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Article

Enhancing Drought Tolerance in Barley through the Application of Watermelon Rind Hydrogels: A Novel Approach to Sustainable Agriculture

by
Bingqin Teng
1,2,
Jun Wu
1,2,*,
Yuan Zhong
2,
Liqun Cai
1,2,
Peng Qi
1,2 and
Zhuzhu Luo
1,2
1
College of Resources and Environment, Gansu Agricultural University, Lanzhou 730070, China
2
State Key Laboratory of Aridland Crop Science, Gansu Agricultural University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(10), 2329; https://doi.org/10.3390/agronomy14102329
Submission received: 14 September 2024 / Revised: 7 October 2024 / Accepted: 7 October 2024 / Published: 10 October 2024
(This article belongs to the Section Farming Sustainability)
Browse Figures
Versions Notes

Abstract

:
Drought stress critically hinders agricultural productivity, especially in arid and semi-arid zones. The results of this study show that the application of watermelon rind-based hydrogels (WR hydrogels), synthesized from a watermelon rind, acrylic acid (AA), and acrylamide (AAm), significantly enhanced soil water retention by 77.46% at a 0.3% concentration, increasing the plant height by 28.98% and biomass by 35.28% under controlled greenhouse conditions (25 °C/20 °C day/night temperature cycle, with a 12 h photoperiod at 400–500 μmol·m−2·s−1 of illuminance and 30–40% relative humidity). The accumulation of proline and soluble sugars decreased, indicating reduced osmotic stress in barley subjected to mild and severe drought conditions (6–15% FC and 17–26% FC). The chlorophyll content rose by 16.36%, boosting photosynthetic activity. A correlation and principal component analysis further highlighted the positive effects of hydrogel addition on plant growth and drought resistance. These findings underscore the potential of WR hydrogels as an effective soil amendment for promoting sustainable agriculture in water-limited conditions.

1. Introduction

The global scenario regarding drought stress reveals its profound impact on agriculture, particularly affecting vast areas dedicated to crop cultivation. An estimated 65 million hectares of wheat production were impacted by drought stress in 2013 alone [1], highlighting the extensive geographical scale of this issue. With the looming threat of global warming and climate change, the frequency of drought conditions is expected to escalate, exacerbating losses in agricultural productivity. In China, a drought is also a recurrent and severe challenge for agriculture. According to data from the Ministry of Water Resources, droughts frequently strike various regions across the country, impacting millions of hectares annually. For instance, during critical periods, drought-prone provinces like Hebei, Shanxi, Shandong, Henan, Shaanxi, and Gansu—key grain-producing areas—often experience water shortages that significantly affect crop yields. The economic losses from a drought can be substantial, reaching billions of yuan per year [2]. In the prevailing scenario of global climate change, drought stress has emerged as one of the most pressing challenges in agricultural production, severely impacting crop growth and yields, particularly among critical food crops such as barley [3,4,5]. A drought not only directly constrains water uptake, stunting plant growth, but also indirectly affects physiological processes, such as photosynthesis and nutrient absorption, leading to a reduced biomass and yields [6,7,8]. As a globally significant cereal crop, barley displays a pronounced sensitivity to droughts, with growth cycles being shortened and grain yields and quality being markedly decreased under drought stress, posing a significant threat to global food security [9,10]. Investigating the mechanisms of crop responses to droughts and developing drought-resilient strategies can provide a scientific foundation for crop improvement and drought management, addressing the challenges posed by climate change.
Drought stress can severely alter a plant’s water status, leading to stomatal closure as a defense mechanism to prevent excessive water loss [11]. This physiological response, however, also restricts CO2 intake, thereby impacting photosynthesis and plant growth [12]. Biochemically, a drought triggers the accumulation of reactive oxygen species (ROS), which can lead to oxidative stress and damage cellular components [13]. To combat this, plants activate antioxidant defense systems to scavenge ROS and maintain a redox balance [14]. At the molecular level, a drought activates a complex signaling network involving various genes and transcription factors. This includes the upregulation of genes involved in osmoprotectant synthesis, which helps in maintaining cell turgor and stability under water-deficit conditions [15].
Barley (Hordeum vulgare L.) is a significant crop globally and particularly in China, playing a pivotal role in both nutrition and the economy. Globally, barley is the fourth most widely cultivated grain, after wheat, rice, and maize, with a total production of around 147 million tons in 2020 [16]. It is a staple food in many regions and a crucial ingredient in the brewing industry. In terms of nutrition, barley is rich in dietary fiber, particularly β-glucans, which have been linked to reduced cholesterol levels and improved blood sugar control [17]. In China, barley cultivation is mainly distributed in the Yangtze River basin, the Yellow River basin and the Qinghai–Tibet Plateau. The total area planted with barley in China in 2014 was approximately 0.5 million hectares, with a production of around 2 million tons [18]. Barley is used in various traditional Chinese dishes and is also an essential raw material for the production of Chinese liquor and beer. Economically, the barley industry contributes significantly to the agricultural sector in China, providing income for farmers and employment in the processing and manufacturing industries. The Chinese government has been promoting the development of the barley industry as part of its strategy to enhance food security and rural economic development [19].
To combat drought stress, innovative approaches have been developed, including traditional plant breeding and, more recently, transgenic methods [20]. Plant breeding has been used to select and cultivate crop varieties that are more resilient to water scarcity [20]. Transgenic approaches, on the other hand, involve the introduction of genes that confer drought resistance, allowing plants to survive and thrive under conditions of reduced water availability [20]. Recent research underscores hydrogel technology’s promise for mitigating drought effects on crops, thanks to their superior water retention [21,22,23,24]. Specifically, lignin and starch-based hydrogels have shown to boost soil moisture conditions, alleviate drought stress on crops such as maize, improve biomass and nutrient uptake, and minimize stress indicators [22]. Polyaspartic acid (PASP) hydrogels have notably enhanced survival rates and water content in Xanthoceras sorbifolia seedlings, improving the fluorescence parameters of chlorophyll [25]. In citrus, PASP hydrogels reduced leaf drop and improved physiological responses [26]. Superabsorbent hydrogels (SAHs) from starch and polyacrylic acid facilitated sunflower growth under drought conditions via enhanced water retention [27]. These studies reveal hydrogel technology’s potential for improving water use efficiency, drought resistance, and yield stabilization [28,29,30,31]. Incorporation into sandy substrates notably boosts critical growth metrics and enhances plant physiology [32]. Beyond water retention, hydrogels promote nutrient availability, aiding seed germination and supporting the overall plant growth [33]. Moreover, they positively affect soil microbial communities, vital for nutrient cycling and soil health, by reducing water and nutrient losses [34]. However, further exploration is required to evaluate hydrogel effectiveness across different crops, soil types, and field conditions for its comprehensive application in drought agriculture management, contributing to global food security and sustainable agriculture.
Against this backdrop, this study aims to explore the potential of watermelon rind-based hydrogels (WR hydrogels) as a novel soil amendment for alleviating the impacts of a drought on the growth and physiology of barley. A watermelon rind, a byproduct of the fruit industry, rich in cellulose and pectin, serves as an ideal precursor for hydrogel synthesis, offering superior biodegradability compared to synthetic hydrogels. Hydrogels were synthesized in the presence of watermelon rinds by a copolymerizing acrylic acid (AA) and acrylamide (AAm), resulting in (AA-co-AAm)/WR hydrogels with favorable water retention properties. This study evaluates the effects of WR hydrogels on the seed germination, seedling growth, and physiological responses of barley under simulated drought stress conditions in a greenhouse-controlled environment. Hydrogels, at different concentrations, were applied to sandy soil, and the germination and growth patterns were monitored under mild and severe drought stress. Additionally, physiological parameters such as the chlorophyll content, proline, and soluble sugars were assessed to understand plant responses to hydrogel applications. The findings of this study hold significant implications for sustainable agriculture, particularly in arid regions. By enhancing soil water retention and improving plant growth and physiological responses, WR hydrogels can contribute to the development of drought-resistant cultivation systems, supporting food security and sustainable land management practices.

2. Materials and Methods

2.1. Experimental Materials

The barley seeds used were of the Ganbeer 8 variety (Gansu Provincial Academy of Agricultural Sciences, Lanzhou, Gansu, China). The sandy soil was collected from a farmland under a mountain in the eastern part of Jinsha Village, Lanzhou City, Gansu Province, with a field water-holding capacity of 29.18%. The soil texture is sandy, and it has a pH of 8.16, an organic-matter content of 0.74 g/kg, and a bulk density of 1.628 g/cm3.

2.2. Synthesis of WR Hydrogels

A novel hydrogel was synthesized from a watermelon rind (WR), rich in cellulose and pectin, using acrylic acid (AA) and acrylamide (AAm) as monomers, ammonium persulfate (APS) as the initiator, and N,N’-Methylenebisacrylamide (MBA) for crosslinking. The (AA-co-AAm)/WR hydrogel was formed, showcasing a robust three-dimensional network structure, as depicted in Figure 1 [35]. (The chemicals were obtained from Shanghai Macklin Biochemical Science and Technology Joint Stock Company, Shanghai, China, and were of analytical grade purity.)
The synthesis involved creating a 5 wt% (total weight is 576 g) of WR slurry, which was heated (70 °C) without oxygen and treated with 0.2 wt% of APS, followed by the sequential addition of 3 wt% of AAm, 0.025 wt% of MBA, and 7 wt% of AA (neutralized with 40% NaOH). The total mixture was adjusted to 576 g by ultrapure water, heated to induce gelation, then washed with ethanol and ultrapure water, and dried at 40 °C.
The hydrogel’s biocompatibility, attributed to the hydrophilic properties of polyacrylic acid (PAA), was confirmed. Despite the potential toxicity of acrylamide (AAm), its polymerized form in the hydrogel significantly mitigates this concern.
The (AA-co-AAm)/WR hydrogel demonstrated an equilibrium swelling capacity of 749 g/g. It also has a high service life with a similar water absorption rate as other hydrogels [35]. This high absorbency is a testament to the hydrogel’s hydrophilic nature and its capacity to retain significant volumes of water, making it a versatile material for agricultural use (Figure 2).

2.3. Measurement of Indicators Related to Seed Germination

Initially, forty disinfected barley seeds were uniformly distributed on petri dishes containing 80 g of sand. The sand was then subjected to WR hydrogel treatments at concentrations of 0%, 0.1%, 0.2%, and 0.3%, aiming to elucidate the role of this polymer in the effects of water scarcity. The experiment incorporated two distinct water regimes to simulate severe stress and a mild stress, respectively. The severe drought stress treatment was calibrated to maintain soil moisture at a range of 20% to 30% of the field capacity, while the mild stress ensured soil moisture levels between 40% and 52% of the field capacity. Each treatment was replicated three times. The germination criterion was defined as the emergence of a shoot length equal to half the length of the seed. The seeds were cultivated under greenhouse conditions meticulously controlled with a diurnal temperature cycle of 25 °C during the day and 20 °C at night. The relative humidity was maintained within the optimal range of 30% to 40%, while the photoperiod was set at 12 h of light and 12 h of dark, with an illuminance level between 400 and 500 μmol·m−2·s−1. Details of specific experimental treatments are shown in Table 1.
The germination potential (GP) was assessed on the third day post-germination, reflecting the initial vigor and germination capacity of the seeds. Seven days after the germination initiation, the germination rate (GR) was determined, offering insights into the overall germination efficiency. To further analyze the physiological performance and stress tolerance of the barley seeds, the germination index (GI), seed vigor index (SVI), and average germination time (AGT) were calculated. These metrics collectively provided a comprehensive understanding of the germination dynamics and seed health under the imposed environmental conditions.
The specific formula is shown below:
Germination rate (GR) = (G7/TS) × 100%
Germination potential (GP) = (G3/TS) × 100%
Germination index (GI) = Σ (Gi/Di)
Seed Viability Index (SVI) = Shoot length × GI
Average germination time (AGT) = Σ Gi × Di/Ni
In the aforementioned equation, G7 and G3 are utilized to represent the cumulative germination counts achieved by the 7th and 3rd days of the germination experiment, respectively. TS is the total seed count subjected to the germination assay. The variable Di signifies the ordinal day of the experiment (where i is a positive integer), Gi denotes the cumulative germination number by the end of the ith day, and Ni specifically quantifies the number of seeds that have germinated on the exact ith day, contributing to the daily increment in germination.

2.4. Measurement of Barley Seedling-Related Indicators

Seeds were surface-sterilized and then placed on a moist filter paper under aseptic conditions to germinate at 23 °C with 40% relative humidity until a radicle length equal to the seed length (for 3 days). Subsequently, the seedlings were transplanted into pots containing sandy soil as five individuals per pot for biological replications. Each treatment was replicated three times. The sandy soil in each pot was mixed with a water gel at concentrations of 0%, 0.1%, 0.2%, and 0.3% and was subjected to two levels of drought stress: severe stress with field water-holding capacity ranging from 6% to 15% and mild stress with field water-holding capacity ranging from 17% to 26%. The experimental conditions of the pots regarding temperature, light, and other factors were identical to those described in Section 2.3. Details of specific experimental treatments are shown in Table 1.
To assess growth parameters and physiological responses, samples were collected 14 days post-treatment. The measured parameters included plant height, root length, the fresh and dry weights of both the shoot and root, leaf saturated fresh weight, chlorophyll content, proline, and soluble sugar content. The leaf saturated fresh weight was determined by immersing the leaves in distilled water under dark conditions for 24 h, followed by gentle blotting to remove excess water from the leaf surface prior to weighing (W). The drying process involved an initial heat treatment at 105 °C for 30 min to cease all metabolic activity, followed by oven drying at 80 °C until a constant weight was achieved. The chlorophyll concentration was quantified using a spectrophotometer (A360, AoYi Instruments Shanghai Co., Ltd., Shanghai, China), with absorbance readings at wavelengths of 665 nm, 649 nm, and 470 nm, and the concentrations of chlorophyll a, chlorophyll b, and carotenoids were calculated using established formulae [36,37]. The proline content was assessed following the method described by Bates et al. [38]. The soluble sugar content was estimated using the phenol-sulfuric acid method, as outlined by Kochert et al. [39]. Finally, the total biomass, root-to-shoot ratio, leaf relative water content, and leaf water saturation deficit were calculated [40]. The specific formulae used for these calculations are as follows:
Chlorophyll a (Ca) = 13.95 × A665 − 6.88 × A649;
Chlorophyll b (Cb) = 24.96 × A649 − 7.32 × A665
Carotenoids (Cx) = (1000 × A470 − 2.05 × Ca − 114.8 × Cb)/245
Relative water content (RWC) = (W1 − W3)/(W2 − W3) × 100%
Water saturation deficit (WSD) = (W1 − W3)/(W2 − W3) = 1 − RWC
Total biomass (TB) = W3 + W4
Root–shoot ratio (R/S) = W4/W3
In the aforementioned equation, W1 denotes the leaf fresh weight, W2 denotes the leaf saturated fresh weight, and W3 denotes the leaf dry weight, and W4 denotes the root dry weight.

2.5. Data Analysis

Data were meticulously organized in Microsoft Excel 2019, then subjected to statistical significance testing in SPSS 23.0 (a single-factor analysis was used). A principal component analysis was executed in CANOCO, elucidating the core structure of the data. Correlation analyses in Origin 9.0 further illuminated variable relationships, while SIMCA 14.1’s OPLS-DA analysis discriminated effectively between groups. A cluster analysis was performed using R (R version 4.3.3).

3. Results

3.1. Germination-Related Indicators

Severe drought stress markedly reduced the germination rate (GR), germination potential (GP), germination index (GI), seed vigor index (SVI), and shoot length (SL) of the barley seedlings by 3.47%, 73.78%, 50.89%, 78.17%, and 53.54%, respectively (p ≤ 0.05), relative to mild stress, extending the average germination time (AGT) by 49.14% (Table 2). WR hydrogels at a concentration of 0.3% under severe stress conditions significantly diminished GR, GP, and GI by 17.65%, 80.72%, and 45.55%, increasing AGT by 20.25% (p ≤ 0.05). Conversely, the same treatment substantially augmented SL by 140.77% (p ≤ 0.05). In the mild stress scenario, hydrogel addition similarly affected these parameters, reducing GR, GP, and GI by 18.52%, 18.81%, and 31.24% while extending AGT and enhancing SL by 14.98% and 48.89%, respectively (p ≤ 0.05). Since SVI integrates the shoot length and germination index, hydrogel addition had no significant effect on SVI in this study.

3.2. Overview Layout of Experimental Design

As depicted in Figure 3a, the addition of hydrogels to the soil resulted in the absorption of soil moisture by the hydrogels. During the initial germination phase, the barley seeds, primarily in contact with the soil rather than the hydrogels, exhibited higher germination rates in the control condition (0% hydrogels added). This is because seeds rely directly on soil moisture, which is less accessible when hydrogels are present, as they act as moisture reservoirs. Figure 3b reveals that once the barley plants developed root systems, the dynamics changed. The roots intertwined with the hydrogels, at which point the hydrogel-amended soil began to provide a continuous supply of moisture to the growing plants. In contrast, the control soil, devoid of hydrogels, experienced moisture loss over time due to evaporation, leading to a significantly inferior growth condition for the barley plants. Further examinations were conducted, and Figure 4 illustrates the intricate details of root–hydrogel interactions at various hydrogel application rates.

3.3. Moisture Content of Sandy Soil

The incorporation of WR hydrogels significantly enhanced the soil water retention capacity, as exemplified in Figure 5. Under severe drought stress conditions, the addition of 0.3% of WR hydrogels to sandy soil boosted the soil water content by 77.46%, in comparison to the soil without hydrogel addition. Similarly, under mild drought stress conditions, a 0.3% hydrogel inclusion resulted in a 72.55% increase in the soil water content relative to the untreated soil. Notably, under severe stress conditions, the soil water content with the addition of hydrogels at concentrations of 0.2% and 0.3% surpassed that of the soil under mild stress conditions without hydrogels by 2.07% and 4.37%, respectively. This highlights the efficacy of WR hydrogels in improving soil moisture retention, particularly in environments subject to water stress.

3.4. Growth Parameters

The incorporation of WR hydrogels (Figure 6a) robustly elevated the total biomass of the barley: a 0.3% addition boosted the biomass by 44.79% under severe drought stress and 25.77% under mild drought stress, relative to untreated soils. The total biomass levels at a 0.2% and 0.3% concentration of WR hydrogels were indistinguishable under both stress conditions.
Severe drought conditions elevated the barley’s root–shoot ratio by 12.42% over mild drought conditions (Figure 6b). A total concentration of 0.3% of WR hydrogels reduced this ratio by 13.09% under severe stress and 11.45% under mild stress, versus untreated controls.
Severe drought stress saw a 28.98% increase in the sprout length of the barley with 0.3% of WR hydrogels (Figure 6c), with no significant effect observed under mild stress. At both 0.2% and 0.3% hydrogel addition levels, there was no significant difference in the sprout length under severe and mild drought conditions.
The root length of the barley under severe drought conditions was 42.11% shorter than under mild drought conditions (Figure 6d). The root length under severe drought stress with 0.3% of the hydrogel was 19.32% shorter, and under mild drought stress, it was 45.02% shorter, compared to no hydrogel.

3.5. Leaf Blade Moisture

The incorporation of 0.3% of the WR hydrogel significantly augmented the leaf fresh and dry weights under varying drought stresses (Figure 7a,b). Specifically, under severe drought conditions, the leaf fresh weight enhanced by 133.08% and the leaf dry weight by 26.63%, in comparison to the control without hydrogels. Under mild drought conditions, these improvements stood at 29.91% for the fresh weight and 9.53% for the dry weight, respectively.
Moreover, the application of 0.3% of the WR hydrogel markedly boosted the leaf relative water content (RWC) and decreased the water saturation deficit (WSD) under different levels of drought stress (Figure 7c,d). Under severe drought conditions, RWC increased by 66.83%, and WSD decreased by 60.88%, while under mild drought stress, RWC rose by 12.21%, and WSD dropped by 40.72%.
At a hydrogel addition rate of 0.3%, there were no significant differences in the leaf moisture metrics under various drought stress conditions (Figure 7a–d).

3.6. Chlorophyll and Carotenoid Analysis

The addition of WR hydrogels did not significantly affect the chlorophyll a content; however, a 0.3% hydrogel supplementation significantly elevated chlorophyll b levels by 60.46% under severe drought stress and by 12.74% under mild drought stress (Figure 8a,b). Similarly, the total chlorophyll content was significantly boosted by 22.92% under severe drought stress and by 9.80% under mild drought stress with a 0.3% hydrogel addition level (Figure 8c).
Conversely, the incorporation of 0.3% of the WR hydrogel significantly reduced the carotenoid content by 62.87% under severe drought stress and by 39.61% under mild drought stress (Figure 8d).

3.7. Proline and Soluble Sugars

Under severe drought stress, the contents of proline and soluble sugars in the barley leaves escalated notably by 52.73% and 114.86%, respectively, compared to those under mild drought stress (Figure 9a,b). The addition of 0.3% of the WR hydrogel significantly attenuated the proline content in the barley leaves under severe drought stress by 54.24% and under mild drought stress by 19.97%. Similarly, the incorporation of 0.3% of the WR hydrogel markedly reduced the soluble sugar content in the barley leaves under severe drought stress by 42.19% and under mild drought stress by 22.39%. Interestingly, under severe and mild drought stress conditions, no significant difference in the proline content was observed when the WR hydrogel was applied at a concentration of 0.3%.

3.8. Correlation Analysis

Through a Pearson correlation analysis (Figure 10), we examined the relationship between environmental factors and barley growth and biochemical traits. 0 visualizes these associations using a color-coded heatmap, where red denotes positive correlations and blue negative correlations, with the color intensity reflecting the correlation coefficient magnitude. Significant correlations are marked with * (p ≤ 0.05), ** (p ≤ 0.01), or *** (p ≤ 0.001).
The addition of WR hydrogels was found to have a strong positive correlation with the soil moisture content (r = 0.57 **), total biomass (r = 0.88 ***), leaf fresh weight (r = 0.72 ***), leaf dry weight (r = 0.72 ***), relative water content (r = 0.70 ***), and sprout length (r = 0.54 **), while showing a negative correlation with the leaf water saturation deficit (r = −0.77 ***), root length (r = −0.58 **), carotenoids (r = −0.75 ***), and proline (r = −0.63 ***). This suggests that WR hydrogels enhance water retention and promote growth under drought conditions, alleviate drought stress, and reduce the accumulation of stress-related metabolites. Notably, a significant negative correlation (r = −0.51 *) with the root–crown ratio was observed, indicating that WR hydrogels may favor shoot development over root growth under stress conditions.
Drought stress, on the other hand, showed a positive correlation with the root–crown ratio (r = 0.74 ***), proline (r = 0.60 **), soluble sugars (r = 0.86 ***), and leaf water saturation deficit (r = 0.44 *), as well as with carotenoids (r = 0.48 *). It was negatively correlated with the soil moisture content (r = −0.77 ***), leaf fresh weight (r = −0.53 **), root length (r = −0.62 **), chlorophyll b (r = −0.83 ***), and total chlorophyll (r = −0.79 ***) and with the leaf relative water content (r = −0.42 *), sprout length (r = −0.48 *), and chlorophyll a (r = −0.47 *). These findings suggest that drought stress triggers adaptive responses to conserve water and maintain the osmotic balance, while negatively impacting plant growth and photosynthetic efficiency.

3.9. Principal Component Analysis and Cluster Analysis

Table 3 demonstrates that the first two principal components encapsulate 91.208% of the total variance, effectively summarizing the core information from the original 14 barley growth and physicochemical indicators. Designated Y1 and Y2, these components offer a reliable and comprehensive evaluation of barley’s growth conditions, ensuring a succinct yet accurate representation of the data.
Figure 11a’s PCA results reveal insightful correlations: the red vectors denote the barley’s growth and physicochemical traits, while the blue vectors signify the environmental influences, including hydrogel addition, drought stress, and soil moisture. Acute angles between vectors indicate positive correlations and obtuse angles negative correlations, with smaller angles and longer vectors denoting stronger relationships. Notably, soil moisture is positively correlated with hydrogel addition and negatively with drought stress. The total biomass, leaf dry and fresh weights, relative water content, sprout length, and chlorophyll content positively correlate with hydrogel addition and soil moisture and negatively with drought stress, with soil moisture showing the strongest association. Conversely, factors like soluble sugars, the root–shoot ratio, proline, carotenoids, and the leaf water saturation deficit exhibited opposite correlations (Figure 11a). Notably, root length is distinct, showing a negative correlation with both hydrogel addition and drought stress level, indicating its independent response mechanism to environmental changes.
The distinct separation observed in the loadings plot was corroborated by a cluster analysis, whose results are presented in Figure 11b, which categorized the variables into two distinct groups, further validating the observed patterns in the PCA results. Complementing the PCA, the cluster analysis, whose results are shown in Figure 11b, distinctly categorized treatments into two, S-0 and S-0.1, versus S-0.2, S-0.3, M-0, M-0.1, M-0.2, and M-0.3, confirming differential responses to water stress and hydrogel applications. The clustering suggests higher drought stress in the S-0 and S-0.1 treatments and lower drought stress in others. This highlights hydrogel’s potential as a stress mitigation tool in agriculture, offering insights into optimizing practices under water scarcity.
Table 4 presents the principal component scores, derived from the principal component equation, alongside the composite score, calculated by weighting each component’s contribution to the total variance of the two principal components. The ranking of treatments based on these scores was as follows: M-0.3, M-0.2, M-0.1, S-0.3, M-0, S-0.2, S-0.1, and S-0. A general observation indicated that treatments subjected to lower drought stress and higher concentrations of added WR hydrogels were ranked higher, reflecting a positive impact on barley growth under these conditions. Notably, S-0.3′s ranking surpassed that of M-0, signifying that under severe drought stress, the addition of 0.3% of the hydrogel to the soil significantly enhanced the barley’s growth to a degree that surpassed its growth under mild stress without hydrogel supplementation. This finding underscores the efficacy of hydrogel applications in mitigating the adverse effects of droughts on barley, providing a strategic approach to agricultural management under water-limited conditions.
Through the application of an OPLS-DA analysis (Figure 12), it was revealed that the treatments S-0.3, S-0.2, S-0.1, and S-0 are dispersed across the plot, indicative of a significant impact of hydrogel addition on barley growth under severe drought stress. In contrast, the treatments M-0.3, M-0.2, M-0.1, and M-0 exhibit a more concentrated distribution, suggesting a lesser effect of hydrogels on barley growth under mild drought stress. Notably, the treatments M-0.3 and M-0.2 are in close proximity, highlighting that under mild drought conditions, the addition of 0.2% and 0.3% of hydrogels has an exceedingly minor influence on barley growth.

4. Discussion

In this study, we successfully demonstrated the potential of watermelon rind hydrogels (WR hydrogels) to enhance growth and development patterns under drought stress conditions. The synthesis of (AA-co-AAm)/WR hydrogels, utilizing agricultural organic waste rich in cellulose and pectin from watermelon rinds, exhibited favorable effects on soil water retention and plant growth. Under both mild and severe drought stress conditions, the hydrogels significantly increased the soil moisture content, indicating their water absorption and retention capabilities, which effectively mitigate the adverse impacts of water scarcity on agricultural productivity.

4.1. WR Hydrogels Increase Barley Shoot Length but Decrease Seed Germination Rate

The WR hydrogels influenced the early development of the barley under water-deficit conditions. While they positively impacted the barley shoot length, they negatively affected seed germination parameters such as the germination rate, potential, and index. As depicted in Figure 3a, the hydrogels absorbed water from the sandy soil, reducing moisture availability for the barley seeds, which rely on surrounding soil moisture for germination. Consequently, the addition of hydrogels to the soil impaired seed germination. However, as the barley seedlings developed roots, they extended and entangled with the hydrogels (Figure 3b). In the absence of hydrogels, the soil moisture rapidly evaporated, whereas the hydrogels retained water, continuously supplying it to the barley roots, ultimately resulting in longer shoots and better growth and development. Therefore, it is not recommended to subject seeds to drought stress during the germination phase when soil is amended with hydrogels, as this may adversely affect seed germination. It is recommended to ensure water irrigation as much as possible during the germination stage to ensure a normal seed germination rate.

4.2. WR Hydrogels Improve Growth Indicators of Barley Seedlings

During the seedling growth and development phase, the barley roots entangled with the hydrogels, which, after absorbing moisture from the sandy soil, prevented rapid evaporation, providing a sustained water supply to the barley (Figure 3b and Figure 4). This nuanced observation underscores the dual impact of hydrogel integration in soil: initially, it may slightly inhibit seed germination due to altered moisture dynamics, but subsequently, it enhances plant growth by serving as a reliable moisture source once the root system is established. The hydrogels’ impact on plant growth and physiological responses, such as an increased total biomass, increased leaf fresh and dry weights, and an increased leaf relative water content, highlights their role in improving the plant water status and overall productivity. Similar findings were reported by Mazloom and Arbona et al. [22,26], who demonstrated that hydrogel addition effectively enhances crop growth and the leaf water content and promotes biomass accumulation. The root-to-shoot ratio is a crucial parameter reflecting the effects of drought stress on plants, with stress increasing the ratio [41]. In our study, the addition of hydrogels reduced the root-to-shoot ratio, decreasing the barley’s stress-related parameters. Notably, hydrogel addition led to a reduction in root length, while a decrease in drought stress severity resulted in an increased root length. The principal component analysis also revealed that the root length was distinct from other factors, suggesting an independent response mechanism to environmental changes. This observation is attributed to the hydrogels’ influence on barley root growth and development, as roots tend to grow and spread towards the hydrogels rather than elongate. Kazemi et al. [41] reported that drought stress results in shorter root lengths, consistent with our findings. Figure 3b and Figure 4 illustrate that the roots without hydrogel addition grew rapidly downward, likely due to the faster evaporation of moisture from the upper soil layers, prompting the roots to grow and extend downward in search of water.

4.3. WR Hydrogels Increase Chlorophyll Content and Reduce Accumulation of Stress Metabolites in Barley Seedlings

The WR hydrogels also contributed to an increased chlorophyll content in the barley seedlings, while reducing the accumulation of stress-related metabolites. The decrease in proline and the soluble sugar content indicates that the hydrogels aided in maintaining the osmotic balance and reducing the accumulation of stress-related metabolites, thereby enhancing the plant’s tolerance to drought stress. Lukić, Chen, and Jin et al. [42,43,44] similarly reported that crops subjected to drought stress exhibit an increased accumulation of stress-related metabolites. The observed decline in the soluble sugar content may be a secondary effect of significant water loss in the leaves, leading to a concentration effect of soluble sugars within the leaf tissues. This accumulation could be a physiological response to cope with the osmotic stress induced by drought stress. This discrepancy may be attributed to differences in the mechanisms of action between salicylic acid and hydrogels, warranting further investigation. The chlorophyll and carotenoid analyses revealed that although the hydrogels did not significantly affect the chlorophyll a content, they significantly increased the chlorophyll b content, thereby enhancing the total chlorophyll content. This increase can improve photosynthetic efficiency under stress conditions. The findings on the chlorophyll content corroborate those of Wei et al. [25], showing that hydrogel addition boosts chlorophyll levels, potentially enhancing photosynthetic efficiency under stress. Carotenoids, key pigments in plants, are known to exhibit increased levels under drought stress, potentially serving as a defense mechanism against oxidative damage. Carotenoids, which are known to rise under drought conditions as a defense against oxidative damage, were also observed to increase, as supported by Islam et al. [45]. This increase in carotenoids might play a critical role in scavenging reactive oxygen species (ROS), maintaining cellular integrity and aiding plant survival under drought conditions. Future research should aim to elucidate the mechanisms by which hydrogels reduce the accumulation of stress metabolites and increase the chlorophyll and carotenoid content. Investigating the interaction between hydrogels and other drought-stress-management strategies could provide a comprehensive approach to enhancing drought tolerance in crops. Additionally, understanding the specific roles of carotenoids in ROS scavenging and their contribution to cellular stability could open new avenues for developing drought-resistant plant varieties.

4.4. Correlation Analysis, Cluster Analysis, and Principal Component Analysis (PCA)

Through a correlation analysis, cluster analysis, and principal component analysis (PCA), we comprehensively examined the relationships between hydrogel addition, drought stress, and various growth patterns and physiological traits. Hydrogel addition was strongly and positively correlated with soil moisture content, total biomass, and leaf weights, while being negatively correlated with stress-related metabolites, highlighting the hydrogels’ role in promoting plant growth and alleviating drought stress. Under drought stress, plants accumulate proline and other stress-related metabolites as a protective mechanism to maintain the cellular osmotic balance. The reduction in proline content with hydrogel applications indicates that plants do not experience severe osmotic stress. This reduction can be attributed to the hydrogels’ ability to maintain a more stable soil moisture level, which in turn reduces the need for plants to produce stress metabolites for osmotic adjustments. The increase in the chlorophyll content, particularly chlorophyll b, in response to hydrogel addition under drought conditions was significant. Chlorophyll b was more efficient in absorbing blue and red light, broadening the spectrum of light that can be used for photosynthesis. This increase in chlorophyll content enhanced the photosynthetic efficiency, which is crucial for plant growth under water-stressed conditions. Moreover, a higher chlorophyll content can also be indicative of reduced photo-oxidative stress, as chlorophylls can help in dissipating excess light energy. Carotenoids, in addition to their role as pigments, are potent antioxidants that protect plant cells from oxidative stress caused by droughts. The increase in the carotenoid content under water-deficient conditions can be seen as a protective mechanism against the overproduction of reactive oxygen species (ROS) that can damage cellular components. By scavenging ROS and protecting the photosynthetic apparatus, carotenoids contribute to maintaining cellular integrity and stability under drought conditions. The observed correlations between hydrogel addition, soil moisture, and physiological traits such as the total biomass and leaf weights underscore the holistic impact of hydrogels on plant growth. These correlations highlight the integrated response of plants to hydrogel applications, where improved soil moisture management leads to better plant health and growth performance.
The ranking of the PCA composite scores indicated that treatments subjected to lower drought stress and higher concentrations of added WR hydrogels were ranked higher, reflecting a positive impact on barley growth and development under these conditions. Notably, the S-0.3 treatment surpassed the M-0 treatment, demonstrating that the addition of 0.3% of hydrogels to soil significantly enhances barley growth and development under severe drought stress, surpassing its growth and development under mild stress without hydrogel supplementation. This finding underscores the efficacy of hydrogel applications in mitigating the adverse effects of droughts on barley, providing a strategic approach to agricultural management under water-limited conditions. The OPLS-DA analysis also revealed that hydrogel addition had a more significant impact on barley growth and development under severe drought stress compared to mild drought stress, further confirming the hydrogels’ efficacy under severe drought conditions.
The observed positive effects of hydrogel addition on plant growth under drought conditions corroborate and extend findings from prior studies [46,47], which have documented the ameliorative role of hydrogels in maintaining soil moisture and enhancing plant performance. Our results, however, uniquely highlight the physiological mechanisms underlying these effects, specifically, a reduction in stress metabolites and an enhancement of the chlorophyll and carotenoid content. This provides new insights into the nuanced ways hydrogels can protect plants from osmotic stress. Future research should explore the long-term viability of hydrogel applications across different crop species and environmental conditions, as well as its potential integration with other water-conservation strategies.

5. Conclusions

In summary, the deployment of WR hydrogels under drought conditions substantially boosted the soil water retention by 77.46%, plant height by 28.98%, and biomass by 35.28%. These hydrogels effectively alleviated osmotic stress, as evidenced by a reduced accumulation of proline and soluble sugars, and significantly enhanced the chlorophyll content by 16.36%, thus stimulating photosynthetic activity. Despite negatively impacting seed germination, the WR hydrogels proved to be an effective soil amendment, enhancing crop resilience and productivity in water-limited settings. This underscores the potential of our technology in transforming agricultural sustainability across diverse climatic zones. These findings are pivotal for the advancement of sustainable agricultural practices and food security initiatives, particularly in drought-prone regions. Future research should aim to optimize application rates to minimize adverse effects on seed germination, thereby maximizing the benefits of WR hydrogels in agricultural applications.

Author Contributions

B.T.: investigation, data curation, writing—original draft, and methodology. J.W.: project administration and funding acquisition. Y.Z.: conceptualization, data curation, and formal analysis. L.C.: conceptualization and resources. P.Q.: software and investigation. Z.L.: software and investigation. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge financial support from the Gansu Agricultural University’s Public Recruitment Doctoral Research Start-up Fund (no. GAU-KYQD-2018-39) and from the Natural Science Foundation of Gansu Province (no. 20JR10RA543 and 21JR7RA811).

Data Availability Statement

No data were used for the research described in this article.

Acknowledgments

I would like to express my deepest gratitude to my father, Zhishun Teng, for providing unwavering confidence during the most challenging times of my research. Additionally, I extend my sincere thanks to Jun Wu for his invaluable assistance and support throughout my research. Lastly, I am deeply appreciative of Jiachen Zhu’s encouragement as my research partner.

Conflicts of Interest

The authors of this article hereby declare that they have no affiliations or involvement with any organization or entity that has a financial interest (such as honoraria, educational grants, participation in speakers’ bureaus, employment, consultancies, stock ownership, or other equity interests, expert testimony, or patent-licensing arrangements) or non-financial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject matter or materials discussed in this manuscript.

References

  1. Lobell, D.B.; Schlenker, W.; Costa-Roberts, J. Climate trends and global crop production since 1980. Science 2011, 333, 616–620. [Google Scholar] [CrossRef] [PubMed]
  2. Xu, Y.; Zhu, X.; Cheng, X.; Gun, Z.; Lin, J.; Zhao, J.; Yao, L.; Zhou, C. Drought assessment of China in 2002–2017 based on a comprehensive drought index. Agric. For. Meteorol. 2022, 319, 108922. [Google Scholar] [CrossRef]
  3. Sallam, A.; Alqudah, A.M.; Dawood, M.F.; Baenziger, P.S.; Börner, A. Drought stress tolerance in wheat and barley: Advances in physiology, breeding and genetics research. Int. J. Mol. Sci. 2019, 20, 3137. [Google Scholar] [CrossRef] [PubMed]
  4. Samarah, N.; Alqudah, A.; Amayreh, J.; McAndrews, G. The effect of late-terminal drought stress on yield components of four barley cultivars. J. Agron. Crop Sci. 2009, 195, 427–441. [Google Scholar] [CrossRef]
  5. Guo, P.; Baum, M.; Grando, S.; Ceccarelli, S.; Bai, G.; Li, R.; Von Korff, M.; Varshney, R.K.; Graner, A.; Valkoun, J. Differentially expressed genes between drought-tolerant and drought-sensitive barley genotypes in response to drought stress during the reproductive stage. J. Exp. Bot. 2009, 60, 3531–3544. [Google Scholar] [CrossRef]
  6. Pinheiro, C.; Chaves, M.M. Photosynthesis and drought: Can we make metabolic connections from available data? J. Exp. Bot. 2011, 62, 869–882. [Google Scholar] [CrossRef]
  7. Reddy, A.R.; Chaitanya, K.V.; Vivekanandan, M. Drought-induced responses of photosynthesis and antioxidant metabolism in higher plants. J. Plant Physiol. 2004, 161, 1189–1202. [Google Scholar] [CrossRef]
  8. Zargar, S.M.; Gupta, N.; Nazir, M.; Mahajan, R.; Malik, F.A.; Sofi, N.R.; Shikari, A.B.; Salgotra, R. Impact of drought on photosynthesis: Molecular perspective. Plant Gene 2017, 11, 154–159. [Google Scholar] [CrossRef]
  9. Elakhdar, A.; Solanki, S.; Kubo, T.; Abed, A.; Elakhdar, I.; Khedr, R.; Hamwieh, A.; Capo-chichi, L.J.; Abdelsattar, M.; Franckowiak, J.D.; et al. Barley with improved drought tolerance: Challenges and perspectives. Environ. Exp. Bot. 2022, 201, 104965. [Google Scholar] [CrossRef]
  10. Cai, K.; Chen, X.; Han, Z.; Wu, X.; Zhang, S.; Li, Q.; Nazir, M.M.; Zhang, G.; Zeng, F. Screening of worldwide barley collection for drought tolerance: The assessment of various physiological measures as the selection criteria. Front. Plant Sci. 2020, 11, 1159. [Google Scholar] [CrossRef]
  11. Zhou, S.; Duursma, R.A.; Medlyn, B.E.; Kelly, J.W.; Prentice, I.C. How should we model plant responses to drought? An analysis of stomatal and non-stomatal responses to water stress. Agric. For. Meteorol. 2013, 182, 204–214. [Google Scholar] [CrossRef]
  12. Brito, C.; Dinis, L.-T.; Moutinho-Pereira, J.; Correia, C.M. Drought stress effects and olive tree acclimation under a changing climate. Plants 2019, 8, 232. [Google Scholar] [CrossRef] [PubMed]
  13. Apel, K.; Hirt, H. Reactive oxygen species: Metabolism, oxidative stress, and signal transduction. Annu. Rev. Plant Biol. 2004, 55, 373–399. [Google Scholar] [CrossRef] [PubMed]
  14. Waszczak, C.; Carmody, M.; Kangasjärvi, J. Reactive oxygen species in plant signaling. Annu. Rev. Plant Biol. 2018, 69, 209–236. [Google Scholar] [CrossRef]
  15. Shinozaki, K.; Yamaguchi-Shinozaki, K. Gene networks involved in drought stress response and tolerance. J. Exp. Bot. 2007, 58, 221–227. [Google Scholar] [CrossRef] [PubMed]
  16. Soare, E.; Bold, N.; Stoicea, P.; David, L.; Dobre, C.-A.; Firățoiu, A.-R. Survey on the worldwide barley production and trade. Bulg. J. Agric. Sci. 2023, 29, 119. [Google Scholar]
  17. Slavin, J. Fiber and prebiotics: Mechanisms and health benefits. Nutrients 2013, 5, 1417–1435. [Google Scholar] [CrossRef]
  18. Yearbook, C.S. China Statistical Yearbook; Zhongguo Tongji Chubanshe, Various Years: Beijing, China, 2014. [Google Scholar]
  19. Zhou, M. Barley production and consumption. In Genetics and Improvement of Barley Malt Quality; Springer: Berlin/Heidelberg, Germany, 2009; pp. 1–17. [Google Scholar]
  20. Krishna, R.; Ansari, W.A.; Soumia, P.; Yadav, A.; Jaiswal, D.K.; Kumar, S.; Singh, A.K.; Singh, M.; Verma, J.P. Biotechnological interventions in tomato (Solanum lycopersicum) for drought stress tolerance: Achievements and future prospects. BioTech 2022, 11, 48. [Google Scholar] [CrossRef]
  21. Saha, A.; Sekharan, S.; Manna, U. Superabsorbent hydrogel (SAH) as a soil amendment for drought management: A review. Soil Tillage Res. 2020, 204, 104736. [Google Scholar] [CrossRef]
  22. Mazloom, N.; Khorassani, R.; Zohury, G.H.; Emami, H.; Whalen, J. Lignin-based hydrogel alleviates drought stress in maize. Environ. Exp. Bot. 2020, 175, 104055. [Google Scholar] [CrossRef]
  23. Tomášková, I.; Svatoš, M.; Macků, J.; Vanická, H.; Resnerová, K.; Čepl, J.; Holuša, J.; Hosseini, S.M.; Dohrenbusch, A. Effect of different soil treatments with hydrogel on the performance of drought-sensitive and tolerant tree species in a semi-arid region. Forests 2020, 11, 211. [Google Scholar] [CrossRef]
  24. Abd El-Aziz, G.H.; Ibrahim, A.S.; Fahmy, A.H. Using environmentally friendly hydrogels to alleviate the negative impact of drought on plant. Open J. Appl. Sci. 2022, 12, 111–133. [Google Scholar] [CrossRef]
  25. Wei, J.; Yang, H.; Cao, H.; Tan, T. Using polyaspartic acid hydro-gel as water retaining agent and its effect on plants under drought stress. Saudi J. Biol. Sci. 2016, 23, 654–659. [Google Scholar] [CrossRef] [PubMed]
  26. Arbona, V.; Iglesias, D.J.; Jacas, J.; Primo-Millo, E.; Talon, M.; Gómez-Cadenas, A. Hydrogel substrate amendment alleviates drought effects on young citrus plants. Plant Soil 2005, 270, 73–82. [Google Scholar] [CrossRef]
  27. Ghobashy, M.M.; Amin, M.A.; Nady, N.; Meganid, A.S.; Alkhursani, S.A.; Alshangiti, D.M.; Madani, M.; Al-Gahtany, S.A.; Zaher, A.A. Improving impact of poly (starch/acrylic acid) superabsorbent hydrogel on growth and biochemical traits of sunflower under drought stress. J. Polym. Environ. 2022, 30, 1973–1983. [Google Scholar] [CrossRef]
  28. El Bergui, O.; Abouabdillah, A.; Bourioug, M.; Schmitz, D.; Biel, M.; Aboudrare, A.; Krauss, M.; Jomaa, A.; Romuli, S.; Mueller, J.; et al. Innovative solutions for drought: Evaluating hydrogel application on onion cultivation (Allium cepa) in Morocco. Water 2023, 15, 1972. [Google Scholar] [CrossRef]
  29. Ghobashy, M.M.; Amin, M.A.; Ismail, M.A.; Nowwar, A.I.; El-Diehy, M.A.; Gayed, H. Radiation cross-linked ultra-absorbent hydrogel to rationalize irrigation water and fertilizer for maize planting in drought conditions. Int. J. Biol. Macromol. 2023, 252, 126467. [Google Scholar] [CrossRef]
  30. Besharati, J.; Shirmardi, M.; Meftahizadeh, H.; Ardakani, M.D.; Ghorbanpour, M. Changes in growth and quality performance of Roselle (Hibiscus sabdariffa L.) in response to soil amendments with hydrogel and compost under drought stress. S. Afr. J. Bot. 2022, 145, 334–347. [Google Scholar] [CrossRef]
  31. Ratke, R.F.; De Sousa, A.; Chaves, D.V.; Zanatta, F.L.; Edvan, R.L.; Sousa, H.R.; Silva-Filho, E.C.; Osajima, J.A.; Nascimento, A.M.S.S.; Aguilera, J.G.; et al. Cashew gum hydrogel as an alternative to minimize the effect of drought stress on soybean. Sci. Rep. 2024, 14, 2159. [Google Scholar] [CrossRef]
  32. Patra, S.K.; Poddar, R.; Brestic, M.; Acharjee, P.U.; Bhattacharya, P.; Sengupta, S.; Pal, P.; Bam, N.; Biswas, B.; Barek, V.; et al. Prospects of hydrogels in agriculture for enhancing crop and water productivity under water deficit condition. Int. J. Polym. Sci. 2022, 2022, 4914836. [Google Scholar] [CrossRef]
  33. Sayed, H.E.; Kirkwood, R.; Graham, N. The effects of a hydrogel polymer on the growth of certain horticultural crops under saline conditions. J. Exp. Bot. 1991, 42, 891–899. [Google Scholar] [CrossRef]
  34. Kaur, P.; Agrawal, R.; Pfeffer, F.M.; Williams, R.; Bohidar, H.B. Hydrogels in agriculture: Prospects and challenges. J. Polym. Environ. 2023, 31, 3701–3718. [Google Scholar] [CrossRef]
  35. Teng, B.; Zhong, Y.; Wu, J.; Zhu, J.; Cai, L.; Qi, P.; Luo, Z. Transforming Watermelon (Citrullus lanatus) Rind into Durable Superabsorbent Hydrogels for Enhanced Soil Water Retention properties and adsorbs dye in water. Heliyon 2024, 10, e38656. [Google Scholar] [CrossRef]
  36. Gr, M. Absorption of light by chlorophyll solutions. J. Biol. Chem. 1941, 140, 315–322. [Google Scholar]
  37. Arnon, D.I. Copper enzymes in isolated chloroplasts. Polyphenoloxidase in Beta vulgaris. Plant Physiol. 1949, 24, 1. [Google Scholar] [CrossRef]
  38. Bates, L.S.; Waldren, R.; Teare, I. Rapid determination of free proline for water-stress studies. Plant Soil 1973, 39, 205–207. [Google Scholar] [CrossRef]
  39. Kochert, G. Carbohydrate determination by the phenol-sulfuric acid method. Chemistry 1978, 95. [Google Scholar]
  40. Čatský, J. Water saturation deficit and photosynthetic rate as related to leaf age in the wilting plant. In Water Stress in Plants: Proceedings of a Symposium Held in Prague, September 30–October 4 1963; Springer: Berlin/Heidelberg, Germany, 1965; pp. 203–209. [Google Scholar]
  41. Kazemi Oskuei, B.; Bandehagh, A.; Farajzadeh, D.; Asgari Lajayer, B.; Rajput, V.; Astatkie, T. Morphological, biochemical, and physiological responses of canola cultivars to drought stress. Int. J. Environ. Sci. 2023, 20, 13551–13560. [Google Scholar] [CrossRef]
  42. Lukić, N.; Kukavica, B.; Davidović-Plavšić, B.; Hasanagić, D.; Walter, J. Plant stress memory is linked to high levels of anti-oxidative enzymes over several weeks. Environ. Exp. Bot. 2020, 178, 104166. [Google Scholar] [CrossRef]
  43. Chen, Z.; Xu, W.; Nie, J.; Khan, A.; Cao, C.; Li, P. Drought stress intensity, duration and its resistance impact on rice (Oryza sativa L.) seedling. Appl. Ecol. Environ. Res. 2020, 18, 469–486. [Google Scholar] [CrossRef]
  44. Jin, E.J.; Yoon, J.-H.; Lee, H.; Bae, E.J.; Yong, S.H.; Choi, M.S. Evaluation of drought stress level in Sargent’s cherry (Prunus sargentii Rehder) using photosynthesis and chlorophyll fluorescence parameters and proline content analysis. PeerJ 2023, 11, e15954. [Google Scholar] [CrossRef] [PubMed]
  45. Islam, M.Z.; Park, B.J.; Jeong, S.Y.; Kang, S.W.; Shin, B.K.; Lee, Y.T. Assessment of biochemical compounds and antioxidant enzyme activity in barley and wheatgrass under water-deficit condition. J. Sci. Food Agric. 2022, 102, 1995–2002. [Google Scholar] [CrossRef] [PubMed]
  46. Oladosu, Y.; Rafii, M.Y.; Arolu, F.; Chukwu, S.C.; Salisu, M.A.; Fagbohun, I.K.; Muftaudeen, T.K.; Swaray, S.; Haliru, B.S. Superabsorbent polymer hydrogels for sustainable agriculture: A review. Horticulturae 2022, 8, 605. [Google Scholar] [CrossRef]
  47. Abdelghafar, R.; Abdelfattah, A.; Mostafa, H. Effect of super absorbent hydrogel on hydro-physical properties of soil under deficit irrigation. Sci. Rep. 2024, 14, 7655. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Roadmap for the synthesis of (AA-co-AAm)/WR hydrogels. AAm denotes acrylamide; AA denotes acrylic acid; MBA denotes N, N′-methylene bisacrylamide; APS denotes ammonium persulfate.
Figure 1. Roadmap for the synthesis of (AA-co-AAm)/WR hydrogels. AAm denotes acrylamide; AA denotes acrylic acid; MBA denotes N, N′-methylene bisacrylamide; APS denotes ammonium persulfate.
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Figure 2. Before and after water absorption of (AA-co-AAm)/WR hydrogels.
Figure 2. Before and after water absorption of (AA-co-AAm)/WR hydrogels.
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Figure 3. Schematic representation of the early germination phase of barley (a) and the subsequent growth phase of barley (b).
Figure 3. Schematic representation of the early germination phase of barley (a) and the subsequent growth phase of barley (b).
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Figure 4. Photographs of hydrogels attached to barley root systems at different concentrations of added hydrogels.
Figure 4. Photographs of hydrogels attached to barley root systems at different concentrations of added hydrogels.
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Figure 5. Effect of WR hydrogels on soil moisture content. Note: Capital letters indicate the significance of the different gradients of added hydrogels, and lowercase letters indicate the significance between different drought stresses (p < 0.05); this convention applies consistently throughout subsequent analyses.
Figure 5. Effect of WR hydrogels on soil moisture content. Note: Capital letters indicate the significance of the different gradients of added hydrogels, and lowercase letters indicate the significance between different drought stresses (p < 0.05); this convention applies consistently throughout subsequent analyses.
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Figure 6. Effect of WR hydrogel addition on barley growth parameters, (a) Total biomass, (b) Root-shoot ratio, (c) Sprout length, (d) Root length.
Figure 6. Effect of WR hydrogel addition on barley growth parameters, (a) Total biomass, (b) Root-shoot ratio, (c) Sprout length, (d) Root length.
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Figure 7. Effect of WR hydrogel addition on leaf blade moisture, (a) Leaf fresh weight, (b) Leaf dry weight, (c) Relative water content, (d) Water saturation deficit.
Figure 7. Effect of WR hydrogel addition on leaf blade moisture, (a) Leaf fresh weight, (b) Leaf dry weight, (c) Relative water content, (d) Water saturation deficit.
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Figure 8. WR hydrogel addition affects chlorophyll and carotenoids, (a) Chlorophyll a, (b) Chlorophyll b, (c) Total chlorophyll, (d) Carotenoids.
Figure 8. WR hydrogel addition affects chlorophyll and carotenoids, (a) Chlorophyll a, (b) Chlorophyll b, (c) Total chlorophyll, (d) Carotenoids.
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Figure 9. WR hydrogel addition affects proline and soluble sugars, (a) Proline, (b) Soluble sugars.
Figure 9. WR hydrogel addition affects proline and soluble sugars, (a) Proline, (b) Soluble sugars.
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Figure 10. Correlation analysis heatmap.
Figure 10. Correlation analysis heatmap.
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Figure 11. Results of a principal component analysis (a) and cluster analysis (b). Note: In this figure, HydrAddt denotes hydrogel addition, SoiMoiCn denotes soil moisture content, DrouStrs denotes drought stress, TotlBiom denotes total biomass, RootRati denotes root–shoot ratio, LeaFrsWe denotes leaf fresh weight, LeaDryWe denotes leaf dry weight, RelWatCh denotes relative water content, WatSatDf denotes water saturation deficit, SproLeng denotes sprout length, RootLeng denotes root length, ChlorA denotes chlorophyll a, ChlorB denotes chlorophyll b, TotlChlr denotes total chlorophyll, Caroteno denotes carotenoids, and SolbSugr denotes soluble sugars.
Figure 11. Results of a principal component analysis (a) and cluster analysis (b). Note: In this figure, HydrAddt denotes hydrogel addition, SoiMoiCn denotes soil moisture content, DrouStrs denotes drought stress, TotlBiom denotes total biomass, RootRati denotes root–shoot ratio, LeaFrsWe denotes leaf fresh weight, LeaDryWe denotes leaf dry weight, RelWatCh denotes relative water content, WatSatDf denotes water saturation deficit, SproLeng denotes sprout length, RootLeng denotes root length, ChlorA denotes chlorophyll a, ChlorB denotes chlorophyll b, TotlChlr denotes total chlorophyll, Caroteno denotes carotenoids, and SolbSugr denotes soluble sugars.
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Figure 12. Results of OPLS-DA analysis.
Figure 12. Results of OPLS-DA analysis.
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Table 1. Seed germination and barley seedling experimental treatments.
Table 1. Seed germination and barley seedling experimental treatments.
TreatmentWR HydrogelDegree of DroughtWater Addition (Seed Germination)Water Addition (Barley Seedling)
S-00%Severe stressWatered to 20–30% FCWatered to 6–15% FC
S-0.10.1%
S-0.20.2%
S-0.30.3%
M-00%Mild stressWatered to 40–52% FCWatered to 17–26% FC
M-0.10.1%
M-0.20.2%
M-0.30.3%
Note: In this table, S-0, S-0.1, S-0.2, and S-0.3 represent treatments with a 0%, 0.1%, 0.2%, and 0.3% addition of WR hydrogels under severe drought stress, respectively. M-0, M-0.1, M-0.2, and M-0.3 represent treatments with a 0%, 0.1%, 0.2%, and 0.3% addition of WR hydrogels under mild drought stress, respectively.
Table 2. Comparative analysis of the germination rate, germination potential, germination index, seed vigor index, and average germination time of wheat seeds under various treatment conditions.
Table 2. Comparative analysis of the germination rate, germination potential, germination index, seed vigor index, and average germination time of wheat seeds under various treatment conditions.
Degree of DroughtTreatmentGR/%GP/%GISVIAGTShoot Length/cm
Severe stressS-085.00 ± 10.00 a47.50 ± 2.50 a25.38 ± 2.38 a32.91 ± 2.88 a3.21 ± 0.04 b1.30 ± 0.10 c
S-0.188.33 ± 1.44 a14.16 ± 3.81 b17.78 ± 0.27 b31.96 ± 4.84 a3.83 ± 0.04 a1.80 ± 0.30 bc
S-0.280.83 ± 3.81 ab14.16 ± 1.44 b16.43 ± 0.84 bc39.13 ± 8.68 a3.82 ± 0.01 a2.36 ± 0.40 ab
S-0.370.00 ± 7.50 b9.16 ± 1.44 c13.82 ± 1.36 c43.68 ± 12.78 a3.86 ± 0.02 a3.13 ± 0.76 a
Mild stressM-090.00 ± 2.50 a84.16 ± 1.44 b44.08 ± 2.46 a158.64 ± 7.84 a2.27 ± 0.15 b3.60 ± 0.10 c
M-0.190.83 ± 1.44 a90.83 ± 1.44 a39.79 ± 4.78 ab175.49 ± 10.75 a2.48 ± 0.18 ab4.43 ± 0.30 b
M-0.281.66 ± 1.44 ab80.83 ± 1.44 b35.31 ± 0.40 bc180.05 ± 8.81 a2.51 ± 0.07 ab5.10 ± 0.26 a
M-0.373.33 ± 8.77 b68.33 ± 3.81 c30.31 ± 4.86 c162.46 ± 24.44 a2.61 ± 0.21 a5.36 ± 0.15 a
Note: In this table, GR denotes germination rate, GP denotes germination potential, GI denotes germination index, SVI denotes seed vigor index, and AGT denotes average germination time. Different lowercase letters indicate significant differences (p ≤ 0.05) among various concentrations of added hydrogels.
Table 3. Total variance explained by principal component results.
Table 3. Total variance explained by principal component results.
ComponentEigenvalueVariance %Cumulative Variance %
111.00378.59278.592
21.76612.61591.208
30.9016.43797.645
40.1401.00398.648
50.1020.72999.377
60.0740.52699.903
70.0140.097100.000
Table 4. Principal component scores and composite scores.
Table 4. Principal component scores and composite scores.
TreatmentY1 ScoreY2 ScoreComprehensive
Score		Rankings
S-0−6.326 0.047 −4.966 8
S-0.1−3.494 0.093 −2.734 7
S-0.20.231 −2.135 −0.088 6
S-0.31.631 −1.597 1.080 4
M-0−0.122 2.004 0.157 5
M-0.12.292 1.036 1.932 3
M-0.22.710 0.308 2.169 2
M-0.33.078 0.245 2.450 1
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Teng, B.; Wu, J.; Zhong, Y.; Cai, L.; Qi, P.; Luo, Z. Enhancing Drought Tolerance in Barley through the Application of Watermelon Rind Hydrogels: A Novel Approach to Sustainable Agriculture. Agronomy 2024, 14, 2329. https://doi.org/10.3390/agronomy14102329

AMA Style

Teng B, Wu J, Zhong Y, Cai L, Qi P, Luo Z. Enhancing Drought Tolerance in Barley through the Application of Watermelon Rind Hydrogels: A Novel Approach to Sustainable Agriculture. Agronomy. 2024; 14(10):2329. https://doi.org/10.3390/agronomy14102329

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Teng, Bingqin, Jun Wu, Yuan Zhong, Liqun Cai, Peng Qi, and Zhuzhu Luo. 2024. "Enhancing Drought Tolerance in Barley through the Application of Watermelon Rind Hydrogels: A Novel Approach to Sustainable Agriculture" Agronomy 14, no. 10: 2329. https://doi.org/10.3390/agronomy14102329

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