Climate-resilient horticulture: Strategies to combat abiotic stress: A review

Introduction

Horticulture is the practice of growing fruits, vegetables, flowers, and herbs, all crops which make substantial contributions to human nutrition and biodiversity. Horticultural crops differ from cereals in being especially sensitive to changes in their environment, hence making them highly vulnerable to climate change. The world has experienced an increase in global temperatures of about 1.1°C relative to the pre-industrial period, with estimates projecting another rise of between 1.5 and 4.5°C by the end of the century (IPCC, 2021) [9] . These changes will be associated with increased cases of drought, extreme precipitation, heat waves, sea-level rise leading to increased soil salinity, and unexpected frosts all abiotic stresses for horticultural crops. Abiotic stress refers to any non-biological factor within the environment that negatively influences plant growth, development, and productivity (Mittler, 2006) [4] . This may involve drought, extreme temperature, salinity, water logging, ultra violet radiation, heavy metal toxicity, and oxidative stress. According to Boyer (Boyer, 1982) [5] , the environmental stresses cause about 30% loss of global crop production below their genetic capacity. Notably, according to recent reports, climate changeinduced disasters have caused annual global crop production to be about 10% to 14% lower from 1964 to 2007 (Lesk et al., 2016) [3] . The future outlook of this trend suggests even greater losses in upcoming years. The horticultural crops are vulnerable to abiotic stress due to the impact of the stress on the quality of fruits and vegetables based on their appearance, flavor, aroma, nutrient content, and storage life. This is because, although environmental factors may influence crop yields of cereals significantly, they may affect other important quality characteristics of of horticultural crops. Heat stress during the anthesis stage in tomatoes can result in complete failure of fruit formation (Bita & Gerats, 2013) [7]. Similarly, chilling of the mangoes, bananas, and cucumbers can lead to irreparable changes in fruit quality (Palta & Weiss, 1993) [28]. The horticultural crops are vulnerable to abiotic stress due to the impact of the stress on the quality of fruits and vegetables based on their appearance, flavor, aroma, nutrient content, and storage life. This is because, although environmental factors may influence crop yields of cereals significantly, they may affect other important quality characteristics of horticultural crops. Heat stress during the anthesis stage in tomatoes can result in complete failure of fruit formation (Bita & Gerats, 2013) [7] . Similarly, chilling of the mangoes, bananas, and cucumbers can lead to irreparable changes in fruit quality (Palta & Weiss, 1993) [28] . Salinity not only reduces yield but also impairs the accumulation of health-promoting secondary metabolites in vegetables (Ashraf & Harris, 2004) [13] . These complex interactions underscore the need for comprehensive, cropspecific strategies to enhance stress resilience. Over the past two decades, numerous findings have advanced our knowledge about the underlying mechanisms of stress responses in plants, ranging from the identification of the signaling processes associated with ABA and non-ABA responses (Yamaguchi-Shinozaki & Shinozaki, 2006) [24] , to the studies on the DREB/CBF family of transcription factors (Agarwal et al., 2006) [65] , ROS signaling pathways (Apel & Hirt, 2004) [35] and ion transport for salt tolerance (Munns & Tester, 2008) [14] . Moreover, significant achievements have been made in such domains as genomics, proteomics, metabolomics, and genome editing with CRISPR-Cas9, opening new opportunities for stressresilience improvements (Zhu, 2016; Varshney et al., 2011) [16,58] . In addition, the developments in agronomy, such as deficit irrigation, mulching, biochar treatment, precision farming, and PGPR, are valuable tools which have proved their effectiveness in practice in different agroecological environments (Farooq et al., 2009) [49] . Finally, osmoprotectant treatments, hormone, and signaling molecule application are effective low-cost strategies for temporary enhancement of tolerance (Choudhary et al., 2012) [73] . Nonetheless, there are still considerable differences between what scientists have discovered in laboratories and how farmers use them in their fields. Tolerance to stress is a multi-gene trait that is situational and often comes at the expense of yield and product quality (Blum, 2005) [57] . In cases where plants experience several stresses at once, the resulting plant response will be entirely different from the response when a single stress factor is present (Zandalinas et al., 2018; Hussain et al., 2019) [2,76] . This review will focus on the current state of knowledge regarding the science of abiotic stress in horticulture and critically assess the range of approaches currently being developed and implemented to create climate-resilient varieties.

Scope and Methodology

The key terms used for the literature search include 'abiotic stress horticulture climate change vegetable crops', 'drought tolerance fruit crops', 'heat stress tomato', 'salinity tolerance horticultural plants', 'ROS signaling plants', 'DREB transcription factors', 'grafting stress tolerance', 'deficit irrigation', and 'CRISPR plant stress'. Only those papers that were related to: (i) the mechanism of abiotic stress response in horticultural plants, (ii) an assessment of breeding/biotechnology strategies for stress tolerance improvement, and (iii) agronomical management practices have been selected for analysis.

Types of Abiotic Stress in Horticultural Crops

Horticultural crops face a diverse array of abiotic stresses that vary in nature, duration, intensity, and combination (Table 1). Each stress type elicits distinct physiological responses while sharing common downstream signaling elements including calcium second messengers, MAPK cascades, and ROS networks (Cramer et al., 2011) [6] . Understanding the specificity and overlap of these responses is crucial for designing broad-spectrum tolerance strategies. [5,6,1] .

Heat Stress

Heat stress arises due to high temperatures beyond the critical limit for physiological activity, usually above 35circ C for most horticultural plants (Bita & Gerats, 2013) [7] . Heat stress affects cellular processes by denaturing proteins, disrupting membrane fluidity, blocking PSII electron transport, and producing reactive oxygen species (ROS) (Wahid et al., 2007) [12] . Heat stress also affects reproductive structures by interfering with pollen tube formation, aborting ovules, and impairing fruit set. The tomato (Solanum lycopersicum) crop is highly sensitive to heat stress: temperatures above 22circ C during the night and above 38circ C during the day can inhibit fruit formation by more than 90% (Bita & Gerats, 2013;

Hasanuzzaman et al., 2013) [11] . Ethylene production triggered by heat increases fruit ripening and decreases their shelf life. Heat stress effects on the molecular level have been extensively studied by Wahid et al. (Wahid et al., 2007).

Drought Stress

Drought is the most damaging abiotic factor for crop yield in the world, resulting in reductions between 20% and 50% for crop plants (Boyer, 1982; Farooq et al., 2009) [5,49] . Water stress leads to stomata closure through ABA signaling, causing reduction in transpirational water loss while simultaneously reducing CO2 absorption and photosynthetic rates (Chaves et al., 2009; Schroeder et al., 2001) [18,55] . Chronic drought stress leads to cellular dehydration, membrane injury, protein denaturation, and activation of proteases. The development of new root architecture, such as deep roots and higher density of root hairs, is an important strategy for avoiding drought conditions (Price et al., 2002) [53] . Fruit quality factors in horticulture crops are highly dependent on water management (Jaleel et al., 2009) [52] .

Salinity Stress

Soil salinization, resulting from irrigation, sea-level rise, and inadequate drainage, currently impacts more than 900 million hectares worldwide and is on the increase at the rate of 1-2% per year (Munns & Tester, 2008) [14] . Salinity causes harm to plants through osmotic stress, which leads to reduced water uptake and ionic toxicity caused by the buildup of Na + and Cl -ions (Munns & Tester, 2008) [14] . For horticultural plants, excess of Na + results in its accumulation in older leaves and consequently in chlorosis, necrosis, and faster senescence (Flowers & Colmer, 2008; Deinlein et al., 2014) [20,30] . The main pathway responsible for Na + extrusion is called SOS, and it consists of the three proteins SOS3-SOS2-SOS1 (Zhu, 2016) [16] . Compartmentation of Na + into vacuoles via NHX antiporters and restricted Na + loading into the xylem via HKT transporters are additional critical mechanisms (Roy et al., 2014) [31] .

Cold and Chilling Stress:

Cold stress is characterized into chilling injury left (0-15circ C right) and freezing injury (< 0circ C), both having different mechanisms. Chilling injury causes damage to the tropical and subtropical horticultural crops like bananas, mangos, cucumbers, and citrus fruits through membrane phase transition into gel form, metabolic malfunction, and loss of ions (Chinnusamy et al., 2007; Palta & Weiss, 1993) [27,28] . Freezing injury results due to the development of ice crystals inside the cell, leading to rupture of the cellular membrane. Cold acclimation is the process of gradual adaptation resulting due to exposure to non-freezing low temperatures and consists of induction of transcription factors, production of cryoprotectants, and membrane lipid modification (Thomashow, 1999; Chinnusamy et al., 2007) [26] .

Waterlogging and Hypoxia

The lack of oxygen results in the change in the cellular metabolism, leading to fermentation of sugars. This causes the formation of harmful products such as ethanol and acetaldehyde (Mittler, 2006) [4] . The formation of aerenchyma in the roots allows oxygen to move from shoots to roots. It is one of the tolerance mechanisms found in plants adapted to flooding conditions. Ethylene gas is synthesized in roots when flooded and triggers the formation of aerenchyma and adventitious roots (Cramer et al., 2011) [6] . Tomato, pea, and bean plants are extremely sensitive to waterlogging beyond 24-48 hours.

Heavy Metal and UV Stress

Metal toxicity (Cd, Pb, As, Hg) constitutes a growing abiotic stress in peri-urban horticulture caused by industry effluents, mining and use of sewage sludge (Ahmad et al., 2010) [37] . Metals interfere with enzymatic activity, impairing photosynthesis while producing reactive oxygen species through Fenton and Haber-Weiss reactions (Apel & Hirt, 2004) [35] . Increased levels of UV-B as a result of the thinning of the ozone layer affect DNA synthesis and impair photosynthesis while deteriorating proteins in horticultural produce (Potters et al., 2007) [39] . On the other hand, moderate exposure to UV-B light may increase phenolic production for quality management.

Physiological and Biochemical Responses to Abiotic Stress

Plant responses to abiotic stress are orchestrated across multiple scales from subcellular biochemistry to whole-plant physiology in a hierarchically organized cascade (Figure 2).

Effective stress tolerance requires rapid perception, efficient signal amplification, coordinated gene expression reprogramming, and ultimately adaptive phenotypic adjustment (Zhu, 2016; Xiong et al., 2002) [16,17] .

Reactive Oxygen Species and Antioxidant Defense

These include O2bullet -, H2O2, OH bullet, and ^ 1O2, which are produced as by-products of aerobic respiration and are highly over-produced under almost all abiotic stresses (Mittler et al., 2004; Apel & Hirt, 2004) [34,35] .

Although excess ROS cause oxidative damage to the cellular constituents (lipids, proteins, and DNA), plants have a fine-tuned ROS signaling gene network and utilize H2O2 for local and long-distance communication in addition to being an ROS molecule (Mittler et al., 2004; Miller et al., 2008) [36] . [33,37] . The ascorbate-glutathione cycle, functioning in chloroplasts, mitochondria, and peroxisomes, plays a crucial role in H2O2 scavenging (Foyer & Noctor, 2009) [38] . Increased activities of antioxidant enzymes are positively correlated with stress tolerance in horticultural species. Many studies have shown that increased antioxidant capacity confers better stress tolerance and stable production (Hasanuzzaman et al., 2020; Ahmad et al., 2010) [33,37] .

Osmotic Adjustment and Compatible Solutes

Osmotic adjustment refers to the active synthesis of compatible solutes within the cell cytoplasm to lower the water potential in the cells without toxicity. Osmotic adjustment is a vital stress tolerance mechanism (Serraj & Sinclair, 2002; Chen & Murata, 2002) [42,43] . Compatible solutes include proline, glycine betaine, trehalose, polyols like mannitol and sorbitol, and sucrose (Bohnert & Jensen, 1996) [41] . Compatible solutes play several roles: They stabilize proteins and membranes, detoxify reactive oxygen species, maintain cytosolic pH balance, and act as reservoirs of reduced forms of carbon and nitrogen for use during recovery after stress (Chen & Murata, 2002; Bartels & Sunkar, 2005) [43., 44] . The accumulation of proline has been highly reported under drought and saline conditions in tomatoes, potatoes, and peppers. Its biosynthesis is controlled by the enzyme Δ¹-pyrroline-5-carboxylate synthetase (P5CS) (Bartels & Sunkar, 2005) [44] . Overexpression of the gene encoding this enzyme, codA gene (encoding choline oxidase for glycine betaine), and trehalose phosphate synthase (TPS) through transgenic technology.

Hormonal Signaling Networks

The key hormone for abiotic stress responses in plants, specifically for drought and salt, is abscisic acid (ABA). In response to the water-deficit conditions, there is a quick accumulation of ABA in the roots and leaves, resulting in stomata closure by controlling ion channels in the guard cells, as well as activation of a wide range of gene expressions through ABA Responsive Element Binding (AREB/ABF) transcription factors (Schroeder et al., 2001) [55] . The molecular components of ABA signaling pathway include PYR/PYL receptors, PP2C repressor phosphatases, and SnRK2 kinases; these have already been thoroughly studied (Zhu, 2016) [16] .

In addition to the role of ABA, salicylic acid (SA), jasmonic acid (JA), brassinosteroids (BRs), ethylene, and nitric oxide (NO) are also involved in stress signaling pathways, each displaying significant cross-talk (Choudhary et al., 2012) [73] . SA priming is responsible for systemic acquired stress resistance, JA regulates wound and cold responses, BRs are involved in multiple stress resistance by enhancing BRI1receptor signaling, while ethylene acts to induce flooding response and promote ripening during heat conditions (Choudhary et al., 2012; Hussain et al., 2019) [76] . [11,12] . The classification of plant HSPs according to their molecular weights divides them into five types: HSP100/Clp B, HSP90, HSP70, HSP60/chaperonins, and small HSPs (s HSPs, 15-42 k Da) (Hasanuzzaman et al., 2013) [11] . Among horticultural plants, s HSPs are abundant and have an important role in the protection of PSII and membranebound proteins from heat stress conditions (Wahid et al., 2007) [12] . Expression of HSPs is controlled by heat shock factors (HSFs), mainly Hsf A1 isoforms, functioning as master controllers of heat stress responses in tomato (Bita & Gerats, 2013; Hasanuzzaman et al., 2013) [7,11] . Overexpression of HSPs constitutively in plants like tomato, pepper, and strawberry results in increased thermal tolerance. [14] ; Mittler et al. (Mittler et al., 2004) [4] ; Hasanuzzaman et al.

Heat

(Hasanuzzaman et al., 2020) [33] ; Chinnusamy et al. (Chinnusamy et al., 2007) [27] ; Wahid et al. (Wahid et al., 2007) [12] ; Zhu (Zhu, 2016) [16] .

Antioxidant [12] . Cold temperature leads to reduced membrane fluidity, phase separation, and affects membrane permeability (Thomashow, 1999) [26] . Plants compensate for the stress caused by high and low temperatures by changing the extent of unsaturation of their fatty acids. Unsaturation decreases under heat stress and increases during cold stress through the action of the enzymes of fatty acid desaturases (FADs) (Thomashow, 1999; Palta & Weiss, 1993) [28] . Transgenic plants engineered to overexpress FAD7 and thus increase the content of UFAs showed increased chilling resistance and may be useful for improving cold-tolerant vegetables (Vinocur & Altman, 2005) [29] . [62] showed that constitutive stress-induced expression of DREB1A in Arabidopsis dramatically improved the resistance of plants to drought, salt, and freezing, making them the major candidates for genetic engineering. In rice, overexpression of OsDREB1 led to increased cold and drought tolerance (Ito et al., 2006) [63] , similarly to results obtained in tomato and potato (Agarwal et al., 2006; Lata & Prasad, 2011) [75] .

Molecular Mechanisms and Gene

NAC and MYB Transcription Factors

NAC transcription factors, including NAM, ATAF, and CUC, represent one of the largest plantspecific families containing more than 100 members in Arabidopsis, most of which are stress inducible (Puranik et al., 2012) [64] . The stress-responsive NACs (SRNACs) play their roles in activation of downstream target genes responsible for osmotic regulation, antioxidants production, and senescence control (Puranik et al., 2012). The function of MYB transcription factors is related to anthocyanins synthesis, cuticle formation, and stomatal closure, all of which can be associated with stress responses in horticultural plants (Agarwal & Jha, 2010) [60] .

Epigenetic Regulation of Stress Responses

Epigenetic processes such as DNA methylation, histone modification, and small RNA-mediated control are playing more and more important roles in the plant response to stress (Hirayama & Shinozaki, 2010). Chromatin modifications in the presence of stress factors that occur at certain loci can be maintained after mitosis or even in seeds during seed transfer across generations (transgenerational epigenetic memory), a process known as stress imprinting (Hirayama & Shinozaki, 2010) [66] . Small RNA molecules, including mi RNAs and si RNAs, regulate gene expression post-transcriptionally by targeting transcription factors responsible for stress responses; for example, mi R169 controls NF-YA transcription factors, and its activity declines during drought to allow better stress gene expression (Hirayama & Shinozaki, 2010) [64] .

MicroRNA Regulation

Micro RNAs (mi RNAs) are natural noncoding RNAs of 20-24 nucleotides, which function as mediators of posttranscriptional gene silencing by virtue of their ability to bind to complementary regions on target m RNAs (Nakashima et al., 2009) [59] . Many mi RNAs have been identified to exhibit differential expression in response to environmental stresses in horticultural crops. For example, mi R156 affects vegetative phase transition and stress tolerance by targeting SPL transcription factors; mi R159 inhibits MYB transcription factors that participate in ABA signaling pathways; and mi R398 targets Cu/Zn-SOD m RNA, whose expression is inhibited under oxidative stress conditions for higher SOD synthesis (Hirayama & Shinozaki, 2010) [66] .

Breeding and Biotechnology Strategies for Stress Tolerance

Breeding for stress tolerance has historically been challenged by the quantitative nature of tolerance traits, genotype × environment interactions, and trade-offs with productivity. Modern molecular tools are increasingly being integrated with conventional breeding to accelerate progress (Table 3). [16,58] Speed Breeding Controlled environment photoperiod

Rapid generation cycling Limited to some species (Tester & Langridge, 2010) [69] Exogenous Elicitors Priming (SA, JA, polyamines)

Broad stress tolerance Dose and timing dependent (Choudhary et al., 2012) [73] Microbial Inoculants PGPR, mycorrhizae Drought, salt tolerance Soil-specific efficacy (Hasanuzzaman et al., 2020) [33] Nano biotechnology Nano-fertilizers, nanosensors Nutrient efficiency, monitoring Ecotoxicological risks (Varshney et al., 2011) [58]

Conventional and Participatory Breeding

The conventional method of breeding including hybridization, selection, backcrossing, and polyploidization continues to serve as the base for crop improvement. Selection for stress-tolerance genes in large germplasm pools followed by introgression in elite germplasm through backcross breeding has led to the development of stressresistant varieties of tomato, pepper, melon, and cucumber (Cattivelli et al., 2008; Tester & Langridge, 2010) [69] . Participatory plant breeding where farmers select and evaluate has been found to be very effective in breeding for locally adapted stressresistant varieties in developing nations (Jain, 2010) [68] . The important problems associated with breeding for stress tolerance are its polygenic nature, genetic uniformity of elite breeding pools, and the lengthy breeding process involved (Cattivelli et al., 2008) [50] .

Marker-Assisted Selection (MAS)

With MAS technology, there is an indirect selection based on flanking DNA markers for selecting the target genes rather than dealing with the problem of phenotyping such stress tolerance traits in an environmental variable condition (Xu & Crouch, 2008) [71] . Some major QTLs that confer resistance against drought stress, salinity stress, and cold stress in tomatoes (fruit setting, ABA concentration), melons (accumulation of ions), and cucumbers (CI index) have been identified and marked by SSR or SNP molecular markers (Collins et al., 2008; Xu & Crouch, 2008). With whole genome sequences available for tomatoes, peppers, cucumbers, watermelons, and strawberries, there is a rapid advancement in QTL-to-gene candidate identification and GWAS for various stresses (Collins et al., 2008) [70] . Genomic Selection (GS) technology, where prediction of the breeding values is made with genomic information without QTL mapping, is becoming increasingly popular (Xu & Crouch, 2008).

Transgenic Approaches

This technique allows the transfer of selected genes imparting stress tolerance ability, which may be obtained from plants, bacteria, or even designed artificially (Vinocur & Altman, 2005; Wang et al., 2003) [29] . [62,67] . It has been found that stacking more transgenes targeting various aspects of the stress pathway gives better results than individual transgene techniques (Wang et al., 2003) [48] . But the regulatory mechanism for GM crops is still a problem in Europe and some developing nations (Varshney et al., 2011) [58] .

CRISPR-Cas9 Genome Editing

CRISPR-Cas9 technology has revolutionized the study of plant stress biology, allowing site-specific and efficient modification of the plant genome (Zhu, 2016) [16] . Some examples of CRISPR applications for the improvement of stress tolerance in horticultural crops include knockout of negative stress regulators (e.g., PP2C in ABA signaling), promoter engineering for enhanced stress inducibility, and base editing to incorporate beneficial mutations discovered via natural variation analysis (Zhu, 2016; Varshney et al., 2011). CRISPR-edited crops without foreign DNA are likely to be regulated less strictly than transgenic plants in some countries, thus providing a more rapid method of implementation than genetic engineering (Varshney et al., 2011) [58] . Some of the potential targets in horticultural crops are Sl ARF4 in tomatoes (drought), Cs SAMDC in cucumber (cold tolerance via polyamines), and Ca NPR1 in pepper (salinity) (Agarwal et al., 2006; Hirayama & Shinozaki, 2010) [60,65,66] .

Grafting as a Stress Tolerance Strategy

The process of grafting involves attaching a piece of shoot from a fruit-bearing species to a rootstock from another stress-resistance species and represents one of the most ancient and frequently used approaches to overcome abiotic stress in plants (Vinocur & Altman, 2005) [29] . Contemporary grafting operations rely on rootstocks that possess deep root networks (drought resistance), ability to exclude Na+ ions (salt resistance), and form aerenchymas (waterlogging resistance) (Roy et al., 2014) [31] . Grafting tomato, cucumber, melon, and watermelon crops onto vigorous rootstocks (hybrids between Cucurbita maxima and C. moschata for cucurbits and S. torvum and S. sisymbriifolium for tomatoes) has become an accepted practice within protected cultivation of valuable crops in the Mediterranean region, East Asia, and the Middle East countries (Roy et al., 2014; Golldack et al., 2014) [32] .

Agronomic Management Strategies 7.1 Deficit Irrigation and Water Use Efficiency

Deficit irrigation (DI), which involves irrigating at less than complete evapotranspiration rates, is a key water management technique used in vegetable crop cultivation in areas where water is limiting (Condon et al., 2004; Blum, 2005) [57] . The principle behind regulated deficit irrigation (RDI) and partial root zone drying (PRD) lies in the effect of root-to-shoot ABA signaling, which leads to partial stomatal closure, thus minimizing the amount of transpirational water loss while not lowering carbon assimilation or yield (Schroeder et al., 2001; Condon et al., 2004) [55] . For example, the use of RDI in tomatoes has been proven effective in maintaining crop yields, while lowering water consumption by 20-40%, improving quality parameters such as lycopene, sugars, and total soluble solids (Condon et al., 2004) [56] . Other irrigation strategies include soil moisture monitoring sensors, ET weather models, and variable rate drip irrigation. drip irrigation enable fine-scale water management tailored to spatial heterogeneity within fields (Araus & Cairns, 2014) [72] .

Biochar and Organic Amendments

Biochar, which is charcoal produced through the pyrolysis process of organic biomass under limited oxygen, improves the soil's water retention, cation exchange capacity, and microbial diversity while decreasing soil bulk density, qualities which are advantageous in situations of drought and salinity stress (Bartels & Sunkar, 2005) [44] . Metaanalysis shows that the use of biochar leads to an increase in crop yield by 10-15% during drought stress and a reduction in soil salinity among saline-infected soils by increasing Na + leaching (Bartels & Sunkar, 2005). In horticulture, the combination of biochar and compost in soil management is associated with improved soil structure and microbial activity under heat and drought stress conditions (Farooq et al., 2009) [49] . Organic mulching with straw, plastic films, or biodegradable materials can lower soil temperature fluctuation, moisture conservation, and weed suppression (Farooq et al., 2009).

Exogenous Chemical Applications

The exogenous application of stress-alleviating compounds represents a flexible, rapid-response tool for horticultural stress management (Table 4). These compounds act through diverse mechanisms: hormonal priming (SA, JA, BRs, ABA), osmotic protection (glycine betaine, proline, trehalose), antioxidant supplementation (ascorbate, tocopherol), signaling modulation (NO, H2O2), and nutritional compensation (Zn, Si) (Choudhary et al., 2012) [73] . [73] ; Ahmad et al. (Ahmad et al., 2010) [37] ; Chen & Murata (Chen & Murata, 2002) [43] ; Vinocur & Altman (Vinocur & Altman, 2005) [29] ; Hasanuzzaman et al. (Hasanuzzaman et al., 2020) [33] ; Hussain et al. (Hussain et al., 2019) [76] . [29] . Hydropriming, halopriming, osmopriming, and biopriming with PGPR are established commercial practices for vegetable seed treatment that can significantly improve performance under field stress conditions (Farooq et al., 2009) [49] . The challenge with exogenous applications lies in translating laboratory-scale efficacy to consistent field performance, accounting for variation in dose, timing, formulation, application method, and crop developmental stage (Choudhary et al., 2012) [73] .

Microbial Inoculants and Biostimulants

Plant Growth Promoting Rhizobacteria (PGPR) and Mycorrhizal Fungi have evolved mutualistic relationships with roots, which offer several advantages during abiotic stress conditions (Hasanuzzaman et al., 2020) [33] . The mechanisms involved in PGPR-induced stress tolerance include the secretion of ACC deaminase (stress ethylene reduction), synthesis of IAA and cytokinins (root morphology), phosphate mobilization (nutrient uptake), osmolyte synthesis, and Induced Systemic Tolerance (IST) through activation of antioxidant and stress-related signaling pathways (Hasanuzzaman et al., 2020). Arbuscular mycorrhizal fungi (AMF) improve drought and salt stress resistance through increased effective root surface area, glomalin synthesis (soil aggregation), and regulation of ABA levels and antioxidants in plants (Hasanuzzaman et al., 2020). Biostimulants based on various combinations of microorganisms have become commercially available for horticultural growers, even though quality control remains an issue (Hasanuzzaman et al., 2020; Farooq et al., 2009) [49] .

Precision Agriculture and Digital Technologies

Advances in precision agriculture technology that include the use of remote sensing by UAVs and satellites, IoT-based sensor technology, machine learning-based crop modeling, and decision support systems are enhancing the ability to identify and mitigate abiotic stresses in real-time (Araus & Cairns, 2014) [72] . Thermal imaging helps in identifying heat and drought stress via measurement of canopy temperature; hyperspectral imaging helps in measuring the level of chlorophyll, water content, and stress metabolites; and electric resistivity tomography can help in assessing soil moisture and salinity levels (Araus & Cairns, 2014). This is further enhanced when combined with climate prediction models and automated irrigation/fertigation systems to provide the necessary technology for horticultural production in climate variable regions (Araus & Cairns, 2014). In order for this technology to be adopted by smallholder farmers in climate vulnerable areas, precision agriculture must become accessible via the use of cheaper sensors and mobile technology (IPCC, 2021; Araus & Cairns, 2014).

Crop-Specific Resilience Strategies in Horticulture

While general principles of abiotic stress tolerance apply broadly, effective resilience strategies must be tailored to the biology, production system, and stress profile of individual horticultural crops. Table 5 summarizes climateresilient cultivars and integrated strategies for ten major horticultural crops, illustrating the diversity of approaches and outcomes achieved. Bita & Gerats (Bita & Gerats, 2013) [7] ; Munns & Tester (Munns & Tester, 2008) [14] ; Vinocur & Altman (Vinocur & Altman, 2005) [29] ; Roy et al. (Roy et al., 2014) [33] ; Araus & Cairns (Araus & Cairns, 2014) [72] . [7,62] . In cucumber, grafting onto Cucurbita rootstocks remains the dominant commercial strategy for chilling tolerance management in heated greenhouse production (Roy et al., 2014) [31] . In potato, integrated approaches combining deficit irrigation with PGPR inoculation and resistant cultivars have demonstrated yield stability under moderate drought stress while reducing water inputs by 30% (Farooq et al., 2009) [49] .

Combined and Multiple Stress Scenarios

As a consequence of climate change, there will be frequent encounters of horticultural crops with multiple or sequential abiotic stresses, especially drought and heat stress that trigger plant responses very different from, and even more deleterious compared to those induced by the two individual stresses (Zandalinas et al., 2018; Mittler, 2006) [2,4] . The limitations of using a single stress for studying the plant responses to stresses were pointed out by Mittler (Mittler, 2006) [4] . Zandalinas et al. (Zandalinas et al., 2018) [2] showed that the response of the plants to the combined drought and heat stress conditions is characterized by novel patterns of transcriptome expression including coactivation of the two pathways, ABA-dependent and ABAindependent.

According to Hussain et al. (Hussain et al., 2019) [76] , under combined conditions of drought and heat stress, the maize suffers larger decrease in photosynthesis rate, antioxidants activities and production as well as reduced yield due to interactions of membrane stability and reactive oxygen species balance. Combined salinity and heat stress affects both ABA signaling in stomata closure and HSPs induction in tomatoes resulting in a decrease in their photosynthesis and fruit quality. Drought and UV-B combination results in premature senescence in lettuce and increased anthocyanins production simultaneously (Potters et al., 2007) [39] Developing tolerance to stress combinations requires identification of genes and pathways that confer broadspectrum protection. [16,58] .

Emerging Technologies and Future Directions 10.1 Nanobiotechnology

The nanoparticles of silicon, zinc, titanium dioxide, and carbon nanotubes have shown stress alleviation capacity in agricultural crops under nano-concentration levels (Varshney et al., 2011) [58] . The modes of action involved include increased antioxidant enzyme activities, ion homeostasis, stimulation of root growth, and gene delivery for genetic modification. Agrochemicals delivered in nanoencapsulation form exhibit greater efficiency and sustained release of stress alleviation chemicals, thereby decreasing the environmental load and increasing plant uptake. However, the potential ecotoxicity issues of nanoparticle effects on soil microbiology, fauna, and food safety need detailed evaluation before their application (Varshney et al., 2011).

Artificial Intelligence and Genomic Big Data

Machine learning models are being used to analyze the relationship between genomic, phenomic, environmental, and management factors to predict the stress resistance phenotype using only genotype data, which would greatly enhance the speed of selection in breeding programs (Araus & Cairns, 2014) [72] . [6,66] .

Speed Breeding and Accelerated Generation Cycling

The speed breeding technologies that employ long day lengths (22 hours) through intense LED light can be used to cut down the generation period of several horticultural crops from 3-4 months to 6-8 weeks, resulting in 4-6 cycles per year (Tester & Langridge, 2010) [69] . If coupled with genomics-based selection and double haploids breeding methods, speed breeding can drastically shorten the process from gene discovery to cultivar development, allowing breeding programs to respond quickly to the fast rate of climate change (Tester & Langridge, 2010).

Plant Microbiome Engineering

The plant microbiome, which includes phyllosphere, rhizosphere, and endophytic microbial populations, has a significant and widely acknowledged impact on plant stress tolerance (Hasanuzzaman et al., 2020) [33] . Modification of the plant microbiome by selective inoculation of beneficial microbial communities, alteration of root exudates to attract certain microbes, and precise microbiome transfer is a unique strategy for dealing with stress that does not depend on host genetic makeup ( [50,70] . Due to its complicated nature, it may be challenging to achieve genetic engineering of stress tolerance while avoiding adverse pleiotropic effects on crop yields, quality, or phenological phase shifts (Blum, 2005) [57] . Another key issue is the discrepancy between laboratory conditions of experiments and field practices (Araus & Cairns, 2014) [72] . Genetic mechanisms of stress tolerance discovered in growth chamber or greenhouse conditions where plants face homogenous and single-stress environments rarely prove to be effective when transferred to actual, more complex growing conditions where plants have to cope with multiple stresses simultaneously (Mittler, 2006; Hussain et al., 2019) [4,76] . Effective methods of fieldbased high-throughput phenotyping are essential to address this problem (Araus & Cairns, 2014) [72] . Third, regulation policies regarding the use of GM crops are quite varied worldwide, limiting the economic viability of transgenic solutions in many regions (Varshney et al., 2011) [58] . Science communication strategies and transparent regulatory policies are needed to positively influence consumer attitudes toward new technologies in plant cultivation, such as GM and CRISPRengineered plants (Tester & Langridge, 2010) [69] . The issues related to intellectual property concerning genomics technology and stress resistance genes may hinder the application of genetic innovations in public crop breeding programs (Varshney et al., 2011) [58] . Fourth, regional and local climate change predictions for the next decades are marked by significant uncertainties, hindering the development of breeding targets that account for future stress tolerance requirements (IPCC, 2021) [9] . Breeding strategies need to take into consideration not only one possible scenario for the future climate but multiple scenarios, thus employing modular tolerance stacking technologies (IPCC, 2021; Cattivelli et al., 2008) [50] . The rise of unprecedented stress combinations, which could not have occurred under historical environmental conditions, complicates the prediction of stress tolerance efficiency (Zandalinas et al., 2018; Hussain et al., 2019) [2,76] .

Conclusion

The existence of abiotic stress remains a significant risk factor for the sustainability of global horticulture in the face of intensifying climate change. This review attempts to collate current knowledge on the field within molecular biology, physiology, genetics, agronomy, and technological innovations to provide an overview of the problem and existing tools of defense. Several important conclusions can be drawn from the analysis. The molecular mechanisms of abiotic stress resistance in plants are inherently multi-tiered, relying on responses at molecular, cellular, organ, and whole plant levels. ROS balance, ABA signaling, osmoprotectant metabolism, HSPdependent protein stability, and ion homeostasis are recurring themes that tie across different types of stresses. The most crucial knowledge gap is the elucidation of how such mechanisms are integrated and modulated under multistress conditions. A single approach will not suffice for achieving climateresilient horticulture. Some of the most effective strategies involve: (i) genetic enhancement using conventional breeding programs and modern molecular tools such as CRISPR/Cas9 technology, marker-assisted selection (MAS), and genomic selection; (ii) exploiting favorable grafting pairings; (iii) the selective application of stressmitigating chemicals; (iv) using microbial biostimulants for promoting soil and root health; (v) optimizing irrigation and fertilization using precision agriculture techniques; and (vi) using digital agriculture for detecting stressors in real-time and adaptive crop management. Some of the key research directions for the future should be:

(1) elucidation of stress tolerance mechanisms in wild relatives of crop plants and rare, but valuable, horticultural species with evolved tolerance to harsh environmental conditions; (2) studies on the functional genomics of combined stresses in priority horticultural crops; (3) development of low-cost, high-throughput field-based phenotyping systems to screen for stress tolerance in crops; (4) testing of integrated management strategy packages under farm-level field trials in climate-sensitive agricultural areas; and (5) creation of regulatory frameworks for new horticultural biotechnologies that accelerate their deployment to farmers.

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