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Article

Repair Effects of Scenedesmus obliquus on Cucumber Seedlings Under Saline–Alkali Stress

by
Zhao Liu
1,
Yanlong Dong
1,2,
Xiaoxia Jin
1,
Yan Liu
1,
Zhonghui Yue
1 and
Wei Li
1,*
1
College of Life Sciences and Technology, Harbin Normal University, No. 1, Shida Road, Limin Economic Development Zone, Harbin 150025, China
2
Horticulture Branch of Heilongjiang Academy of Agricultural Sciences, Harbin 150069, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(6), 1468; https://doi.org/10.3390/agronomy15061468
Submission received: 13 May 2025 / Revised: 6 June 2025 / Accepted: 11 June 2025 / Published: 16 June 2025
(This article belongs to the Section Plant-Crop Biology and Biochemistry)
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Abstract

:
In this study, cucumber seedlings were treated with Scenedesmus obliquus at different concentrations (0.25, 0.50, 0.75, 1 g·L−1) under saline–alkali stress (60 mM and 90 mM). The effects of Scenedesmus obliquus on the repair of cucumber seedlings under saline–alkali stress were explored from physiological and morphological perspectives by measuring growth physiological indices and observing microstructure. It provides a cytological basis for the development of microalgae biofertilizer. The results showed that the addition of Scenedesmus obliquus effectively alleviated the physiological and structural damage in cucumber seedlings caused by saline–alkali stress, with the best mitigation effect at 0.75 g·L−1. More specifically, the addition of Scenedesmus obliquus significantly improved seedling fresh weight and plant height under saline–alkali stress, increased stem vascular vessel diameter, thickened vessel walls, reduced structural damage, the structural recovery of mitochondria, nuclei, and other organelles in the phloem; The results showed that root xylem vessel distribution became more centralized, vessel diameter decreased, and wall thickness decreased, with other changes similar to those in the stem; The number and volume of mesophyll cells increased, chloroplast morphology recovered, and chlorophyll content rose, effectively alleviating the impact of saline–alkali stress on photosynthesis. MDA content decreased, mitigating oxidative damage caused by saline–alkali stress.

1. Introduction

In China, facility agriculture has developed rapidly [1]. However, during this development, secondary soil salinization has become increasingly serious due to excessive chemical fertilizer application and improper irrigation practices [2]. Cucumber (Cucumis sativus L.) is one of the most widely cultivated vegetables globally and has a large planting area in China’s facility agriculture [3]. However, cucumbers have a shallow root system with limited volume, require specific environmental conditions, and are particularly sensitive to saline–alkali stress [4]. When soil salinity and alkalinity exceed tolerance levels, cucumber growth and development are inhibited, reducing yield and quality, which ultimately constrains the development of China’s cucumber industry [5].
Salt stress affects morphological characteristics, physiological metabolism, and photosynthesis in plants, inhibiting their growth and development [6,7]. Studies show that salt stress inhibits seedling growth in rice (Oryza sativa L.), citrus (Citrus reticulata Blanco), and oilseed rape (Brassica campestris L.), reducing plant height and fresh weight [8,9,10]. In soybean (Glycine max), red sandalwood (Reaumuria soongorica), and tomato (Solanum lycopersicum), chlorophyll, malondialdehyde, as well as SOD and POD activities, were altered under saline stress [11,12,13]. Salinity stress significantly decreases chlorophyll content in leaves of tomato (Solanum lycopersicum), rice (Oryza sativa L.), and oleander (Cerasus humilis), and may disrupt the balance of chlorophyll biosynthesis and degradation by enhancing chlorophyll-degrading enzyme activity, thereby altering chlorophyll content [14,15,16].
During the long evolutionary process, plants and their growing environment have always formed an integrated system. The long-term environmental effects have shaped plant morphological structures, which are inherently adapted to their surroundings [17]. Salinity stress induces significant changes in the microstructure and ultrastructure of plant roots, stems, and leaves. The structural modifications observed in root systems encompass the following: In Acacia auriculiformis roots, the mean vascular column thickness (as a percentage of root diameter) gradually increases under salt stress [18]; in maize (Zea mays L.) and cotton (Gossypium hirsutum L.) roots, excessive salt stress leads to a marked increase in mitochondria within xylem parenchyma cells. Cotton roots also exhibit thickened cell walls, but their nuclei show swollen nuclear membranes with blurred boundaries, indicating cellular damage [19,20]. The stem undergoes structural modifications, including salt stress-induced thickening in the cuticular layer, epidermal tissue, and mechanical structures of Thalassia hemprichii stems, serving as an adaptive mechanism to minimize transpirational water loss and improve stem stability [21]. Similarly, oilseed rape (Brassica campestris L.) stems accumulate larger starch granules in plastids, increase mitochondrial numbers, and form cytoplasmic crystals alongside vesicular inclusions [22]. Leaf tissues exhibit structural modifications, including apple (Malus pumila Mill.) seedlings exhibiting leaf thinning, with palisade tissue cells shrinking significantly. Intercellular spaces in both palisade and spongy tissues expand and become disorganized [23]; in tomato (Solanum lycopersicum), chloroplasts aggregate within mesophyll cells, while the grana and thylakoid structures degrade, impairing photosynthetic function [24]; and sweet potato (Ipomoea batatas) leaves exhibit mitochondrial vacuolization and reduced cristae number [25].
Studies have shown that microalgae can alleviate saline stress damage [26]. Salt-tolerant cyanobacteria isolated from saline soils were used as biofertilizers in a 240-day pot experiment. The treated soils showed significantly increased ion-exchange capacity (carbon, nitrogen, phosphorus, potassium, and magnesium) and water-holding capacity, while sodium ion content and electrical conductivity decreased [27]. Extracellular polysaccharides from Dunaliella salina modulate metabolic pathways related to salt tolerance, reducing negative effects on tomato (Solanum lycopersicum) [28]. However, current research has two major limitations: First, most studies focus solely on the soil amelioration effects of microalgae, while the mechanisms by which they enhance salt tolerance through regulating plant cellular structures (such as chloroplasts, mitochondria, and other organelles) remain unexplored. Second, although existing research on microalgae predominantly concentrates on their lipid production and environmental protection functions, there remains substantial untapped potential in developing microalgae-based products for agricultural applications. This research gap highlights the innovative value of our work—by systematically elucidating the mechanisms through which microalgae influence plant cellular ultrastructure under salt stress, we can provide a theoretical foundation for developing novel eco-friendly biofertilizers. Such advancements would play a significant role in addressing large-scale soil salinization and mitigating the detrimental effects of salt stress on plants.
This study investigated the growth physiological indices, microstructure, and ultrastructure of cucumber seedlings under saline–alkali stress with microalgae treatment. Structural modifications were analyzed at both cellular and organelle levels, with subsequent integration of physiological and morphological characteristics. The study aims to (1) Elucidate the cytological mechanisms by which Scenedesmus obliquus mitigates saline–alkali stress in cucumber and (2) Provide theoretical support for developing Scenedesmus obliquus as an environmentally friendly biofertilizer.

2. Materials and Methods

2.1. Experimental Materials

The cucumber (Cucumis sativus L.) seeds used in this study were of the conventional commercial cultivar ‘Green Sword’ and exhibit sensitivity to saline–alkali stress. provided by the Horticultural Branch of Heilongjiang Provincial Academy of Agricultural Sciences. The plants were cultivated, collected, and fixed at the Key Laboratory of Harbin Normal University. Scenedesmus obliquus was provided by the Aquatic Biology Group at Harbin Normal University. The cultures were grown at 25 °C under a 12/12 h light/dark photoperiod with 4000 lx illumination.

2.2. Experimental Design

In March 2024, cucumber seeds were surface-sterilized, soaked, and germinated before being transplanted into 1/2 Hoagland nutrient solution (pH 6.0 ± 0.1). Plants were cultivated under controlled conditions (12 h photoperiod, 28 ± 1 °C/25 ± 1 °C day/night temperature, 4000 lux light intensity, and 65–75% relative humidity) until reaching the two-leaf one-heart stage. A mixed saline solution (NaCl:NaHCO3 = 1:2) was used to simulate stress conditions. Fifteen treatment groups were established with two stress concentrations (60 and 90 mmol·L−1) (Table 1). To avoid osmotic shock, the saline solution was added gradually in 30 mmol·L−1 increments at 12 h intervals. Sampling was conducted on the 4th day of stress treatment. All treatments were replicated three times. Scenedesmus obliquus (0.75 g·L−1) was applied 12 h prior to saline–alkaline stress exposure.

2.3. Experimental Methods

On the 4th day of post-hydroponic stress treatment, three uniformly grown cucumber seedlings were collected from each experimental group, rinsed with distilled water, and gently dried with filter paper. Plant height was measured using a vernier caliper, while fresh weight was recorded using an electronic balance (AR-210, OHAUS Instruments (Shanghai) Co., Ltd. Shanghai, China). Physiological parameters were analyzed in triplicate using a UV spectrophotometer (UNIC-7200, Shanghai Mapada Instruments Co., Ltd. Shanghai, China). Peroxidase (POD) activity was determined by the guaiacol method [29], reagents used: Guaiacol, 30% hydrogen peroxide, and phosphate buffer (pH 6.0–7.0); superoxide dismutase (SOD) activity by the nitroblue tetrazolium (NBT) reduction method [30], reagents used: Nitroblue tetrazolium (NBT), riboflavin, methionine (Met), EDTA-Na2, and phosphate buffer (pH 7.8); catalase (CAT) activity by ultraviolet absorption [31], reagents used: 30% hydrogen peroxide (H2O2) and phosphate buffer (pH 7.0); malondialdehyde (MDA) content by the thiobarbituric acid (TBA) method [29], reagents used: Thiobarbituric acid (TBA), trichloroacetic acid (TCA), and phosphate buffer; and chlorophyll levels by UV spectrophotometry [32], reagents used: Acetone, calcium carbonate (CaCO3), and quartz sand.
For semi-thin and ultra-thin section preparation, fresh leaf and stem samples were cut using double-sided razor blades. The samples underwent conventional processing involving dual fixation with glutaraldehyde (3%) and osmium tetroxide (1%), followed by phosphate buffer rinses and embedding in Epon812. Semi-thin sections were cut using an ultramicrotome (Leica-UC6, Leica Microsystems GmbH, Wetzlar, Germany), stained with toluidine blue, and examined under an optical microscope (Nikon Eclipse E600w, Nikon Corporation, Tokyo, Japan). Ultra-thin sections were stained with uranyl acetate and lead citrate, then observed under a transmission electron microscope (Hitachi HT7800, Hitachi High-Tech Corporation, Tokyo, Japan).

3. Results

3.1. Effect of Scenedesmus obliquus Addition on Plant Height and Fresh Weight of Cucumber Seedlings Under Saline–Alkali Stress

Under saline–alkali stress, the phenotypes of S. obliquus were similar across different treatment groups. Compared to the control group, stressed seedlings exhibited reduced height, which further decreased with increasing saline–alkali stress intensity. Additionally, root systems became shorter or even detached, and leaves showed yellowing—particularly under 90 mM saline–alkali stress. However, application of S. obliquus at varying concentrations alleviated these stress symptoms in all treatment groups (Figure 1).
Under non-stressed conditions, the application of S. obliquus did not significantly affect cucumber seedling growth, with plant height and fresh weight showing no statistically meaningful changes (p > 0.05). Saline–alkali stress caused significant reductions in both plant height and fresh weight, exhibiting a concentration-dependent response. Compared to the unstressed control (0 mM), the 60 mM stress treatment decreased plant height by 7.26% and fresh weight by 17.22%, while the 90 mM treatment resulted in greater reductions of 12.43% and 32.23% for plant height and fresh weight, respectively. The addition of S. obliquus at 0.75 g·L−1 showed the most substantial mitigation effects, increasing plant height by 5.55% and fresh weight by 14.38% under 60 mM stress and by 7.52% and 20.88% under 90 mM stress. These findings clearly demonstrate that S. obliquus application can effectively alleviate the inhibitory effects of saline–alkali stress on cucumber seedling growth parameters (Figure 2).

3.2. Effects of Scenedesmus obliquus on Antioxidant Enzyme Activities in Cucumber Seedlings Under Saline–Alkali Stress

Under non-stressed conditions, different concentrations of S. obliquus showed no significant effects on SOD, POD, and CAT activities in cucumber seedling leaves (p > 0.05). Saline–alkali stress significantly reduced antioxidant enzyme activities in a concentration-dependent manner. Compared to the 0 mM control, 60 mM stress decreased SOD, POD, and CAT activities by 42.09%, 56.45%, and 38.83%, respectively, while 90 mM stress caused greater reductions of 62.44%, 63.38%, and 53.45%. Scenedesmus obliquus supplementation restored these activities, with the 0.75 g·L−1 treatment showing the most pronounced effects: 60 mM stressed plants exhibited increases of 23.33% (SOD), 24.24% (POD), and 13.45% (CAT), while 90 mM stressed plants showed recoveries of 38.83%, 32.74%, and 25.86%, respectively. These results demonstrate that 0.75 g·L−1 S. obliquus effectively mitigates saline–alkali stress-induced inhibition of the antioxidant enzyme system in cucumber seedlings (Figure 3).

3.3. Effects of Scenedesmus obliquus on MDA Content in Cucumber Seedlings Under Saline–Alkali Stress

Under non-stressed conditions, application of S. obliquus showed no significant effect on MDA content in cucumber seedling leaves (p > 0.05). Saline–alkali stress significantly increased MDA content in a concentration-dependent manner. Compared to the 0 mM control, 60 mM saline–alkali stress increased MDA content by 48.40%, while 90 mM stress caused a greater increase of 51.65%. The addition of 0.75 g·L−1 S. obliquus effectively reduced MDA content by 25.91% under 60 mM stress and by 30.86% under 90 mM stress. These results demonstrate that S. obliquus can significantly alleviate saline–alkali stress-induced membrane lipid peroxidation in cucumber seedlings. (Figure 4).

3.4. Effects of Scenedesmus obliquus on Photosynthetic Pigment Content in Cucumber Seedlings Under Saline–Alkali Stress

Under non-stressed conditions, application of S. obliquus showed no significant effect on chlorophyll a, chlorophyll b, or total chlorophyll content in cucumber seedling leaves (p > 0.05). Saline–alkali stress significantly reduced chlorophyll content in a concentration-dependent manner. Compared to the 0 mM control, 60 mM saline–alkali stress decreased chlorophyll a, chlorophyll b, and total chlorophyll content by 42.57%, 37.35%, and 40.98%, respectively, while 90 mM stress caused greater reductions of 59.42%, 68.98%, and 62.34%. The addition of 0.75 g·L−1 S. obliquus effectively restored chlorophyll content under both stress conditions. Under 60 mM stress, chlorophyll a increased by 37.06%, chlorophyll b by 33.12%, and total chlorophyll by 35.84%. Similarly, under 90 mM stress, the treatment increased chlorophyll a by 37.17%, chlorophyll b by 52.31%, and total chlorophyll by 41.82%. These results demonstrate that S. obliquus can significantly mitigate saline–alkali stress-induced chlorophyll degradation in cucumber seedlings (Figure 5).

3.5. Effects of Scenedesmus obliquus on Microstructure of Cucumber Seedlings Under Saline–Alkali Stress

3.5.1. Effects of Scenedesmus obliquus on Microstructure of Cucumber Seedling Leaves Under Saline–Alkali Stress

The saline–alkali stress effect at 90 mM and the mitigation effect of S. obliquus were more pronounced, with 0.75 g·L−1 being the optimal concentration. Therefore, subsequent experiments used 90 mM saline–alkali stress and 0.75 g·L−1 S. obliquus (C3), with four treatment groups: CK, C3, SA2 CK, and SA2 C3.
Under non-stress conditions, the upper and lower epidermal cells exhibited regular shapes, the palisade tissue consisted of two layers of elongated, orderly arranged cells, and both palisade and sponge tissue cells maintained plump morphology (Figure 6A). The addition of 0.75 g·L−1 S. obliquus showed no significant changes compared to the control (Figure 6B). Under saline–alkali stress, palisade tissue cells became disorganized and narrowed, while both palisade and sponge tissue cells decreased in size (Figure 6C). However, adding S. obliquus under stress restored these structural alterations (Figure 6D).
Statistical analysis of cucumber seedling mesophyll cell sizes revealed no differences in palisade or sponge tissue cell length/width when S. obliquus was added under stress (p > 0.05). Saline–alkali stress reduced palisade and sponge tissue cell length by 48.66% and 36.14% (p < 0.05), respectively, and width by 42.09% and 34.53% (p < 0.05). After S. obliquus application, cell length increased by 25.89% and 16.79% (p < 0.05), and width increased by 23.09% and 18.76% (p < 0.05) (Figure 7).

3.5.2. Effects of Scenedesmus obliquus on the Microstructure of Cucumber Seedling Stem Under Saline–Alkali Stress

Under non-saline–alkali stress conditions, the stem structure remained intact, with cortical parenchyma cells appearing turgid and vessel elements maintaining regular morphology (Figure 6E). The addition of 0.75 g·L−1 S. obliquus showed no significant structural alterations compared to the control (Figure 6F). When subjected to saline–alkali stress, slight damage occurred to the central cortex parenchyma cells, accompanied by a significant reduction in vessel elements (Figure 6G). However, supplementation with S. obliquus under stress conditions effectively restored these structural parameters (Figure 6H).
Statistical analysis of vessel element dimensions in cucumber seedling stems revealed no significant differences (p > 0.05) in lumen length or width between S. obliquus-treated and control groups under saline–alkali stress. Stress treatment alone significantly reduced vessel element dimensions by 21.11% in length and 26.24% in width (p < 0.05). Following S. obliquus application, these parameters showed significant recovery, increasing by 8.74% in length and 14.18% in width (p < 0.05) (Figure 8).

3.5.3. Effects of Scenedesmus obliquus on the Microstructure of Cucumber Seedling Roots Under Saline–Alkali Stress

Under saline–alkali-free stress, the root structure remained intact, with cortical parenchyma cells appearing plump and compact and vessel distribution being concentrated (Figure 6I). The addition of 0.75 g·L−1 S. obliquus showed no significant changes compared to the control (Figure 6J). Under saline–alkali stress, some cells were damaged, cortical parenchymal cells expanded, their number decreased with loose arrangement, and vessel distribution became dispersed while significantly increasing (Figure 6K). However, when S. obliquus was added under saline–alkali stress, these structural changes were restored (Figure 6L).
Statistical analysis of cucumber seedling root canal aperture dimensions revealed no significant differences in length or width between the S. obliquus treatment and the control under saline–alkali stress (p > 0.05). Saline–alkali stress significantly increased vessel length and width by 14.84% and 16.60%, respectively (p < 0.05), while the addition of S. obliquus significantly reduced these dimensions by 9.57% and 6.04%, respectively (p < 0.05) (Figure 8).

3.6. Effects of Scenedesmus obliquus on the Ultrastructure of Cucumber Seedlings Under Saline–Alkali Stress

3.6.1. Effects of Scenedesmus obliquus on the Ultrastructure of Mesophyll Cells in Cucumber Seedlings Under Saline–Alkali Stress

Under saline–alkali-free conditions, the cytoplasm of palisade and spongy tissue cells appeared thin, with chloroplasts (Ch) arranged regularly along the cell wall (Cw). The chloroplasts exhibited plump oval or spindle shapes, with clear chloroplast membranes (Chm) and tightly stacked thylakoid lamellae (Th) (Figure 9A,E). No significant differences were observed between the S. obliquus-treated group and the control group (Figure 9B,F). Under saline–alkali stress, both palisade and spongy tissue cells displayed morphological alterations: chloroplasts became detached from the cell wall, showed swelling with membrane dissolution, and exhibited disorganized thylakoid structures with indistinguishable grana morphology. Additionally, starch granules and osmiophilic granules increased in spongy tissue cells (Figure 9C,G). When S. obliquus was added under saline–alkali stress, these structural changes were effectively restored (Figure 9D,H).

3.6.2. Effects of Scenedesmus obliquus on the Ultrastructure of Cucumber Seedling Stems Under Saline–Alkali Stress

Under saline–alkali-free stress, cortical parenchyma cells contained few organelles, most of which were arranged adjacent to the cell wall (Figure 10A). The vessels exhibited distinct wall thickening (Figure 10E). Phloem cells displayed abundant organelles, with intact nuclear membrane structures, homogeneous nucleoplasm, and well-defined internal organization. Mitochondria showed intact structures, dense matrices, and clearly visible cristae (Figure 10I). No significant differences were observed after the addition of S. obliquus (Figure 10B,F,J). Under saline–alkali stress, cortical parenchyma cells exhibited significantly thickened cell walls and contained deformed plastids (Figure 10C); Vessels showed reduced wall thickening but increased expansion, along with elevated internal electron density, the appearance of salt crystal-like deposits, and higher cytoplasmic increased electron density in surrounding cells (Figure 10G); In phloem cells, nuclei were displaced toward the cell wall, nuclear membranes were partially dissolved, and chromatin became highly condensed. Mitochondria displayed fewer cristae and degrading outer membranes (Figure 10K). When S. obliquus was added under saline–alkali stress, these structural alterations were substantially restored (Figure 10D,H,L).

3.6.3. Effects of Scenedesmus obliquus on the Ultrastructure of Cucumber Seedling Roots Under Saline–Alkali Stress

In the absence of saline–alkali stress, cortical parenchymal cells contain only a few organelles (Figure 11A). The visible wall thickening of the catheter is observed (Figure 11E). Phloem cells display intact nuclei and mitochondria, with clear nuclear membrane structure, uniform nucleoplasm, and complete internal organization. Mitochondria exhibit abundant matrix and distinct inner cristae (Figure 11I). The addition of S. obliquus caused no significant differences (Figure 11B,F,J). Under saline–alkali stress, the cell wall of cortical parenchymal cells thickened significantly (Figure 11C), and duct wall thickening became more pronounced (Figure 11G). Phloem cells shrank, with partial nuclear disintegration, chromatin condensation, and nuclear membrane dissolution. Mitochondria showed reduced inner cristae and degraded outer membranes (Figure 11K). When S. obliquus was added under saline–alkali stress, these structural changes were restored (Figure 11D,H,L).

4. Discussion

4.1. Effects of Scenedesmus obliquus on the Growth and Development of Cucumber Seedlings Under Saline–Alkali Stress

Saline–alkali stress in the environment will affect the normal growth of plants, resulting in crop yield reduction [33]. This experiment demonstrates that saline–alkali stress causes cucumber leaves to yellow, stunts plant growth, and significantly reduces fresh weight and plant height, indicating inhibition of growth, development, and biomass accumulation. These results align with studies on tomato, rice, and other crops [34,35,36]. Cyanobacterial extracellular metabolites alleviate salt stress-induced suppression of the rice germination index [37]. Similarly, brown algal oligosaccharides mitigate salt stress damage in rice by activating signal transduction, regulating photosynthesis, promoting cell wall formation, and enhancing antioxidant pathways [38]. In this study, treatment with 0.75 g·L−1 S. obliquus restored the phenotype of cucumber seedlings under saline–alkali stress, significantly increasing fresh weight and plant height, whereas no significant changes occurred in the non-stressed group, consistent with prior research on mung bean and cucumber [39]. As a salt-tolerant microalga, S. obliquus absorbs and adsorbs salt ions (e.g., Na+ and Cl−) through intrinsic regulatory systems [40]. The mechanism may involve algal cell desalination through the binding of proteins and fatty acids to saline–alkali ions [41], further supporting microalgae’s role in enhancing plant salt tolerance.

4.2. Effects of Scenedesmus obliquus on Physiological Indices of Cucumber Seedlings Under Saline–Alkali Stress

Reactive oxygen species (ROS) maintain a dynamic balance in plants and regulate growth and development [42]. Saline–alkali stress destroys the electron transport chain, leading to ROS accumulation, inducing membrane lipid peroxidation, inhibiting antioxidant enzyme activity, and reducing ROS scavenging capacity [43]. In this study, saline–alkali stress significantly reduced the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in cucumber seedling leaves while increasing malondialdehyde (MDA) content, indicating intensified membrane lipid peroxidation. However, S. obliquus treatment significantly increased antioxidant enzyme activity and decreased MDA content in the stressed group, consistent with the mitigation effects observed in salt-stressed tomatoes [44]. Additionally, saline–alkali stress significantly inhibited chlorophyll a, chlorophyll b, and total chlorophyll synthesis, whereas S. obliquus treatment markedly increased their content. This aligns with the restorative effects of Dunaliella salina, Chlamydomonas reinhardtii, and Anabaena azotica on salt-stressed wheat [45]. The improvement of chlorophyll synthesis by S. obliquus may be attributed to the following several mechanisms: (1) the reduction of reactive oxygen species (ROS), thereby decreasing oxidative damage to chlorophyll; (2) the regulation of light intensity through colony formation, preventing chlorophyll synthesis inhibition under strong light; and (3) the release of oxygen via photosynthesis [46], alleviating local hypoxia and enhancing photosynthetic efficiency. In conclusion, S. obliquus effectively alleviates oxidative damage and photosynthetic inhibition in cucumber seedlings under saline–alkali stress by enhancing antioxidant enzyme activity, reducing ROS accumulation and MDA content, and increasing chlorophyll content.

4.3. Effects of Scenedesmus obliquus on Microstructure and Ultrastructure of Cucumber Seedling Roots and Stems Under Saline–Alkali Stress

Under salt stress, Puccinellia tenuiflora stems exhibit decreased xylem vessel diameter alongside increased vessel numbers, with concurrent thickening of vessel walls, inter-vessel walls, and enhanced mechanical strength. Under mild to moderate salt stress conditions, these adaptive changes—particularly the increased vessel density—compensate for reduced vessel diameter, thereby maintaining xylem water transport efficiency. The augmented wall thickness and mechanical strength simultaneously improve hydraulic safety, ensuring normal growth. However, under severe salt stress conditions, xylem embolism incidence rises markedly, substantially impairing water transport effectiveness and safety while diminishing self-regulatory capacity, ultimately causing rapid decline in conduction rates [47]. The present study reveals distinct responses in cucumber seedlings under saline–alkali stress, characterized by significant reduction in stem vessel diameter accompanied by decreased vessel wall thickness, yet exhibiting expanded wall thickening range and elevated electron density. These structural modifications parallel observations in cotton, where smaller-diameter vessels with thicker walls demonstrate enhanced water cohesion and negative pressure resistance, thereby maintaining efficient water and nutrient transport under stress conditions [48]. In this study, the cortical parenchyma cell walls were thickened, consistent with findings in cotton [48], while the organelle structure in phloem cells was damaged, similar to observations in tomato and rice [49,50,51]. The reduction in vessel diameter may result from the following three adaptive mechanisms: (1) plants reducing transpiration water loss through narrower vessels, (2) callose and lignin deposition enhancing mechanical strength while limiting cell ductility; and (3) improved water cohesion and negative-pressure resistance to maintain water transport [52]. The thickening of vessel walls may be due to: (1) increased mechanical strength to prevent collapse; (2) reduced water seepage loss; (3) protection against Na+ and Cl− toxicity; and (4) osmotic pressure adjustment.
In this study, saline–alkali stress caused loose distribution of root canals and significantly increased pore size, consistent with findings in Arabidopsis thaliana [53]. This adaptation may enhance resistance to salt stress by accelerating water transport and promoting organic matter accumulation [53]. The application of 0.75 g·L−1 S. obliquus (C3) under saline–alkali stress conditions resulted in significant improvements in both the microscopic and ultrastructural characteristics of cucumber stems and roots. This novel discovery indicates that S. obliquus may mitigate the inhibitory effects of saline–alkali stress on hydraulic conductance and reduce growth suppression by optimizing the vascular system architecture—specifically through restoring xylem vessel morphology and modulating secondary cell wall deposition patterns. More specifically, the restoration of xylem vessel structure likely enhances water transport efficiency [54], while the regulation of secondary wall thickening may reinforce mechanical stability and ion exclusion capacity. These coordinated modifications in vascular anatomy collectively alleviate the detrimental impacts of saline–alkali stress on plant growth and development. These results demonstrate that S. obliquus effectively mitigates saline–alkali stress-induced damage to cucumber stem and root cellular structures, representing a previously unreported protective mechanism.

4.4. Effects of Scenedesmus obliquus on Microstructure, Ultrastructure, and Physiology of Cucumber Seedling Leaves Under Saline–Alkali Stress

Saline–alkali stress affects plants through osmotic stress and ion toxicity (excessive accumulation of Na+), which inhibits photosynthesis. Since leaves are the primary organs for photosynthesis, changes in their microstructure serve as an important indicator of stress response [55]. Additionally, saline–alkali stress damages the ultrastructure of plant cells. Chloroplasts, the organelles most sensitive to salt, are particularly vulnerable; their structural integrity directly impacts photosynthetic efficiency [56]. This study found that saline–alkali stress significantly reduced the cell size of palisade and spongy tissues in cucumber leaves, consistent with observations in tobacco [57] and cotton [58]. Furthermore, chloroplasts in cucumber seedlings exhibited dissolved outer membranes, disorganized thylakoid arrangements, and blurred grana lamellae, aligning with findings in maize [59]. Saline–alkali stress also significantly decreased photosynthetic pigment content in cucumber seedling leaves, corroborating earlier research [60].
In this study, the addition of 0.75 g·L−1 S. obliquus (C3) increased leaf tissue cell size, restored mesophyll chloroplast structure, and significantly increased chlorophyll content, effectively improving cucumber seedling photosynthetic efficiency—consistent with previous findings [61].
The mechanisms by which algae alleviate saline–alkali stress may include the following: (1) Na+ adsorption by algal extracellular polysaccharides [62]; (2) Na+/Cl− binding and removal through algal cell wall functional groups [63]; (3) synthesis and secretion of growth-promoting substances like auxin, gibberellin, and cytokinin [64]. Wei J et al. found that S. obliquus cell wall functional groups effectively bind Na+ and Cl−, with both adsorption and absorption contributing to salt removal [63]. This study observed no significant differences in leaf physiological indices or structure when S. obliquus was added under non-stress conditions, suggesting its stress-alleviating mechanism primarily involves salt ion absorption/adsorption rather than direct plant growth promotion.

5. Conclusions

Under saline–alkali stress, the addition of S. obliquus significantly increased seedling fresh weight and plant height, enlarged the stem vessel diameter, and moderated tube wall thickening. It also restored the phloem organelle structure. Root vessel distribution returned to a clustered pattern, with decreased vessel diameter and wall thickening. Other root changes mirrored those in stems. The number and volume of mesophyll cells increased, chloroplast morphology recovered, and chlorophyll content rose, effectively mitigating the impact of saline–alkali stress on photosynthesis. The reduction in MDA content alleviated oxidative damage. In summary, S. obliquus effectively alleviated physiological and structural damage in cucumber seedlings under saline–alkali stress, with 0.75 g·L−1 as the optimal concentration. We propose that the mechanism involves Na+ adsorption by algal extracellular polysaccharides and Na+/Cl− binding to functional groups on algal cell walls, thereby alleviating saline–alkali stress. These results provide a cytological basis for using S. obliquus to enhance crop resilience.

Author Contributions

Z.L.: Writing—Original Draft, Validation, Formal analysis, Visualization, Software, Methodology, Investigation, Conceptualization, and Data Curation. W.L.: Methodology, Writing & review and Editing, Funding Acquisition, Resources, Supervision, and Project Administration. Y.D., X.J., Y.L. and Z.Y.: Resources, Data Curation, Visualization, Investigation, Formal analysis, Validation, Software, and Funding Acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Phenotypic effects of different concentrations of Scenedesmus obliquus on cucumber seedlings. Note: CK: Control, do not add S. obliquus; C1: Add 0.25 g·L−1 S. obliquus; C2: Add 0.50 g·L−1 S. obliquus; C3: Add 0.75 g·L−1 S. obliquus; C4: Add 1 g·L−1 S. obliquus; SA1: 60 mmol·L−1 saline–alkali stress; and SA2: 90 mmol·L−1 saline–alkali stress.
Figure 1. Phenotypic effects of different concentrations of Scenedesmus obliquus on cucumber seedlings. Note: CK: Control, do not add S. obliquus; C1: Add 0.25 g·L−1 S. obliquus; C2: Add 0.50 g·L−1 S. obliquus; C3: Add 0.75 g·L−1 S. obliquus; C4: Add 1 g·L−1 S. obliquus; SA1: 60 mmol·L−1 saline–alkali stress; and SA2: 90 mmol·L−1 saline–alkali stress.
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Figure 2. Effects of Scenedesmus obliquus on plant height (a) and fresh weight (b) of cucumber seedlings under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
Figure 2. Effects of Scenedesmus obliquus on plant height (a) and fresh weight (b) of cucumber seedlings under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
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Figure 3. Effects of Scenedesmus obliquus on SOD activity (a), POD activity (b), and CAT activity (c) in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
Figure 3. Effects of Scenedesmus obliquus on SOD activity (a), POD activity (b), and CAT activity (c) in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
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Figure 4. Effects of Scenedesmus obliquus on MDA content in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
Figure 4. Effects of Scenedesmus obliquus on MDA content in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
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Figure 5. Effects of Scenedesmus obliquus on chlorophyll a (a), chlorophyll b (b), and total chlorophyll (c) contents in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
Figure 5. Effects of Scenedesmus obliquus on chlorophyll a (a), chlorophyll b (b), and total chlorophyll (c) contents in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: 0 g·L−1 S. obliquus (CK); 0.25 g·L−1 S. obliquus (C1); 0.50 g·L−1 S. obliquus (C2); 0.75 g·L−1 S. obliquus (C3); 1 g·L−1 S. obliquus (C4).
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Figure 6. Effect of Scenedesmus obliquus supplementation on the microstructure of cucumber seedling leaves, stems, and roots under saline−alkali stress. Note: (A–D): Leaf; (E–H): Stem; (I–L): Root; (A,E,I) ((a,e,i) are amplifications of (A,E,I), respectively): Control treatment (CK); (B,F,J) ((b,f,j) are amplifications of (B,F,J), respectively): Control treatment + 0.75 g·L−1 S. obliquus (C3); (C,G,K) ((c,g,k) are amplifications of (C,G,K), respectively): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L) ((d,h,l) are amplifications of (D,H,L), respectively): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 6. Effect of Scenedesmus obliquus supplementation on the microstructure of cucumber seedling leaves, stems, and roots under saline−alkali stress. Note: (A–D): Leaf; (E–H): Stem; (I–L): Root; (A,E,I) ((a,e,i) are amplifications of (A,E,I), respectively): Control treatment (CK); (B,F,J) ((b,f,j) are amplifications of (B,F,J), respectively): Control treatment + 0.75 g·L−1 S. obliquus (C3); (C,G,K) ((c,g,k) are amplifications of (C,G,K), respectively): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L) ((d,h,l) are amplifications of (D,H,L), respectively): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Figure 7. Effect of Scenedesmus obliquus on palisade tissue cell (a) and spongy tissue cell (b) size in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: Control treatment (CK); Control treatment + 0.75 g·L−1 S. obliquus (C3); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 7. Effect of Scenedesmus obliquus on palisade tissue cell (a) and spongy tissue cell (b) size in cucumber seedling leaves under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: Control treatment (CK); Control treatment + 0.75 g·L−1 S. obliquus (C3); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Figure 8. Effect of Scenedesmus obliquus addition on stem (a) and root (b) canal diameters in cucumber seedlings under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: Control treatment (CK); Control treatment + 0.75 g·L−1 S. obliquus (C3); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 8. Effect of Scenedesmus obliquus addition on stem (a) and root (b) canal diameters in cucumber seedlings under saline−alkali stress. Data is average, bars represent mean ± standard error (n = 3). mean values followed by different letters show significant differences at p < 0.05. Note: Control treatment (CK); Control treatment + 0.75 g·L−1 S. obliquus (C3); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Figure 9. Effects of Scenedesmus obliquus on the ultrastructure of palisade and spongy tissues in cucumber seedling leaves under saline−alkali stress. Note: (a–d): Palisade tissue; (e–h): Sponge tissue; (A–D): Chloroplast inside palisade tissue; (E–H): Chloroplast inside sponge tissue; (A,E): Control treatment (CK); (B,F): Control treatment + 0.75 g·L−1 S. obliquus (C3); (C,G): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); (D,H): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 9. Effects of Scenedesmus obliquus on the ultrastructure of palisade and spongy tissues in cucumber seedling leaves under saline−alkali stress. Note: (a–d): Palisade tissue; (e–h): Sponge tissue; (A–D): Chloroplast inside palisade tissue; (E–H): Chloroplast inside sponge tissue; (A,E): Control treatment (CK); (B,F): Control treatment + 0.75 g·L−1 S. obliquus (C3); (C,G): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); (D,H): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Agronomy 15 01468 g010
Figure 10. Effects of Scenedesmus obliquus on the ultrastructure of xylem vessels in cucumber seedlings under saline−alkali stress. Note: (A–D): Cortical parenchyma cells; (E–H): Vessel; (I–L): Phloem cell; (i–l): Mitochondria in phloem cells; (A,E,I): Control treatment (CK); (B,F,J): 0.75 g·L−1 of S. obliquus (C3); (C,G,K): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 10. Effects of Scenedesmus obliquus on the ultrastructure of xylem vessels in cucumber seedlings under saline−alkali stress. Note: (A–D): Cortical parenchyma cells; (E–H): Vessel; (I–L): Phloem cell; (i–l): Mitochondria in phloem cells; (A,E,I): Control treatment (CK); (B,F,J): 0.75 g·L−1 of S. obliquus (C3); (C,G,K): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Figure 11. Effect of Scenedesmus obliquus on the ultrastructure of cucumber seedling roots under saline−alkali stress. Note: (A–D): Cortical parenchyma cells; (E–H): Vessel; (I–L): Phloem cell; (i–l): Mitochondria in phloem cells; (A,E,I): Control treatment (CK); (B,F,J): 0.75 g·L−1 of S. obliquus (C3); (C,G,K): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
Figure 11. Effect of Scenedesmus obliquus on the ultrastructure of cucumber seedling roots under saline−alkali stress. Note: (A–D): Cortical parenchyma cells; (E–H): Vessel; (I–L): Phloem cell; (i–l): Mitochondria in phloem cells; (A,E,I): Control treatment (CK); (B,F,J): 0.75 g·L−1 of S. obliquus (C3); (C,G,K): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress (SA2 CK); and (D,H,L): 90 mmol·L−1 NaCl/NaHCO3 saline−alkali stress + 0.75 g·L−1 S. obliquus (SA2 C3).
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Table 1. Treatment group treated with cucumber seeds.
Table 1. Treatment group treated with cucumber seeds.
Treatment GroupNaCl
(mmol·L−1)		NaHCO3
(mmol·L−1)		Concentration of Scenedesmus obliquus (g·L−1)
CK000
C10.25
C20.5
C30.75
C41
SA1 CK20400
SA1 C10.25
SA1 C20.5
SA1 C30.75
SA1 C41
SA2 CK30600
SA2 C10.25
SA2 C20.5
SA2 C30.75
SA2 C41
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MDPI and ACS Style

Liu, Z.; Dong, Y.; Jin, X.; Liu, Y.; Yue, Z.; Li, W. Repair Effects of Scenedesmus obliquus on Cucumber Seedlings Under Saline–Alkali Stress. Agronomy 2025, 15, 1468. https://doi.org/10.3390/agronomy15061468

AMA Style

Liu Z, Dong Y, Jin X, Liu Y, Yue Z, Li W. Repair Effects of Scenedesmus obliquus on Cucumber Seedlings Under Saline–Alkali Stress. Agronomy. 2025; 15(6):1468. https://doi.org/10.3390/agronomy15061468

Chicago/Turabian Style

Liu, Zhao, Yanlong Dong, Xiaoxia Jin, Yan Liu, Zhonghui Yue, and Wei Li. 2025. "Repair Effects of Scenedesmus obliquus on Cucumber Seedlings Under Saline–Alkali Stress" Agronomy 15, no. 6: 1468. https://doi.org/10.3390/agronomy15061468

APA Style

Liu, Z., Dong, Y., Jin, X., Liu, Y., Yue, Z., & Li, W. (2025). Repair Effects of Scenedesmus obliquus on Cucumber Seedlings Under Saline–Alkali Stress. Agronomy, 15(6), 1468. https://doi.org/10.3390/agronomy15061468

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