDA contents showed upward trends, there were no significant differences among the treatments. It might because that cadmium stress had a certain effect on the peroxidation of the inner membrane of sassafras seedlings, resulting in damage to the cell membrane structure. Some studies have noted that an increase in the activity of the protective enzyme system reduces lipid membrane peroxidation and maintains the integrity of the membrane system [32]. The primary antioxidant enzymes are SOD, CAT and POD. When plants are subjected to heavy metal stress, the activity of antioxidant enzymes increases gradually with increasing heavy metal concentration, but when the heavy metal concentration becomes too high, the protective enzyme system is destroyed, and enzyme activity decreases [33]. The resistance of antioxidant enzymes to heavy metal stress is a complex physiological process that is influenced by the plant species and by the concentration and properties of the heavy metal. In Ozfidan-Konakci C’s research, the POD content in wheat leaves increased significantly under cadmium stress, but the SOD content did not change [34]. In this study, SOD activity increased in the Cd50 and Cd100 treatments, while POD activity increased steadily with increasing cadmium concentration. These results suggest that POD was likely the main protective enzyme involved in the reactive oxygen removal system in sassafras. The results also indicate that sassafras adapts to the increase in reactive oxygen species (ROS) and enhances its tolerance to cadmium by adjusting the activities of SOD and POD in its organs under cadmium stress. However, when the cadmium content exceeded a certain range, the activities of SOD, CAT and POD in the stressed plants were inhibited, resulting in a limited ability to remove ROS and serious damage to the functional membranes and enzyme systems of the plant tissues and cells.
Pro is an important osmotic protective substance that plays an indispensable role in maintaining the normal function of cells [35]. Heavy metal stress impacts the water balance in plants and induces a large increase in Pro, which is involved in the osmotic regulation of cells [36]. Some studies have shown that a variety of abiotic stresses tend to make plants produce a large amount of Pro and accumulate Pro in their organs [37, 38]. Plants have different pathways for the synthesis and degradation of Pro in different situations, so the effects of Pro are not completely consistent [39, 40]. In this study, the content of Pro increased with increasing cadmium concentration, indicating that cadmium stress induced sassafras to synthesize more Pro to resist osmotic stress. SS is not only the product of plant photosynthesis but also participates in the process of plant photosynthesis, which can provide energy for the growth and development of plant organs and plays an important role in plants. SS can reduce the osmotic potential in cells and maintain cell water potential and normal metabolism [41, 42]. In this study, compared with that in CT, the SS content declined under different concentrations of cadmium. This result may have been due to the destruction of chloroplasts and the subsequent decrease in photosynthesis with the aggravation of the degree of stress. At the same time, to resist the toxicity of heavy metals, plant tissue cells enhance their metabolic activities and consume some of the SS. Cd stress can affect the synthesis of normal proteins and the production of stress proteins [43]. According to this research, as the concentration of cadmium increased, the SP content first increased and then decreased, which was similar to the research results of Ge Wei et al. [44]. It may be that when the cadmium content is low, mild cadmium stress causes young trees to produce more antioxidant proteins and stress proteins in response to cadmium toxicity. However, with the aggravation of cadmium stress, the protein synthesis system is damaged to a certain extent. In addition, cadmium stress inhibits plant photosynthesis, resulting in a decrease in the contents of proteins involved in ATP activities; therefore, the content of SP decreases [45, 46].
Photosynthesis is an important physiological process in plants. Many studies have shown that cadmium stress can inhibit plant photosynthesis and is significantly related to the degree of heavy metal stress [47, 48]. Its effects on plant photosynthesis are mainly reflected in the destruction of chlorophyll structure and the reduction in photosynthetic pigment content, which can affect the ability of plants to photosynthesize [49]. The chlorophyll content is directly related to the intensity of photosynthesis and can reflect the ability of leaves to absorb and transform light energy. As the main pigment involved in photosynthesis, chlorophyll a is responsible for converting light energy into chemical energy in the light reaction center, while chlorophyll b is responsible for capturing and transmitting light energy [50]. In this research, the chlorophyll content first increased and then declined at increasing cadmium concentrations, which indicated that a low concentration of cadmium promotes an increase in chlorophyll content, while a high concentration of cadmium inhibits the formation of chlorophyll. This was probably due to the low concentration of Cd complexes accelerating the absorption of Mg, Fe, K and P nutrients from the soil, promoting the formation of leaf porphyrin rings, and thus increasing the chlorophyll content [51].
Studies have shown that the Pn of plants generally decreases with increasing heavy metal concentration [52], which is in line with our experimental results. Factors that affect the photosynthetic rate of plant leaves can be divided into stomatal factors and nonstomatal factors. Farquhar et al. proposed that to judge the factors that affect the photosynthetic rate of plants, changes in stomatal conductance and intercellular carbon dioxide concentration should be observed at the same time [53]. In this experiment, the Pn and Gs of sassafras leaves decreased, while the Ci increased, indicating that the photosynthetic rate of sassafras leaves was limited by nonstomatal factors.
Photosynthetic light-response parameters can reflect the photosynthetic potential, light energy utilization and light inhibition of plants under adverse conditions [25, 54]. Pnmax can reflect the potential photosynthetic capacity of plants. The greater the value of Pnmax, the greater the photosynthetic potential of plant leaves, indicating that leaves can synthesize more photosynthetic products under the appropriate light conditions [55]. In this study, the Pnmax of sassafras leaves decreased significantly with increasing Cd stress, indicating that heavy metal stress significantly affected the photosynthetic capacity of the sassafras leaves. The LCP and LSP reflect the ability of plants to utilize weak light and strong light, respectively [56]. In this study, with the increase in Cd concentration, the LCP and LSP of sassafras leaves generally showed a decreasing trend, indicating that Cd stress weakened the ability of sassafras leaves to use both weak light and strong light. Qi’s research showed that reducing the LCP was an adaptive response of plants to a low light environment and was conducive to maintaining the carbon balance of plants under low light intensity [57]. Lower Rd and LCP in leaves are more conducive to reducing the consumption of photosynthetic products in leaves, thus increasing the net photosynthetic accumulation to obtain the maximum carbon accumulation [55]. Under cadmium stress, sassafras leaves maintained normal photosynthesis by reducing the Rd and LCP, which reflected the mechanism of the response of sassafras to cadmium stress.
Cd stress had negative influences on sassafras growth, as it reduced the net growth of plant height and the biomass of leaf, branch, and root. H2O2 and MDA increased with the aggravation of Cd stress, which indicated that cadmium stress had a certain effect on the peroxidation of the inner membrane of sassafras seedlings, resulting in damage to the cell membrane structure. SOD activity increased in treatments Cd50 and Cd100, while POD activity increased steadily with increasing cadmium concentration, indicating that POD was likely the main protective enzyme involved in the reactive oxygen removal system in sassafras. The increase in Pro content indicated that cadmium stress induced sassafras to synthesize more Pro to resist osmotic stress. Compared to that in CT, the SS content declined under the different Cd treatments. The SP content first increased and then decreased. At increasing levels of cadmium stress, the chlorophyll content of the seedlings first increased and then decreased, and it was higher in the Cd5 and Cd20 treatments than in the CT treatment. These results reflect that Cd has photosynthesis-promoting effects at low Cd concentrations and photosynthesis-suppressing effects at high Cd concentrations. The photosynthetic gas exchange parameters and photosynthetic light-response parameters showed downward trends in the Cd treatments compared with those in CT, which reflected the inhibition of photosynthesis in sassafras due to Cd stress.
Author Contributions: HY developed the project and write the manuscript, which was read and revised by all co-authors. Fieldwork was performed by HY, JL, QL and XY. Laboratory analyses were performed by HY and XY. HY, JL and QL performed the statistics and data evaluation. HL and JZ were both corresponding authors who gave advices and provided funding for this project. All authors listed have made substantial, direct and intellectual contributions to the work, and have approved it for publication.
Funding: The project has been funded by the Breeding Research Project of Sichuan Province during the 13th Five-year Plan period (2016 NZ0098-10), Key Project of Sichuan Education Department (13ZA0246).
Acknowledgments: The authors would like to thank Prof. Hu and Prof. Zhang for critical reading of the manuscript and fruitful corrections. We would also thank all the participants in this project.
Conflicts of Interest: All authors declare no conflicts of interest.
|
Cd |
cadmium |
|
Chl a |
chlorophyll a |
|
Chl b |
chlorophyll b |
|
Car |
carotenoids |
|
Chl |
chlorophyll |
|
PAR |
photosynthetically active radiation |
|
Pn |
net photosynthetic rate |
|
Gs |
stomatal conductance |
|
Tr |
transpiration rate |
|
Ci |
intercellular CO2 concentration |
|
LSP |
light saturation point |
|
Rd |
dark respiration rate |
|
LCP |
light compensation point |
|
SOD |
superoxide dismutase |
|
POD |
peroxidase |
|
Pro |
proline |
|
SS |
soluble sugar |
|
SP |
soluble protein |
|
CAT |
catalase |
|
MDA |
malondialdehyde |
|
ROS |
reactive oxygen species |
- Ali B, Qian P, Jin R, Ali S, Khan M, Aziz R, Tian T, Zhou W (2014) Physiological and ultra-structural changes in Brassica napus seedlings induced by cadmium stress. Biol Plant 58(1):131–138.
- Van Assche F and Clijsters C 1990a Effects of metals on enzyme activity in plants. Plant Cell Environ. 13, 195–206.
- Jia L, Liu Z, Chen W, Ye Y, Yu S, He XY (2015) Hormesis effects induced by cadmium on growth and photosynthetic performance in a hyperaccumulator, Lonicera japonica, Thunb. J Plant Growth Regul 34(1):13–21.
- Ahammed GJ, Choudhary SP, Chen SC, Xia XJ, Shi K, Zhou YH, Yu JQ (2013) Role of brassinosteroids in alleviation of phenanthrenecadmium co-contamination-induced photosynthetic inhibition and oxidative stress in tomato. J Exp Bot 64(1):199–213.
- Rizwan, M., Ali, S., Abbas, T., Zia-ur-Rehman, M., Hannan, F., Keller, C., Al-Wabel, M.I., Ok, Y.S., 2016. Cadmium minimization in wheat: a critical review. Ecotoxicol. Environ. Saf. 130, 43–53.
- Zou, J., Wang, G., Ji, J., Wang, J., Wu, H., Ou, Y., Li, B., 2017. Transcriptional, physiological and cytological analysis validated the roles of some key genes linked Cd stress in Salix matsudanaKoidz. Environ. Exp. Bot. 134, 116–129.
- Chen, H.C., Zhang, S.L., Wu, K.J., Li, R., He, X.R., He, D.N., et al., 2020. The effects of exogenous organic acids on the growth, photosynthesis and cellular ultrastructure of Salix variegata Franch. Under Cd stress. Ecotoxicol. Environ. Saf. 187, 1–10.
- Sarvajeet, S.G., Nafees, A.K., Narendra, T., 2011. Cadmium at high dose perturbs growth, photosynthesis and nitrogen metabolism while at low dose it up regulates sulfur assimilation and antioxidant machinery in garden cress (Lepidium sativum L.). Plant Sci. 182, 112–120.
- Daniel, H., Tereza, C., Tom´a, V., Radka, P., 2019. The effect of nanoparticles on the photosynthetic pigments in cadmium-zinc interactions, 26 (4), 4147–4151.
- Tian, X., Dumat, C., Pierart, A., et al., 2016. Measurement of metal bioaccessibility in vegetables to improve human exposure assessments: field study of soil–plant–atmosphere transfers in urban areas, South China. Environ. Geochem. Health 38 (6), 1283–1301.
- He, J., Li, H., Luo, J., Ma, C., Li, S., Qu, L., Gai, Y., Jiang, X., Janz, D., Polle, A., Tyree, M., Luo, Z., 2013a. A transcriptomic network underlies microstructural and physiological responses to cadmium in Populus× _canescens. Plant Physiol. 162, 424–439.
- He, J., Ma, C., Ma, Y., Li, H., Kang, J., Liu, T., Polle, A., Peng, C., Luo, Z.B., 2013b. Cadmium tolerance in six poplar species. Environ. Sci. Pollut. Res. 20, 163–174.
- He, N., Zhang, C., Qi, X., Zhao, S., Tao, Y., Yang, G., Lee, T., Wang, X., Cai, Q., Li, D., et al., 2013c. Draft genome sequence of the mulberry tree Morus notabilis. Nat. Commun. 4, 1–9.
- Wu Pinglan, LUO Zuyun. Precious sassafras of Guizhou[J]. Guizhou Forestry Science and Technology, 1981.
- Flora of China, 1982, vol. 31, p. 238.
- Chen Xiyou. Study on Growth of Sassafras in different Mixed ways[J]. Anhui Forestry Science and Technology, 2015(4):9–11.
- Cheng Yong, Wu Jiyou, Tang Chun, et al. Storage Test of Sassafras Seeds[J]. Hunan Forestry Science and Technology, 2014(2):28–30.
- Shen Y, Tian D L, Xiang H, et al. Study on biomass and productivity of natural secondary Sassafras Mixed Forest[J]. Journal of Central South University of Forestry and Technology, 2011, (5):26–30.
- Li Hesheng, Sun Qun, Zhao Shijie, et al. Principles and Techniques of plant Physiological and biochemical experiments [M]. Beijing: Higher Education Press,2000:164–165,258–260.
- Xiong Q E. Experimental course of Plant Physiology [M]. Chengdu: Sichuan Science and Technology Press,2003:72–73,85–86,126–127. (in Chinese)
- Wang Li. Comparative study on the determination of sod activity in different extraction stages of maize [D]. Shenyang Agricultural University,1999.
- Hu Xiaojuan. Study on the interaction between several active substances and superoxide dismutase [D]. Anhui University,2012.
- Liu Jun, LV Bo, XU Langlai. An improved method for the determination of hydrogen peroxide content in plant leaves [J]. Progress in Biochemistry and Biophysics.2000(05):548–551.
- Ye Zipiao. Advances in models of photosynthetic response to light and CO2[J]. Chinese Journal of Plant Ecology, 2010(06):727–740.
- Zhao FJ, Jiang RF, Dunham SJ, McGrath SP (2006) Cadmium uptake, translocation and tolerance in the hyperaccumulator Arabidopsis halleri. New Phytol J 172:646–654.
- Saidi I, Ayouni M, Dhieb A, Chtourou Y, Cha.bi W, Djebali W (2013) Oxidative damages induced by short-term exposure to cadmium in bean plants: protective role of salicylic acid. S Afr J Bot 85:32–38.
- Anwaar SA, Ali S, Ishaque W, Farid M, Farooq MA, Najeeb U, Abbas F, Sharif M (2014) Silicon (Si) alleviates cotton (Gossypium hirsutum L.) fromzinc (Zn) toxicity stress by limiting Zn uptake and oxidative damage. Environ Sci Pollut Res 22:3441–3450.
- Wu Fuzhong, Yang Wanqin, Zhang Jian et al. Effects of cadmium stress on the growth, nutrient accumulation, distribution and utilization of Osmanthus fragrans. Journal of Plant Ecology, 2010, 34(10): 1220–1226.
- Wang H, ZHAO S C, XIA W J, et al. Effects of cadmium stress at different concentrations on photosynthesis, lipid peroxidation and antioxidant enzyme activities in maize seedlings [J]. Journal of Plant Nutrition and Fertilizer, 2008, 14(01): 36–42.
- Pan Xiaowu, LI Yong-chao, LI Xiaoxiang, et al. Metabolism and signal transduction of H2O2 in plants under abiotic stress [J]. China Agricultural Science and Technology Herald, 2010, 12(2): 38–43.
- MengJie An1 · HaiJiang Wang1, Effects of Modifiers on the Growth, Photosynthesis, and Antioxidant Enzymes of Cotton Under Cadmium Toxicity, Journal of Plant Growth Regulation (2019) 38:1196–1205.
- Wei X, Wei YX, Guo D, Sun B, Wang XD, Liu C (2015) Effects of different breaking dormancy ways on the photosynthetic characteristics and activities of protective enzymes of ‘misty’ blueberry leaves. Sci Agric Sin 48(22):4517–4528.
- Chaabene Z, Hakim I R, Rorat A, et al. Copper toxicity and date palm(Phoenix dactylifera)seedling tolerance: monitoring of related biomarkers. Environmental Toxicology and Chemistry, 2018, 37(3):797–806.
- Ozfidan-Konakci C, Yildiztugay E, Bahtiyar M, et al. The humic acid-induced changes in the water status, chlorophyll fluorescence and antioxidant defense systems of wheat leaves with cadmium stress. Ecotoxicology and Environmental Safety, 2018, 155:66–75.
- Peng Z H, Peng K Q, HU J J, et al. Research progress of plant proline accumulation under osmotic stress [J]. Chinese Agricultural Science Bulletin, 2002, 18(4): 80–83.
- Rady M M, Hemida K A. Modulation of cadmium toxicity and enhancing cadmium-tolerance in wheat seedlings by exogenous application of polyamines. Ecotoxicol Environ Saf, 2015, 119:178–185.
- Chen Y H, Shi L, Li Y H, et al. Study on the characteristics of proline and active oxygen metabolism in red sea under salt stress [J]. Journal of Xiamen University (Natural Science), 2004, 43(03): 402–405.
- Niu M G, WANG X, Chen L, et al. Effects of drought, waterlogging and low temperature stress on physiological and biochemical characteristics of wheat [J]. Seed, 2003(04): 17–19.
- Deng Feng-fei, Yang Shuang-long, GONG Ming. Regulation of proline metabolism in abiotic plants by cell signaling molecules [J]. Journal of Plant Physiology, 2015, 51(10): 1573–1582.
- SAMUEL D, KUMAR TK, GANESH G, et al. Proline inhibits aggre-gation during protein refolding[J]. Protein Sci, 2010, 9(2): 344–352.
- Abd Allah E F, Hashem A, Alqarawi A, et al. Calcium application enhances growth and alleviates the damaging effects induced by Cd stress in sesame (Sesamum indicum L.). Journal of Plant Interactions, 2017, 12(1):237–243.
- Zhang X D, Meng J G, Zhao K X, et al. Annotation and characterization of Cd-responsive metal transporter genes in rapeseed (Brassica napus). Bio Metals, 2018a, 31(1):107–121.
- Yuan Qiuhong. Effects of heavy metals on antioxidant system of aquatic plants and study on heavy metal stress proteins [D]. Nanjing: Nanjing Normal University, 2008.
- Ge Wei, JIAO Yunqiu. Changes of soluble protein content of two poplar trees under cadmium stress [J]. Modern Agricultural Science and Technology, 2012(1): 199–200.
- AINA R, LARBRA M, FUMAGALLI P, et a1. Thiol-petide level and proteomic changes in response to cadmium toxicity in Oryza sativa L. rotts[J]. Environmental and Experimental Botany, 2007, 59(3): 381–392.
- AHSAN N, LEE SH, LEE DG, et a1. Physiological and protein profiles alternation of germinating rice seedlings exposed to acute cadmium toxicity[J]. Comptes Rendus Biologies, 2007, 330(10): 735–746.
- Hendrik Küpper, Frithjof Küpper, Martin Spiller. Environmental relevance of heavy metal-substituted chlorophylls using the example of water plants[J]. J. Exp. Bot,1996.
- Kalaji M H, Loboda T. Photosystem II of Barley seedlings under cadmium and lead stress[J]. Plant Soil & Environment, 2007, 53(12):511–516.
- Chatzistathis T A, Papadakis I E, Therios I N, et al. IS CHLOROPHYLL FLUORESCENCE TECHNIQUE A USEFUL TOOL TO ASSESS MANGANESE DEFICIENCY AND TOXICITY STRESS IN OLIVE PLANTS? [J]. Journal of Plant Nutrition, 2010, 34(1):98–114.
- Sun Xiaolin, Xu Yuefei, Ma Luyi, et al. Response of photosynthetic pigments in plant leaves to shading[J]. Chinese Journal of Plant Ecology, 2010,34(8):989–999.
- Chen xia-xia, Pu gao-zhong, Huang yu-qing, et al. Effects of cadmium stress on growth and photosynthetic characteristics of asparagus spears[J]. Plants of Guangxi, 2019,39(6)):743–751.
- Lu yan, Li Xinrong, He Mingzhu, et al. Effects of heavy metals on photosynthetic and physiological growth characteristics of halophytes[J]. Acta Botanica Northwestern Sinica, 2011,31(2):370–376.
- Farquhar, G D, Sharkey, T D. Stomatal Conductance and Photosynthesis[J]. Annu.rev.plant Physiol,1982,33(1):317–345.
- Wang Haizhen, Han Lu, Xu Yali, et al. Response of chlorophyll fluorescence characteristics to high temperature in heteromorphous leaves of Populus eureka [J]. Acta Ecologica Sinica, 2011, (9):100–109.
- Li Liyuan, Li Jun, Tang Xiao-juan, et al. Photosynthetic light response simulation of leaves of Quercus variabilis and Robinia pseudoacacia under different light environments[J]. Chinese Journal of Applied Ecology, 2016,29(7):2295–2306.
- Wang Fu-kong, XU Wei-zhou, Huang Fu-ren, et al. Photosynthetic light response curve of Populus microphylla under different slope orientation[J]. Water and Soil Conservation Research, 2015, V. 22; No. 113 (6): 182–187.
- Qi Xin, Cao Qunfang, Feng Yulong. Adaptation of photosynthesis to growth light intensity in seedlings of three tree species of Putaoia in tropical rain forest [J]. Chinese Journal of Plant Ecology, 2004, (01):34–41.