An Investigation into Drought Tolerance: The Case of the Beet

Drought stress is a major environmental factor limiting global sugar beet production. Water deficit impairs photosynthesis, disrupts osmotic balance, induces oxidative stress, and disturbs source–sink relationships, ultimately leading to inhibited root growth, reduced sucrose accumulation, and decreased overall yield.

Introduction

Sugar beet (Beta vulgaris L.) is the world’s second most important sugar crop, with an annual global yield of approximately 281 million tons. Beyond its primary role in sugar production, it serves as a versatile feedstock for various industries, yielding products such as biodegradable polymers, biofuels (via ethanol fermentation), food additives, and animal feed. Notably, sugar beet contains a higher sucrose concentration than sugarcane and requires less time and water, positioning it as an efficient and water-wise alternative for sugar production. Grown in over 50 countries, mainly across the temperate zones of Europe and North America, sugar beet contributes significantly to global sugar security. However, the yield potential of sugar beet is highly sensitive to environmental stresses, with drought being one of the most critical limiting factors. Drought currently affects nearly 50% of the world’s arable land and can reduce sugar beet yield by 5–30%.

Role of phytohormones in drought stress adaptation in sugar beet

Sugar beet perceives drought stress through complex hormonal signaling cascades involving phytohormones such as ABA, gibberellic acid (GA3), and jasmonic acid (JA), which integrate growth regulation with stress response mechanisms. In addition to their roles in developmental regulation, these hormones function as key modulators of defense pathways under water deficit conditions. During the onset of drought, plants activate interconnected signal transduction networks that enhance stress tolerance by hormonally regulating stomatal conductance, osmolyte synthesis, and antioxidant defense mechanisms. Phytohormones in drought stress adaptation in sugar beet are presented in Fig. 1.

Abscisic acid in drought stress

ABA is a central phytohormone in drought signaling and a key coordinator of plant water status maintenance under stress. It triggers stomatal closure to reduce water loss and promotes the accumulation of osmolytes for osmotic adjustment, thereby collectively safeguarding leaf water status and cellular turgor under drought stress. Although ABA-mediated stomatal closure reduces CO₂ uptake and photosynthetic rates, it significantly decreases transpirational water loss, thereby improving water conservation. Exogenous application of ABA through foliar spraying or seed priming enhances drought resistance in sugar beet by maintaining higher photosynthetic activity and more effective stomatal regulation under water deficit conditions. Furthermore, crosstalk between ABA and other hormones (e.g., GA3 and JA), as well as interactions with osmolyte biosynthesis and ROS scavenging pathways, reinforces ABA’s role as a master regulator of drought stress adaptation in sugar beet.

Gibberellic acid (GA3) in mitigating drought effects

Exogenous application of GA3 improves sugar beet performance under drought stress by promoting photosynthetic pigment accumulation, growth, and plant water status. GA3 treatment enhances leaf number, biomass production, root yield, sucrose content, and RWC, while also strengthening vascular development and xylem patterning. Proline accumulation and osmotic adjustment are significantly enhanced by foliar application of GA3 at a concentration of 150 mg L⁻¹, which alleviates the detrimental effects of drought on growth and yield. Although GA3 often exhibits antagonistic interactions with ABA in growth regulation, exogenous GA3 application supports recovery and enhances drought tolerance in sugar beet by improving physiological efficiency under water-limited conditions.

Role of jasmonic acid (JA) in drought stress tolerance

Several studies have reported that JA enhances drought tolerance in sugar beet by inducing antioxidant defense systems, promoting chlorophyll biosynthesis, and improving WUE. These effects collectively contribute to improved root development and increased sucrose production under drought stress. Exogenous application of JA promotes plant growth and confers drought tolerance by modulating stress-responsive gene expression and increasing the accumulation of protective metabolites. JA also regulates the expression of its own biosynthetic genes, further supporting its role in adaptive stress signaling. Methyl jasmonate (MeJA), the most biologically active derivative of JA, protects the photosynthetic apparatus from drought-induced oxidative damage. Application of MeJA delays leaf wilting and alleviates drought symptoms in young sugar beet plants. Foliar application of JA at a concentration of 10 μM L⁻¹ increases LAI and dry matter accumulation under drought conditions. Although JA is a key inducer of growth and stress resistance during plant development, its post-harvest application does not prevent dehydration or storage losses in sugar beet taproots. Overall, both JA and MeJA act as potent signaling molecules that regulate antioxidant enzyme activity, osmolyte accumulation, and cellular homeostasis under water-deficit conditions. Their application holds promise for improving drought resistance in sugar beet cultivated in arid and semiarid regions.

Strategies for alleviating the consequences of drought on sugar beet

Sugar beet drought tolerance can be improved through precision agriculture and smart irrigation to optimize water use, soil amendments to enhance water retention, foliar application of biostimulants to maintain osmotic balance and redox homeostasis, and molecular approaches targeting stress-responsive genes, and pathways to enhance cellular protection and water transport. Several strategies to improve sugar beet resistance to drought are discussed below.

Role of soil amendments and nanoparticles in enhancing sugar beet drought tolerance

Soil amendments and nanoparticles (NPs) provide innovative approaches to mitigate drought stress and enhance sugar beet’s resilience. Biochar, as a promising soil amendment, improves sugar beet performance under drought, particularly in water-limited sandy loam soils. It enhances soil properties such as water retention, hydraulic conductivity, and cation exchange capacity, leading to better nutrient uptake and improved root development. In sandy soil, a 20% biochar amendment increases total soil water storage by 27–35% and reduces soil bulk density by 19.3%, thereby enhancing root-zone moisture retention and extending irrigation intervals. Morphologically, biochar promotes root system development, delays premature senescence, and optimizes plant architecture. Root activity in biochartreated plants is only 4.8–18.5% lower than under optimal conditions, compared to a reduction of 42.9–66.7% in untreated stressed plants, which helps sustain canopy function and biomass accumulation. Physiologically, biochar improves water relations, membrane stability, and nitrogen assimilation, with nitrogen assimilation increasing by 111.1% at 50 days after sowing (DAS) but decreasing to 42.9% by 130 DAS. This boost in nitrogen uptake is coupled with increased chlorophyll synthesis–chlorophyll b rose by 23.9–41.2%, supporting photosynthesis and carbohydrate production under drought stress. Biochar application also promotes the accumulation of osmoprotectants like proline and soluble sugars, aiding in maintaining cell turgor and improving nutrient use efficiency under drought stress. Importantly, biochar treatment increases root yield by 48.3–49.8%, improves sugar content by 11.2–12.7%, and enhances peak dry matter accumulation by up to 42.1% at 70 DAS, highlighting its significant role in boosting yield and water use efficiency under stress. These improvements in yield are supported by biochar’s positive effects on photosynthesis, stomatal conductance (increased by 29.8–55.0%), and overall plant metabolic efficiency, making it a sustainable strategy for sugar beet production in water-limited environments.

In addition to biochar, vermicompost has shown promise in enhancing sugar beet’s drought tolerance. Vermicompost tea (VCT) applied at 27–54 L ha⁻¹ significantly improves root yield by 20% and sugar yield by 33% under severe drought conditions (50% irrigation requirement). This effect is attributed to upregulation of the anti-oxidant defense system, including enhanced activities of catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POX), which help mitigate oxidative stress by reducing H₂O₂ and malondialdehyde (MDA) levels. VCT’s enriched nutrient content, humic acids, and beneficial microbes increase soil water-holding capacity and promote mycorrhizal associations, maintaining photosynthetic efficiency under drought stress. Nanoparticles (NPs), such as fullerene NPs and silicon (Si) NPs, provide another promising strategy for drought mitigation in sugar beet. Fullerene NPs function as intracellular water reservoirs, forming hydrogen bonds with water molecules and creating additional intercellular water reserves that reduce drought stress. Interestingly, fullerene NPs reduce proline accumulation by 78–79% under severe drought compared to untreated controls, suggesting that these NPs may partially replace the need for osmolyte synthesis. The hygroscopic properties of fullerenol NPs allow them to penetrate plant tissues effectively, as evidenced by reduced proline accumulation in roots following foliar application, demonstrating their systemic mobility. Fullerenol efficiency is dose-dependent, with 0.01 nmol mm⁻² optimally enhancing antioxidant enzyme activities (CAT and APX) under severe drought, while lower doses are less effective, highlighting the importance of nanoparticle concentration for optimal plant stress response.

Similarly, Si-NPs enhance plant functioning under drought through several physiological mechanisms. Foliar application of Si increases leaf calcium and magnesium content, with approximately 70% of absorbed Si accumulating in roots, contributing to structural support, while 30% remains in leaves to maintain cellular stability. Si NPs also improve water relations and nutrient uptake, with sugar beet accumulating approximately 75 kg Si ha⁻¹, exceeding phosphorus uptake by 3.5-fold and magnesium by 20% under field conditions. Sugar yield improvements result from enhanced metabolic efficiency and plant functioning under water-limited conditions. Despite these promising results, determining the optimal concentration of NPs is critical, as overexposure may negatively affect plant stress responses and antioxidant defense mechanisms.

Recent works has demonstrated that engineered nanomaterials can enhance drought tolerance in sugar beet through multiple physiological and biochemical pathways. Foliar spraying of silica nanoparticles in combination with brassinolide was shown to improve cell membrane stability, increase antioxidant enzyme activities and osmolyte accumulation, and consequently enhance leaf water status and water use efficiency under drought conditions in sugar beet, indicating synergistic benefits of nanomaterial and phytohormone treatments in drought mitigation. Additionally, foliar application of zinc oxide nanoparticles (ZnO-NPs) combined with biostimulants such as proline and chitosan increased chlorophyll content, antioxidant enzyme activities (e.g., catalase, superoxide dismutase, and ascorbate peroxidase), and root yield under water deficit stress, while reducing oxidative stress markers such as malondialdehyde and abscisic acid, demonstrating that ZnO-NPs can fortify drought defense mechanisms in sugar beet. These emerging findings underscore the potential of nanomaterials as part of integrated strategies for enhancing drought tolerance in sugar beet, while also pointing to the need for further work on dosage optimization, long-term effects, and field-level validation.

While both biochar and nanoparticles have shown significant potential in mitigating drought stress in sugar beet, further research is needed to evaluate the long-term viability, environmental persistence, and potential phytotoxicity of NPs in soil ecosystems and groundwater. Additionally, exploring the interactions between organic amendments like vermicompost and engineered NPs could provide valuable insights for developing integrated drought management strategies for sustainable sugar beet cultivation.

Advances in developing drought-tolerant sugar beet

Breeding for drought tolerance in sugar beet is constrained by its limited genetic base. This bottleneck stems from its domestication from its domestication history, which possesses lower allelic richness and reduced heterozygosity compared to wild relatives. To address this limitation, wild relatives such as Beta maritima and fodder beets have been effectively utilized in breeding programs to augment genetic diversity and enhance stress tolerance. Physiological differences in drought tolerance have been observed among wild beet populations, with key discriminating variables between genotypes being leaf temperature and osmotic adjustment. Other relevant physiological traits, such as green leaf cover, wilting score, and stomatal conductance, are considered reliable indirect selection parameters for drought-tolerant genotypes. Preservation of green foliage during drought correlates strongly with drought tolerance indices, whereas higher wilting scores and increased leaf senescence are negatively associated with stress resilience. Hybrids are particularly promising for introgressing drought tolerance, often using wild species such as B. corolliflora or B. maritima. Portuguese wild beet populations, particularly the Vaiamonte ecotype from dry inland habitats, maintain consistent biomass production even under severe water deficit and exhibit smaller increases in leaf temperature compared to other genotypes. In addition, drought-tolerant mutants have been produced through induced mutagenesis using ionizing radiation (e.g., gamma rays) to alter antioxidant enzyme activities. Resistance lines with higher chlorophyll, carotene, and antioxidant activity have also been identified through in vitro selection using polyethylene glycol or sodium chloride, effectively alleviating oxidative damage under water stress. These in vitro selection methods facilitate rapid detection of stress-tolerant genotypes with enhanced enzymatic antioxidant levels.

Genetic engineering further expands opportunities for improving drought tolerance. Transgenic sugar beet lines expressing genes such as Bacillus subtilis SacB or Helianthus tuberosus 1-SST accumulate fructans, enhancing osmotic adjustment and promoting water retention in cells. Expression of a hemoglobin-like protein gene (Beta vulgaris He- moglobin 2 (BvHb2)) increased iron concentration in leaves, reduced leaf wilting, and sustained photosynthetic activity under drought. Genome editing approaches, including virus vector-based systems such as Beet Necrotic Yellow Vein Virus (BNYVV), provide platforms for dissecting and precisely manipulating stress-responsive genes, revealing novel genes to increase drought tolerance.

Recent advances in molecular breeding and genomics have provided new insights into the genetic basis of drought tolerance. A genome-wide association study using 328 sugar beet germplasms identified 11 significant loci and multiple candidate genes associated with drought tolerance traits, offering molecular markers that can be used in breeding programs. Genome-wide analyses of transcription factor families such as the DREB genes have further revealed key regulators of drought stress responses, laying the foundation for targeted breeding to improve drought tolerance. Genome editing tools like clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) and related systems present powerful strategies for precise modification of drought resistance genes in sugar beet and other sugar crops. Recent reviews highlight successful use of CRISPR-based approaches to edit genes controlling abiotic stress responses and discuss strategies for accelerating the development of stress-tolerant sugar beet. While these results are encouraging under controlled conditions, more extensive testing and regulatory approval are needed before genetically engineered drought-tolerant sugar beet cultivars can be widely applied in agricultural practice.

Using beneficial microbes in enhancing drought tolerance of sugar beet

The increasing frequency of drought due to climate change poses a major threat to sugar beet cultivation. To address this challenge, beneficial microorganisms such as Plant Growth Promoting Rhizobacteria (PGPR) and Arbuscular Mycorrhizal Fungi (AMF) have emerged as promising agents for enhancing drought tolerance. PGPR, including Pseudomonas fluorescens and Bacillus subtilis, can stimulate drought-protective mechanisms by enhancing nutrient uptake, activating the antioxidant system, and modulating plant growth. These bacteria can also induce osmotic adjustment by synthesizing compatible solutes such as proline, which helps maintain cellular structure under stress. Furthermore, PGPR can stimulate the production of phytohormones such as auxins and gibberellins, which promote root system development and improve water accumulation under drought stress.

In contrast, mycorrhizal fungi enhance water and nutrient uptake in surface roots through their mutualistic relationship with plants, which is essential for improving WUE. AMF inoculation under drought has been reported to increase RWC retention and reduce oxidative damage in sugar beet. Additionally, AMF enhance the plant’s antioxidative capacity by increasing the activity of enzymes such as SOD and CAT, which scavenge ROS generated under drought stress. The synergistic relationship with these microbes helps reduce water deficit and directly supports plant growth and stability of the photosynthetic machinery, as indicated by higher chlorophyll content and maintenance of photosynthetic activity under drought. Combined application of PGPR and AMF may serve as a promising strategy to improve sugar beet performance under water-limited conditions. These microorganisms complement each other and enhance both the physical and biochemical processes of sugar beet plants under normal and stress conditions, resulting in improved drought tolerance and higher yield.

However, current literature is limited by variability in microbial efficacy, which is often condition-dependent and strain-specific. Most studies have been conducted under controlled experimental conditions, and relatively little is known about their long-term performance in the field. Future research should focus on elucidating the synergistic interactions between PGPR and AMF, optimizing their combined application across different soil types, and developing inoculant formulations suitable for general field use. Additionally, studies are needed on the economic feasibility of these biological inputs for large-scale production and on their molecular mechanisms in enhancing drought tolerance. An overview of integrated approaches for drought stress mitigation in sugar beet is presented in Fig. 2.

Summary and future perspectives

In response to drought stress, sugar beet initiates a series of adaptive mechanisms. Morphologically, it develops deeper roots and reduces leaf area to optimize water uptake and conservation. Physiologically and biochemically, it accumulates osmoregulatory compounds such as proline and glycine betaine to maintain cellular turgor and enhances the activity of antioxidant enzymes to mitigate oxidative damage. In addition to these inherent tolerance mechanisms, external management practices can enhance drought resistance. These include optimized irrigation scheduling, soil amendments such as biochar and vermicompost, foliar application of plant hormones (e.g., abscisic acid, gibberellic acid, and jasmonic acid), and the use of nanoparticles such as fullerenol and silicon.

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