Responses of Gardenia jasminoides Ellis Leaf Traits and Anatomical Structures to Drought Stress in P

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  Abstract [Objectives] This study was conducted to investigate the response of Gardenia to purple soil drought stress, hoping to provide a reference for the selection of plants for vegetation restoration in purple soil regions.
  [Methods]The pot-weighing water control method was used to apply different degrees of drought stress to Gardenia seedlings in purple soil, and the effects of drought stress on the electrical conductivity, chlorophyll content, leaf morphology and structure of Gardenia leaves were explored.
  [Results] The leaf electrical conductivity increased with the increase of drought stress intensity, and the leaf electrical conductivity under severe drought stress increased by 59.93% compared with the control; the chlorophyll content of Gardenia showed a single-peak changing trend that increased and then decreased with the development of drought stress, and it was the highest in each stress stage under severe drought stress; the leaf thickness, palisade tissue thickness and sponge tissue thickness of Gardenia were reduced with the stress degree increasing, and showed the largest decreases under severe stress; the stomatal length, stomatal width and stomatal opening of Gardenia gradually decreased with the increase of stress, while the stomatal density gradually increased.
  [Conclusions]This study provides a technical and resource basis for vegetation restoration in purple soil.
  Key words Purple soil; Drought stress; Gardenia jasminoides Ellis; Anatomical structure; Leaf traits
  Received: June 2, 2020  Accepted: August 1, 2021
  Supported by Hunan Forestry Science and Technology Innovation Project (XLK201971); Changsha Science and Technology Program (kq1801028).
  Yan YANG (1980-), female, P. R. China, associate researcher, devoted to research about forest tree breeding and cultivation and vegetation restoration on difficult sites.
  *Corresponding author. E-mail: 1726027015@qq.com.
   Purple soil has shallow soil layer, poor water permeability and weak water storage and water retention capacity, which, combined with its darker color, strong heat absorption, and poor thermal conductivity, result in high soil temperature in high temperature weather. Therefore, there is less natural vegetation and poor growth in purple soil, which is thus a difficult area for vegetation restoration[1]. Hunan Province is one of the provinces in China where the distribution of purple soil is relatively concentrated, and Hengyang is the area with the largest area of purple soil slopes in Hunan Province. Due to the high temperature and rain in summer and abundant rainfall, people did not take corresponding water and soil conservation measures during the development and utilization process, which has caused serious soil erosion. As a result, the purple soil slopes in Hengyang have suffered serious soil erosion and seasonal drought for a long time, making local ecological environment more fragile, and severely restricting the development of local production, life and economy[2-3]. In view of this, it is an inevitable trend for the construction of local ecological civilization to discover an ecological restoration model that is suitable for local vegetation restoration and can help local economic development.   Gardenia jasminoides Ellis is an evergreen shrub with green leaves, full floral fragrance, and festive fruit color. The fruit contains high medicinal active ingredients and has high ornamental and medicinal value[4]. G. jasminoides Ellis has a developed root system with strong soil-fixing and water-retaining capacity, and possesses good economic, social and ecological benefits[5-6]. Relevant studies also show that G. jasminoides Ellis is suitable for cultivation in a warm, humid, and sufficient light environment, and is relatively resistant to drought[7-8]. Therefore, it can be preliminarily judged that G. jasminoides Ellis is an ideal tree species for ecological restoration in purple soil regions. In this study, in order to explore the drought tolerance of G. jasminoides Ellis and the adaptability mechanism of purple soil, the response of G. jasminoides Ellis to purple soil drought stress was investigated, hoping to provide reference for the selection of plants for vegetation restoration in purple soil regions, and to provide a technical and resource basis for vegetation restoration in purple soil.
  Materials and Methods
  Materials
  The experimental material was one-year-old robust disease-free G. jasminoides Ellis seedlings with consistent growth.
  Experimental design
  At the beginning of June 2020, the G. jasminoides Ellis seedlings were transplanted into 17 cm high flower pots, and the cultivation soil substrate was purple soil. There was one G. jasminoides Ellis seedling per pot, totaling 120 pots. The transplanted potted seedlings were placed in a rain shelter for routine management, and the drought stress test was started after 1 month of pre-cultivation.
  This experiment adopted the pot-weighing water control method, and was set with five treatment groups: control (CK), normal watering, mild drought stress (Z1, soil relative water content being 80% of field water holding capacity), moderate drought stress (Z2, soil relative water content being 60% of field water holding capacity), and severe drought stress (Z3, soil relative water content being 40% of field water holding capacity). The detailed test design is shown in Table 1. The plants were watered fully before the stress test until the relative water content of the soil was saturated, and the soil was allowed to dry naturally. The pot soil was weighed with an electronic scale at 17:00 every day. After the relative water content of the soil was reduced to the drought condition required for the test, the pot soil was weighed and added with water every 2 d to keep the relative soil water content within the ranges of the drought stress treatments. The water condition was controlled continuously for 90 d until the drought stress ended on October 15, 2020.   Determination of related indicators
  Determination of relative conductivity of leaves
  With reference to the method of Chen et al.[9], a conductivity meter (type: DDS-307) was used to determine the relative conductivity of the leaves.
  Determination of chlorophyll content
  With reference to Wang[10], five mature leaves were randomly selected for each of the control and drought stress treatments, washed and dried, cut into pieces and mixed (removed the main vein). Then, 0.2 g was weighed from each sample, placed in a test tube with a stopper, added with 25 ml of 95% ethanol, and soaked in a dark place at 25 ℃ until the material turned white completely. The extract was measured for the absorbance at 663 and 645 nm using an ultraviolet spectrophotometer (UV5100B). The chlorophyll content was calculated as follows:
  Chlorophyll a content (mg/g)=[(13.95A665-6.88A649)×V]/(m×1 000)
  Chlorophyll b content (mg/g)=[(24.96A649-7.32A665)×V]/(m×1 000)
  Total chlorophyll content (mg/g)=Chlorophyll a content+Chlorophyll b content
  In the formulas, V is the volume of the extract (ml); m is the mass of the sample leaf (g); and 1 000 means 1 L=1 000 ml.
  Observation of leaf microstructure
  Leaf thickness, palisade tissue thickness and sponge tissue thickness were observed by paraffin sections. The leaves were cut into pieces of 0.5 cm×0.5 cm and placed in an FAA fixing solution. After fixing for 48 h, five pieces were taken from each treatment and placed in a weighing bottle, which was then added with the mixed solution of glacial acetic acid and 30% hydrogen peroxide in a volume ratio of 1∶1. The weighing bottle was put in a constant temperature incubator at 40 ℃ until the cut leaf pieces turned white, and the segregation solution was poured out. The upper and lower epidermis was peeled off with tweezers and dissecting needles, and prepared into slides.
  The slides prepared above were observed and photographed with a digital microscope (Motic Images Advanced 3.0), and the images were measured under the image measurement software (Digimizer 4.0). The measured indexes included leaf thickness, palisade tissue thickness and sponge tissue thickness, as well as stomatal length, stomatal width, stomatal opening and stomatal density. Each treatment was repeated 15 times and the average value was taken.
  Data processing and analysis
  Excel 2010 was used to organize the data and make charts, and SPSS18.0 was used to perform one-way analysis of variance and Duncan multiple comparisons for each index (P<0.05).   Results and Analysis
  Effects of drought stress on relative electrical conductivity of G. jasminoides Ellis leaves
  The relative conductivity of plant leaves refers to the membrane permeability of plant cells. The greater the relative conductivity, the greater the permeability of the plant cell plasma membrane, indicating that the plant suffers more damage. According to Fig. 1, it can be seen that in the early stage of drought stress, the relative conductivity of G. jasminoides Ellis leaves rose slowly under mild drought (Z1) and moderate drought (Z2), and rose rapidly under severe drought (Z3), which increased by 37.50% compared with the control. In the mid-stage of drought stress, the relative conductivity of G. jasminoides Ellis rose significantly, increasing by 11.97%, 27.03% and 46.25% respectively compared with the control. In the late stage of drought stress, the relative conductivity of G. jasminoides Ellis with a greater degree of stress. Among them, the relative electrical conductivity of G. jasminoides Ellis leaves under severe drought was 32.93%, which was 59.93% higher than that of the control.
  Effects of drought stress on chlorophyll content of G. jasminoides Ellis
  Chlorophyll mainly includes chlorophyll a and chlorophyll b, which are important pigments for photosynthesis of plants, and their content reflects the photosynthetic capacity and growth of plants. In the early stage of drought stress, the chlorophyll content of G. jasminoides Ellis maintained a relatively stable increase, which increased by 38.33%, 24% and 22.35% respectively compared with the control. In the mid-stage of drought stress, the chlorophyll content of G. jasminoides Ellis reached the peak of the whole stress period, and that of severe drought stress increased the most, which was 17.71% higher than the control. When the drought stress entered the later stage, the chlorophyll content decreased significantly. The chlorophyll contents of the control, mild drought and moderate drought decreased significantly, and that of the severe drought stress decreased to a small extent, only decreased by 0.25 mg/g compared with the mid-stage drought stress.
  Effects of drought stress on mesophyll structure of G. jasminoides Ellis
  In order to further explore the effects of drought stress on the mesophyll structure of G. jasminoides Ellis, this study was carried out by making slides and carrying out microscopic determinations, as shown in Table 2. According to Table 2, it can be seen that the leaf thickness, palisade tissue thickness and sponge tissue thickness of G. jasminoides Ellis gradually decreased with the increase of the stress level, and there were no significant differences in the parameters of the anatomical structure between the mild drought stress and the control; and with the increase of drought stress, there were significant differences in leaf structure indexes, and under severe drought stress, leaf thickness, palisade tissue thickness, and sponge tissue thickness were reduced by 33.49%, 31.83%, and 42.63%, respectively, compared with the control.   Effects of drought stress on leaves stomata of G. jasminoides Ellis
  Stomata are the main channels for plants to exchange gas with the external environment, and they are also the main gateways for plants to carry out transpiration. The morphological characteristics of stomata are closely related to the growth and metabolism of plants. It can be seen from Table 3 that the stomatal length, stomatal width and stomatal opening of G. jasminoides Ellis gradually decreased with the increase of the stress, while the stomatal density gradually increased; under mild drought stress, the stomatal length, stomatal width, and stomatal opening of G. jasminoides Ellis were not significantly different from those of the control, and under severe drought stress, the stomatal length, stomatal width and stomatal opening were reduced by 13.45%, 10.68% and 28.67% respectively compared with the control; and compared with the control, the stomatal density increased by 13.00%, 19.88% and 45.32%, respectively, under various stress levels.
  Yan YANG et al. Responses of Gardenia jasminoides Ellis Leaf Traits and Anatomical Structures to Drought Stress in Purple Soil
  Conclusions and Discussion
  Under normal growth of plant, the active oxygen in their bodies is in a state of equilibrium between production and removal, which will not cause damage to the membrane system. However, under drought stress, the activity of active oxygen scavenging enzymes in the plant decreases, and active oxygen cannot be cleared in time. A large amount of accumulated reactive oxygen species will peroxidize cell membrane lipids, cause damage to the membrane system and increase membrane permeability[11]. When plants suffer from drought stress, the balance between the production and removal of free radicals is broken, membrane lipid peroxidation occurs, a large amount of free radicals accumulate, peroxides such as malondialdehyde (MDA) are produced, which are toxic to the plant membrane system, and membrane permeability increases, leading to an increase in the relative electrical conductivity of plants[12]. Studies have shown that the relative electrical conductivity and MDA content in plant leaves represent the degree of damage to the plant membrane system[13]. In this study, it was found that the relative electrical conductivity and MDA content about leaves of G. jasminoides Ellis gradually increased with the aggravation of drought stress and the extension of time. It indicated that under drought stress, the cell membrane structure of G. jasminoides Ellis was destroyed or its function was impaired, causing the water-soluble substances in the cell to leak out of cells, which is consistent with the research results of Reyiza[14] and Wang et al.[15].   The chlorophyll content of plants can reflect to a certain extent the ability of plants to use light energy, produce organic matter and adapt to and utilize environmental factors. Under drought stress, the chlorophyll content of different plants changes differently. Studies have shown that under drought stress, a large number of free radicals are produced to peroxidize the chloroplast membrane, the membrane system is damaged, causing chlorophyll degradation, and the balance between chlorophyll synthesis and degradation in plants is disrupted, resulting in a decrease in chlorophyll content[16-17]. However, some studies have also found that plant chlorophyll content will increase under drought stress, which may be due to that drought stress reduces the relative water content of leaves, hinders leaf expansion and growth, and increases the chlorophyll content per unit area of leaves, which ensures the full use of light energy by plant leaves, improves the conversion efficiency to ensure carbon assimilation, enhances metabolic activity in the body, and enhances drought resistance. Although the chlorophyll content increases, the decrease in water content reduces the activity of chlorophyll[18]. The results of these two studies indicate that the changes in chlorophyll content of plants under drought stress may be related to the response strategies of plants to drought stress. In this study, we found that the chlorophyll content about leaves of G. jasminoides Ellis gradually increased with the stress increasing, indicating that G. jasminoides Ellis is more sensitive to drought stress. It mainly resisted drought stress by increasing the chlorophyll content; and with the extension of the stress time, the chlorophyll content showed a law of first rising and then falling. The reason that the chlorophyll content about leaves of G. jasminoides Ellis decreased in the later period of stress might be the damage of the chloroplast membrane system due to the aggravation of the stress. It might also be related to the season.
  Leaves are the main place for plants to perform photosynthesis and transpiration, and they are also the center of energy conversion with the external environment. The contact area of leaves with the surrounding environment is relatively large, so plants’ response to the environment is more reflected in the morphological structure of leaves. Studies have found that the greater the thickness of a plant’s leaves, the stronger its water storage capacity, so leaf thickness can be used as one of the indicators to measure the drought resistance of plants[19]. In addition, the highly developed palisade tissue and sponge tissue make the leaves have high-strength mechanical support and sufficient water reserves, which can effectively protect mesophyll cells from damage caused by high temperature and strong light, prevent leaf wilting under drought conditions, and help plants to improve the utilization of water and light energy to resist drought stress[20]. The results of this study showed that with the intensification of drought stress, the thickness about leaves of G. jasminoides Ellis gradually decreased. The reason might be that drought stress reduced the water potential about leaves of G. jasminoides Ellis, which led to the imbalance of water metabolism about leaves of G. jasminoides Ellis, which in turn affected the growth and division of leaf cells[21]. Under normal water supply conditions, the sponge tissue thickness of G. jasminoides Ellis was greater than the thickness of palisade tissue, which indicates that G. jasminoides Ellis has a strong water storage capacity and a high utilization rate of water and light energy. With the intensification of drought stress, the palisade tissue thickness and sponge tissue thickness of G. jasminoides Ellis gradually decreased, and the heavier the drought stress, the more the decrease, which is consistent with the research results of Cui et al.[22].   Stomata are the windows for plant water diffusion and gas exchange. The size, opening, and density of stomata have important effects on photosynthesis and transpiration of plants. Stomatal regulation is one of the important regulatory mechanisms for plants to respond to changes in the external environment[23-24]. The results of this study showed that with the intensification of drought stress, the stomatal length, width, and opening of G. jasminoides Ellis were reduced, but the stomatal density increased. Small and dense stomata can make the adjustment of stomata more flexible, quickly adjust the transpiration rate, reduce water loss, maintain normal physiological metabolism, and help improve the resistance of G. jasminoides Ellis seedlings[25].
  In summary, G. jasminoides Ellis has strong adaptability under drought conditions and can be popularized and applied in areas with seasonal droughts, which has important practical significance for helping the construction of ecological civilization in purple soil areas and the construction of rural revitalization.
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