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Underlying Mechanisms of Action to Improve Plant Growth and Fruit Quality in Crops under Alkaline Stress

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Fabián Pérez-Labrada, José Luis Espinoza-Acosta, Daniel Bárcenas-Santana, Elizabeth García-León and Mari Carmen López-Pérez

Submitted: 22 December 2023 Reviewed: 21 February 2024 Published: 24 April 2024

DOI: 10.5772/intechopen.114335

Abiotic Stress in Crop Plants IntechOpen
Abiotic Stress in Crop Plants Edited by Mirza Hasanuzzaman

From the Edited Volume

Abiotic Stress in Crop Plants [Working Title]

Prof. Mirza Hasanuzzaman and MSc. Kamrun Nahar

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Abstract

The high content of carbonates (CO32−), bicarbonate (HCO3−), and high pH (>7.5) causes environmental pressure and alkaline stress, impairs plant growth and development, and limits fruit quality by causing osmotic alterations and hindering nutrient absorption. Because of alkaline stress, plants are in an oxidative environment that alters their metabolic processes, impairing their growth, development, and fruit quality. In response to this situation, plants use several mechanisms to cope, including the alteration of osmolytes, induction of transcription factors, signal transduction, hormone synthesis, alteration of the antioxidant system, and differential gene expression. Current knowledge and understanding of the underlying mechanisms that promote alkalinity tolerance in crops may lead to new production strategies to improve crop quality under these conditions, while ensuring food security.

Keywords

  • antioxidants
  • calcium
  • pH
  • oxidative stress
  • potassium
  • transcriptomic

1. Introduction

Natural alkalinity (associated with arid and semi-arid regions) or alkalinity induced by agricultural activity is serious because it affects food productivity [1, 2]. Of the total global cultivated land, this stress affects 30–37% (438 million ha) [3]. High carbonate (CO32−) and bicarbonate (HCO3−) contents [4] promote the soil buffering capacity, inducing a high pH [5]. Natural highly alkaline conditions (pH ≥ 9) are associated with parts of Western Asia, Eastern Europe, southern South America, and southern Australia, with increasing incidence in India and Egypt as a consequence of irrigation by the Ganges River and the Nile (Figure 1) [6]. This alkaline environment impairs plant and fruit growth and quality by negatively altering the root system, modifying the nutrient uptake system, and altering the relative water, chlorophyll (Chl), and soluble sugar contents [7]. In addition, there is usually a reduction in the microbiological activity and bioavailability of mineral nutrients [5, 8]. Likewise, plants show damage to electron transport, and thus, photosynthetic activity [9]. Similarly, high pH induces the synthesis of reactive oxygen species (ROS) and malondialdehyde, which cause significant damage to the cell membrane and intracellular components [10].

Figure 1.

Probable distribution of alkaline soils.

Alkalinity has diversified impacts on plants, so the response and tolerance mechanisms are more complex [10]. Initially, the plant senses the stressful condition (elevated pH derived from carbonate content) via root apoplast, thanks to a sulfotyrosine of the endogenous peptide root growth factor (RGF) being deprotonated, destabilizing the RGF-RGFR receptor-SERK coreceptor complex. Considering that the target of RGF is a master regulator of root development (PLETHORA) [11], inhibition of this complex reduces root apical meristem growth [12, 13]. The same authors pointed out that this pH sensing system procures a growth or defense system, which significantly affects plant quality.

To combat this stress, plants develop mechanisms that allow them to cope with high pH, high carbonate content, ionic toxicity, and oxidizing environments. Initially, they promote an osmoregulatory system to reduce cellular Ψ [9] and maintain a high K+/Na+ ratio [14]. Likewise, it promotes the activity of enzymatic and non-enzymatic antioxidant systems to combat oxidative stress [15, 16] as well as the expression of hormones and a diversity of metabolites [17] derived from transcriptional modifications [18, 19, 20, 21]. Alternatives in the management of crops developed in these environments include grafting [22], application of Ca2+-based fertilizers (to replace Na+ ions) [23], micronutrients [24], nanotechnology [25], biochar [26], microorganisms [27], growth regulators [28], and genetic engineering [29, 30].

In that sense, this study aimed to establish the main impacts of alkalinity on plants and to review the underlying mechanisms that could promote tolerance to this type of stress. It also highlights management alternatives that can potentiate crop quality in the face of environmental stress. This knowledge seeks to promote the development of innovative strategies to mitigate the negative impacts of alkalinity and safeguard the production of high-quality food.

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2. Alkalinity environments

Alkaline stress in soils is characterized by a high pH (generally >7.5) caused by the dominance of [HCO3−] + 2[CO32−], salts, and carbonate alkalinity [8, 31]. These anions naturally originate from the dissolution of carbonate minerals from the parent material and by the combination of water and (H2O + CO2disolved + CO32− ↔ 2HCO3−) which in turn can react with calcium (Ca2+) in the soil: Ca2+ + CO32− ↔ CaCO3; [Ca2+][CO32−] [8, 32]. Another pathway is high evaporation and low precipitation [33], which are common in arid and semi-arid areas. In soil solutions, when organic species are omitted, the alkalinity can be defined as [HCO3−] + 2[CO32−] + [H2PO4−] + 2[HPO4−] + 3[PO43−] + [B(OH)4−] + [OH−] − [H+] [8, 34]. Generally, soils can have between 0.4 and 3 mM HCO3−, a condition that increases and buffers the edaphic pH and alkalinizes the xylem medium [13]. However, high alkalinity (pH ≥ 9) can occur in soil solutions with HCO3− + CO32− > Ca2+ + Mg2+. Likewise, when sodium chloride (NaCl) salts react in the soil to release H+, they tend to react with CaCO3 to form sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3) salts. In addition to increasing exchangeable Na+, it is important to mention that alkalinity is regulated by the concentrations of HCO3− and CO32− and not by Na+ [8, 35]. In this context, the increase in soil pH is associated with an increase in Na+ concentration, an environment that reduces Ca2+ and magnesium (Mg2+) cations, causing the dispersion of clays and loss of soil structure [8].

In water bodies, alkalinity is defined as [HCO3−] + 2[CO32−] + [OH−] − [H+] and is regulated by the dynamics of CO2dissolved, carbonic acid (H2CO3) [CO2 + H2O = H2CO3], HCO3−, and CO32−, as H2CO3 ↔ H+ + HCO3− and HCO3− ↔ H+ + CO32−, and CO2 can react with CaCO3 + H2O dissociating into Ca2+ + 2HCO3− [36]. Therefore, water alkalinity is a determinant of agricultural production, as it can modify the pH of the substrate (H2O + CO2dissolved + CO32− ↔ 2HCO3−), altering the growth, development, and quality of plants and fruits [37]. Although the negative effects of carbonates vary according to pH, the greatest damage by HCO3− occurs at pH 8–9 while CO32− occurs at pH >9 [8]. When alkaline soils (with a sodic subsoil) present transient waterlogging or anoxic environments, the CO2 concentration increases with increasing pH because of the increase in HCO3− + CO32− > Ca2+ + Mg2+ [8]. Waterlogging can induce toxicity in Mn, Fe, Al, and B.

Plants in alkaline environments are simultaneously exposed to salt action and elevated pH [9], causing osmotic stress, ionic toxicity, and elevated pH, which severely affect plant growth [1, 38]. High pH reduces cellular water content by altering the root system by reducing turgor pressure, which minimizes the extensibility processes of microfibrils and thus cell extension, water and nutrient uptake, and solute accumulation [39]. Similarly, the buffering capacity of HCO3− affects the solubility of essential nutrients, inducing deficits in iron (Fe2+), manganese (Mn2+), boron (B), zinc (Zn2+), and phosphate (PO43−) [8, 9, 40]. However, attenuation of alkaline effects in soil can be mitigated when pH < 9; soils with pH 8.5 or > 9 demand less H+ to induce a reduction due to lower CaCO3 content, which buffers the pH. In contrast, soils with pH ≈ 8.5 demand more H+ because the concentration of CaCO3 dissolves and buffers pH alteration [8, 41]. This is because CaCO3 ↔ Ca2+ + CO32−, and CO32− + H2O ↔ HCO3− + OH−. Thus, one of the most feasible criteria for mitigating the negative effects of alkalinity is HCO3− + CO32− > Ca2+ + Mg2+, where one would ideally seek to reduce HCO3− + CO32− and maintain soil pH ≤ 8.5 [8].

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3. Impact of alkalinity on agricultural crops and fruit quality

Alkaline stress has a significant impact on yield, plants, and fruit quality because it hinders the smooth uptake of nutrients [42]. The damage caused varies according to the type and phenological stage of the plant, time of exposure, and level of alkalinity of the medium; however, the main disorders are oxidative damage caused by alterations in ionic homeostasis and impairment of the photosynthetic process (Figure 2a) [43, 44].

Figure 2.

Endogenous alterations (a) and response mechanisms (b) in plants under alkaline stress.

3.1 Process photosynthetic alterations

The photosynthetic capacity of plants under alkaline stress is negatively altered by reduced leaf area and Chl content [25, 45]. In Medicago sativa plants, Chl synthesis and photosystem activity (PSII/PSI) are altered, promoting non-photochemical quenching (NPQ) and cyclic electron transfer, and negative regulation of PetA, PetB, petE, and petF genes associated with the photosynthetic mechanism was observed [46]. A decrease in the maximum quantum yield of PSII photochemical parameters (Fv/Fm), PI, Fv, Fm, Fv/Fo, RC/ABS, and (1-Vj)/Vj, Chl fluorescence, and relative leaf water content in relation to NaHCO3 levels during the vegetative and reproductive growth stages has been documented in strawberry plants. These conditions reduce fruit quality parameters, such as electrical conductivity, pH, total soluble solids, and color [9]. Likewise, a decrease in gas exchange rates (reduced transpiration, stomatal conductance, and CO2 assimilation) can occur, reducing the maximum quantum yield of PSII primary photochemistry (φPo) and causing an increase in PSII energy dissipation and inhibiting PSII electron transport. This environment leads to overproduction and overaccumulation of ROS, mainly hydrogen peroxide (H2O2), hydroxyl ions (OH•), singlet oxygen (1O2) [47], and superoxide radicals (O2−). Increased concentrations of H2O2, OH•, 1O2, and O2•− cause lipid peroxidation of the thylakoid and chloroplast membranes, degrading Chl and inhibiting the electron transport chain, leading to the collapse of the photosynthetic apparatus.

The growth, development, and quality of plants and fruits strongly depend on the synthesis of sugars (carbohydrate metabolism), such as hexose, glucose, and fructose, as they are the basis for the generation of metabolites involved in plant growth [48, 49]. The collapse of the photosynthetic process causes a reduction or no synthesis of sugars; therefore, alkalinity decreases the chemical characteristics of the fruit, which is cited in strawberry fruits under alkaline conditions, where the increase in NaHCO3 concentration was associated with a decrease in the content of soluble sugars [9] coupled with intercellular ion imbalance [50]. Alkalinity stress decreases production, as well as total soluble solids and acidity, in Blanc Du Bois vines with their own roots, derived from the low nutritional status [51]. Similarly, reduction of the photosynthetic process by carbonate salts minimizes the synthesis of some pigments, such as anthocyanins, which may affect the quality of leaves, flowers, and fruits [9, 52]. In addition, the aforementioned soluble sugars participate in carbon stores and are critical for osmoregulation, osmotic adjustment, and ROS scavenging [14, 53].

3.2 Ionic disbalance

The affection in the ionic balance of the plant by alkalinity (NaHCO3) is due to a differential modification in the absorption and storage of calcium (Ca2+), potassium (K+) and sodium (Na+) or by the increase in pH that reduces the availability and absorption of phosphorus (H2PO4−), magnesium (Mg2+), K+, iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) [25, 54, 55, 56]. For example, in oilseed rape (Brassica napus L.), inhibition of Ca2+ uptake in stems and leaves has been reported, which leads to ionic imbalance between K+ and Na+ [57].

Under 40 mM Na2CO3, a reduction in the growth of B. napus shoots was documented because of the high pH and pH-salt ion interaction, showing a high K+ concentration and K+/Na+ ratio in leaves as well as an increase in Ca2+ and Mg2+ in roots [58]. In blueberry “Jewel” (Vaccinium corymbosum) plants, a reduction in H+ flux from the root was documented, which may be associated with inhibition of the VcHA gene encoding H+-ATPases derived from the alkaline condition, pH 8.0 [59]. In strawberry plants, increased Na+ absorption was observed, accompanied by a decrease in K+, Ca2+, Mg2+, and Fe, which negatively affected fruit color [47]. In Ricinus communis L. seeds exposed to 50, 100, and 150 mM NaHCO3, the germination rate was reduced by an increase in Na+ content and reduction in K+ [60].

3.3 Oxidative stress

The burst in ROS production and concentration is a generalized response in plants subjected to alkaline stress, which causes lipid peroxidation, increases malondialdehyde (MDA) concentration, and electrolyte leakage [16, 28, 61], creating an oxidative environment [56] that can damage plant genetic material by arresting the cell cycle in the root [14]. This oxidative environment can reduce the germination process and root development in seedlings [62]. In this regard, inhibition of the germination power of Hordeum vulgare L. under alkaline stress has been cited, decreasing seedling vigor, which may be due to starch degradation, suppression of sugar metabolism, and reduction of enzymes involved in the Embden-Meyerhof-Parnas process, tricarboxylic acid cycle, pentose phosphate pathway, and the enzymes hexokinase and malic dehydrogenase [63].

In maize, increased lipid peroxidation, higher Na+ content, and elevated H2O2 levels were documented, which is in agreement with the reduction of K+ and Chl, decreased shoot and root fresh weight, root dry weight, plant length, number of leaves, and maximum leaf width, when grown under NaHCO3:Na2CO3 (9:1) [64]. The oxidative environment prevailing in plants under alkaline stress can significantly reduce the level of phosphatidylcholine (the main phospholipid of the cell membrane) in addition to inducing alterations in the expression of non-specific phospholipase A and phospholipase D/phospholipase C encoding genes [65]. This phenomenon can lead to marked deterioration in plant and fruit quality.

Under alkaline conditions (Na2CO3 75 mM), Trifolium alexandrinum L. showed oxidative damage by H2O2, O2•−, methylglyoxal, MDA, and oxidized glutathione, reducing shoot and root length, as well as Chl content, water, and nutrient uptake [54], reducing plant quality. In Onobrychis viciaefolia, there was an increase in MDA derived from the generation of ROS (mainly O2•− and H2O2), accompanied by an increase in proline content. This environment reduces fresh and dry biomass, Chl content, K+ concentration, K+/Na+ ratio, and fructose and glucose contents [66]. This stress also affects fruit quality; it decreases nucleic acids, lipids, hydroxycinnamoyl derivatives, organic acids and derivatives, and vitamins in wolfberry (Lycium barbarum) under saline-alkaline conditions [67].

3.4 Enzyme activity and transcriptomic responses

Under alkaline stress, radical cells are severely damaged by the accumulation of ROS (mainly O2•− and H2O2), which inhibit plant growth. Although this condition increases the activation of antioxidant enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11), ascorbic acid (AsA), and glutathione (GSH), their synthesis is overcome by excess ROS. This phenomenon was associated with the positive expression of cell death-associated genes OsKOD1, OsHsr203j, OsCP1, and OsNAC4 and negative regulation of the cell death suppressor gene (OsBI1) [68].

Suppression of enzyme activity implies a potential reduction in plant and fruit growth, development, and quality, although this response is cultivar- and alkalinity level-dependent. In mulberry seedlings, oxidative stress (O2•− and H2O2) induced by NaHCO3 (100 mM) reduced CAT activity and CAT gene expression (W9RJ43), but improved POD and the AsA-GSH acid cycle, allowing the elimination of H2O2. The genes involved in the expression of the electron donor ferredoxin-thioredoxin reductase (FTR) and proteins such as thioredoxin M (TrxM), thioredoxin M4 (TrxM4), thioredoxin X (TrxX), TrxF, and Trx CSDP decreased their expression, that is, there was an inhibition of the thioredoxin-peroxiredoxin (Trx-Prx) elimination pathway, which did not decrease oxidative damage [69]. Higher POD, SOD, and CAT activities have been reported in maize plants [70].

Transcriptomic regulation related to the oxidation-reduction process and lipid metabolism, as well as the expression of genes encoding D-3-phosphoglycerate dehydrogenase 1, glutamyl-tRNA reductase 1, fatty acid hydroperoxide lyase, ethylene-insensitive protein 2, metal tolerance protein 11, and magnesium-chelatase subunit ChlI, are altered in sugar beets under this stress [10]. These transcriptional modifications can significantly reduce crop quality; however, some red cherry tomato varieties susceptible to alkaline soils (pH 7.9) exhibit acceptable fruit quality [71].

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4. Plant mechanism against alkalinity stress

Plants sense alkaline environments through the root, where the sulfotyrosine of the endogenous peptide RGF is deprotonated and destabilizes the RGF-RGFR receptor-SERK receptor complex, causing alterations in the root apical meristem [12, 13]. Thus, RGF is a pH sensor that reacts to H+ concentration in the medium, which could be the first signaling mechanism for plants under alkaline stress. Considering that this stress impacts plants in a multifaceted manner, plant response mechanisms are complex [10]. However, as shown in Figure 2b, the tolerance systems developed by plants can be generalized to osmoregulation, induction of the antioxidant defense system, and metabolites through transcriptomic modifications.

4.1 Osmoregulation

As previously mentioned, alkaline environments cause alterations in ionic homeostasis, and plants can extrude hydrogen ions (H+) into the rhizosphere by means of proton pumps associated with the root plasma membrane (PM H+-ATPases - HAs) acidifying the rhizosphere zone, allowing nutrient uptake [59] to achieve an adequate ionic balance. Although Na+ can enter through non-selective K+ channels, the latter (K+) tends to be translocated to the cytoplasm to generate osmotic regulation [54], thereby reducing toxicity and generating an adequate flow of water and nutrients [72].

In canola roots under alkaline conditions (Na2CO3, 40 mM), the concentrations of K+, Ca2+, and Mg2+ are elevated, similar to the K+/Na+ ratio in the leaves [58]. In alkaline soils, Mn deficiency triggers the kinase pathway (CPK21 and CPK23) and increases Ca2+ uptake mediated by the NRAMP1 transporter [73], safeguarding the cation balance. A good ionic supply and K+/Na+ regulation can improve transpiration, Chl content, PSII quantum efficiency, relative water content, and water saturation [74], in addition to photosynthetic indicators [18].

Another osmoregulation alternative can be the increase in sugar concentration, in this regard it is mentioned that in fruits of Jujube “Junzao” irrigated with a moderate alkaline solution (90 mM of NaCl:NaHCO3 (3:1)), an increase in sucrose, glucose, and fructose content was observed, in addition to an increase in sucrose phosphate synthase (SPS) and sucrose synthase (SS-I) activities [53] allowing a partial attenuation of stress and maintaining plant and fruit quality. In addition to the accumulation of soluble sugars [14], proline is associated with osmoregulatory mechanisms in plants under alkaline conditions [46].

4.2 Stimulation of antioxidant biocompounds

One of the mechanisms developed by plants to census and scavenge ROS under alkaline stress conditions is the promotion of the antioxidant system by SOD, POD, CAT, APX, glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) [16], as well as phenolic acids and flavonoids. Increased SOD, CAT, POD, and APX enzyme activities are in line with higher levels of SOD, CAT, POD, and APX gene expression [64]; however, this mechanism only operates when the ROS burst is below the enzyme response threshold and in line with osmotic regulation. For example, some cultivars of broom millet show tolerance to alkaline stress (80 and 40 mM) because of high antioxidant enzyme activity that reduces MDA concentration, as well as a high content of soluble osmolytes, which reduces electrolyte leakage [15]. Similarly, in soybeans grown in hydroponics (NaHCO3 and Na2CO3; Na+ from 25 to 50 mM), activation of the antioxidant enzymes SOD, POD, and APX was found [14]. In contrast, rapeseed line 2205 (B. napus) showed reduced plasma membrane damage due to osmoregulation and higher enzyme activity [75]. In the case of autotetraploid rice 93-11 T grown on saline-alkali soil, tolerance was found to be derived from soluble sugar accumulation and increased SOD and POD activity in leaves [17]. It appears that the level of polyploidization in rice may trigger alkaline tolerance by stimulating peroxide-associated genes, transcription factors [76], and possibly the antioxidant system.

Another type of compound that promotes ROS attenuation is phenolic acids, polyphenols, flavonoids, and alkaloids; in grapes, their content increases under alkaline conditions [16]. In addition to these compounds, some essential oils [25], amino acids, and organic acids (in Lycium barbarum L. berry) promote regulatory mechanisms [67]. In alkalinity-tolerant Arabidopsis thaliana, an increase in methionine-derived glucosinolates via jasmonate and salicylate has been reported [77]. Likewise, salicylic acid (SA) can increase alkalinity by promoting signaling mechanisms [17].

4.3 Transcriptomic regulation

Plants under alkaline stress can exhibit alterations in signaling and response processes through a complex transcriptomic network. In sugar beet (Beta vulgaris L.), an alkaline salt (Na2CO3) attenuating mechanism, differential expression of genes related to cutin, suberin, wax biosynthesis, sesquiterpenoid and triterpenoid biosynthesis, flavonoid biosynthesis and monoterpenoid biosynthesis, amino acid metabolism, and starch and sucrose metabolism have been documented [18]. Histologically, B. napus showed more integrated mesophyll cells as well as higher expression of genes associated with carbohydrate metabolism, photosynthetic processes, and ROS regulation [75].

Inhibition of ion uptake could be a mechanism used by plants to signal alkaline environments through the differential expression of genes related to hormone signal transduction, mitogen-activated protein kinase (MAPK) signaling, phenylpropanoid biosynthesis, and ATP-binding cassette (ABC) transport. Although phenylpropanoid and flavonoid biosynthesis pathways are exacerbated, the accumulation of metabolites varies according to the level of alkalinity in the environment [78]. Likewise, there may be positive regulation of specific root transporters (e.g., NTR1, NRT2.1, and NRT2.5) [79].

Surprisingly, alkalinity of alfalfa (Medicago truncatula) generates a redistribution of nutrients, a mechanism controlled by the differential expression of 985 genes associated with nutrient cycling, cellular amino acid metabolic, and catabolic processes of organic substances, and 1180 genes associated with the oxidoreductase complex, aerobic respiration, and ion transport. In addition to several transcription factors, MYB, WRKY, bHLH, and NAC are involved in stress resistance and senescence regulation [80]. In contrast, recombinant inbred lines (RILs) of rice showed tolerance to alkalinity due to a low Na+/K+ ratio derived from a reduction in Na+ accumulation and increased K+ accumulation in roots and shoots. Tolerance may also be due to the expression of expressed protein genes, glucan endo-1,3-beta-glucosidase precursor, F-box domain-containing proteins, double-stranded RNA-binding motif-containing proteins, aquaporins, receptor kinase-like proteins, semialdehyde, hydrogenase, and NAD-binding domain-containing protein genes [81].

Sodium-hydrogen exchangers (NHX) are fundamental regulators of cellular Na+/K+ and pH homeostasis. In soybean, the expression of the GmNHX6 gene encodes a sodium/hydrogen exchanger (NHX) in the Golgi apparatus via a single nucleotide polymorphism in the NHX6 promoter region. In A. thaliana, the expression of GmNHX6 enhances tolerance to alkaline salts derived from a high K+ content and low Na+/K+ ratio [31, 82]. In contrast, the Gγ protein seems to regulate sensitivity to alkalinity, with the alkaline tolerance 1 (AT1) locus, which encodes the γ subunit of the atypical G protein. In this context, a model for the regulation of crop alkaline tolerance mediated by the AT1 subunit of the G protein is proposed because under alkaline stress conditions, artificially or naturally removing AT1, as well as its homologs, modulates the phosphorylation of PIP2, which allows the outflow of H2O2, reducing its cellular concentration, improving tolerance, and promoting crop yield [61]. Several aquaporins can interact with AT1, including SbPIP2;1 with SbAT1 (sorghum) and OsPIP2 with 1-OsGS3, a homolog of AT1 (rice). These findings indicate a vital role for aquaporins in ROS regulation. Thus, SbAT1, which encodes the γ subunit of the G protein, inhibits aquaporin phosphorylation, thus regulating H2O2 concentration and levels under alkaline stress [83]. This information applies to monocotyledonous crops; therefore, future research should be conducted on dicotyledonous plants. Similarly, when TaAT1 was modified, higher wheat yields were observed in alkaline soils [84]. In sorghum plants tolerant to alkaline stress, ATP1 (alkaline tolerance 1) encodes the G protein by negatively modulating the phosphorylation level of PIP2 (aquaporin with H2O2 transport activity), that is, when ATP1 is eliminated, PIP2 is phosphorylated from the cell membrane, allowing greater transport of H2O2 from the cytosol to the apoplast, reducing oxidative stress [85].

Oilseed rape grown in alkalinized soil (Na2CO3; 8 g  kg−1) shows strong genetic regulation associated with ionic K+ transport as well as alterations in the amino acid metabolic pathway, leading to reduced Na+, MDA, and relative conductance (REC), increased K+ and Mg2+, and the enzymes SOD, POD, and CAT. Interestingly, the positive regulation of phenylalanine ammonia lyase (PAL) transcription factors shows greater importance in mitigating alkaline stress [86]. Similarly, higher nitrogen content, soluble sugar, proline, and higher SOD and POD activities have been reported in plants under alkaline conditions [79].

In soybean (Glycine max, cultivar Talar), exposure to alkalinity (50 mmol L−1 NaHCO3) affected the transcriptomic level associated with abiotic stress response processes, metabolic regulation, response to cellular stimuli, and signaling, highlighting 321 overexpressed genes [87]. A. thaliana is sensitive to alkalinity and induces genes associated with Fe deficiency in leaves [77]. Molecular regulation enhances the defense system and osmotic adjustment, by modifying genes associated with ubiquitin-mediated proteolysis, glycolysis/gluconeogenesis pathways, transcription factors (C2H2, bZIP, and bHLH), and protein kinases CaMK (calcium/calmodulin-dependent protein kinase), CDPK (calcium-dependent protein kinase), MAPK (mitogen-activated protein kinase), and RLK (receptor-like kinase) [88]. In wheat plants, approximately 20 genes are associated with salt stress control, abiotic stress tolerance, response to oxidative stress, and response to osmotic stress, including TraesCS2B02G276500, TraesCS2B02G504900, and TraesCS2B02G276300 [55].

Hormone content may also be regulated in response to alkalinity; cotton plants differentially express 18,230 and 11,177 genes in the root and leaf tissue, respectively. These genes are associated with plant hormone signal transduction, amino acid biosynthesis, and secondary metabolites, the latter of which is related to ROS scavenging [89]. In broomcorn millet (Panicum miliaceum L.), there is negative regulation of gibberellic acid (GA) synthesis genes and positive regulation of GA inactivation and abscisic acid (ABA) synthesis genes; therefore, resistant genotypes have higher GA/ABA ratios, in addition to having a more robust antioxidant system [90]. Similarly, polyploid rice plants under high salt–alkali stress conditions (50 mM Na2CO3, pH = 11.39) showed positive hormone regulation, particularly of genes associated with auxin (AUX), SA signal transduction (roots), lignin biosynthesis (leaves or roots), and wax biosynthesis (leaves) [17].

Some plant 3-ketoacyl-CoA synthase gene families (KCS; GBKCS3, GBKCS8, GBKCS20, GBKCS34) may be involved in hormonal signaling, defense, and stress responses that allow Gossypium barbadense to improve its fiber quality [91]. In lentil plants grown in alkaline soil, an increased number of genes related to ABA signaling (dehydrin 1, 9-cis-epoxycarotenoid dioxygenase, ABA-responsive protein 18, and BEL1-like homeodomain protein 1) as well as increased synthesis of secondary metabolites were found [20], which could promote a higher mitotic rate, allowing the organization of the internal structures (root and shoot) of the plant, avoiding the disintegration of the internal structure and the distortion of the root and shoot architecture. This environment allows for an increase in K+ accumulation, osmolytes, antioxidant enzymes, relative water content, membrane stability index, and abscisic acid. In canola, tolerance to alkaline salts is mediated by modifications in Ca2+ signaling, ABA, and ROS pathways, which together promote gene regulation, organic acid synthesis (osmoregulators), and the transcription factors bHLH, WRKY, ERF, MYB, and NAC [92]. In the cereal Chenopodium quinoa Willd, the differential expression of genes associated with ROS homeostasis, hormone signaling (auxin and ethylene), cell wall synthesis (hemicellulose), and transcription factors has been documented [93].

Metabolite accumulation is a plant response to alkaline events; however, these transcriptomic alterations can reduce plant production and fruit quality [28]. In this regard, canola plants stressed with Na2CO3 showed accumulation of L-proline, L-glutamate, L-histidine, L-phenylalanine, L-citrulline, L-tyrosine, L-saccharopine, L-tryptophan, linoleic acid, dihomo gamma linolenic acid, alpha linolenic acid, Eric acid, oleic acid, neuronic acid, sucrase, alpha, alpha trehalose, and polyol (ribitol), associated with a decrease in UDP-D-galactose, D-mannose, D-fructose, and D-glucose 6-phosphate [21]. In broom millet, differential accumulation of volatile and non-volatile metabolites has been reported, namely phenylpropanoids, flavonoids, flavones, flavanols, valine, leucine, isoleucine, arginine, proline, tryptophan, and ascorbate [78]. Under alkaline conditions (50, 100, and 150 mM NaHCO3), castor seeds promoted between 31 and 113 differential metabolites and 138–694 genes associated with amino acid, carbohydrate, lipid, and nucleotide metabolism. In this sense, castor seeds in alkaline conditions improve amino acid metabolism and the tricarboxylic acid cycle, allowing the regulation of ROS levels and promotion of osmotic balance [60].

Alkalinity induced higher expression of GmABF4, GmAAO3, GmABI5, GmNCED5, and GmNCED9 genes in soybean [62]; however, ribonucleic acid editing in rice chloroplast and mitochondria mRNA increased under alkaline stress; four types of RNA editing were notable—A-G (A-I), C-T (C-U), G-A, and T-C. Pentatricopeptide repeats (PPR) and organelle zinc-fingers (OZI) are involved in various biological functions and molecular processes. This phenomenon may contribute to amino acid changes in genes, which could lead to tolerance to this type of stress [94].

In Glycine max (L.) Merri, the gene coding for DNAJ chaperone proteins (coding for GmDNAJC7 gene) showed overexpression allowing stress tolerance; higher Chl content and relative water content [82]. In soybean, the gene associated with chaperone proteins GmDnaJA6 seems to coordinate resistance to alkaline stress; likewise, DnaJ interacting with Hsp70 and GmHsp70 generates greater tolerance to alkaline stress (100 mmol L−1 NaHCO3) [95]. For Gossypium hirsutum under alkaline conditions, differential transcriptional regulation is associated with carbohydrate metabolism and the oxidation-reduction process (stage SS48), while in roots, there is an alteration of the oxidation-reduction process (stages SS3, SS12, and SS48). Specifically, the GhSOS3/GhCBL10-SOS2 network is notoriously related to the response to this type of stress [96]. Finally, in apples, overexpression of the MdATG18a gene under alkaline conditions provides tolerance to alkaline stress by increasing biomass, photosynthetic rate, and antioxidant capacity. Similarly, an increase in glutamate promotes the derivation of γ-aminobutyric acid. Similarly, upregulation of MdATG18a increases autophagosomes, allowing the recovery of small molecules from substances damaged by alkaline stress [2].

Recently, it has been reported that long non-coding RNAs (lncRNAs) can act as competitive endogenous RNAs (ceRNAs) that participate in the response to alkaline stress. Rice plants showed alterations in differentially expressed lncRNAs, miRNAs, and mRNA (75, 34, and 1725, respectively). Likewise, the triplet NONOSAT000455-osa_miR5809b-LOC_Os11g01210 was found to be a central node in the response to saline-alkali stress. These lncRNAs act as competitive endogenous RNAs (ceRNAs) and constitute a novel set of genetic controls under saline-alkaline stress conditions [97]. It is important to note that the response to this type of stress may exhibit differences according to the level of polyploidization and expression of transcription factors (phytohormone pathways, salt resistance, signal transduction, and physiological and biochemical metabolism) [98].

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5. Alkalinity management strategies

Several alternatives have been studied to reduce alkali stress damage in crops. The use of grafting ameliorates this stress by enhancing the desired characteristics of the rootstock and improving the Ca, Mg, P, S, Cu, Fe, and Mn contents [22]. The use of rootstocks can increase the total soluble solids and nutritional status of grapevine fruits grown in alkaline environments [51]. In contrast, the use of light-emitting diodes in monochromatic blue (460 nm), monochromatic red (660 nm), and dichromatic blue/red (1:3) light at 200 μmol m−2 s−1 enhances electron transport per reaction center and the probability of transferring electrons from the exciton into the electron transport system, promoting gas exchange in alkalinity-stressed strawberry plants [47, 99].

The efficient use of fertilizers (macro- and micronutrients) can ameliorate the negative effects of alkalinity by stimulating plant and soil enzymatic activity, improving nutrient content, microbiological activity, total Chl, protein [26, 42, 100], antioxidant activity [70], ion homeostasis (K+/Na+) [57], photosynthetic activity [24, 101, 102], and gene expression [103]. Potassium-ferrite-nanocoated diammonium phosphate [104], organic polymer composite materials [105], carbon nanodots [106], zinc nanocomplexes [25], and nano-Ca2+ [107] can ameliorate alkalinity damage. Similarly, biochar combined with daily fertigation in alkaline soil can improve cucumber yield [101]. Similarly, microorganisms are another window of action to mitigate alkalinity damage, as they increase antioxidant enzymes and modify plant gene expression [108], regulate enzyme activity and soil microbial load [27, 109], decrease pH [110], and regulate soil ionic balance [111, 112], improving nutrient availability [113]. All of these management techniques improve plant productivity and promote better fruit quality by increasing the total soluble solids, acid ratio, sugar, and phenol content [42, 114].

Some compounds, such as spermine [115], taurine [54], auxin (IAA, indole-3-acetic acid)/TIBA (2,3,5-triiodobenzoic acid, which inhibits the transport of auxins) [116], hormone-like growth regulator (AUT) [117], yeast extract and gibberellic acid (GA) [118], ABA [119], strigolactone [62], and brassinosteroid [28] improve plant development under alkaline conditions by inducing alterations at the transcriptomic level, promoting Chl content, photosynthetic rate, enzymatic activity, and reduction of prooxidant compounds. Finally, transgenic plants can show stress tolerance [29, 30, 56, 61, 83, 84, 120, 121], which is a further strategy to promote quality foods under alkaline conditions. The use of vinas and molases favors yield, total soluble solids, and total acidity in strawberries grown in calcareous soils [122]. In addition, several genotypes (VRO-112, VRO-110, Kashi Kranti, VROB-178, AE-70, and VRO-108) can tolerate alkaline conditions (pH 8–9.5), thereby increasing yield [123].

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6. Conclusions and outlook

Alkalinity stress as an environmental stress, which is associated with high pH and high CO32− and HCO3− content, causes considerable decreases in crop quality. To attenuate stress, plants censor the edaphic environment and promote osmotic regulation (soluble sugars and proline), ionic homeostasis (K+), activation of secondary messengers, differential gene expression (associated with the antioxidant system), and accumulation of secondary metabolites. Such mechanisms may promote tolerance to alkalinity by reducing the burst and accumulation of ROS, consistent with the high efficiency of the photosynthetic process. Plant type, phenological stage, and the level and time of exposure to an alkaline environment are fundamental to the response to this stress.

The climatic alterations of the last decades, the excessive use of fertilizers, and poor management of water use can increase alkalinity stress, so it is important to understand the underlying mechanisms that plants develop to cope with this stress. Future research should be conducted to mitigate the negative effects of alkaline salts and produce high-quality crops in a sustained manner.

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Acknowledgments

The authors are highly thankful to the researchers for their findings in this area, which served directly or indirectly in the preparation of this manuscript.

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Conflict of interest

The authors declare no conflicts of interest.

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Funding

The authors received no direct funding for this study.

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Data availability

All the data generated are included in this article reference′s part.

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Fabián Pérez-Labrada, José Luis Espinoza-Acosta, Daniel Bárcenas-Santana, Elizabeth García-León and Mari Carmen López-Pérez

Submitted: 22 December 2023 Reviewed: 21 February 2024 Published: 24 April 2024

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.