Drought Stress in Lentil (Lens culinaris, Medik) and Approaches for Its Management

Abstract

Lentil plays an important role for food and nutritional security. It is a sustainable source of protein, zinc, iron, prebiotic carbohydrates, and diverse health-promoting nutrients. This crop is widely cultivated in semi-arid marginal areas and exposed to various environmental stressors. Drought stress is the major abiotic stress that causes serious effects on lentil growth and development. Thus, it is imperative to set up innovative and sustainable solutions to reduce the adverse effects of drought on lentil crop. In this review, the agro-morphological, physiological, and biochemical effects of drought on lentil were highlighted. Furthermore, breeding and agronomic interventions to improve lentil performance in drought-prone environments were also discussed. Overall, drought disturbs lentil germination, photosynthesis, water relations, shoot and root growth, thereby reducing final yield. Conventional breeding programs have identified several sources of drought tolerance; however, modern biotechnological tools could be adopted to decipher the genetic architecture of drought tolerance in lentil to accelerate the genetic progress. Cost-affordable and eco-friendly agronomic practices may also contribute to minimize the negative consequences of drought stress. Smart exploitation of breeding approaches and agronomic practices could help overcome drought, improve lentil productivity, and increase the profitability of farmers in dry areas.

1. Introduction

Grain legumes are a versatile, inexpensive, and sustainable source of protein, minerals, carbohydrates, and an arsenal of health-promoting bioactive compounds. Global food security is a challenge that requires huge investments in all agricultural research-related fields, including technologies development, sociology, and economy [1,2]. Collaborative efforts are very critical to establish new agricultural policies with a clear vision that may help in the enhancement of crop productivity in limited available land superficies, and overcoming negative effects of climate change, thereby ensuring increased global food demand. In this context, lentil (Lens culinaris, Madik) is among the important food legumes contributing to global food security. In 2020, the global production of lentil was about 6.54 million tons [3]. The major lentil producer is Canada (2.9 million tons), followed by India (1.2 million tons), Australia (0.5 million tons), and Turkey (0.3 million tons). Morocco has produced 9044 tons in 2020 [3], which remains low due to diverse production constraints, especially drought and heat stress [4,5].

Abiotic stress is the dominant limiting factor of crops yield and productivity [6,7]. These environmental constraints disturb crop growth and development processes at physiological, biochemical, and molecular scales. Lentil is not spared from these stressors, as it has been described that lentil is adversely impacted by a range of abiotic stresses including drought [4,8,9], waterlogging [10,11], heat [12,13,14,15], cold [16,17], salinity [18,19,20,21], alkalinity [22], heavy metal [23,24], nutrient deficiency [25,26], and nutrient toxicity [27,28,29].

Drought stress is the principal constraint for lentil production in arid and semi-arid areas [8,9]. In lentil, drought stress reduces germination [30], shoot and root growth [19,31,32], leaf area [33,34,35], relative water content [36,37], membrane stability [38,39], perturbs the photosynthesis process [40,41], induces oxidative stress [42,43], leading ultimately to considerable reduction in biomass production and severe yield losses [41,44]. Additionally, drought may occur simultaneously with other stressors, such as heat stress, which further increases losses severity especially at reproductive stage [13,14,45,46].

Climate change will increase the severity and intensity of environmental stresses which constitute a challenge for global food security [47]. Lentil, as a nutritious and affordable component of crop rotation, will play a pivotal role in global food and nutritional security. Therefore, development of climate-change-ready lentil cultivars for enhancing lentil productivity in harsh environments is a fascinating task that might contribute to overcoming the effects of climate change. Improvement of lentil productivity in such conditions requires integrated approaches that include the development of climate-resilient cultivars accompanied with innovative and sustainable agronomic practices [1,48,49,50]. Such a task could be achieved by a deeper understanding of lentil response and tolerance mechanisms to drought stress. Besides, efficient exploitation of genetic resources using traditional genetic improvement practices and current technological advances in crop genomics and system biology may accelerate the development of drought-smart and high-productive lentil cultivars. In this review, we described the consequences of drought on lentil, and highlighted relevant management strategies to improve lentil performance in drought-prone environments.

2. Morphological, Physiological, and Biochemical Effects of Drought Stress on Lentil

Drought disturbs cell division and hampers plant growth and development at different growth stages, from germination to flowering and seed formation [48]. Grain yield, as the ultimate objective of crop growers, is reduced by drought as a result of the alteration of crop morphological, physiological, and biochemical processes [1]. Germination is a critical phase in the crop lifecycle which strongly influences the crop stand establishment, and final yield [42]. At early development stage, drought stress impaired lentil seed germination by reducing germination rate, and root and shoot growth [51]. The activity of enzymes involved in germination metabolism was also reduced under drought stress which consequently restricted seed germination [30]. Besides, root and shoot height, dry weight, and seedling growth rate were negatively influenced by drought stress [32]. Drought stress at reproductive stage reduced plant height, leaf area, and dry matter production in lentil [52]. In a recent study, cultivated lentil and wild accessions (Lens orientalis, Lens tomentosus, Lens odemensis, Lens lamottei, and Lens ervoides) revealed different root and shoot responses to drought stress [53]. In fact, it has been reported that the response behavior of lentil wild relatives to drought stress is not related to ecological conditions of their site of origin [54]. Nevertheless, drought adaptation mechanisms of wild lentil accessions were attributed to environmental conditions at their native habitats, especially average amount of rainfall during the growing season [53].

Photosynthesis is an important plant metabolic process that plays a pivotal role in plant growth and development. In lentil, drought stress (50% field capacity (FC)) at reproductive stage decreased stomatal conductance, efficiency of photosystem II (Fv/Fm) ratio, RuBisCo activity, photosynthetic rate (Pn), and chlorophyll (Chl) concentration [55]. Drought stressed-sensitive lentil cultivar revealed reduced photosynthetic parameters including efficiency of photosystem II (Fv/Fm), operating efficiency of photosystem II (ΦPSII), photochemical quenching (qP), rate of photosynthetic electron transport (ETR); however, it showed increased photochemical quenching (φNPQ) and non-regulated energy dissipation (φNO) after 4 days of stress [40]. Under drought stress conditions, plants close their stomata to prevent water losses through transpiration, which decrease CO₂ diffusion into chloroplast, thus reducing photosynthetic activities [50]. In fact, reduction of photosynthetic rate is linked to water use efficiency in plant [1]. In another way, reduction of photosynthesis performance is attributed to overproduction of reactive oxygen species (ROS) that interact with cell biomolecules and photosynthesis components [48]. Additionally, inferior photosynthetic activity leads to insufficient assimilates mobilization which in turn influences negatively the global growth and development [1,55].

Plant relative water content is also affected by drought stress, and it can serve as a pertinent indicator to evaluate plant response and adaptation to water deficit [36,37]. In lentil, a decline of this parameter has been reported under drought stress in many studies [38,40,44,55]. Nevertheless, under progressive drought stress, lentil wild genotypes (Lens orieltalis) recorded high relative water content compared to cultivated lentil [36]. Besides, other important traits such as leaf water potential, transpiration rate and water use efficiently (WUE) are also influenced by drought stress. Drought negatively impacts the transpiration rate and leaf water potential, which leads to warm plant canopy [56,57]. In lentil, increased canopy temperature has been observed under water-limited conditions [36,37]. In addition, reduction of leaf area and stomatal conductance is an important adaptation strategy that allows the management of water resources in water-stressed environments [58].

The ultimate effect of the majority of abiotic stresses is the induction of oxidative stress. Drought stress increased the amount of ROS including peroxyl radical (ROO^•), superoxide radical (O₂^•−), singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), alkoxyl radicals (RO^•) which badly alters cell biomolecules (i.e., proteins, lipid, acid nucleic). ROS are produced in cell organelles, especially chloroplast and mitochondria [56,59]. To deal with ROS, plants are endowed with an arsenal of biochemical compounds that help them maintain the different cellular functions. The upregulation of antioxidant defense system and different osmoprotectants are common strategies adopted by plants to cope with adverse effects of environmental stresses [60].

In lentil, drought stress increased lipid peroxidation and the production of malondialdehyde (MDA) as a result of oxidative stress [38,61]. In addition, drought-stressed lentil increased the synthesis of compatible solutes such as proline, glycine betaine, and soluble sugars [30,55,62,63]. The same trend was also observed in drought-stressed chickpea (40% FC), where drought increased proline and soluble sugar contents, but resulted in reduced relative water and chlorophyll contents [64]. The antioxidant defense system was also reported to be altered by drought stress in lentil [38,42,61,65]. More recently, a study reported that drought stress (20% polyethylene glycol (PEG)) alone or in combination with heat stress increased the activity of antioxidant enzymes such catalase and peroxidase, with higher activity under combined stress compared to single stress [65].

In fact, yield losses are strongly associated with stress intensity and duration, crop phenological stage, tolerance or sensibility of the genotype, and the occurrence of other abiotic/biotic stresses [8,13,48,52,56]. In lentil, it has been recorded that the reproductive stage is highly sensitive to drought stress [62]. Drought stress under reproductive stage resulted in significant reduction in yield (70%) and harvest index (17–32%); additionally, at such condition, seed yield is greatly defined by the number of flowers, pods, and seeds produced per plant [52]. Lentil type has been reported to influence yield under drought stress conditions; in fact, macrosperma-type lentil is less tolerant to drought than microsperma-type lentil [66,67]. Adversely, in another study, no relation has been found between seed size and yield reduction [52].

In addition to seed yield, seed quality is also important and should be considered when examining the effect of drought stress on lentil, as seed quality is a determinant factor of lentil market value. However, studies related to drought stress effects on lentil quality are scarce. For instance, drought stress (50% FC) alone or in combination with heat stress (temperatures > 30/20 °C day/night) showed different effects on lentil at the reproductive stage [55]. Heat stress impacted yield formation associated traits (number of seed produced per plant, and seed weight per plant), while drought stress showed a major effect on individual seed weight; nevertheless, combined stress induced harmful effects on all yield attributes [55]. Likewise, combined heat (33/28 °C day/night) and drought stress (50% FC) decreased both lentil yield and nutritional profile, while drought stress induced severe effects than heat stress [68]. Drought stress reduced seed weight per plant, individual seed weight, protein content, minerals (zinc, iron, calcium, potassium, phosphorous, and magnesium), carbohydrates, and carbohydrate metabolism associated enzymes. Interestingly, drought stress and heat stress increased the content seed amino acids such as proline, alanine, lysine, leucine, isoleucine, and glycine; however, they lowered the content of arginine, tryptophan, histidine, threonine, etc., while combined stress resulted in decreased content of all amino acids examined [68]. However, the degree of alteration is genotype-specific; in fact, drought-tolerant cultivars showed better adaptation under all stressed conditions (i.e., drought, heat and combined stress) [68]. This highlights the possibility for developing lentil cultivars adapted to combined stress. In another investigation, terminal drought (no irrigation was supplied) and heat stress (induced by late sowing) significantly reduced iron, boron, and seed protein contents [41]. Additionally, the combination of drought with heat stress resulted in a great decline in lentil grains’ zinc, iron and protein contents in comparison with heat stress alone [13,46].

It can be concluded that drought stress reduces lentil seed yield by altering various morphological, physiological, and biochemical traits (Figure 1). Lentil grain quality is also impacted by drought stress due to the decline of various biological processes such as photosynthesis and nutrient use efficiency. Biological nitrogen fixation is also affected by drought stress [69], which may impact negatively yield and seed quality. However, limited studies have examined the effect of drought on biological nitrogen fixation in lentil; hence, future investigations are highly needed. In addition, extensive research is required to evaluate the response of lentil to different drought scenarios, at different growth stages, using diverse genetic material from the cultivated and wild gene pools. Molecular mechanisms behind lentil response and tolerance to drought stress are not fully explored and warrant additional investigations.

3. Management of Drought Stress in Lentil

3.1. Conventional Breeding for Improving Lentil Drought Tolerance

3.1.1. Exploitation of Genetic Variation for Selecting Potential Sources of Drought Tolerance

Breeding for drought tolerance may contribute to the improvement of lentil production in drought-prone areas [36,70]. Such a task might be achieved through conventional and modern breeding strategies. Traditional breeding approaches have contributed to developing high-yielding cultivars adapted to different environmental conditions [5,50,71]. For example, in Morocco, breeding efforts at the National Institute of Agricultural Research (INRA-Morocco) have resulted in the release of improved varieties with great yield stability and adaptability to different Moroccan agro-environments [4,5,72,73].

Exploration of the genetic variability for drought tolerance is an important step for selecting genotypes with better performance in water-stressed environments. Genetic variation for drought tolerance have been explored in cultivated lentil as well as in its wild relatives [9,13,14,31,44,53,54,67,74,75,76,77,78,79,80], and potential sources of tolerance were identified (

Table 1. Potential sources of drought tolerance identified in lentil.

Accession	References
ILL 6002	[9]
JL 1, IPL 98/193, DPL 53	[77]
ILL 123613, ILL 123466	[74]
ILL 7835, ILL 7835, ILL 6075, ILL 7814, ILL 7804, ILL 7833, ILL 8029, ILL 6338, ILL 6104, ILL 7814, ILL 6362	[14]
ILL-10700, ILL-10823, FLIP-96-51	[81]
BM-1247, BM-1227, BM-981, BM-502	[31]
IC559713, IC559696, IC560051, IC560246, IC559647, IC560032, IC559769, IC559757, IC559744, IC835822, IC560337	[78]
Digger, Cumra, Indianhead, ILL5588, ILL6002, ILL5582	[37]

).

3.1.2. Potential Traits for Screening Drought Tolerance in Lentil

Exploitation of drought-adaptive traits is a pragmatic approach for overcoming the adverse effects of water deficit [58,82]. In lentil, traits such as early flowering and maturity have been proven to be useful for escaping terminal drought stress [53,77,83]. Thus, these phenological traits can be targeted to improve lentil production in regions where terminal drought and heat stress are frequent.

Under water deficit, seed yield was reported to be associated with seed size, and the number of pod and seed per plant [84]. Furthermore, seed yield revealed positive correlation with seedling vigour, harvest index, and biological yield (r = 0.44, 0.62 and 0.75, respectively) under drought stress conditions [77]. Interestingly, Sarker et al. [9] reported high positive correlation (r = 0.93) between stem length and seed yield per plant in lentil under rainfed conditions. Additionally, significant variation was reported among 30 lentil genotypes for different agro-morphological traits such as yield per plant, number of primary and secondary branches per plant, number of pods and seeds per plant, and 100-seed weight under rainfed [31], and irrigated conditions [85]. These Agro-morphological traits could be an important target for selecting drought-tolerant genotypes.

On the other hand, it is well-understood that root system plays a pivotal role in the adaptation of plant to drought stress, and it can be, therefore, targeted for improving lentil productivity in water-stressed environment. The lentil has a slender taproot system with the presence of a mass of fibrous lateral roots [86]. In terms of depth and lateral proliferation, lentil root system may be shallow, intermediate, or deep [86]. According to Gorim & Vandenberg [87], lentil roots can be categorized into very fine, fine, and small diameter classes, with the predominance of fine diameter class. Interestingly, significant genetic variability was observed among different lentil genotypes for root traits such as root dry weight, taproot length, lateral root number, total root length, average taproot diameter, mean taproot diameter, root surface area, and root–shoot ratio [8,9,31,77,81]. This variability is of utmost importance, indicating the feasibility of selection and improvement of belowground traits conferring drought tolerance, which are generally neglected compared to the aboveground traits.

Physiological traits such as stomatal conductance, stomatal density, leaf area, relative water content, transpiration rate, water use efficiency, water losing rate, seedling vigor, SPAD value (chlorophyll content), canopy temperature, and canopy temperature depression were used to quantify drought tolerance in lentil [8,36,37,67,70,88]. Biju et al. [37] reported that infrared thermal imaging (IRTI) could be exploited as a rapid, simple, and non-destructive tool for assessing drought tolerance in lentil based on the canopy temperature (indicator of plant water status), canopy temperature depression (CTD), and crop water stress index (CWSI). They suggested IRTI as a powerful tool for screening a large number of lentil germplasm. More recently, Ben Ghoulam et al. [36] have used IRTI and other morpho-physiological attributes (relative water content, water losing rate, root–shoot ratio, harvest index, and cell membrane stability) to describe the response of lentil elite advanced lines, landraces and wild relatives (Lens orientalis) to drought stress. Additionally, Sehgal et al. [55] reported positive correlation of relative leaf water content, osmotic potential and stomatal conductance with biomass and seed yield per plant in lentil under stressed conditions. Sánchez-Gómez et al. [67] found that earliness, high stomatal density, stomatal conductance, leaf area, and antioxidant potential (high polyphenol and carotenoid contents) are useful traits for escaping early drought stress in lentil. Cell membrane stability, an indicator of membrane damage, can also be utilized to quantify drought tolerance in lentil [36,89]. Additionally, osmotic adjustment and epicuticular wax could also be employed as selection criteria to assess drought tolerance in lentil [90].

Unfortunately, photosynthetic performance is primordial for adapting to drought stress. Therefore, understanding photosynthesis traits associated with drought adaptation might help breeders to assess drought tolerance in different lentil genetic backgrounds. Photosynthetic attributes such as chlorophyll content, Pn, Fv/Fm, and RuBisCo activity are essential indicators of photosynthesis performance under abiotic stress [91]. In lentil, Mishra et al. [62] reported that chlorophyll stability index (CSI), an indicator of chlorophyll sensitivity to stress, is associated with drought tolerance. Sehgal et al. [55] showed that drought reduced chlorophyll concentration, Pn, Fv/Fm ratio, and RubisCo activity in lentil, but the reduction is lower in tolerant than in sensitive genotypes. Thus, these traits might be targeted during screening processes for drought tolerance in lentil. However, physiological attributes such as photosynthetic parameters are not widely adopted in breeding programs due to the lack of simple and efficient methods to screen a large number of genotypes [92]. Thus, there is a need to develop pertinent phenotyping methods to develop next-generation drought-smart cultivars.

Drought stress induces various metabolic modifications in plants, and the investigation of these biochemical and metabolic changes might be useful to screen drought-tolerant genotypes, and to identify biochemical traits associated with drought tolerance. For example, proline is an important compatible solute produced by plants to overcome damages induced by water deficit. Accumulation of free proline and other compounds such as glycine betaine, soluble sugars, and sugar alcohols contributes greatly to the tolerance of plants to abiotic stresses, including water deficit [1]. In lentil, drought stress alters the production of different metabolites such as proline, total soluble sugars, and antioxidant enzyme activity [42,63,93]. Drought-tolerant genotypes showed higher proline and anthocyanin contents, increased superoxide dismutase activity, and lower lipid peroxidation (malondialdehyde content) than drought-sensitive genotypes [63]. Likewise, Muscolo et al. [30] reported that lentil genotypes (Eston and Castelluccio) exhibited higher proline and total soluble sugars contents compared to drought-sensitive genotypes (Ustica and Pantelleria). In another study, drought-tolerant lentil genotypes showed higher enzymatic antioxidants activity (i.e., catalase, superoxide dismutase, ascorbate peroxidase, peroxidase), higher content of proline, glycine betaine, and soluble sugars, but lower amount of hydrogen peroxide (H₂O₂), superoxide anion (O₂^•−), and MDA when compared to drought-sensitive genotypes [42]. On the other hand, starch, sucrose, reducing sugars content, and activity of sucrose synthase and acid invertase were reported to be correlated with seed yield under combined heat-drought stress [55]. Thus, drought-tolerant lentil produces a wide range of osmoprotectants, i.e., proline, glycine betaine, soluble sugar, and different enzymatic antioxidants, to reduce the adverse effects of drought-induced oxidative stress, and to sustain growth and development under stressful environments. However, for efficient integration of these biochemical traits as selection criteria in a lentil breeding program, further research is needed to clarify the variation of these metabolites under different drought scenarios and phenological stages in a variety of lentil genotypes, and to investigate their relationships with seed yield. Additional studies are also needed to ascertain the molecular mechanisms governing theses biochemical responses.

In another way, drought tolerance indices such as stress tolerance index (STI), drought susceptibility index (DSI), geometric mean productivity (GMP), mean productivity (MP), and harmonic mean (HM) have been proposed to assess lentil drought tolerance [62,88,94,95]. Recently, it was reported that GMP, STI, and MP are the best indices for selecting lentil genotypes that could withstand combined heat and drought stress [14].

Despite the progress achieved by conventional breeding to breed drought-tolerant lentil varieties, these approaches remain a slow process to cope with the challenges of current climate change. Therefore, in this context, biotechnological tools may offer new opportunities to circumvent the limits of conventional breeding, and accelerate varietal development for boosting lentil productivity worldwide, thereby ensuring global food security.

3.2. Biotechnological Approaches for Improving Lentil Drought Tolerance

3.2.1. Identification of Quantitative Trait Loci (QTLs)

Drought tolerance is a polygenic trait controlled by various genes/QTLs and is highly influenced by genotype and environment interaction [1]. As a result, direct manipulation of this trait by conventional approaches is a challenging task. Conventional breeding approaches are labor-intensive and time-consuming limiting, therefore, the genetic gain [33]. However, with advances in molecular biology and genomics area, conventional breeding programs can be updated to accelerate the variety development. Using theses molecular approaches, QTLs/genes governing morphological, physiological and biochemical traits imparting drought tolerance can be identified and transferred to elite cultivars via marker-assisted selection strategies [33,96]. For example, root traits are very important for avoiding drought stress; however, screening of these traits is very intensive and difficult, especially when a large number of genotypes are targeted; however, identification of QTLs or genes linked to root traits can facilitate their integration in breeding program for improving drought tolerance. The advances in high-throughput genotyping technologies have provided opportunities to enrich lentil genomics resources that will be very helpful for future breeding programs. Interestingly, these advances have led to the development of draft genome sequence of lentil [97,98].

In the past years, various traits of interest have been targeted using different molecular markers. In lentil, QTLs linked to stemphylium blight resistance [99], days to 50% flowering, seed diameter, and 100-seed weight [100], ascochyta blight resistance [101,102,103,104,105], fusarium wilt resistance [106], anthracnose resistance [107,108], aphanomyces root rot resistance [109], tolerance to boron toxicity [27,28], tolerance to aluminum toxicity [110,111], salt tolerance [20], heat tolerance [112], and selenium and manganese uptake [113,114] have been successfully identified.

Nevertheless, few studies have been undertaken to understand the genetic basis of drought tolerance in this crop. For instance, the first study on the genetic control and association of molecular markers to drought tolerance trait in lentil was reported by Singh et al. [115]. They used seven simple-sequence repeat (SSR) markers, and 101 F2 mapping population derived from JL-3 (drought-sensitive parent) and PDL-1 (drought-tolerant parent) to map the locus associated with seedling survival drought tolerance using bulk segregation analysis. They documented SSR markers linked to a single major gene Sdt connected with seedling survival drought tolerance which explains 69.7% of phenotypic variation [115].

In another study, Idrissi et al. [70] assessed genetic diversity and drought tolerance in 70 Mediterranean lentil landraces using SSR and amplified fragment length polymorphism (AFLP) markers. Interestingly, using the Kruskal–Wallis test, the authors documented 6 SSRs and 91 AFLPs associated with relative water content, 4 SSRs and 105 AFLPs associated with water losing rate, and 5 SSRs and 71 AFLPs linked to wilting severity [70].

Although significant progress has been made in lentil genomics and breeding, studies that address lentil drought tolerance using these advanced high-throughput technologies are very limited. In contrast, in other food legumes, such as chickpea, drought tolerance has been widely studied and various QTLs have been documented [116]. Interestingly, a QTL-hostspot genomic region harboring various QTLs associated with drought tolerance, which explains up to 58.2% of phenotypic variation, was introgressed into elite chickpea cultivars using marker-assisted backcrossing approach and allowed the development of improved lines with better yield and drought tolerance compared to their recurrent parents [117,118].

In lentil, QTLs mapping of different shoot and root traits linked to drought tolerance was firstly addressed by Idrissi et al. [119] using 132 lentil recombinant inbred line (RIL) population derived from ILL6002 (drought-tolerant) and ILL5888 lines (drought-sensitive). Using composite-interval mapping (CIM), the authors identified 18 QTLs controlling 14 root and shoot traits linked to drought tolerance. Interestingly, a QTL controlling root–shoot ratio, which explains 27.6% and 28.9% of phenotypic variance, was identified in two successive seasons. Additionally, a QTL-hostspot genomic region harboring QTLs associated with several shoot and root traits (dry root weight, dry shoot weight, lateral root number, root surface area, and shoot length) was also detected [119]. Nevertheless, additional research is needed to validate the stability of identified QTLs in other genetic backgrounds, and in different environments and drought stress scenarios.

In addition to QTLs analysis, genome-wide association studies (GWAS) have now become a robust approach in plant genetic to dissect the genomic variation associated with agronomic traits of interest. It exploits the historical recombination existing in diverse genetic material including landraces, breeding lines, and wild accessions to identify QTLs/genes with high resolution. Importantly, GWAS has been used previously to elucidate the genetic architecture of drought adaptive traits in various crops including wheat [120], barley [121], maize [122], and soybean [123]. Although the importance of this approach, few studies have been reported in lentil [109,124]; however, with the availability of lentil reference genome [97,98] and the recent advances in high-throughput genotyping platforms, GWAS will be a common procedure in lentil breeding programs. It has been reported that the combination of conventional QTLs mapping and GWAS improves the power of detection of QTLs associated with complex traits [109,125], including drought tolerance [120]. In lentil, Ma et al. [109] used a bi-parental mapping population and an association mapping population to dissect the genetic architecture of Aphanomyces root rot resistance, and they identified 19 QTLs using the former approach and 38 QTLs by the later one. In addition, the authors highlighted the importance of the integration of both approaches (QTL mapping and GWAS) for comprehensive dissection of QTLs associated with Aphanomyces root rot resistance in lentil. Hence, the same approach could be used to investigate the genetic basis of drought tolerance in lentil.

3.2.2. High-Throughput Phenotyping

The accuracy and the precision of QTLs/genes detection are highly dependent on the quality of phenotypic data. Thus, phenotyping the trait of breeders’ interest such as drought tolerance should be performed with appropriate approaches that can enable efficient and effective screening of a large number of germplasm and mapping populations. Conventional phenotyping methods are error-prone, laborious, and time-consuming. Furthermore, screening of some drought-adaptive traits, such as root system architecture, using destructive and low-throughput conventional phenotyping tools is more difficult [119]. Indeed, the evaluation of a large set of genotypes for root traits is very challenging, limiting the progress toward identification of QTLs/genes that could be used in marker-assisted breeding [126]. However, the recent advances in automated high-throughput phenotyping platforms may hold tremendous opportunities to address the constraints related to conventional phenotyping. These platforms are under progress in various breeding programs around the world and allow precise and accurate evaluation of drought adaptive traits in large germplasm collections. Moreover, the data provided by such phenomics technologies can be integrated with genomics data through appropriate bioinformatics tools to improve the precision and efficiency of breeding program [127].

Phenomics studies require multidisciplinary approaches to collect, precisely and rapidly, the multidimensional whole phenotypic data in order to quantify growth, development and the response of plant to environmental stresses [128]. For instance, high-throughput phenotyping can offer great opportunities for large-scale screening of trait imparting drought tolerance in large germplasm collections and mapping populations under controlled and field conditions. In fact, these technologies have been used to assess drought tolerance in several crops such as maize [122], rice [129,130], wheat [131], rapeseed [132], mungbean [133], and chickpea [134]. However, limited studies have been conducted in lentil. For instance, lentil resistance to Aphanomyces root rot was assessed in controlled conditions and in field using different image-based phenotyping tools [135]. Importantly, the results showed moderate to strong correlation between image-derived traits and conventional scoring, highlighting, therefore, the importance of image-based phenotyping as a powerful approach to dissect the biotic stress tolerance in lentil [135].

In another study, 276 lentil accessions were evaluated for salt tolerance using a high-throughput image-based phenotyping approach [136]. The authors used non-destructive traits such as height, compactness, projected shoot area, convex hull area, and green and non-green color to investigate salt tolerance among lentil accessions, and they reported significant correlation between conventional screening and image-guided phenotyping. Additionally, the phenotypic data obtained were combined with genotypic data using GWAS, and markers and genes linked with salt tolerance in lentil were successfully documented [137].

Waterlogging is a serious problem that affects the lentil productivity and needs to be addressed for improving lentil production in waterlogging-prone areas. Recently, waterlogging tolerance of 111 lentil genotypes was assessed using plant growth rate which is estimated by Canopeo [10]. Canopeo app is a powerful, simple, and efficient tool that could be used in smartphones, and enable rapid and easy phenotyping of green canopy cover of different crops [138]. Recently, Canopeo app was employed for screening a lentil recombinant inbred line population for drought tolerance [139]. Interestingly, green canopy cover showed significant correlation with wilting score (r = −0.60) and relative water content (r = 0.55) which supports the possibility of employing Canopeo app to quantify drought tolerance in lentil [139]. Thus, Canopeo app could be used to follow the growth of lentil under drought stress in controlled as well as in field conditions.

Recently, a first-time screening of diverse lentil genetic material consisting of a recombinant inbred line population, landraces, and wild accessions, under a high-throughput phenotyping platform, showed high genetic variation and high heritability for root biomass, shoot biomass, and root–shoot ratio [140]. The incorporation of these new phenomics tools in lentil breeding programs targeting root traits can provide meaningful data which could facilitate the documentation of key players underlying root system growth dynamic and flexibility under different drought scenarios.

3.2.3. Transcriptomics

Omics tools including transcriptomics, proteomics and metabolomics are of utmost importance, and provide insight into drought-stress-associated genes, proteins, and metabolites that could assist in exploiting genomics data and bridging the gap between genotype and phenotype. Interestingly, the integration of different omics approaches (multi-omics) was exploited to investigate growth and development of various crops and their adaptation to biotic and abiotic stresses [141].

The transcriptomics term was coined by Charles Auffray in 1999 to refer to a complete set of transcripts expressed in a cell, tissue, or organ at a given time under particular conditions. Transcriptomics allows the possibility to decipher gene and gene regulatory networks that underlay plant response and tolerance to stress [142]. Many technologies have been developed to investigate variation at transcriptomics level under specific conditions, which include expressed sequence tags (ESTs), microarray, serial analysis of gene expression (SAGE), and RNA sequencing (RNA-seq). RNA-seq technology has emerged as an inexpensive and fascinating platform for deciphering underlying mechanisms of drought tolerance [38,142]. Hence, such an approach is important to probe the mechanisms behind drought tolerance in lentil. For this end, the transcriptomic response of drought-tolerant (PDL2) and drought-sensitive (JL3) lentil cultivars, subjected to drought stress at seedling stage, was executed using Illumina HiSeq 2500 sequencing platform [38]. Analysis of differentially expressed genes (DEGs) indicated an upregulation of 11,435 transcripts and a downregulation of 6934 ones under drought stress conditions. Besides, transcriptome analysis revealed upregulation of DEGs linked to the metabolism of glucose, TCA cycle, ABC family protein, and ion channel transport in both tolerant and sensitive genotype under drought stress conditions. Genes connected with organ senescence, TCA cycle electron transport chain, oxidation-reduction process, correct folding of protein, and reduction of stomatal conductance were upregulated in tolerant genotype compared to sensitive genotype; alternatively, genes involved in GABA, negative regulation of abscisic acid, synthesis of cell wall protein have exhibited downregulation in tolerant genotype than sensitive genotype [38].

In another investigation, the root and leaf transcriptomic response of drought-sensitive lentil cultivar “Sultan”, subjected to short- and long-term drought stress treatments (15% PEG 6000) at the seedling stage, was examined using an Illumina HiSeq 4000 sequencing platform [40]. Results showed 6949 and 2915 DEGs in leaf and roots, respectively, under short-term drought, while 8306 and 18,327 DEGs were revealed in leaf and roots, respectively, under long-term drought. Under short-term drought, in roots, genes involved in regulation of transcription, response to salt stress, flower development, response to abscisic acid, response to water deprivation, etc., were all upregulated, while genes that play a role in protein phosphorylation, cell wall organization, tyrosine kinase signaling pathway, etc., were downregulated; whereas, in leaf, genes associated with biological processes such as regulation of transcription, protein ubiquitination, response to abscisic acid, flower development, etc., were upregulated, but genes associated with protein phosphorylation, defense response to bacterium, transport, etc., showed downregulation. Under long-term drought, in roots, genes involved in protein phosphorylation, protein ubiquitination, cell wall organization, seed dormancy, cell division, DNA repair, root development were upregulated, whereas genes related to regulation of transcription, response to salt, response to water deprivation, etc., were downregulated; in leaf, genes linked to regulation of transcription, protein ubiquitination, response to abscisic acid, etc., showed upregulation, but genes involved in protein phosphorylation, tyrosine kinase signaling pathway, circadian rhythm, chloroplast organization, etc., showed downregulation [40].

Recently, lentil transcriptomic response to drought (20% PEG 6000 for 3 days), heat (40 °C for 4 h) and combined stress was examined using Illumina HiSeq 2500 [65]. Results showed 1702 DEGs under drought, 4327 DEGs under heat stress, and 14,167 DEGs under combined drought-heat stress. Interestingly, transcriptomic analysis showed several genes that were specifically altered under combined stress in comparison with individual stress. Under drought stress, pathways such as photosynthesis, plant hormone signal transduction, glutathione metabolism, phenylpropanoid biosynthesis, etc., were significantly enriched, while under heat stress the results revealed the enrichment of pathways such as transporters, chromosome and associated proteins, exosome, chaperones and folding catalysts, etc.; however, under combined stress, the results revealed the significant enrichment of pathways such as ribosome, spliceosome, RNA transport, protein processing in endoplasmic reticulum, starch and sucrose metabolism, oxidative phosphorylation, etc. Genes that showed common expression under drought, heat, and combined stress such as AP2, CCA1, FKF1, P5CS2, and TOC1 could be promising targets to improve the performance of lentil crop under stressful conditions [65].

More recently, the response of lentil to several abiotic stresses including drought (exposition of lentil seedling to air for 4 h for 3 consecutive days), heat (35/33 °C), salt stress (120 mM NaCl), and alkalinity (40 mM NaHCO₃) at the seedling stage was investigated using physio-biochemical indicators and RNA-seq [143]. Illumina HiSeq 2500 sequencing platform was used to sequence cDNA of salinity, alkalinity, and drought-stressed seedling, while Illumina HiSeq 2000 sequencing was used in the case of heat stress. The comparison of transcripts expressed in tolerant genotypes compared to sensitive revealed 51 DEGs in the case of drought stress, 720 DEGs under heat stress, 1060 DEGs for salt treatment, and 1358 DEGs in the case of alkalinity stress. Interestingly, common upregulated DEGs under salinity and alkalinity stress were involved in ABA signaling, regulation of cation transport at the root symplast-xylem interface, epigenetic regulation, transport of sucrose, and vesicular trafficking, whereas genes related to synthesis of organic acids and translational regulation were found to be upregulated under both drought and heat stress [143]. Under drought and heat, citrate synthase gene, NADP-dependent malic enzyme, and 50S ribosomal protein were upregulated, while chalcone synthase genes were downregulated [143]. Citrate synthase is an important enzyme of tricarboxylic acid (TCA) cycle; it catalyzes the reaction of condensation of acetyl coenzyme A with oxaloacetic acid to form citrate acid which is the first reaction in TCA cycle. In a recent study, the overexpression of citrate synthase gene of Rhododendron micranthum Turcz in tobacco improved tolerance of transgenic tobacco to several abiotic stresses including drought [144]. Besides, transgenic tobacco plants showed higher proline content, and higher amounts of organic acids (i.e., citric acid, malic acid, and tartaric acid) in the roots; however, they revealed reduced height, leaf area, and leaf MDA content compared with wild-type [144]. Thus, there is a need for functional characterization of citrate synthase genes in lentil at different growth stages and stress treatments. Furthermore, the functional characterization of NADP dependent malic enzyme, 50S ribosomal protein, and chalcone synthase genes could be also helpful for identifying molecular mechanisms for drought tolerance in lentil. Moreover, such drought-responsive genes may be targeted to develop gene-based markers which could assist in lentil molecular breeding.

3.2.4. Proteomics

In addition to transcriptomics studies, proteomics (study of the proteome that is a set of proteins of a cell, tissue, or organ expressed under specific conditions) is another omics approach that permits the identification of structure and function of proteins, their interactions, and their role under stressful conditions [145]. The study of plant response to drought stress using proteomics is a promising way to shed light on the mechanisms of response and tolerance at proteome level, and identifying new players associated with plant adaptation under unfavorable conditions. The application of proteomics is required to describe abiotic stress-responsive proteins which could be altered to modulate lentil behavior in harsh environments and maintaining grain yield. Nevertheless, to date, no study was reported in relation to lentil proteomic response under abiotic or biotic stresses [146]. In contrast, in other food legumes such as chickpea, soybean, pigeon pea, pea, and common bean, proteomics has been successfully applied, and allowed the documentation of proteins altered by a variety of abiotic stress [6]. Thus, future proteomic-based studies are strongly recommended in order to establish a comprehensive picture of the lentil proteome response to drought stress.

3.2.5. Metabolomics

Metabolomics has emerged as a powerful tool for studying the variation of metabolome, i.e., a set of metabolites of a biological sample collected under specific conditions. Metabolomics helps in investigating metabolites associated with the adaptation of plant to abiotic stress [147]. Generally, plants respond to abiotic stress by triggering diverse biochemical processes; therefore, the quantification of these chemical compounds using metabolomics approach is a pragmatic way for the efficient and accurate investigation of lentil genetic materials for selecting superior genotypes and identifying the most important metabolites and metabolic pathways imparting drought tolerance. Besides, breeding practices assisted by high-throughput metabolomics technologies can help in detecting traits linked to high yield and adaptation to environmental stress [148]. Metabolomic-assisted breeding that smartly combines metabolomics with other genomics studies under the same umbrella could be a fascinating approach to accelerate gene discovery and developing climate-resilient crops [149].

In food legumes, metabolomics has allowed comprehensive characterization of metabolites imparting drought tolerance such as proline, sugars, and γ-aminobutyric acid (GABA) [142]. However, lentil studies dealing with metabolomics response to drought stress are limited. For instance, the response of lentil genotypes to drought stress (18% PEG 6000), and to salinity (150 mM NaCl) at seedling stage was assessed using morph-physiological traits and gas chromatography-mass spectrometry (GC-MS) [19]. The authors suggested ornithine and asparagine as biomarkers for drought tolerance, while they reported alanine and homoserine as key markers for salt stress tolerance [19]. More recently, metabolomics response of drought-tolerant (Elpida) and drought-sensitive (Flip03–24L) lentil cultivars to drought stress (2.5% and 5% PEG-6000) at seedling stage was examined, and D-fructose, α,α-trehalose, myo-inositol, and L-thryptophan were suggested as potential biomarkers to be used to assess the response of lentil to drought stress [150]. However, metabolite fluctuation in plants is hugely affected by various factors such as analytical tool adopted, plant tissue analyzed, growth stage, genotype, stress imposed, and statistical tools used for data analysis and interpretation. Therefore, extensive research is needed to pinpoint robust biomarkers which can be exploited for developing lentil cultivars with tailored metabolic pathways. Furthermore, the integration of metabolomics with transcriptomics can help unravel influential metabolic pathways and molecular mechanisms governing lentil response and tolerance to drought stress.

3.2.6. Genetic Engineering

Genetic engineering could be a valuable option to develop drought-smart lentil cultivars. In fact, genetic manipulation of lentil has been reported [33]. Interestingly, dehydration-responsive element binding (DREB1A) gene under control of rd29A promoter has been introduced in lentil using Agrobacterium tumefaciens-mediated transformation [151]. The transgenic lines revealed improved tolerance to salt stress compared to non-transgenic lines [151]. Thus, it is important to exploit these tools to investigate the nature of expression of drought-responsive genes. For example, genes encoding antioxidants, osmoregulators, plant growth regulators, transcription factors, and late embryogenesis abundant proteins are potential targets to induce drought tolerance using transgenic approach [152]. However, no study was conducted in lentil using this tool; thus, efforts are needed to confirm the utility of transgenic in improving lentil drought tolerance. Nevertheless, the public acceptance, and stringent regulation of transgenic crops are other challenges to large application of transgenic approaches for lentil genetic improvement against drought stress.

In recent years, advances in genome editing (GEd) technologies have allowed precise alteration of plant genome. Genome editing is a valuable tool for designing plants with suitable drought tolerance. Zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) are among the major genome editing methods in plants [153]. CRISPR/Cas9-mediated genome editing is the technology most applied due to its simplicity, accuracy, efficiency, and ability to induce targeted alteration in crop genome [154]. Application of CRISPR/Cas9 has been reported in a variety of crops, including food legumes, for optimizing a wide range of traits [153,154,155]. Until now, no genome-edited lentil line was reported. However, despite to its recalcitrant nature to genetic transformation and regeneration, promising results have been recorded [151,155], and could promote future application of CRISPR/Cas9 technology. For example, decapitated embryo explant has provided positive results for in vitro generation of lentil shoots after genetic transformation by agrobacterium [156]. In another study, in vitro flowering and seed formation in lentil were recorded [157]; this finding may help in avoiding constraints related to in vitro rooting induction and could enable in vitro production of seeds of transformed lines. Thus, these advances may pave the way for the improvement of lentil using transgenic and genome editing tools. However, there is a dire need to optimize in vitro regeneration protocols by examining different combination of culture medium ingredients such as mineral composition and plant growth regulators [155]. Additionally, the first protocol enabling the production of transgenic hairy roots in lentil was established recently using Agrobacterium rhizogenes-mediated transformation which could be used to assess the root development and behavior under biotic and abiotic stress [158].

3.2.7. Integration of Biotechnological Tools with Speed Breeding

In recent years, speed breeding has shown its potential to reduce the breeding cycle by allowing the realization of several generations per year. Under speed breeding conditions, up to six generations can be obtained in one year in crops such as wheat, chickpea, pea, and barley rather than one to two generations in conventional breeding programs [159]. In this technology, accurate control of temperature, day length, light intensity and quality helps to accelerate plant growth, enhancing photosynthetic activity, and induce early flowering. Speed breeding can be used to accelerate different breeding activities such as crossing, development of mapping populations, and the identification and exploitation of genes associated with traits of breeders’ interest [160]. In lentil, up to eight generations per year were obtained through extended photoperiod, application of plant growth regulator, and immature seeds cultivation [161]. Likewise, another system consisted of extended photoperiod (20 h light–4 h dark) and immature seed germination delivered six generations per year, and permitted screening and selection of Aphanomyces root rot resistance in lentil [162]. Besides, cultivation of lentil under an extended photoperiod (20 h light and 4 h dark; 23 °C)-based hydroponic system with flurprimidol (0.6 μM) supplementation allowed the achievement of six generations per yield [163]. In another investigation, extended photoperiod combined with single seed descent (SSD) method accelerated growth and development of F2 population and resulted in three generations per year rather than one generation under conventional greenhouse conditions [164]. Recently, an efficient speed breeding protocol based on extended photoperiod regime of 22 h resulted in up to four generations per year [165]. Besides this, it allowed the reduction of flowering time in the lentil wild relative, Lens orientalis, for a better exploitation in the pre-breeding program; nevertheless, this protocol needs to be optimized with respect to light intensity, spectral compositions, and red to far-red ratio (R/RF) to increase seed viability and reduce plant mortality [165]. In fact, it has been recorded that light quality attributes such as R/RF have determinant effect on lentil morphology and phenology, while R/RF of 3.1 was reported to be the critical threshold for flowering initiation in lentil [166].

Overall, the integration of omics approaches using robust bioinformatics pipelines can allow high-throughput assessment of lentil genetic resources and rapid identification of genes linked to drought tolerance. Identified genes could be exploited using marker-assisted selection and genetic engineering tools to breed superior drought-tolerant cultivars. In addition, speed breeding can speed up the entire breeding process; for example, the integration of speed breeding with marker-assisted breeding approaches (“marker-assisted speed breeding”) may accelerate cultivar development through rapid identification and introgression of traits of breeders’ interest (Figure 2). Marker-assisted speed breeding is an innovative and robust concept for fast-track introgression of QTLs or genes controlling drought tolerance in lentil. This can be achieved through the development of robust markers tightly linked to traits that impart drought tolerance in lentil using both QTLs mapping and GWAS; after which, crossing could be performed under a speed breeding environment to accelerate generation turnover. Besides, progenies harboring traits of interest can be detected using molecular markers. Such a process shows the potential of the establishment of “marker-assisted speed breeding” in empirical lentil breeding programs, especially those targeting complex and polygenic traits such as drought tolerance.

3.3. Agronomic Interventions to Induce Drought Stress Tolerance in Lentil

Adoption of pertinent agronomic practices is another potential way for minimizing yield reductions in lentil fields prone to drought stress. In fact, it has been recorded that agronomic interventions such as optimization of sowing time and rate, seed enhancement, exogenous application of plant growth regulators and osmoprotectants, adequate nutrients supplementation, application of plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF), adoption of conservation agriculture practices (i.e., no-till, crop rotation and crop residue management) can minimize the detrimental effects of drought stress [1,48,50,59,82,167,168,169,170,171].

The plantation in a relevant time is an effective strategy for escaping drought stress, and ensuring optimal conditions for crop growth. The best sowing time allows better exploitation of rainfall throughout the crop cycle. In fact, relevant sowing time can reduce the effects of drought, particularly at flowering and pod felling stage [168]. In Syria, delayed sowing resulted in 20% grain yield losses in comparison with early sowing in lentil [172]. Likewise, delaying sowing resulted in a short period of maturity and reduced yield due to the occurrence of drought at flowering and pod filling stage [173]. In southwestern Australia, early sowing of lentil resulted in prolonged vegetative and reproductive growth phases, rapid canopy development, higher assimilation of photo-synthetically active radiation, and more efficient use of water, leading ultimately to higher biomass accumulation and increased yield [174]. In another study, normal lentil sowing resulted in higher dry matter, net assimilation rate, crop growth rate, leaf area index, prolonged growth period, lower canopy temperature, and 30% more grain yield compared to late planting [39]. Water use efficiency, biomass production, and grain yield in lentil were significantly influenced by cultivars and sowing time in southern and western Australia [175]. Plant density is another parameter to be considered for the efficient use of water and increased yield in drought-prone environments [168].

Exogenous application of plant growth regulators, or phytohormones, (i.e., ethylene, salicylic acid, melatonin, abscisic acid, jasmonic acid, brassinosteroids, cytokinins, gibberellic acid) [59,176,177], and osmoprotectants (i.e., proline, glycine betaine, trehalose, polyamines) can help plant to withstand adverse effects of drought stress through the modulation of physiological and biochemical processes. Nevertheless, limited studies have been conducted in lentil in this area; thus, further research is needed to evaluate the potential effects of osmoprotectants and growth regulators in inducing drought tolerance in lentil.

Adequate nutrient management is crucial to improve lentil productivity under optimal and sub-optimal conditions. It is known that drought stress limits nutrient uptake and use efficiency [168]; however, adequate application of zinc has improved chickpea adaptation to drought and heat stress [82]. Furthermore, soil and foliar application of micronutrient such as iron, zinc, and selenium improves plant tolerance to abiotic stress [178]. In lentil, silicon (2 mM) supplementation has modulated osmolytes and antioxidant defense system metabolism, and improved drought tolerance of drought-stressed lentil genotype at the germination stage [42]. The same authors observed that silicon application (2 mM) improved drought tolerance capacity of drought-stressed lentil genotypes (50% and 25% of FC for moderate and severe drought, respectively) at flowering stage by increasing the antioxidant defense system, relative water content, chlorophyll content, silicon content, and biomass accumulation; meanwhile, it reduced osmolytes content, reactive nitrogen species (RNS), ROS, and hence oxidative stress damages [179]. Besides, silicon supplementation reduced anti-nutritional factors, improved nutritional profile, and increased sensorial quality of lentil seeds [180]. In another study, combined foliar application of (boron (0.2%) + iron (0.5%)) and (boron (0.2%) + iron (0.5%) + zinc (0.5%)) at pre-flowering and pod development mediated the alleviation of terminal drought and heat stress in lentil by the modulation of antioxidant metabolism, increase of chlorophyll content, and improvement of photosynthetic capacity [41]. Furthermore, selenium supplementation improved the tolerance of lentil genotypes treated with combined drought-heat stress (50% FC; 32/20 °C day/night) at flowering stage by the optimization of physiological (photosynthesis, water relation), and biochemical responses (antioxidant defense, osmolytes accumulation), which in turn improved seed yield [181].

Plant-promoting rhizobacteria and arbuscular mycorrhizal fungi have also showed positive impacts on lentil under drought condition in different studies [43,182,183,184,185]. ACC-deaminase producing rhizobacteria application in combination with caffeic acid (20 ppm) alleviated adverse effects of drought (50% FC) and improved lentil growth and productivity by improving physiological and biochemical attributes associated with drought tolerance [43]. In another investigation, mycorrhizal fungi (Glomus intraradices) and Nitrogen-Fixing (Azotobacter)-based biofertilizer enhanced lentil chlorophyll and leaf water content, reduced proline content, and increased seed yield and protein content under rainfed and irrigated conditions, which may be attributed to improved water and nutrient uptake [182].

Seed priming is an efficient seed enhancement method to accelerate germination process, and improve crop stand establishment and adaptation to environmental stress [186]. Osmopriming (1% CaCl₂) improved germination, soluble sugar, amylase activity, and chlorophyll content of drought-stressed lentil [187]. In addition, hydroperming increased stand establishment of field-sown lentil [188], including under water-stressed conditions [189].

Conservation agriculture practices such as reduced or no-till (zero-till), and crop residues conservation allowed efficient crop production by the reduction of investments associated with tillage operation. For example, in Morocco, studies reported that shifting to no-till resulted in reduced soil erosion, improved soil physicochemical quality and water holding capacity, and better and more stable yield [190]. However, crop yield under no-till is linked to other factors such soil type, climatic conditions, residue incorporation, crop rotation, and weed management. In lentil, minimum till and no-till with residue retention conserved soil moisture, and enhanced lentil relative water content, chlorophyll content, growth rate, biomass, and provided high yield compared to conventional tillage [191]. Mulch application resulted in 36.5–66.6% more yield compared to residue removal [192]. Moreover, no-till increased lentil yield by improving carbohydrate content, relative water content, leaf area index, and favored soil microbial activity, which was attributed to soil moisture conservation; thus, no-till could provide a buffer system to prevent adverse effects of drought on rainfed lentil if harmonized with other agronomic practices such as the time of sowing [35].

4. Conclusions and Perspectives

It is believed that climatic fluctuations have significant effects on agricultural production and global food security. Drought stress is the major destructor of crop growth and productivity in arid and semiarid areas. In fact, lentil is not an exception, and its morphology, physiology, and productivity are badly hampered by drought stress alone, or in combination with other stressors, especially heat stress in many cases.

Biotechnological tools could be exploited to modernize the lentil breeding programs. Such advances may be endorsed by the development and enrichment of lentil genetic and genomic resources. Next-generation sequencing platforms have revolutionized genomic research in lentil through the development of draft genome, high density linkage maps, and allowing high resolution detection of QTLs/gens using GWAS and QTL mapping. In addition, other omics tools, especially transcriptomics and metabolomics, revealed significant results with respect to genes and metabolites associated with drought acclimation in lentil; nevertheless, extensive research is required to exploit their full potential. The major bottleneck for identifying trait-imparting drought tolerance is the phenotyping, as imprecise and inaccurate phenotyping impedes genotyping progress; for this, high-throughput phenotyping platforms have emerged as a robust tool for accurate and rapid capturing of phenotypic features that play a role in plant adaptation to drought. Thus, the combination of biotechnological tools will provide meaningful data that could enable high-throughput dissection of traits connected with high-performance of lentil crop in dry areas. Furthermore, identified traits may be targeted through marker-assisted breeding and genetic engineering approaches. In another way, speed breeding is a new asset to improve the genetic gain in breeding programs. Merging speed breeding and other biotechnological tools may speed up cultivar development to increase lentil yield in arid and semi-arid areas.

Agronomic innovation could further enhance lentil yield, and should be integrated with breeding strategies for exposing the full potential of cultivars developed. Agronomic practices, such as conservation agriculture practices, adequate fertilization and sowing date, seed enhancement, and application of plant-promoting rhizobacteria and arbuscular mycorrhizal fungi, have proven to be useful in overcoming the adverse effects of drought stress. However, limited research has been reported in lentil, highlighting the importance of future studies in this direction.

Finally, smart adoption of advanced breeding platforms and innovative agronomic practices may be a fascinating way for large production of lentil in arid and semi-arid cropping systems to contribute in global food and nutritional security, especially under current and prospected fluctuating environmental conditions.