Introduction
Drought stress is one of the environmental factors that most severely affects agricultural productivity worldwide. As the frequency and intensity of droughts increase due to climate change, understanding and enhancing drought resistance of vegetable crops has become even more important. Most vegetable crops are primarily composed of water, making them particularly vulnerable to drought stress that affects their growth and development, and water deficits during critical growth stages result in significant reductions in yield and quality. Vegetables are an essential food resource for humans, playing a key role in providing nutrients, vitamins, dietary fiber, antioxidants, and other beneficial phytochemicals. However, despite the increase in global production, the supply remains insufficient.
Drought stress induces osmotic and oxidative stress in crops, leading to damage in cellular organelles caused by reactive oxygen species (ROS). In response to drought stress, vegetable crops engage complex mechanisms, including the hormonal regulation of key gene expression/signaling pathways, the activation of antioxidant enzymes, and protein synthesis and degradation processes.
Climate change and drought stress: impacts on vegetable crops
Climate conditions significantly influence the yield and quality of vegetable crops. Recently, abnormal weather patterns due to global warming have become more frequent, and global warming due to the increase in atmospheric carbon dioxide is expected to further alter precipitation patterns and distribution. Typically, drought stress results from insufficient rainfall, but it can also be triggered by prolonged high temperatures, intense sunlight, and soil drying caused by dry winds.
Developing countries are facing severe food security crises, partly due to declining annual rainfall, which leads to reduced agricultural productivity. Impacts are also seen in temperate zones such as the UK, where changes in rainfall patterns due to climate change are also expected to increase the frequency and intensity of droughts. Additionally, traditional agricultural regions are moving because of more frequent droughts and water shortages. In Korea, climate simulation studies predict significant changes in rainfall from September to November between 2030 and 2050, which will impact Chinese cabbage production.
Drought stress refers to the physiological and biochemical responses of plants when they experience water deficits. It occurs mostly from reduced rainfall that leads to insufficient soil moisture, severely affecting crop growth, development, productivity, and quality. For example, with Chinese cabbage (Brassica rapa subsp. pekinensis), insufficient water supply during the early heading stage after sowing, leads to retarded leaf development and significantly reduced growth rates. Leaves also become smaller in size and number, and tend to yellow, wilt or become necrotic. Similarly, tomato (Solanum lycopersicum L.) requires adequate water supply, especially during early growth and fruit formation. In addition, water availability is essential to the growth of pepper (Capsicum annuum), with higher water requirement during flowering and fruit-set. The duration, intensity, and frequency of drought stress also affect the growth and productivity of crops such as cabbage (Brassica oleracea var. capitata), that under prolonged drought stress causes decreased leaf size, resulting in reduced photosynthesis and a significant reduction in final yield.
Climatic conditions affect the yield of various vegetables, with insufficient water supply inhibiting the growth and development of plant organs such as leaves, stems and roots, negatively impacting overall plant growth. Thus, to maintain agricultural sustainability under abnormal weather patterns including drought caused by climate change, it is imperative to further research drought resistance in plants; including a deeper understanding molecular/physiological mechanisms in response to drought stress, to the development of optimized agricultural practice and breeding of drought tolerant crops.
Physiological response mechanisms
Vegetable crops exhibit physiological changes in response to environmental stimuli, such as drought stress, which significantly impact crop quality and yield. Plants lose more water through transpiration from their leaves than they can take in through their roots when soil moisture levels drop as a result of less rainfall, heat, sunlight, or dry winds. In response, plants expand their root systems to absorb more water and minimize water loss by closing stomata on their leaves. Understanding the mechanisms behind root expansion, including increasing root length and growing deeper into the soil is critical for developing agricultural strategies to enhance crop productivity. By optimizing water absorption and minimizing water loss, plants can better adapt to drought stress.
Under drought stress, plants commonly exhibit leaf changes, including reduced growth, curling, yellowing, tip burn, and permanent wilting. Stomatal closure is a defense mechanism that plants use to minimize water loss in drought conditions, but it can also reduce photosynthetic efficiency. Research on tomatoes (Solanum lycopersicum L.) demonstrated that plants close their stomata to conserve water, but this also lowers internal carbon dioxide levels, leading to reduced photosynthesis under drought stress. As photosynthesis declines, energy and carbohydrate production needed for growth decrease, ultimately inhibiting overall plant development(Figure 1).
In addition, plants maintain cellular ion concentration and osmotic balance through a process called osmoregulation, enhancing their physiological adaptability and increasing the chances of survival under stress. Thus, osmoregulation serves as an important defense mechanism; however, its effectiveness decreases under prolonged stress conditions, as observed in Welsh onion (Allium fistulosum), where extended drought stress (28 days) led to a decrease in osmolyte accumulation and an inability to restore physiological homeostasis, ultimately reducing plant survival and productivity. Drought stress-induced physiological changes in plants directly affect agricultural productivity, making it increasingly difficult to cultivate vegetable crops in water-scarce regions.
Biochemical response mechanisms
4.1 Antioxidant defense system
Drought stress leads to an overproduction of ROS in plant cells, causing oxidative stress that significantly affects physiological functions. ROS, such as hydrogen peroxide (H2O2), superoxide ion (O2-), hydroxyl radical (OH-), and singlet oxygen can cause detrimental effects on essential cellular components like membranes, proteins, and nucleic acids. To counteract this, plants activate their biochemical defense mechanisms through antioxidant enzymes. Superoxide dismutase (SOD) converts superoxide ions into H2O2 and oxygen (O2), which are then further broken down into H2O by other antioxidant enzymes. Peroxidase (POD) is another key antioxidant enzyme whose activity increases in response to various abiotic stresses, playing a critical role in scavenging ROS and protecting cells. POD also contributes to lignin biosynthesis, which helps maintain the cell wall and reinforces plant structure. Ascorbate peroxidase (APX) also plays a significant role in removing ROS; APX uses ascorbic acid as a substrate to reduce hydrogen peroxide to water, generating dehydroascorbic acid (DHA). DHA can be recycled back into ascorbic acid, allowing continuous antioxidant action.
Together with SOD, POD, and APX, catalase (CAT), another crucial antioxidant enzyme, converts hydrogen peroxide into oxygen and water, rapidly lowering elevated ROS levels. These enzymes are particularly active in cell organelles like peroxisomes, and mitochondria, where ROS are predominantly generated, minimizing cell damage. In Chinese cabbage (Brassica rapa subsp. pekinensis), activation of SOD was shown to suppress ROS accumulation, thereby reducing cell damage and improving drought tolerance and productivity. Similarly, in tomato (Solanum lycopersicum), drought and heat stress significantly increased ROS levels, which were counteracted by the activation of SOD, POD, APX, and CAT. These antioxidant enzymes played a crucial role in maintaining ROS homeostasis and preventing oxidative damage, particularly in drought-tolerant varieties. Likewise, in soybean (Glycine max), drought-tolerant cultivars exhibit enhanced antioxidant enzyme activities, effectively mitigating ROS-induced oxidative stress. Under split-root drought conditions, soybean plants significantly increased the activities of SOD, CAT, APX, and POD, which correlated with reduced H2O2 and MDA levels. Notably, drought-tolerant cultivars exhibited stronger antioxidant responses, maintaining higher chlorophyll content and photosynthetic efficiency compared to susceptible cultivars.
Through the activation of these antioxidant enzymes, plants enhance their biochemical defense systems to mitigate environmental stresses and help maintain internal homeostasis (Figure 2).
4.2 Accumulation of osmolytes
Plants accumulate osmolytes as a biochemical mechanism to defend themselves against various stresses. Osmolytes are small organic compounds that help plants adapt to various abiotic stresses. These osmolytes include proline, glycine betaine, polyamines, glycerol, and mannitol, which play key roles in osmotic regulation, cell stabilization, and protein protection, allowing plant cells to maintain their function even under stress conditions. Proline accumulation occurs in response to a variety of stress conditions, such as drought, salinity, cold, and heavy metals, and has been reported to correlate with drought tolerance in several plant species. Additionally, it scavenges ROS, mitigating the harmful effects of abiotic stresses. In lettuce (Lactuca sativa), drought stress resulted in a significant accumulation of proline, with levels increasing approximately 660-fold on day 8 compared to control conditions. This dramatic increase suggests that proline plays a crucial role in osmotic adjustment and adaptation to drought stress by reducing oxidative damage and stabilizing cellular structures. In eggplant (Solanum melongena) and its wild relatives, proline accumulation increased significantly under drought stress; tolerant genotypes such as S. incanum and S. pyracanthos showed levels more than 79- and 95-fold higher, respectively than control plants, whereas susceptible genotypes showed only an 8-fold increase. In drought-tolerant soybean (Glycine max) varieties, the accumulation of proline was significantly higher under drought stress in a split-root system compared to that in susceptible varieties. This increased concentration of proline is associated with enhanced water retention and osmotic regulation, both of which contribute to improved drought tolerance. However, recent findings indicate that the relationship between proline accumulation and drought tolerance may not be universally positive, as it can vary depending on the plant species and developmental stage. For example, in pepper (Capsicum spp.), bell pepper varieties such as Green Wonder exhibited higher proline levels under drought stress, but showed increased drought susceptibility rather than improved tolerance, suggesting a more complex interaction between osmotic regulation and stress adaptation. Glycine betaine, another important osmolyte, accumulates primarily in chloroplasts in response to drought stress and helps maintain photosynthetic efficiency through osmotic regulation. Its primary functions include stabilizing proteins, regulating osmotic pressure, and scavenging ROS, thereby protecting plant cells and contributing to stress tolerance. Despite the diverse biochemical/physiological effects of osmolytes on plants, further research is required for a deeper understanding of their impact on stress responses. Given their potential to mitigate the negative effects of ROS, osmolytes play an essential role in reducing agricultural losses caused by various environmental stresses (Figure 2).
Molecular biological response mechanisms
5.1 Hormonal regulation
Plants respond to various environmental stresses through hormonal regulation, controlling specific growth patterns and physiological changes. In drought stress environments, hormones such as abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) play complex roles in regulating stress responses. ABA is a major hormone involved in abiotic stress response, influencing plant growth, germination, aging, and stress tolerance. ABA regulates drought stress responses by inducing stomatal closure through its signaling pathway and activating the expression of drought response genes. ABA binds to multiple ABA receptors (PYR/PYL/RCARs), which inhibit the activity of PP2C, leading to the activation of SnRK2 and affecting downstream signaling pathways; the ABA receptors regulate the ABA signaling pathway by binding to ABA and inhibiting PP2C, thereby facilitating SnRK2 activation under stress conditions(Figure 3). On the other hand, gibberellin (GA), which regulates growth processes such as seed germination and flowering is suppressed under drought conditions. The deceleration of GA biosynthesis retards plant growth, thereby bolstering their chances of survival. JA is primarily known for its role in defense against biotic stresses such as pests and pathogens, but it also plays a crucial role in abiotic stresses including drought, salinity, and heat. JA accumulates during drought stress and promotes stomatal closure to reduce water loss. It also activates antioxidant responses and regulates ion balance to minimize cell damage. SA contributes to drought tolerance by modulating ROS levels and regulating the expression of genes involved in stomatal regulation, often working in coordination with other hormones. ET, while primarily involved in fruit ripening and leaf aging also plays a crucial role in drought stress by balancing growth promotion and stress defense through stomatal regulation, aging acceleration, and stress-responsive gene expression. These hormonal regulations are vital for plants to enhance their survival under extreme environmental conditions.
5.2 Drought stress response genes
Although plant hormones typically regulate similar physiological processes, each hormone functions through distinct transcription factors (TFs) or gene networks, ensuring non-redundant activity. Under drought stress, many genes are regulated by ABA, highlighting its importance in the stress responses. Drought stress responses are generally divided into ABA-dependent and ABA-independent pathways (Figure 3). The TFs, AREB/ABFs (ABRE-binding protein/ABRE-binding factors) function within the ABA-dependent pathway by binding to the ABRE (ABA-responsive element) cis-acting element, while DREBs (DRE-/CRT-binding proteins) operate within the ABA-independent pathway by binding to the DRE/CRT (Dehydration-responsive element/C-repeat) cis-acting element present in the promoters of stress-responsive genes. AREB1, AREB2, and ABF3 play critical roles in activating the expression of genes in the ABA-dependent pathway by binding to ABRE. When all of these TFs are functional, plants exhibit significantly enhanced drought tolerance. AREB/ABFs induce the expression of downstream genes such as RD22 (Responsive to Desiccation 22) and RD29B (Responsive to Desiccation 29B), which are crucial for stress adaption by protecting cells and preserving intracellular water. Some LEA (Late Embryogenesis Abundant) genes contain ABRE sequences, which are bound by AREB/ABF transcription factors and increase the production of LEA proteins in response to drought stress. LEA proteins stabilize cell structures and protect critical molecules during cellular dehydration. Through the ABA-independent pathway, plants can react to drought stress without ABA signaling thanks to available DREB (Dehydration-Responsive Element-Binding) TFs that are a member of the plant-specific AP2/ERF (APETALA2/ethylene-responsive factor) family.
Specifically, DREB2A (Dehydration-Responsive Element-Binding protein 2A) controls the expression of genes that respond to drought stress, enhancing the capacity of plants to retain water. During the early phases of leaf turgor reduction, the NCED3 (9-cis-Epoxycarotenoid Dioxygenase) gene, which is crucial for ABA biosynthesis, rapidly increases its expression, raising ABA levels under drought stress. The P5CS (Pyrroline-5-Carboxylate Synthetase) gene is involved in proline biosynthesis, contributing to the antioxidant system of plants.
TF families such as NAC (NAM, ATAF1/2, and CUC2), MYB (myeloblastosis), and WRKY are also involved in controlling drought tolerance (Figure 3). The NAC TFs regulate downstream stress-responsive genes, helping plants adapt to drought and salinity (Nakashima et al., 2012); for example, overexpression of NAC gene family members including ANAC019, ANAC055, and ANAC072 in Arabidopsis conferred drought tolerance (Tran et al., 2004). Similarly, transgenic plants overexpressing AtMYC2 and/or AtMYB2 showed higher sensitivity to ABA and the ABA-induced gene expression of RD22 was enhanced in those transgenic plants. Furthermore, transgenic plants overexpressing both AtMYC2 and AtMYB2 displayed upregulated expression levels of several ABA-inducible genes, contributing to drought stress resistance. WRKY TFs, as plant-specific TFs are strongly and rapidly induced in response to certain abiotic stresses, such as drought and salinity. Moreover, it is known that they play crucial roles in both biotic and abiotic stress responses in plants. Overexpression of GmWRKY54 in Arabidopsis and soybean (Glycine max) plants has been shown to enhance drought and salinity resistance through activating genes in ABA and Ca2+ signaling pathways.
Strategies for overcoming drought damage: Use of various biostimulants
The key strategy to reduce drought damage to vegetable crops is the use of biostimulants (Figure 4). The market of plant biostimulants is expanding, and they are being regarded as innovative agricultural tools. Agricultural biostimulants can be used as substitutes for synthetic chemical pesticides like insecticides and herbicides, helping to make agriculture more resilient and sustainable. Plant biostimulants, which can improve plant growth and production under abiotic stress conditions are made from a variety of organic and inorganic materials as well as microorganisms. By improving soil conditions, biostimulants have a direct impact on plant physiology and metabolism; increasing the efficiency of water and nutrient usage, promoting plant growth, and enhancing primary and secondary metabolism to assist plants cope with abiotic stress.
Humic substances (HSs) such as humic acid, fulvic acid, and humin are naturally occurring components of soil organic matter and have been shown to enhance drought resistance when applied to plants. For example, when humic acid-based biostimulants were applied to lettuce (Lactuca sativa), the plants showed improved nutrient absorption capacity, leading to enhanced quality and increased resistance to abiotic stress. In melon (Cucumis melo L.), the application of humic acid increased the accumulation of potassium (K) and calcium (Ca) ions, chlorophyll content, and the activity of antioxidant enzymes like SOD and CAT, thereby improving drought resistance. As soil conditioners and biostimulants, seaweed extracts were also utilized in agriculture because they included compounds that functioned similarly to plant hormones to stimulate plant growth. Applying seaweed extracts to broccoli (Brassica oleracea var. cymosa L.) enhanced resilience to abiotic stresses and improved production and quality. These results suggest that seaweed extracts can be useful for enhancing the productivity of high-value crops such as broccoli. Microorganisms also play roles in improving soil health and promoting plant growth. PGPB (Plant Growth Promoting Bacteria) enhance the production of osmolytes and regulate hormonal pathways in plants. They also help plants withstand abiotic stresses such as heat, salinity, and drought by improving root development and water uptake. Similarly, γ-glutamic acid (γ-PGA), a biodegradable and non-toxic polymer produced by microorganisms, has attracted attention for its potential applications in agriculture. γ-PGA improves nitrogen absorption under soil conditions, enhances water retention capacity in Brassica napus L., and promotes the removal of ROS while facilitating the accumulation of the osmoprotectant proline under drought stress. Biostimulants not only promote plant growth but also modulate stress signaling and regulate gene transcription, thereby fundamentally strengthening crop resilience. In particular, γ-PGA regulates plant stress response pathways by activating the expression of ABA biosynthesis-related genes and contributes to enhancing the activity of antioxidant enzymes such as SOD, CAT, APX, and POD. Thus, the mechanisms of various biostimulants are involved in enhancing cellular and metabolic responses, such as stress signaling, ROS scavenging, and transcriptional activation. Nanoparticles have recently attracted more attention from researchers due to their functional roles that are comparable to those of biostimulants. The application of proper concentration of titanium dioxide (TiO2) and zinc oxide (ZnO) nanoparticles was shown to promote photosynthesis and nutrient absorption in tomato (Solanum lycopersicum L.), demonstrating the potential to enhance abiotic stress tolerance. However, high concentrations of nanoparticles can cause physiological disorders and inhibit growth, so further research is needed to establish optimal concentrations for application across various crop species. Another approach to confer drought resistance is the application of exogenous hormones or bioactive substances, which can induce positive changes in crops under stress conditions (Table 1).
| Biostimulant/Exogenous Application | Crops | Effects of crops under drought stress |
| Abscisic Acid | Lettuce, Sweet potato | Increased sugar content, antioxidant enzyme activity, improved drought tolerance |
| Ascorbic Acid | Pepper, Sweet pepper | Improved antioxidant defense, drought tolerance, enhanced physiological responses |
| Benzoic Acid | Soybean | Improved gas-exchange and chlorophyll contents |
| Brassinosteroid | Chinese cabbage | Improved osmotic regulation, antioxidant defense |
| Y-glutamic acid | Rapeseed | Enhanced antioxidant enzyme activity, activated ABA biosynthesis |
| L-glutamic acid | Tomato | Increased proline content, reduced ROS accumulation, enhancedantioxidant enzyme activity |
| Humic Acid | Lettuce, Melon | Improved nutrient absorption, increased K and Ca ion accumulation, improved nutrient absorption capacity, enhanced drought resistance |
| Hydrogen Peroxide | Cucumber | Improved drought tolerance |
| Jasmonic Acid | Chinese cabbage, Strawberry | Improved osmotic regulation, antioxidant defense |
| Kinetin | Lettuce | Enhanced productivity, improved drought tolerance |
| Melatonin | Pea | Improved antioxidant defense, growth and photosynthetic eficiency |
| PGPB (Plant Growth Promoting Bacteria) | Broccoli | Improved growth, yield, antioxidant enzyme activity |
| Common bean | Improved root growth, water absorption, osmolyte synthesis, anddrought tolerance | |
| Pea | Improved growth, yield, antioxidant enzyme activity | |
| Protein Hydrolysate Based Biostimulant | Paper, Tomato | Enhanced root growth, improved shoot biomass, increased water-useeficiency, and elevated antioxidant enzyme activity |
| Salicylic Acid | Chinese cabbage, Cucumber, Sweet potato | Improved drought tolerance, enhanced growth and yield |
| Seaweed Extracts | Broccoli, Tomato | Improved drought tolerance, production and quality |
| Serotonin | Tomato | Increased antioxidant activity |
| Zinc Oxide Nanoparticles | Eggplant, Melon | Improved photosynthetic efficiency, growth, yield, and antioxidant defense |
Conclusions
The sustainable production of vegetable crops, which is essential for ensuring global food security, is seriously threatened by the increasingly extreme weather patterns brought on by climate change. Among the many impacts of climate change, drought stress, interferes with physiological, biochemical, and molecular mechanisms of plants from germination to maturation, resulting in decreased productivity.
For key vegetable crops, drought stress has been shown to cause leaf senescence and reduce photosynthetic efficiency. In response to drought-induced ROS, the activity of antioxidant enzymes such as SOD, POD, CAT, and APX was increased. These enzymes contributed to reducing ROS accumulation and minimizing cellular damage, thereby enhancing crop survival. Through the ABA signaling pathway, stomatal closure and the activation of drought-responsive gene expression were triggered, while other hormones like GA and JA also induced various physiological changes, increasing plant tolerance to drought.
Optimizing biostimulant applications, nanoparticle utilization, and smart irrigation systems will be crucial for maximizing their impact under drought conditions. An integrated approach that combines gene editing, molecular breeding, smart irrigation, and biostimulant applications will be essential for climate-resilient vegetable cultivation and production, thereby ensuring long-term global food security.
The drought stress worsened hunger, price volatility, and low productivity. Biostimulants offer a solution via stress tolerance and nutrient efficiency. Dora agri aims to support future research into sustainable agricultural development to enhance drought tolerance in vegetable production.
See more details of Dora Fulvic HQ (OMRI) and Dora Humate HQ (OMRI)
See more details of Dora γ-PGA
See more details of Dora Seaweed Fertilizer
See more details of Dora Amino Acids Fertilizer