Grafting in Pepper to Overcome Drought, Salinity, and High Temperature

Abstract

Since the twentieth century, pepper production and consumption have increased worldwide. However, pepper-harvested area decreases every year, which is partly associated with climate change effects such as extreme temperatures, salinity, and drought. These abiotic stresses affect pepper plants by limiting photosynthesis, growth, and development; increasing reactive oxygen species (ROS); and blocking metabolic processes, among others, leading to reduced production and fruit quality. Grafting rises as an effective technique to cultivate in unfavorable environmental conditions, because crops yields increase when tolerant rootstocks are employed due to vanished stress perception in the scion. Tolerant rootstocks favor water and nutrients uptake, photosynthesis maintenance, antioxidant system and hormonal signaling activation, and gene expression regulation, facilitated by the bidirectional signal transmission between rootstock and scion. This chapter summarizes the latest advances in pepper abiotic stress mitigation by grafting: how tolerance is achieved with the help of tolerant pepper rootstocks under heat, salt, and water stress.

1. Introduction

The genetic pool of Capsicum annuum includes lots of artificial and natural hybrids, breeding lines, cultivars, and landraces [1], as well as the most relevant commercial pepper varieties, such as bell peppers. In line with Ref. [2], the harvested pepper area worldwide in 2021 was 2.055.310 ha, with 36286.643 tons of pepper production all around the world. For continents, Asia was the first producer (67.9%), followed by the Americas (11.5%) and Europe (10.8%), in 2021 [2]. In the last two decades, pepper production has grown, but farmland areas diminish annually. This scenario can be partly owing to climate change because it accelerates land degradation, caused by drought, desertification, extreme temperatures, among other socio-environmental constraints [3].

Pepper is normally eaten fresh or processed as a spice or vegetable, and can also be used as cosmetic and medical applications from its extract, and also for ornamental purposes [4]. Global pepper success can be partly due to its nutritional composition, which comprises fiber, tocopherols like vitamin E, ascorbic acid (vitamin C), minerals, carotenoids, and flavonoids. Its vitamin C concentration is sufficiently high to be made available to consumers [5, 6].

One serious threat to the world’s agriculture is abiotic stress, and it is the first source of crop productivity loss that, for major crops, can be as high as 50% [7, 8]. This stress includes extreme temperatures, salinity, and drought, among others, whose prevalence and severity rise with today’s expected climate change conditions (Figure 1) [9]. Indeed, salinity and drought are expected to transform more than half the world’s arable land into unproductive land by 2050 [8].

Figure 1.

Frequency of natural disasters in the world (1980–2022). International Monetary Fund indicators dataset: Climate-related disasters frequency (2022). Blue bars refer to drought events, and orange bars depict extreme temperature events.

Certain strategies that aim to solve the problems caused by abiotic stress could include either conventional or molecular breeding. Nevertheless, tolerance to abiotic stress results from numerous quantitative characters controlled by several minor genes [8, 10]. Ameliorating abiotic stress by grafting on Solanaceae crops is an effective strategy, especially for heat stress, drought, and salinity conditions [11]. With horticultural crops, the basis of grafting for cultivation under abiotic stress conditions is rootstocks’ ability to survive undesirable stressors, in other words, their resistance [12]. Tolerant pepper rootstocks could attenuate the impact of abiotic stresses. Hence, the mechanisms implicated in this tolerance response should be studied.

2. Salinity

Salinity stress markedly limits plant growth and productivity, and influences water quality, soil biodiversity, and soil erosion [13]. Of every salt type that brings about salinity stress, plants must cope with the most soluble and ubiquitous form: sodium chloride (NaCl) [14]. All development stages from germination to reproduction are affected by salt stress because it causes wide-ranging symptoms: more succulent and thicker but smaller leaves that are abscised with necrosis on roots and shoots, and consequences for several physiological processes such as transpiration and photosynthesis. This stress interferes with the metabolism of phytohormones and other metabolic pathways, and can affect protein functions and disturb genes [15].

Plants generally respond to salinity in two phases: by one first quick response, which begins a few minutes after being exposed to salinity and lasts some days. It is known as the osmotic phase, when stomatal closure is promoted and young leaf growth is inhibited, followed by a delayed response, which can last for days, or even weeks. It is called the ionic phase, and it aims to stop metabolic processes, induce mature leaf senescence, and even cause cell death [14, 16].

The osmotic phase that is driven by salinity destroys water potential homeostasis due to the excessive concentration of ions in soil and/or water outside roots [17]. To successfully prevail osmotic stress, plants carry out osmotic adjustment to lower the osmotic potential in the cytosol. Next, they activate water transport systems and synthesize compatible solutes. All this involves less energy use for plant growth and for survival under stress conditions [18, 19]. Compatible solutes are also known as osmoprotectants or osmolytes, which consist of a substantial quantity of compounds, including sugar alcohols (myo-inositol), sugars (raffinose, trehalose), quaternary ammonium compounds (glycine betaine), and amino acids (proline) that operate to diminish oxidative damage from drought stress [20, 21].

The imbalance of ionic distribution and ionic homeostasis owing to not only the cellular toxicity of sodium (Na⁺) and chloride (Cl⁻) ions that accumulate but also an unbalanced K⁺/Na⁺ (potassium ion/sodium ion) ratio inside plants occurs in the ionic stress phase [15, 22, 23]. To reduce ionic stress, plants attempt to avoid high Cl⁻ and Na⁺ concentrations in the cytoplasm via two distinct mechanisms: transferring salt from the cytoplasm to vacuoles (sequestration) and/or preventing salt uptake by roots and it being transported across plants (exclusion) [18].

As salt stress triggers secondary stresses, mainly oxidative stress, to restore the reactive oxygen species (ROS) balance, plants develop detoxification strategies [24] to promote the synthesis of antioxidants and antioxidant enzymes activities to scavenge ROS, regulate ROS-responsive genes expression, and balance ROS generation [19, 24]. To overcome salinity, a molecular response occurs that involves salt-responsive genes, which carry out lots of functions, such as molecular chaperones (HSPs), ion homeostasis or transport (SOS, NHX1, H⁺-ATPase), and transcription factors (DREBs), among others [25]. This response is frequently mediated by Ca²⁺ accumulation sensing. This, in turn, begins complex signal transduction pathways and/or abscisic acid (ABA) because these genes can contain ABA-responsive elements [26].

In circumstances in which peppers face salt stress, the authors note less growth with salinity because of the toxic effect of ions and the limited capacity to make osmotic adjustments [23]. According to Suarez et al. [27], tolerant pepper cultivars limit the root-to-shoot Na⁺ flux. This involves the lesser gene expression of ion transporters and lower Na⁺ concentrations in leaves. These authors conclude that in pepper plants’ response to salt stress, Na⁺ exclusion is a crucial trait, with wide variability among genotypes related to salt tolerance [27]. The work of Penella et al. [28] establishes that one reliable trait for choosing pepper cultivars for their salt stress tolerance is maintaining the net photosynthetic rate.

In order to collect more in-depth knowledge about the tolerance mechanisms in pepper, the authors investigate the response of two pepper genotypes with different salt tolerances [29]. They find that the lower relative water content (RWC) in the tolerant genotype is lesser than in the sensitive genotype. The tolerant genotype maintains the chlorophyll concentration; the increase in lipid peroxidation is not so severe, and the increases in both superoxide dismutase activity and glutathione content are more marked than in the sensitive genotype [29]. The authors in Ref. [30] detect that tolerant pepper cultivars have less disturbed photosynthesis, a higher proline content, and a bigger leaf/and root biomass than the sensitive genotype under salinity conditions.

3. Heat stress

As Figure 1 shows, extreme temperature events are becoming increasingly frequent due to climate change. Heat stress is complex stress of variable intensity and duration that is frequently deemed to be heat shock when temperatures are 10–15°C above the normal crop development range for sufficiently long periods to irreversibly cause plant damage [31]. Rises in temperature can be transitory (heat wave or heat shock) or constant (prolonged heat stress), and can result in detrimental effects for plant development, plant growth, and yield losses, and can even cause plants to die in drastic circumstances [32, 33]. Thus, heat stress can impact plants at physiological, biochemical, reproductive, and morpho-anatomical levels [34].

Plants exposed to high temperatures can suffer cellular damage, including interference with enzyme functions by modifying protein activity or alterations to lipid membrane integrity by changing fluidity [35]. The impact of high temperatures on plant development depends on the growth stage, that is, delayed seed germination, which can lead to completely inhibited germination, as determined by heat stress intensity and plant species [31]. In succeeding stages, chloroplasts become extremely vulnerable to rising temperatures, and damage to chloroplasts signifies CO₂ assimilation, electron transport between both photosystems, and disturbed chlorophyll biosynthesis, among other photosynthesis-related processes [35]. Heat stress interferes with other relevant processes such as water relations, respiration, and primary/secondary metabolisms, which trigger changes in protein regulation and gene expression toward plant tolerance [31, 35].

Plant strategies to successfully overcome heat stress include changing leaf orientation to prevent direct solar radiation; the promotion of cooling via transpiration; ROS scavenging by means of antioxidants synthesis; preserving plant membrane stability via alteration to its lipid composition; the activation of heat response-related factors and genes, including heat shock factors (HSFs) and heat shock proteins (HSPs); the generation of compatible osmolytes; and several signaling strategies such as mitogen-activated protein kinase (MAPK), chaperones, and calcium-dependent protein kinase (CDPK) cascades [31, 34, 36].

In HSP terms, the pepper heat stress response is investigated in Ref. [37]. The authors put forward the principal role in thermotolerance for the Hsp70-1 gene. This gene is induced by putrescine, hydrogen peroxide (H₂O₂), and calcium ion (Ca²⁺). This induction is more marked with heat stress. The authors of Guo et al. [38] confirm the implication of HSP70s in the pepper heat stress response via the regulation of stress-related genes expression, plus their connection with plant development and growth. The glutathione metabolism has been related to thermotolerance in pepper because the increase in the metabolites and genes involved on this pathway in the tolerant pepper genotype under heat stress is greater than that in the sensitive genotype [39]. The authors of Rajametov et al. [40] suggest that the photosynthetic rate is maintained via improved transpiration, along with a higher proline concentration, as an effective strategy for thermotolerant peppers in the seedling stage.

For the reproductive stage, some authors have indicated a relation between flower abscission with heat stress and ethylene production [41]. In other words, flower abscission depends on flowers’ susceptibility to ethylene, which generates these stress conditions. Some studies note and confirm the effects of high temperatures on male pepper gametophytes, which are more sensitive than female gametophyte, and not only during their formation and development but also in the later germination and pollen tube growth stages [42, 43]. Fruit set at high temperature can be affected during fruit development. This leads to fruit weighing less and being smaller in size, and is closely related to pollination efficiency and subsequent ovule fertilization, which determine the number of seeds per fruit [44, 45]. In pepper fruit, some authors have run multi-omic analyses (proteomics, metabolomics, and transcriptomics) and noted how heat stress inhibits secondary metabolite synthesis, that is, ascorbic acid and capsaicin [46]. Three transcription factors (TFs) have been found to be involved in capsaicin synthesis regulation under heat stress conditions [46].

4. Drought

Water is an essential resource. It is threatened by an ever-growing population and urban areas emerging in arid areas under climate change conditions. In turn, the span, prevalence, and frequency of droughts all increase (Figure 1) [47]. FAO [47] indicates that agriculture is the most, and the first, sector to be affected by drought. Drought impacts on crops can result from many factors, including high light intensity, scarce rainfall, extreme (high and low) temperatures, or salinity [48].

The drought impact on plants has several adverse effects that influence all plant levels: biochemical, morphological, physiological, molecular, and cellular. Ultimately, this implies smaller crop yields [49]. Drought-related morphological alterations mostly include delayed growth and shorter plant height, which are consequences of slower cell expansion on roots, stems, and senescing leaves, whose number and size diminish [49, 50].

When the CO₂ assimilation rate lowers, it might be accompanied by a reduction in photosynthesis and transpiration [49, 50, 51]. This is when phytohormones play a role in the drought response, particularly ABA, whose synthesis is promoted under drought conditions in roots, and is transported to leaves and shoots to induce mainly stomatal closure and to avoid transpiration-related water loss [48].

Changes in water relations favor root growth over shoot growth due to rapid drops in the water potential (Ψ_w) in roots. This promotes water uptake, while the Ψ_w in leaves lowers at a slower rate [52]. In connection with rapid Ψ_w decline to allow water to enter, osmotic adjustment is normally made by accumulation and osmoprotectant synthesis, which make the concentration of the solutes inside cells rise [53].

Oxidative damage takes place by drought exposure and is generated by excess ROS. All this brings about harmful effects, such as DNA nicking, photosynthesis restriction, lipid peroxidation, and protein denaturation, among others [50]. Plants attempt to counteract oxidative damage by antioxidant systems being activated, which involve enzymatic activities such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), catalase (CAT), dehydroascorbate reductase (DHAR), superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferase (GST), ferritin, thioredoxins, glutaredoxins, as well as antioxidant compounds (i.e., carotenoids, glutathione, ascorbic acid, α-tocopherol flavonoids, and phenolic acids) [54, 55].

ROS can act as secondary messengers during drought signaling with other secondary messengers like ABA and Ca²⁺ to regulate the expression of the drought-induced proteins and drought-responsive genes implicated on signaling pathways [56]. Certain TFs like WRKY, dehydration-responsive element binding (DREB), NAC, MYB, HSFs, and basic leucine zipper (bZIP) are key regulators of the downstream genes involved in the drought molecular response [57]. Some drought-responsive proteins are aquaporins, which are water channels that transport metal ions, small solutes and gases, dehydrins (promote osmolyte accumulation, water retention, ROS scavenging, and photosynthetic activity), and late embryogenesis abundant (LEA) proteins (protect plant membranes to avoid water loss, among other functions) [58, 59, 60].

In the study by Delfine et al. [61], drought effects are studied on pepper plants. Here the authors report photosynthetic limitations via lesser stomatal conductance (g_s) and lower leaf Ψ_w which lead to reductions in plant growth, plant productivity, and fruit quality in relation to well-irrigated control plants. In the study by Hu et al. [62], the authors offer similar results in two pepper cultivars under drought conditions: a lower photosynthetic rate, increases in ROS, and then antioxidant enzymes APX and SOD being activated.

The authors of Ref. [63] describe the behavior of two pepper cultivars with differing degrees of tolerance. They also note opposite responses to progressive drought stress. Cultivar Shanshu-2001 better tolerates drought and obtains higher growth and yield values. This is probably the result of a higher RWC in leaves, lower lipid peroxidation, and a prolonged rise in the proline concentration and electrolyte leakage than cultivar Nongchengjiao-2, which apparently shows a sensitivity response [63]. It is noteworthy that the more tolerant cultivar performs greater constitutive SOD, CAT, and peroxidase activities than the more sensitive cultivar, which may confer better drought resistance ability [63].

Some authors have investigated the response of two pepper genotypes (one tolerant, one sensitive) under water stress and report comparable results: The RWC and photosynthetic rate are higher and g_s is lower in the tolerant genotype [64]; moreover, there are changes in gene expression; in other words, aquaporin genes are downregulated in the sensitive genotype and upregulated in the tolerant one. Some works have studied the transcriptomic response of water-stressed pepper plants, and Park et al. [65] have identified seven rapidly induced cDNAs. One (Ca-LEAL1) encodes for an LEA-like protein and is induced by ABA application. The authors in Ref. [66] identify and describe a DREB-like gene, which is rapidly induced by water stress in pepper roots, and they put forward a role in transcription activation.

Drought affects pepper fruit in several ways: delayed ripening, less production, and modified volatile and bioactive compounds. As such alterations depend on the plant stage, these mechanisms are required to adapt to drought conditions [67].

5. Grafting technique to overcome abiotic stresses

Grafting as an agricultural technique involves joining two different plant parts: the scion, which provides the shoot, and the rootstock, which supplies the root system (Figure 2). The scion and the rootstock are connected via the union of their vascular systems to become a single plant after being joined [69].

Figure 2.

Grafted pepper seedlings [68].

For more than 2000 years, grafting has been an ancient technique applied to fruit trees. In the last century, this practice was extended to vegetable crops [70]. Grafting in vegetables began in Korea and Japan to restore crop loss, which was caused by soilborne pathogens and led to grave diseases, intensified by serial cropping [71]. In the 1950s, tunnels and greenhouses became popular, and most farmers performed protected cultivation to prolong crop seasons and left the rotating crop system behind. Therefore, grafting a variety onto tolerant rootstocks is required to control soilborne infections, among other diseases [70].

Grafting has many other uses than soil disease mitigation applications, of which cultivation performed under adverse environmental conditions (biotic-abiotic stresses) is highlighted, as are improved crop yields that result from applying vigorous rootstocks [71, 72]. In the present day, most research centers on environmental stress resistance and vegetable grafting physiology, as well as on two families, that is, Solanaceae and Cucurbitaceae, which are cultivated as grafted plants in North America, Europe, and Asia [73, 74].

Grafting amends the scion stress perception of an unfavorable environment, that is, a big robust root system from rootstocks, which involves a better water relations balance, better ability for water and nutrients uptake in relation to the latter, enhanced photosynthetic activity and antioxidant system, hormonal signaling network activation, and, finally, long-distance signal transmission by mRNAs, small RNAs, and proteins [75]. There is a general consensus that bidirectional genetic exchange between the rootstock and scion occurs in grafting plants by the grafting union [76]. Long-distance signals particularly appear to be a promising strategy for modifying physiological functions and regulating rootstock-scion relations [74, 77] to move toward higher production.

5.1 Grafting in pepper to counteract abiotic stress

Abiotic stress amelioration by grafting on Solanaceae crops has been proven to be an effective strategy, mainly under salinity, heat stress, and drought conditions. Regarding salinity, the authors in Ref. [78] reveal that grafted plants mitigate growth reduction owing to enhanced photosynthesis and antioxidant enzyme activities.

On peppers that face salt stress, some authors attribute grafted plant tolerance to rootstocks’ ability to maintain ion homeostasis in the scion. This leads to less disrupted photosynthesis, lipid peroxidation, and nitrate reductase activity on leaves, as reflected by bigger marketable fruit yields [79]. In Ref. [80], the authors observe that the impact of salinity is milder on the biomass of the plants grafted onto the NIBER^® rootstock (F1 hybrid), with 32–80% higher yields than the ungrafted pepper plants previously employed in experiments under salinity field conditions for 3 years. This lesser impact is associated with lower reductions in nitrate reductase, photosynthetic rates, and stomatal conductance compared to the ungrafted and self-grafted plants. These authors also point out a signaling role for H₂O₂, which activates antioxidant mechanisms, as reflected by increased proline and phenols contents [80].

Finally, some other authors reveal that the response of grafted pepper plants to salinity is rootstock-dependent. They indicate three differing strategies: (i) The Creonte rootstock (from Ruiter seeds) increases fruit yield and the reproductive/vegetative ratio is not disturbed thanks to better biomass distribution, and also to opposite cytokinins and ethylene precursor regulation; (ii) the Terrano rootstock (from Syngenta) stunts plant height by non-gibberellins accumulation and induces ABA to maintain the reproductive and vegetative biomass; and (iii) the Atlante rootstock (from Ramiro Arnedo) and its invigorating effect that result from improving the ABA/cytokinins balance [81].

The impact of heat stress on pepper grafted plants is investigated in Ref. [82]. These authors indicate that employing the Creonte rootstock (from Ruiter seeds) under heat stress allows higher fruit yields and lowers the percentage of sunscalded fruits, whereas the photosynthetic rate rises. In a controlled environment, other authors have studied the response of grafted pepper to high temperatures. They have verified that marketable pepper yields increase (> 50%) grafting onto a thermotolerant rootstock under greenhouse conditions versus the ungrafted variety [43, 83]. These higher yields depend on mechanisms in the vegetative (i.e., undisturbed chlorophyll and carotenoids, more ascorbic acid, and less electrolyte leakage) and reproductive (enhanced pollen germination and fruit set and improved anthers proline content) stages.

Heat stress also affects pepper roots growth, and a bigger number of roots appeared on the peppers grafted onto a hybrid rootstock bred toward abiotic stress tolerance [84]. The combination of the variety with the hybrid rootstock obtained better results at the two evaluated upper depths (0–15 cm and 15–30 cm) compared to the self-grafted and ungrafted variety under heat stress [84].

With drought, grafted pepper responses under water stress induced by PEG are studied in Ref. [85]. For all the grafted plants, the authors observe diminished photosynthetic activity and NO₃⁻ uptake and transport to leaves. The photosynthesis of the plants grafted onto tolerant pepper accessions is less disturbed. The authors propose a protective role of osmotic adjustment and proline accumulation to overcome water stress [85]. Some other authors report a protective role for the tolerant pepper rootstock tested under short-term water stress against oxidative stress for being capable of alleviating oxidative stress in the scion and might balance the C and N biomass distribution, which results in larger crop yields [86].

Oxidative stress mitigation by the antioxidant system and its relation with photosynthesis maintenance with water stress in pepper grafted plants is studied in Ref. [87]. The authors show that pepper plants grafted onto tolerant hybrid rootstock H92 under water stress are able to avoid oxidative damage and sustain the photosynthetic rate thanks to enhanced proline and ascorbic acid contents.

The work in Ref. [88] puts the NIBER^® pepper tolerant rootstock under deficit irrigation conditions for consecutive years to the test. It reports the milder impact of water stress on the plants grafted onto the NIBER^® rootstock in photosynthesis, fruit production, and plant biomass terms (marketable yield losses were up to 50% lower) versus the ungrafted variety. With other studies, the authors confirm their previous results obtained in the pepper plants grafted onto the NIBER^® rootstock (50% higher marketable yield under water stress) and they note better root biomass distribution compared to the ungrafted variety [89].

Regarding roots implication in pepper water stress tolerance, in Ref. [90], the authors describe constitutive differences in the roots gene expression between pepper-sensitive accession A10 and pepper-tolerant rootstock NIBER^®. These differences under the control conditions were associated with the detoxification system and ABA induction in the NIBER^® rootstock. Under short-term water stress conditions in the NIBER^® rootstock, TFs like DREBs and MYC, chaperones, and aquaporins were upregulated, while the concentration of auxins, abscisic acid, jasmonic acid, trehalose, raffinose, and spermidine increased [90]. The authors point out that these mechanisms, together with lower oxidized glutathione, indicate the ability to prevent oxidative damage and water stress tolerance in the very early phase of stress.

The signal transmission from roots to shoots, and vice versa, is key for water stress tolerance achievement [91]. The phytohormones network regulation of roots and shoots during the water stress response in pepper grafted plants is studied in Ref. [92]. The authors quantify a higher ABA concentration in the variety grafted onto NIBER^® roots at 4 h after water stress and suggest the transport of this hormone from roots to shoots because the ABA concentration lowered in the roots and rose in the leaves of this plant combination after 48 h of water stress. Moreover, the authors link the ABA levels at 48 h with the higher WUE (water use efficiency) in the variety grafted onto NIBER^® under water stress, which is favored by major stomata closure in relation to the self-grafted variety. Simultaneously, the concentrations of the ethylene precursor ACC, jasmonic acid, and salicylic acid increased in this plant combination under water stress, which the authors explain by their role in abiotic stress signaling and tolerance [92].

6. Conclusion

In summary, grafting is a proven effective strategy for mitigating abiotic stress effects on horticultural plants, specifically on pepper plants. In this chapter, some abiotic stress tolerance mechanisms of grafted pepper plants are presented, which lead to better production and fruit quality (Figure 3). Tolerant rootstocks are able to modify the stress perception of the grafted variety by means of different tolerance strategies to overcome high temperature, salinity, or drought. Given a trend toward sustainable approaches to fight climate change, the grafting technique has a lot to offer. Recent efforts have been made to study the molecular mechanisms involved in grafting-acquired tolerance, but a lot of work remains to be done.

Figure 3.

Abiotic stress tolerance mechanisms in roots and leaves of grafted pepper plants. The drop icon depicts drought stress; the sun icon depicts high temperature stress, and the salt shaker icon refers to salt stress. The blue squares include the tolerance mechanisms found in one stress condition (one icon) or common to more than one stress condition individually (two or three icons). Arrows depict an increase (up) or decrease (down) under stress conditions in relation to control conditions, and the equal sign reflects no changes. This figure was designed using Freepik illustrations.

Acknowledgments

This work has been financed by Grant PID2020-118824RR-C21 funded by MCIN/AEI/10.13039/501100011033. Ramón Gisbert-Mullor is a beneficiary of a doctoral fellowship (FPU-MEFP (Spain)). Yaiza Gara Padilla is a beneficiary of grant PRE2018-086374 funded by MCIN/AEI/10.13039/501100011033 and, as appropriate, by “ESF Investing in your future.” Parts of this chapter were previously published in the doctoral thesis by the same author: Yaiza G. Padilla. Study of the Physiological, Metabolomic and Transcriptional Changes Mediated by Rootstocks to Explain the Water Stress Tolerance of Grafted Pepper Plants. 2023. Universitat Politècnica de València. Available from: https://riunet.upv.es/handle/10251/199992.