Abstract
Soil salinization is an environmental problem globally. Bermudagrass (Cynodon dactylon) has long been used for soil restoration in saline-alkali land. Urbanization and the compound planting pattern combining trees, bushes, and grasses induced shading are becoming one of the most significant environmental constraints on the management of bermudagrass, which directly affects photosynthetic characteristics. Salinity and shade have become the most important environmental constraints on lawn development and implementation. Previous studies have shown that the plant physiological response under combined stress was different from that under single stress. The purpose of this research was to investigate the effects of salinity stress, shade stress, and the combined stress on bermudagrass. Shade nets were used to simulate shade stress to 85% shade. The NaCl concentration gradient for salinity stress was 1.0% for 7 days, 1.5% for 7 days, and 2.0% for 13 days, respectively. The combined stress combines the two approaches mentioned previously. The results showed that the salinity stress significantly inhibited the plant height, leaf relative water content, chlorophyll content, the chlorophyll a fluorescence induction (OJIP) curve and other photosynthetic parameters of bermudagrass while increasing electrolyte leakage when compared with control. Shade stress significantly enhanced the plant height, chlorophyll content, electrolyte leakage, the OJIP curve, and other photosynthetic parameters. Under the combined stress, the plant height and relative water content did not change significantly, but the photosynthetic parameters such as chlorophyll content and the OJIP curve increased. Furthermore, under the combined stress, the photosynthesis-related genes were regulated. Salinity stress inhibited the photosynthetic ability of bermudagrass more than shade stress, while the combined stress exhibited a considerably better photosynthetic ability. These findings provide information for the usage of bermudagrass in salinized shade conditions.
With qualities including quick growth, a dense root system, a strongly developed stolon, and the capacity to consolidate embankments, bermudagrass (Cynodon dactylon) is one of the most lucrative warm-season turfgrasses in the world (Shi et al. 2014). Bermudagrass is frequently used in parks, soccer fields, and soil rehabilitation areas because of its high yield, excellent quality, and resilience (Fan et al. 2014). Bermudagrass has both turf type and forage type. With the increase in production and popularity of bermudagrass, it is crucial to determine its growth response in stress, thereby improving its stress tolerance.
Soil salinization intensifies with global warming, and soil problem has become one of the most contentious issues worldwide (Jose et al. 2017). The total area of saline soil on the planet is ∼950 million hectares, which makes up 7.62% of the Earth’s land areas. Salinity stress reduces water absorption, causes leaf yellowing, and increases electrolyte leakage, all of which have impacts on carbon-nitrogen balance, secondary pigment metabolism, and plant photomorphogenesis (Clevidence 2010; Gorham et al. 1985). Salinity stress affects the morphology and photosynthetic physiological characteristics of bermudagrass (Singh et al. 2013). According to Hu et al. (2012), salinity stress reduces the lawn quality and canopy height of bermudagrass cultivar C43, increasing the root number, root length, and root/shoot ratio. Yu et al. (2015) found that salinity stress usually leads to a significant reduction in the net photosynthetic rate of hybrid bermudagrass (Cynodon transvaalensis × Cynodon dactylon) cultivar Tifway. Bizhani and Salehi (2014) found that the leaf area, chlorophyll content, photosynthetic rate, and starch level of bermudagrass cultivar California Origin decreased with the increasing of salinity level. However, the extent to which salinity stress affects photosynthetic pigment, thylakoid membrane protein, and photosynthetic rate varies by plant species, salinity stress severity, and duration (Megdiche et al. 2008; Misra et al. 1997). The growth and photosynthetic characteristics of bermudagrass under salinity stress require further systematic research.
Furthermore, as urbanization expands, a growing number of skyscrapers and overpasses have resulted in landscape plants being planted in low-light conditions, including shade by trees and surrounding buildings. When compared with normal growing conditions, shade-stressed plants have larger and thinner leaves, lower stomatal density, and decreased total amount of light energy captured, which leads to a decrease in photosynthetic rate, but an increase in photosynthetic pigment content and photosystem II efficiency, increasing their ability to capture light energy (Jiang et al. 2011; Marchiori et al. 2014). It has been reported that bermudagrass is sensitive to low-light stress, and the low light causes a significant decrease in the turf quality, photosynthetic rate, and carbohydrate content of bermudagrass (Jiang et al. 2004). Shade affects the biomass, shoot length, and turf quality of turfgrass (Sladek et al. 2009; Trappe et al. 2011); however, the performance of bermudagrass under shade varies by cultivars. Tetraploid bermudagrass cultivar Chuanxi exhibits higher shade tolerance, a higher net photosynthetic rate, and total chlorophyll than three triploid hybrid bermudagrass cultivars Tifdwarf, Tifsport, and Tifway under shade stress (Cao et al. 2022).
Photosynthesis is the most important biochemical reaction in nature, and it is essential for plant growth, production, carbon allocation, and organ formation (Zlatev and Lidon 2012). To elucidate photosynthesis and its regulation mechanism, scientists have combined theoretical and practical aspects of photosynthesis research, such as the investigation of effective usage of crop light energy and artificial photosynthesis (El-Khouly et al. 2017; Li et al. 2011). Nonetheless, photosynthesis is influenced by several environmental factors, including temperature, light intensity, soil salinity, and carbon dioxide concentration (Bunce 1998; Kitta et al. 2020; Zhang et al. 2014). Plants respond to salinity stress and shade stress by adjusting their photosynthetic physiological responses. Plants may, for example, respond to salinity stress by increasing the root-shoot ratio to absorb more water (Bazihizina et al. 2017; Handa et al. 1986). Plants may also repair salt-damaged photosynthetic membrane proteins while maintaining photosystem stability by transporting reactive oxygen species to other areas via channel proteins (Bienert et al. 2007; Huang et al. 2006; Li et al. 1994). Plants adapt to shade by delaying flowering and limiting branching (Yañez et al. 2012). Following that, the chlorophyll content of plants tends to increase, the photochemical quenching efficiency (Fv/Fm) is maximized, and the plant’s photosynthetic properties are improved (Liu et al. 2004). Shade also increases the activity of phytochrome-interacting factors 4 (PIF4) and 5 (PIF5), which are associated with the shade avoidance response, and increases the production of cell elongation-promoting genes, growth hormone–related genes, and other genes that promote plant growth (Peng et al. 2018). However, the photosynthetic characteristics of bermudagrass under the combined stress of salinity and shade are unknown.
Bermudagrass is inevitably stressed by salinity and low-light combined stress when applied to landscaping and soil improvement. Systematic studies on the responses of bermudagrass photosynthetic properties to shade, salinity, and combined stress are still lacking. In this study, we measured the photosynthetic physiological indexes of bermudagrass ‘A12359’ under simulated shade, salinity, and combined stress to enrich information on the combined stress of salinity and shade.
Materials and Methods
Plant materials and growth conditions.
Bermudagrass ‘A12359’ was used as the plant materials in this study. The plant materials were obtained from the Academy of Coastal Grass Science and Technology, Ludong University, Yantai, Shandong, China (lat. 37°31′N, long. 121°21′E, elevation 110 m). On 8 Jul 2020, a small piece of sod was harvested from the lawn and placed using a special instrument into a black culture tube (25.52 cm depth, 6.42 cm diameter) that was filled with loam (sand loam): the contents of sand, silt, and clay were 65%, 20%, and 15%, respectively; 20 culture tubes in total were transplanted. This bermudagrass was kept in a controlled greenhouse with a temperature of 25/30 °C (night/day) and 72.8% relative humidity before experimental treatment. In addition, the bermudagrass was fertilized every 4 d with 150 mL half-strength Hoagland’s nutrient solution (Hoagland and Arnon 1950). On 20 Aug 2020, 16 tubes of bermudagrass were selected and cut consistently to 8 cm, then the experimental treatment was started.
Experimental design and treatment.
Control (CK), shade, salinity, and the combined stress (SS) were the four experimental treatments each with four tubes of bermudagrass. A randomized complete block design was used to minimize the impact of environmental conditions. After treatments were initiated, all culture tubes with the bermudagrass were placed in a large water tank filled with half-strength Hoagland’s nutrient solution (total 2 L). The photosynthetically active radiation under shade and combined treatments were measured 16.8 μmol·m−2·s−1 (85% shade) on average, whereas the others were 112 μmol·m−2·s−1. In the salinity and combined treatments, NaCl was used to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d (20–27 Aug 2020), 1.5% (257 mm) for 7 d (27 Aug to 3 Sep 2020), and 2% (342 mm) for 13 d (3–16 Sep 2020). In the controlled greenhouse, the plants were kept at a temperature of 25/30 °C (night/day) and 72.8% relative humidity. Fully extended leaves were collected on day 27 after treatment (16 Sep 2020) for relative water content, electrolyte leakage, chlorophyll content, and chlorophyll a fluorescence analysis. Meanwhile, leaves were collected at 6 h (20 Aug 2020), and on days 7 (27 Aug 2020), 14 (3 Sep 2020), and 27 (16 Sep 2020) for gene expression analysis.
Plant height measurement.
Plant height was determined by the distance between the plant base and the highest point measured with a ruler, and was averaged by four replicates.
Relative water content and electrolyte leakage.
After 27 d of treatment, ∼0.15 g leaves were cut into 0.5-cm-long segments and transferred into a 50-mL plastic centrifuge tube filled with 25 mL of deionized water. The tubes were shaken at 25 °C for 24 h before measuring the initial conductivity (Ci) with a conductivity meter (model 3173; Jenco Instruments, Inc., San Diego, CA, USA). Following that, the leaf tissues in the tube were heated for 15 min at 121 °C. The maximum conductivity (Cmax) of the samples was measured after they had been cooled to room temperature.
Electrolyte leakage (percent) = (Ci/Cmax) × 100%
Chlorophyll content.
Measurement of chlorophyll fluorescence parameters.
A pulse amplitude modulation fluorometer (PAM 2500; Heinz Walz GmbH, Efeltrich, Germany) was used to measure the chlorophyll fluorescence transient curves. Three replicates of fully unfolded functioning leaves were subjected to 30 min dark adaptation. To obtain a true maximal fluorescence excitation intensity, the instrument’s red light of 3000 mol·m−2·s−1 turns off all the reaction centers of the leaf photosystem II (PSII). Following that, chlorophyll fluorescence transients were triggered by a strong light, measured between 10 s and 320 ms, and digitized with data-handling software (OriginPro version 9.0; MicroCal Corp, Northampton, MA, USA). The OJIP transients were examined followed the method discussed in Chen et al. (2013).
RNA extraction and cDNA synthesis.
Approximately 0.1 g of leaves were collected for gene expression analysis. The RNA extraction kit (RNAprep Pure Plant Kit; Tiangen Biotech Co., Ltd., Beijing, China) was used to isolate total plant RNA. To evaluate the quality of the RNA preparations, gel electrophoresis was performed in 1.5% agarose gel.
The first strand was created from 2.5 g total RNA using reverse transcriptase (Hifair® Reverse Transcriptase; Yeasen Biotech Co., Ltd., Shanghai, China) and an oligo (dT) primer [Oligo (dT)18 primer, Yeasen Biotech Co., Ltd.], then stored at −20 °C.
Quantitative real-time polymerase chain reaction analysis.
A quantitative real-time polymerase chain reaction (PCR) analysis was performed using real-time PCR system (ABI QuantStudio 6; Thermo Fisher Scientific, Waltham, MA, USA) detection equipment to determine the level of expression of each gene examined. The fluorescent marker was SYBR Green real-time PCR master mix (SYBR® Premix Ex TaqTM RR420A; Takara, Shika, Japan). The total system volume of the reaction was 20 μL, which included 10 μL 2 × SYBR mix, 0.5 μL primer F, 0.5 μL primer R, 2 μL complementary DNA templates, and 7 μL double distilled H2O (the primer design is reported in Table 1).
Gene descriptions and primer sequences for quantitative real-time PCR amplification analysis in bermudagrass.
Data analysis.
Each treatment was repeated three or more times, and values were reported as mean ± SD. Statistical software (IBM SPSS Statistics version 22.0; IBM Corp., Armonk, NY, USA) and spreadsheet software (Microsoft Excel; Microsoft Corp., Redmond, WA, USA) were used to perform the statistical analysis, which included one-way analysis of variance with Duncan’s test to separate means at a significance level of P < 0.05.
Result
Bermudagrass plant height.
To elucidate the effects of stress on plant growth, plant height was measured after the experiment (16 Sep 2020). As shown in Table 2, there were no differences in plant heights between the combined stress and control. However, compared with the control, shade stress resulted in an 8.43% increase in the plant height and salinity stress led to a 5.38% decrease.
Effects of shade and salinity stress on the plant height (Ht), relative water content (RWC), electrolyte leakage (EL), and concentrations of chlorophyll a (Chla), and chlorophyll b (Chlb) of bermudagrass.
Effect of salinity and shade treatment on relative water content and electrolyte leakage of bermudagrass.
As shown in Table 2, after 27 d of stress (16 Sep 2020), the shade treatment leaves retained 16.03% more water content than the control. Under the salinity stress, the leaves were dehydrated, and the relative water content was reduced by 25.10% when compared with the control. However, the relative water content of the combined stress was reduced by only 4.58% compared with the control. Table 2 shows that plants in each treatment group had higher electrolyte leakage levels during the 27-d stress period. After salinity treatment, the electrolyte leakage of salinity-treated plants was 2.18 times greater than that of control plants. The electrolyte leakage of the plants was 1.24 times and 1.74 times higher in the shade and combined treatment, respectively, than that in the control plants. Our findings show that separate salinity treatment caused more damage to bermudagrass.
Effect of salinity and shade treatment on photosynthetic pigment content of bermudagrass.
Table 2 shows that the salinity and shade treatment had a significant influence on the photosynthetic pigments of the bermudagrass leaves. Chlorophyll a and chlorophyll b were significantly altered when compared with the control. The concentration of photosynthetic pigments increased significantly during shade treatment, with increases in chlorophyll a and chlorophyll b of 25.47% and 23.08%, respectively. Each photosynthetic pigment’s content was significantly reduced by salinity treatment, with chlorophyll a and chlorophyll b decreasing by 15.53% and 20.51%, respectively, compared with the control. The combined stress increased the concentration of photosynthetic pigments, with chlorophyll a and chlorophyll b increasing by 26.09% and 17.95%, respectively, compared with the control.
OJIP fluorescence transient and parameter analysis.
The chlorophyll a fluorescence transient test was used to determine the effect of moderate shading on the photochemistry of PSII of bermudagrass leaves treated with different concentrations of salt. The results showed that moderate shading could mitigate the effect of salinity stress on bermudagrass. The chlorophyll fluorescence response of bermudagrass under various treatment conditions is depicted in Fig. 1. The shade treatment outperformed the control after continuous treatment, followed by the combined treatment, the OJIP transient curve in salinity-treated bermudagrass was the lowest, and exhibited the most damage.
A line plot shows the influences of the shade and salinity stress on the alterations of chlorophyll fluorescence transients (OJIP curve) in leaves of bermudagrass [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment uses NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment.
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
A line plot shows the influences of the shade and salinity stress on the alterations of chlorophyll fluorescence transients (OJIP curve) in leaves of bermudagrass [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment uses NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment.
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
A line plot shows the influences of the shade and salinity stress on the alterations of chlorophyll fluorescence transients (OJIP curve) in leaves of bermudagrass [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment uses NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment.
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
The light energy absorbed per unit area (ABS/CSO) and light energy captured per unit area (TRO/CSO) of the salinity treatment decreased by 5.67% and 7.69%, respectively, compared with the control treatment; ABS/CSO of the combined treatment decreased by 1.89%, compared with the control treatment. Furthermore, when compared with the control treatment, the maximum photochemical efficiency (φPo) in the shade and combined treatments increased by 3.31% and 3.07%, respectively (Table 3).
The determination of photosynthetic parameters of bermudagrass under different treatments.
All treatment groups had lower photochemical quenching coefficients (qP) than the control group, with reductions of 17.16%, 40.69%, and 27.45% in the shade treatment, salinity treatment, and combined treatment, respectively. Shade and salinity stress reduced the electron transfer rate (ETR) of bermudagrass, but the salinity treatment had the most significant drop in ETR, which was 37.23% lower than the control. Furthermore, the real photochemical quantum yield of PSII, Y(II), was lower in all treatments than in the control. However, Y(II) in the salinity treatment was ∼36.36% lower than the control treatment (Table 3).
Effect of shade and salinity treatments on the expression of photosynthesis-related genes in bermudagrass.
According to quantitative reverse transcriptase-polymerase chain reaction, the expression of most photosynthesis-related genes increased significantly after 6 h of shade treatment (Fig. 2). Further investigation revealed that, when compared with the unshaded experimental materials, the expression of glutamyl–transfer RNA reductase gene (HEMA1) and PIF4 genes in the leaves of the combined treatment was lower than that in the shade treatment but significantly higher than that in the salinity treatment as salt concentration and treatment time were increased (Fig. 2A and 2B).
Bar plots show the influences of the shade and salinity stress on photosynthesis-related gene expression in bermudagrass leaves [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment use NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment. HEMA1 (A) is a key gene in the early stage of chlorophyll synthesis, and the PIF4 (B), COP1 (C), and HY5 (D) are light signal transduction-related genes. Different lowercase letters indicate significant differences under four different treatments based on Ducan’s test combining one-way analysis of variance (P < 0.05).
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
Bar plots show the influences of the shade and salinity stress on photosynthesis-related gene expression in bermudagrass leaves [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment use NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment. HEMA1 (A) is a key gene in the early stage of chlorophyll synthesis, and the PIF4 (B), COP1 (C), and HY5 (D) are light signal transduction-related genes. Different lowercase letters indicate significant differences under four different treatments based on Ducan’s test combining one-way analysis of variance (P < 0.05).
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
Bar plots show the influences of the shade and salinity stress on photosynthesis-related gene expression in bermudagrass leaves [CK = control (i.e., no shade and salinity stress), SS = combined stress]. The shade treatment was 85% shade. The salinity treatment use NaCl to simulate salt environment, with a salt concentration gradient ranging from 1.0% (171 mm) for 7 d, 1.5% (257 mm) for 7 d, and 2% (342 mm) for 13 d. The combined treatment integrated the salinity and shade treatment. HEMA1 (A) is a key gene in the early stage of chlorophyll synthesis, and the PIF4 (B), COP1 (C), and HY5 (D) are light signal transduction-related genes. Different lowercase letters indicate significant differences under four different treatments based on Ducan’s test combining one-way analysis of variance (P < 0.05).
Citation: Journal of the American Society for Horticultural Science 148, 1; 10.21273/JASHS05240-22
The expression of the constitutive photormorphogenic1 (COP1) and elongated hypocotyl5 (HY5) genes increased significantly after 6 h of shade when compared with the control group. With increasing salt concentration and treatment duration, COP1 gene expression increased after a brief decrease in shade treatment but remained constant after a slight decrease in combined treatment. However, in the shade and combination treatments, the HY5 gene, which interacts with the COP1 gene, shows a continuous drop in expression while remaining relatively high in the control and salinity treatments (Fig. 2C and D).
Discussion
The effects of environmental stress on plant development vary. The most direct indicators of a plant’s stress tolerance level are morphological changes (Li et al. 2018). In this study, salinity stress reduced bermudagrass growth while shade increased height (Robinson et al. 2019; Wang et al. 2020). The combined treatment slowed bermudagrass growth insignificantly, demonstrating that the interaction of shade and salt did not exacerbate the plant’s unfavorable effects. Environmental stress can cause varying degrees of damage to plant cell membranes, and electrolyte leakage is a sign of membrane damage. According to the study findings, the salt-treated regimen had the highest electrolyte leakage values and the greatest degree of cell membrane damage. Similarly, Hniliková et al. (2019) discovered that when lettuce (Lactuca sativa) was exposed to salt, its electrolyte leakage values increased significantly with increasing salt concentration. In contrast, the combined treatment caused less cell membrane damage to bermudagrass than the salinity treatment. Furthermore, the relative water content changed under shade and combined treatment, although the differences were minor. In contrast, after salinity treatment, the relative water content of leaves was significantly reduced. Shade reduces the possibility of leaf water loss, maintains a relatively stable water content, and demonstrates a strong water-retention ability to ensure the supply of water required by bermudagrass by reducing the degree of damage to the leaf cell membrane of bermudagrass under salinity stress.
Plant development is altered to varying degrees when the growing environment changes, which is mostly due to variations in the ability to synthesize photosynthetic compounds. Chlorophyll is a component of chlorophyll-protein complexes on the thylakoid membrane, which represents the photosynthetic potential of the plant (Procházková et al. 2013). Salinity stress has been reported with inhibiting chlorophyll synthesis or accelerating chlorophyll breakdown (Jia et al. 2019; Stpień and Kobus 2006), reducing plants’ ability to receive and transfer light energy and altering their photosynthetic function. Plants can improve their ability to collect and use light energy in low-light conditions by increasing the concentration of photosynthetic pigments and then adapting to low-light environments (Alvarenga et al. 2003; He et al. 1996).In this study, the chlorophyll a and chlorophyll b contents of bermudagrass were significantly reduced by salinity treatment. The chlorophyll concentration was significantly higher in the shade and combined treatments than in the control, which is consistent with Lima’s findings (Lima et al. 2011). This means that when exposed to salinity stress, shadow increases the chlorophyll content of bermudagrass leaves.
Chlorophyll content changes are frequently associated with changes in PSII-mediated photochemical processes (Sharma and Hall 1992). To investigate the PSII-mediated photochemical reactions, we assessed the yield of chlorophyll fluorescence emission in this study. Chlorophyll fluorescence provides critical information on photochemical quantum yield and is commonly used as a photosystem efficiency metric (Strasser and Govindjee 1992; Strasser et al. 2004). The OJIP curve in this experiment revealed that after continuous stress treatment, bermudagrass performance was best in the shade treatment, followed by the combined treatment, and the lowest in the salinity treatment group. This is the same with other studies in which a consistent trend was revealed on the OJIP transitory outcomes under shade and salinity treatment (Wei et al. 2010; Zushi and Matsuzoe 2017). Many parameters of the OJIP curve were investigated further to determine the effects of stress on plants. As salt concentration increased, the energy capture efficiency of PSII in wheat (Triticum aestivum) leaves decreased significantly (Mehta et al. 2010). Furthermore, previous studies revealed an increasing trend of energetic parameters, such as ABS/CSO and TRO/CSO, in each PSII reaction center of Physocarpus amurensis leaves under low light (Zhang et al. 2016). Moreover, when compared with controls, ABS/CSO and TRO/CSO were lower in salinity treatments, whereas ABS/CSO and TRO/CSO were higher in shade treatments. The ABS/CSO and TRO/CSO ratios increased in the combined treatment compared with the salinity treatment, indicating that shade improved the light energy absorption and capture ability of bermudagrass in the salinity treatment. There was no difference in φPo in the salinity treatment compared with the control, which contradicted the findings of Shin et al. (2020) on lettuce. This discrepancy may be attributed to differences in the degree of reaction to salinity stress in different species. However, φPo in the shade treatment was significantly higher than that in the control, consistent with the results reported by others (Yang et al. 2019). The combined treatment had much higher φPo than the salinity treatment, indicating that shade increased maximum photochemical efficiency under salinity stress. Scholars discovered that qP, ETR, and Y(II) levels were significantly lower in the salinity treatment than in controls (Zhao et al. 2019), which is consistent with our findings. The ETR and Y(II) of the shade and combined treatments were not significantly different from the control. Finally, shade increased the chlorophyll content of bermudagrass leaves under salinity stress, which improved the absorption and capture of light energy in bermudagrass leaves under salinity stress. Shade also increased the maximum photochemical efficiency and electron transmission rate under salinity stress, which alleviated the negative effects of salinity stress on bermudagrass photosynthesis.
The COP1, PIF4, HY5, and HEMA1 genes are essential components of the optical signaling cascade. When plants are exposed to shade, PIF4, HEMA1, and COP1 are rapidly increased (Li et al. 2020; Pacín et al. 2013; Zhang et al. 2018), but HY5 is initially strongly elevated and then destroyed by COP1 (Osterlund et al. 2000). In conclusion, the significant activation of the PIF4, HEMA1, and COP1 genes in response to shade stress demonstrate their critical role in dealing with shade stress and increasing plant photosynthetic performance (Leivar et al. 2008; Liu et al. 2020; Nozue et al. 2007; Pacín et al. 2013). In the current study, the expression of PIF4, HEMA1, and COP1 was substantially increased under shade treatment, and the results of these gene expressions under shade treatment were similar to the results of the others described previously. PIF4, HEMA1, and COP1 expressions were significantly up-regulated after 6 h of combined stress, and PIF4 and HEMA1 expression was significantly higher after treatment than that in the control and salinity stress. PIF4 and HEMA1 were discovered to be closely related to plant elongation and chlorophyll production (Peng et al. 2018; Sun et al. 2020; Tanaka and Tanaka 2006). This finding was consistent with the plant height, chlorophyll content, and chlorophyll fluorescence of bermudagrass under combined stress in this study. The preceding gene expression level was increased in the combined treatment, so that the growth status and photosynthetic features of bermudagrass were better than those in the single salinity treatment, demonstrating that shading can mitigate the negative effects of salinity stress on bermudagrass.
Conclusions
Abiotic stresses, including salinity, shade, and the combined stress, changed the relative water content and electrolyte leakage of bermudagrass leaves, causing a change in photosynthetic capability, with salinity stress causing the most severe damage. The growth status and photosynthetic features of bermudagrass were better than those under the salinity treatment alone. To our knowledge, this is the first study to look at the effects of low light and salt on bermudagrass. These findings improve the theoretical system of mixing shade and salt and provide theoretical support for the use of bermudagrass in the shaded environment of saline soils.
References Cited
Alvarenga, A.A.D., Castro, E.M.D., Junior, É.D.C.L. & Magalhães, M.M. 2003 Effects of different light levels on the initial growth and photosynthesis of Croton urucurana Baill. in southeastern brazil Rev. Arvore 27 1 53 57 https://doi.org/10.1590/S0100-67622003000100007
Arnon, D.I. 1949 Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgaris Plant Physiol. 24 1 1 15 https://doi.org/10.1104/pp.24.1.1
Bazihizina, N., Veneklaas, E.J., Barrett-Lennard, E.G. & Colmer, T.D. 2017 Hydraulic redistribution: Limitations for plants in saline soils Plant Cell Environ. 40 10 2437 2446 https://doi.org/10.1111/pce.13020
Bienert, G.P., Møller, A.L., Kristiansen, K.A., Schulz, A., Møller, I.M., Schjoerring, J.K. & Jahn, T.P. 2007 Specific aquaporins facilitate the diffusion of hydrogen peroxide across membranes J. Biol. Chem. 282 2 1183 1192 https://doi.org/10.1074/jbc.M603761200
Bizhani, S. & Salehi, H. 2014 Physio-morphological and structural changes in common bermudagrass and Kentucky bluegrass during salt stress Acta Physiol. Plant. 36 3 777 786 https://doi.org/10.1007/s11738-013-1455-y
Bunce, J.A. 1998 The temperature dependence of the stimulation of photosynthesis by elevated carbon dioxide in wheat and barley J. Expt. Bot. 49 326 1555 1561 https://doi.org/10.1093/jxb/49.326.1555
Cao, Y., Yang, K., Liu, W., Feng, G., Peng, Y. & Li, Z. 2022 Adaptive responses of common and hybrid bermudagrasses to shade stress associated with changes in morphology, photosynthesis, and secondary metabolites Front Plant Sci. 13 817105 https://doi.org/10.3389/fpls.2022.817105
Chen, K., Chen, L., Fan, J. & Fu, J. 2013 Alleviation of heat damage to photosystem II by nitric oxide in tall fescue Photosynth. Res. 116 1 21 31 https://doi.org/10.1007/s11120-013-9883-5
Clevidence, B.A. 2010 Tropical and subtropical fruits: Phytonutrients and anticipated health benefits Acta Hortic. 864 485 498 https://doi.org/10.17660/ActaHortic.2010.864.66
El-Khouly, M.E., El-Mohsnawy, E. & Fukuzumi, S. 2017 Solar energy conversion: From natural to artificial photosynthesis J. Photochem. Photobiol. Chem. 31 36 83 https://doi.org/10.1016/j.jphotochemrev.2017.02.001
Fan, J., Ren, J., Zhu, W., Amombo, E., Fu, J. & Chen, L. 2014 Antioxidant responses and gene expression in bermudagrass under cold stress J. Am. Soc. Hortic. Sci. 139 6 699 705 https://doi.org/10.21273/JASHS.139.6.699
Gorham, J., Jones, R.G.W. & Mcdonnell, E. 1985 Some mechanisms of salt tolerance in crop plants Biosalinity in action: Bioproduction with saline water. 17 15 40 https://doi.org/10.1007/978-94-009-5111-2_2
Handa, S., Handa, A.K., Hasegawa, P.M. & Bressan, R.A. 1986 Proline accumulation and the adaptation of cultured plant cells to water stress Plant Physiol. 80 4 938 945 https://doi.org/10.1104/pp.80.4.938
He, J., Chee, C.W. & Goh, C.J. 1996 Photoinhibition of Heliconia under natural tropical conditions: The importance of leaf orientation for light interception and leaf temperature Plant Cell Environ. 19 11 1238 1248 https://doi.org/10.1111/j.1365-3040.1996.tb00002.x
Hniliková, H., Hnilika, F., Orsák, M. & Hejnák, V. 2019 Effect of salt stress on growth, electrolyte leakage, Na+ and K+ content in selected plant species Plant Soil Environ. 65 2 90 96 https://doi.org/10.17221/620/2018-PSE
Hoagland, D.R. & Arnon, D.I. 1950 The water culture method for growing plants without soil Calif Agric Exp Stn Circ. 347 1 32 https://doi.org/10.1016/S0140-6736(00)73482-9
Hu, L., Huang, Z., Liu, S. & Fu, J. 2012 Growth response and gene expression in antioxidant related enzymes in two bermudagrass genotypes differing in salt tolerance J. Am. Soc. Hortic. Sci. 137 3 134 143 https://doi.org/10.21273/JASHS.137.3.134
Huang, F., Fulda, S., Hagemann, M. & Norling, B. 2006 Proteomic screening of salt stress induced changes in plasma membranes of Synechocystis sp. strain PCC 6803 Proteomics 6 3 910 920 https://doi.org/10.1002/pmic.200500114
Jia, X.M., Wang, H., Sofkova, S., Zhu, Y., Hu, Y., Cheng, L., Zhao, T. & Wang, Y. 2019 Comparative physiological responses and adaptive strategies of apple Malus halliana to salt, alkali and saline-alkali stress Scientia Hort. 245 154 162 https://doi.org/10.1016/j.scienta.2018.10.017
Jiang, C.D., Wang, X., Gao, H.Y., Shi, L. & Chow, W.S. 2011 Systemic regulation of leaf anatomical structure, photosynthetic performance, and high-light tolerance in sorghum Plant Physiol. 155 3 1416 1424 https://doi.org/10.1104/pp.111.172213
Jiang, Y.W., Duncan, R.R. & Carrow, R.N. 2004 Assessment of low light tolerance of seashore paspalum and bermudagrass Crop Sci. 44 2 587 594 https://doi.org/10.2135/cropsci2004.5870
Jose, A.M., Maria, O., Agustina, B.V., Pedro, D.V., Maria, S.B. & Jose, H. 2017 Plant responses to salt stress: Adaptive mechanisms Agronomy (Basel) 7 1 18 https://doi.org/10.3390/agronomy7010018
Kitta, E., Katsoulas, N. & Kittas, C. 2020 Effect of shading on photosynthesis in greenhouse hydroponic cucumber crops Acta Hortic. 1320 167 172 https://doi.org/10.17660/ActaHortic.2021.1320.21
Leivar, P., Monte, E., Oka, Y., Liu, T., Carle, C., Castillon, A., Huq, E. & Quail, P.H. 2008 Multiple phytochrome-interacting bHLH transcription factors repress premature seedling photomorphogenesis in darkness Curr. Biol. 18 23 1815 1823 https://doi.org/10.1016/j.cub.2008.10.058
Li, C., Wang, Y., Liu, L., Hu, Y., Zhang, F., Mergen, S., Wang, G., Schläppi, M.R. & Chu, C. 2011 A rice plastidial nucleotide sugar epimerase is involved in galactolipid biosynthesis and improves photosynthetic efficiency PLoS Genet. 7 7 e1002196 https://doi.org/10.1371/journal.pgen.1002196
Li, H.M., Kaneko, Y. & Keegstra, K. 1994 Molecular cloning of a chloroplastic protein associated with both the envelope and thylakoid membranes Plant Mol. Biol. 25 4 619 632 https://doi.org/10.1007/BF00029601
Li, L., Tian, S.L., Jiang, J. & Wang, Y. 2020 Regulation of nitric oxide to Capsicum under lower light intensities S. Afr. J. Bot. 132 268 276 https://doi.org/10.1016/j.sajb.2020.05.020
Li, Q., Huang, W., Xiong, C. & Zhao, J. 2018 Transcriptome analysis reveals the role of nitric oxide in Pleurotus eryngii responses to Cd2+stress Chemosphere 201 294 302 https://doi.org/10.1016/j.chemosphere.2018.03.011
Lima, M.C., Amarante, L.D., Mariot, M.P. & Serpa, R. 2011 Crescimento e produção de pigmentos fotossintéticos em Achillea millefolium L. cultivada sob diferentes níveis de sombreamento e doses de nitrogênio Cienc. Rural 41 1 45 50 https://doi.org/10.1590/S0103-84782011000100008
Liu, H., Huang, Q. & Chen, R. 2004 Effects of shade on photosynthetic characteristics in chieh-qua Acta Hortic. 659 799 804 https://doi.org/10.17660/ActaHortic.2004.659.103
Liu, L., Lin, N., Liu, X., Yang, S., Wang, W. & Wan, X. 2020 From chloroplast biogenesis to chlorophyll accumulation: The interplay of light and hormones on gene expression in Camellia sinensis cv. shuchazao leaves Front Plant Sci. 11 256 https://doi.org/10.3389/fpls.2020.00256
Marchiori, P.E.R., Machado, E.C. & Ribeiro, R.V. 2014 Photosynthetic limitations imposed by self-shading in field-grown sugarcane varieties Field Crops Res. 155 30 37 https://doi.org/10.1016/j.fcr.2013.09.025
Megdiche, W., Hessini, K., Gharbi, F., Jaleel, C.A., Ksouri, R. & Abdelly, C. 2008 Photosynthesis and photosystem 2 efficiency of two salt-adapted halophytic seashore Cakile maritima ecotypes Photosynthetica 46 3 410 419 https://doi.org/10.1007/s11099-008-0073-1
Mehta, P., Jajoo, A., Mathur, S. & Bharti, S. 2010 Chlorophyll a fluorescence study revealing effects of high salt stress on photosystem II in wheat leaves Plant Physiol. Biochem. 48 16 20 https://doi.org/10.1016/j.plaphy.2009.10.006
Misra, A.N., Sahu, S.M., Misra, M., Singh, P., Meera, I., Das, N., Kar, M. & Shau, P. 1997 Sodium chloride induced changes in leaf growth, and pigment and protein contents in two rice cultivars Biol. Plant. 39 2 257 262 https://doi.org/10.1023/A:1000357323205
Nozue, K., Covington, M.F., Duek, P.D., Lorrain, S., Fankhauser, C., Harmer, S.L. & Maloof, J.N. 2007 Rhythmic growth explained by coincidence between internal and external cues Nature 448 358 361 https://doi.org/10.1038/nature05946
Osterlund, M.T., Hardtke, C.S., Wei, N. & Deng, X.W. 2000 Targeted destabilization of HY5 during light-regulated development of Arabidopsis Nature 405 462 466 https://doi.org/10.1038/35013076
Pacín, M., Legris, M. & Casal, J.J. 2013 COP1 re-accumulates in the nucleus under shade Plant J. 75 4 631 641 https://doi.org/10.1111/tpj.12226
Peng, M., Li, Z., Zhou, N., Ma, M., Jiang, Y., Dong, A., Shen, W.H. & Li, L. 2018 Linking phytochorme-interacting factor to histone modification in plant shade avoidance Plant Physiol. 176 2 1341 1351 https://doi.org/10.1104/pp.17.01189
Procházková, D., Haisel, D., Wilhelmová, N., Pavlíková, D. & Száková, J. 2013 Effects of exogenous nitric oxide on photosynthesis Photosynthetica 51 4 483 489 https://doi.org/10.1007/s11099-013-0053-y
Robinson, J.C., Yang, G., Gu, S. & Lu, Z. 2019 Plant growth response of black cohosh to shade levels in a high tunnel HortScience 54 12 2178 2181 https://doi.org/10.21273/HORTSCI14472-19
Sharma, P.K. & Hall, D.O. 1992 Changes in carotenoid composition and photosynthesis in sorghum under high light and salt stresses J. Plant Physiol. 140 6 661 666 https://doi.org/10.1016/S0176-1617(11)81020-5
Sharp, R.E., Hsiao, T.C. & Silk, W.K. 1990 Growth of the maize primary root at low water potentials: II Role of growth and deposition of hexose and potassium in osmotic adjustment Plant Physiol. 93 4 1337 1346 https://doi.org/10.1104/pp.93.4.1337
Shi, H., Ye, T., Zhong, B., Liu, X. & Chan, Z. 2014 Comparative proteomic and metabolomic analyses reveal mechanisms of improved cold stress tolerance in bermudagrass (Cynodon dactylon) by exogenous calcium J. Integr. Plant Biol. 56 11 1064 1079 https://doi.org/10.1111/jipb.12167
Shin, Y.K., Bhandari, S.R., Jo, J.S., Song, J.W. & Lee, J.G. 2020 Response to salt stress in lettuce: Changes in chlorophyll fluorescence parameters, phytochemical contents, and antioxidant activities Agronomy (Basel) 10 11 1627 https://doi.org/10.3390/agronomy10111627
Singh, K., Pandey, V.C. & Singh, R.P. 2013 Cynodon dactylon: An efficient perennial grass to revegetate sodic lands Ecol. Eng. 54 32 38 https://doi.org/10.1016/j.ecoleng.2013.01.007
Sladek, B.S., Henry, G.M. & Auld, D.L. 2009 Evaluation of zoysiagrass genotypes for shade tolerance HortScience 44 5 1447 1451 https://doi.org/10.21273/HORTSCI.44.5.1447
Stpień, P. & Kobus, G. 2006 Water relations and photosynthesis in Cucumis sativus L. leaves under salt stress Biol. Plant. 50 4 610 616 https://doi.org/10.1007/s10535-006-0096-z
Strasser, R.J. & Govindjee 1992 The Fo and the OJIP fluorescence rise in higher plants and algae Regulation Chloroplast Biogenesis. 226 423 436 https://doi.org/10.1007/978-1-4615-3366-5_60
Strasser, R.J., Tsimilli-Michael, M. & Srivastava, A. 2004 Analysis of the chlorophyll a fluorescence transient Chlorophyll a Fluorescence. 19 321 362 https://doi.org/10.1007/978-1-4020-3218-9_12
Sun, W., Han, H., Deng, L., Sun, C., Xu, Y., Lin, L., Ren, P., Zhao, J., Zhai, Q. & Li, C. 2020 Mediator subunit MED25 physically Interacts with phytochrome interacting factors4 PIF4 to regulate shade-induced hypocotyl elongation in tomato Plant Physiol. 184 3 1549 1562 https://doi.org/10.1104/pp.20.00587
Tanaka, A. & Tanaka, R. 2006 Chlorophyll metabolism Curr. Opin. Plant Biol. 9 3 248 255 https://doi.org/10.1016/j.pbi.2006.03.011
Trappe, J.M., Karcher, D.E., Richardson, M.D. & Patton, A.J. 2011 Shade and traffic tolerance varies for bermudagrass and zoysiagrass cultivars Crop Sci. 51 2 870 877 https://doi.org/10.2135/cropsci2010.05.0248
Wang, H., Wang, R., Liu, B., Yang, Y., Ling, Q., Chen, E., Zhang, H. & Guan, Y. 2020 QTL analysis of salt tolerance in Sorghum bicolor during whole-plant growth stages Plant Breed. 139 3 455 465 https://doi.org/10.1111/pbr.12805
Wei, Z., Jeranyama, P., Zhang, F., Demoranville, C. & Hou, H.J.M. 2010 Probing the mechanisms of the yellow vine syndrome development in the American cranberry: Shade effect HortScience 45 9 1345 1348 https://doi.org/10.21273/HORTSCI.45.9.1345
Yañez, P., Chinone, S., Hirohata, R., Ohno, H. & Ohkawa, K. 2012 Effects of time and duration of short-day treatments under long-day conditions on flowering of a quantitative short-day sunflower (Helianthus annuus) ‘Sunrich Orange’ Scientia Hort. 140 8 11 https://doi.org/10.1016/j.scienta.2012.03.008
Yang, M., Liu, M., Lu, J. & Yang, H. 2019 Effects of shading on the growth and leaf photosynthetic characteristics of three forages in an apple orchard on the loess plateau of eastern Gansu, China PeerJ 7 e7594 https://doi.org/10.7717/peerj.7594
Yu, J., Sun, L., Fan, N., Yang, Z. & Huang, B. 2015 Physiological factors involved in positive effects of elevated carbon dioxide concentration on bermudagrass tolerance to salinity stress Environ. Exp. Bot. 115 20 27 https://doi.org/10.1016/j.envexpbot.2015.02.003
Zhang, H., Zhong, H., Wang, J., Sui, X. & Xu, N. 2016 Adaptive changes in chlorophyll content and photosynthetic features to low light in Physocarpus amurensis Maxim and Physocarpus opulifolius “Diabolo” PeerJ 4 e2125 https://doi.org/10.7717/peerj.2125
Zhang, K., Zheng, T., Zhu, X., Jiu, S., Liu, Z., Guan, L., Jia, H. & Fang, J. 2018 Genome-wide identification of PIFs in grapes (Vitis vinifera) and their transcriptional analysis under lighting/shading conditions Genes (Basel) 9 9 451 https://doi.org/10.3390/genes9090451
Zhang, M., Hu, C., Sun, X., Zhao, X., Tan, Q., Zhang, Y. & Li, N. 2014 Molybdenum affects photosynthesis and ionic homeostasis of Chinese cabbage under salinity stress Commun. Soil Sci. Plant Anal. 45 20 2660 2672 https://doi.org/10.1080/00103624.2014.941855
Zhao, H., Liang, H., Chu, Y., Sun, C., Wei, N., Yang, M. & Zheng, C. 2019 Effects of salt stress on chlorophyll fluorescence and the antioxidant system in Ginkgo biloba L. seedlings HortScience 54 12 2125 2133 https://doi.org/10.21273/HORTSCI14432-19
Zlatev, Z. & Lidon, F.C. 2012 An overview on drought induced changes in plant growth, water relations and photosynthesis Emir. J. Food Agric. 24 1 57 72 https://doi.org/10.9755/ejfa.v24i1.10599
Zushi, K. & Matsuzoe, N. 2017 Using of chlorophyll a fluorescence OJIP transients for sensing salt stress in the leaves and fruits of tomato Scientia Hort. 219 216 221 https://doi.org/10.1016/j.scienta.2017.03.016
