**1. Introduction**

Salinity in soil or water is a serious threat to plant growth that prevents plants in achieving their genetic potential. Salinity annually damages about 20% of the world's crops grown under irrigation [1]. The high salt content of productive arable lands could render half of this land unusable for agriculture by 2050 [1]. In the world's arid and semiarid areas, less rain, high evaporation, saline irrigation water and inefficient management of water leads to salinity problems. Approximately 20% of the total cultivated lands and 33% of the irrigated agricultural lands worldwide are afflicted by high salinity and such areas are increasing at a rate of 10% per year [2]. It is estimated that about 3 ha of productive land per minute is lost due to salinity [3]. Thus, it is imperative to take appropriate measures timely to improve the salt tolerance of crops [4], especially tomato (*Solanum lycopersicum*

L.), which enjoys the prime position worldwide among different vegetables owing to having high nutritional and economical significance.

Tomato is a member of the nightshade family and cultivated for fresh and processing purposes as an annual crop [5]. Additionally, this crop is used as a model plant to study physiology to molecular genetics and genomics in the angiosperms [6,7]. Salinity represents a substantive threat to tomato production [8]; it causes considerable reductions in tomato growth and yield [2]. Most commercial cultivars of tomato are considered to be moderately sensitive to salt stress, which affects seed germination and the vegetative and reproductive stages of growth [5,9,10]. Ongoing efforts to improve salt tolerance in tomato using plant breeding, biotechnological approaches and other management practices have met with limited success due to the genetic and physiological complexity of the traits involved in salt tolerance [4,7,10,11]. According to Rao et al. [12], improvement of salt tolerance using plant breeding is an intricate task because of the high number of traits involved, their quantitative nature, epistatic gene action, low to moderate heritability and high sensitivity to the environment. Genetic engineering has been used to increase salt tolerance in plants and has claimed some success [11,12], but its commercial success is still to be witnessed.

An environmentally friendly, sustainable and effective method is grafting that enables to exploit the benefit of resistant genotypes (as rootstocks) to improve the performance of commercial cultivars (as scion) that are susceptible to (a)biotic stresses [13]. Grafting offers an alternative to breeding and biotechnological approaches to rapidly enhance salt tolerance in vegetable plants [8,11,14–16]. This technique in woody perennial fruit plants has been a routine practice in Asia for more than 2000 years [14]. In herbaceous vegetables, grafting initially was practiced in 1920s in watermelon to increase resistance to soil-borne diseases [17]. After the first scientific publication about grafting [14,17], use of the technique spread to other cucurbitaceous and solanaceous vegetable crops to address soil-borne diseases and other environmental stresses [18,19]. Commercial tomato grafting became popular in the 1960s in East Asia, Europe and later in North America [20,21]. Application of vegetable grafting has spread in many countries by the end of the 1990s, but it got momentum only after the banning of methyl bromide (MB) in the Montreal protocol in 2005, particularly in developed countries. Although, the protocol was authorized until 2015 in developing countries, with the efforts of United Nations Industrial Development Organization (UNIDO), United Nations Development Program (UNDP), World Bank and lateral agencies from Europe, many developing countries (some of Latin America, Africa, Middle East and Asia) were able to control the use of MB well before 2015, and started using grafting as an alternative to MB [18]. MB was the most commonly used soil fumigant against deadly soil borne pathogens prevailing in intensive vegetables production in protected cultivation [22]. Besides, being a sustainable practice, grafting is a useful component of organic vegetable production, particularly of tomato. In recent pasts, the horizon of grafting use expanded to abiotic stresses too and among which, salinity got the most priority. For instance, watermelon produced in countries like Japan, Korea, Turkey, Greece and parts of Spain and Italy with almost 100% grafted seedlings, and the use of grafted plants is also increasing immensely in other vegetables like tomato, eggplant, pepper, cucumber and melons across the world [23]. In major countries where grafting is a popular technique, out of the total tomato cultivation proportion share of grafted tomato seedlings use is around 1% in China, though it also ranks first in the number of grafted seedlings used, 25% in Korea, 33% in Vietnam, 40% in Japan, 50% in France and 75% in Netherlands. In other countries like the USA, Italy, Morocco and Spain the uses of grafted seedlings were available in numbers, i.e., 18, 15.1, 44, 72.8 and millions [24,25]. In this review, we present an outline of the potential of a grafting tool to enhance salt tolerance in a tomato based on recent researches done across the world. We also propose a strategy for future research as well as adoption for its better exploitation for the growth of the agriculture sector.

#### **2. E**ff**ect of Salinity on Tomato Plants**

Salinity is the most serious of all environmental stresses [26] and poses a great threat to agricultural sustainability [27]. It occurs when there is an excessive accumulation of salts (especially high Na+, Cl<sup>−</sup> and SO4 −) in the soil [28] or irrigation water [11,16]. The elevated level of salts generally causes a reduction of water potential in the root medium, thereby leading to a water deficit within plants [5,29], besides their excess level can cause ion-toxicity and nutrient imbalance, especially of K<sup>+</sup>, Ca2<sup>+</sup> and Mg2<sup>+</sup> by disturbing their uptake and/or transport to the shoots [30].

In general, the main factor of inhibiting growth of salt stressed plants is elevated levels of Na<sup>+</sup> and Cl−; with roots remaining the primary sites for stress perception [31] and the subsequent responses at the cell, organ or whole plant levels [32]. According to the two-phase model of salt-induced growth reduction, plants suffer initially due to osmotic stress impairing their ability to absorb sufficient water, and then from salt specific injuries ascribed to, in general, toxic levels of Na<sup>+</sup> and Cl<sup>−</sup> interfering with key cellular processes and causing damage to the cell membranes and organelles; altering nutrient ratios, endogenous growth regulator concentrations and enzymatic activities and suppressing photosynthetic assimilation and causing plant death in the extreme cases [33]. In fact, the second phase of salinity stress, i.e., ion-specific toxicity, is a long-term process and depends on the intracellular salt ion levels, which mostly tend to increase with an increase in the magnitude and duration of salinity stress.

Salinity has been reported to disturb plants physiological and biochemical processes and induce changes in morphological characteristics that finally lead to losses in yield [2,27,28,34]. Salt stress causes decrease in plant height, shoot and root biomass and root length in tomato plants [35]. According to Najla et al. [36] salinity adversely affected plant height and leaf area, and the overall development process of tomato plants. However, the salinity induced decrease in plant height was more related to reduced internodal length than the number of nodes, and lower plant growth rate was associated with a decrease in leaflet growth and the number of leaflets per leaf in salt stressed plants [36]. The adverse effects of salinity on shoot and root morphology were the result of an alteration of plant physiology that includes altered absorption of water and nutrients, hormonal production and disrupted root to shoot signals [37]. Salt induced inhibition in photosynthesis and oxidative stress has been widely documented. Salt stress can also alter leaf metabolites concentrations as have been reported by Khavari-Nejad and Mostofi [38]. They observed a notable decrease in the level of chlorophyll and β-carotene contents, along with an increase of soluble sugars and total saccharides in the leaves of tomato plants treated with 100 mM NaCl. In addition, aggregated chloroplasts and distorted and wrinkled cell membranes were also noticed in tomato leaves by these researchers. In spite of inhibiting plant height and shoots and roots dry matter contents, excess salt concentration in water reduced water use efficiency (WUE) and amount of K<sup>+</sup> and K/Na ratio in all studied tomato cultivars [39]. The accumulation of monovalent and bivalent Na+, K<sup>+</sup> and Ca2<sup>+</sup> ions in foliage and roots under saline conditions was, though, genotype-dependent [40]. Further, salinity affects almost all the plant growth stages, but the severity depends on the growth stage, salinity level and cultivar [35]. However, despite the inhibitory effects of salinity on growth and yield, the enhancement of some fruit quality characteristics (i.e., higher levels of sugars and organic acids) in tomato have been reported [41]. An increase of up to 40% carotenoid content in fruits of tomato plants exposed to moderate salinity has also been observed (4.4 dS m<sup>−</sup>1) [42].

#### **3. Grafting to Improve Salt Tolerance in Tomato**

#### *3.1. Growth and Yield*

The persuasive response of grafting on plant growth and yield characteristics under saline conditions can vary; this can be the result of intrinsic characteristics of scion, rootstock and their functional interactions, and severity of saline stress. A wide range of studies suggests that the adverse effects of salt stress on vegetable plants can be mitigated by grafting. Improved plant growth and yield performance of susceptible tomato cultivars under salt stress is the manifestation of positive response

of grafting; these ascribed to the right choice of scion-rootstocks combination [11]. Concerning the response of scion-rootstock combination on tomato growth and yield, grafting 'Cuore di Bue' scion onto 'Arnold' rootstock was found superior to either non-grafted or those grafted onto other rootstocks ('Maxifort' and 'Armstrong') under a moderate salinity level (20 mM NaCl), while the response of rootstock grafting was not evident at higher NaCl concentration (40 mM) [43]. The response of different rootstocks to salt susceptible scion 'Moneymaker' varied for yield and fruit parameters under saline conditions; graft combination involving 'Beaufort' and 'He-man' rootstocks was more productive under a mild saline condition. However, between two, only 'He-man' rootstock could provide sustenance to susceptible scion 'Moneymaker' for fruit yield under a high level of salinity. Contrarily, 'HPG' and 'Energy' rootstocks grafted tomatoes showed negative effect and produced lower yields [15].

Salt stress decreased both vegetative growth (i.e., stem diameter, plant height and shoot fresh weight) and fruit yield of both grafted and non-grafted plants. At this salinity level (3.76 dS m−1), no significant decrease in fruit yield was noticed in grafted plants ('Faridah' scion onto 'Unifort' rootstock) [10]. Concerning scion-rootstock combinations, a significant variation in plant growth (e.g., stem growth rate) was observed under saline conditions by Balliu et al. [44], who revealed that 'Charlotte' grafted onto 'Cyndia' exhibited higher mean stem growth rate than non-grafted plants. However, no difference in this parameter was noted between grafted and non-grafted tomato plants under normal (non-saline) condition. These authors, further stated that 'Bona' plants grafted onto 'Energy' rootstock produced higher mean yield compared to non-grafted plants, while this combination displayed lowest stem growth rate. The other scion, 'Charlotte', gave the maximum yield with rootstock 'Prospero' under saline conditions. The performance of salt sensitive cultivar 'Moneymaker' improved in terms of both plant growth and fruit yield under salt stress (50 mM NaCl), when it was grafted onto the salt tolerant rootstock 'Pera' [45]. Santa-Cruz et al. [46] reported that the 'UC-82B'/'Kyndia' (scion/rootstock) combination showed a tolerance response to excess salt (100 mM NaCl) by producing higher shoot growth and fruit yield than the rest of the other graft combinations. Among yield-contributing traits, fruit weight was found to be the single factor to determine yield, while the number of fruits affected was not significantly affected by salt stress; as also was obvious in this study where graft combination 'UC-82B'/'Kyndia' exhibited higher fruit weight. Al-Harbi et al. [10] concluded that tomato could be grown successfully with a satisfactory yield by grafting onto suitable rootstock under salt stress (EC 3.76 dS m<sup>−</sup>1). While comparing the response of tomato grafting onto different rootstocks under saline conditions in indoor or outdoor grown plants, Voutsela et al. [47] revealed that tomato fruit yield was significantly higher in grafted plants than non-grafted plants in both indoor and outdoor grown plants under salt stress (6.0 mS/cm). The benefit of grafting for increased fruit yield over non-grafted plants under salt stress was more pronounced in indoor condition (from 208% to 259%) than outdoor condition (from 0% to 149%). These workers have also reported significantly higher fruit yield in salt stressed self-grafted plants than non-grafted plants regardless of growing conditions. However, Iseri et al. [48] stated that enhanced salt tolerance and adaptive response of tomato scions was rootstock dependent rather than graft-induced changes per se. These reports indicate that selection of rootstocks and scion cultivars to be made reasonably for harnessing the benefit of grafting in tomato under saline conditions.

#### *3.2. Fruit Quality*

Despite the reduction of the fruit yield, enhancement of fruit quality traits is a general response of mild stress (i.e., water and salt); these are due to the accumulation of more metabolite contents under stress conditions. The interaction between grafting and salinity for fruit quality traits may be positive, negative or even neutral under stress conditions. The response of grafting on growth and fruit yield was positive when tomato cultivar 'Cuore di Bue' grafted onto 'Arnold' rootstock, whereas no obvious effect of grafting was observed in this graft combination on fruit quality traits (i.e., total soluble solids, fruit dry matter percentage, titratable acidity and TA) at any of the levels of NaCl (i.e., 0, 20 or 40 mM) added medium [43]. Similarly, grafting 'Durinta' onto 'He-man' rootstock showed promising response for yield traits, especially under high saline medium (8.8 dS m<sup>−</sup>1), but the fruits of this graft combination had low titratable acidity content [15]. Turhan et al. [49] have also reported a reduction in the tomato fruit quality in grafted plants than non-grafted plants under salt stress. Contrary to these, findings of Balliu et al. [44] reported positive response of both grafting and salinity level; the fruit quality characteristics namely fruit dry matter percentage, vitamin C and total soluble solids contents increased in grafted plants with the increase of NaCl concentration from 0 to 5 mM. Flores et al. [50] noticed that grafting 'Moneymaker' onto 'UC82B' did not increase fruit yield either under optimal or saline condition, fruit yield rather decreased in grafted plants, but fruit quality parameters (i.e., soluble solid content and titratable TA) were higher in grafted plants, more conspicuously under salt stress condition (50 mM). However, grafting 'Moneymaker' onto 'Radja' rootstock was found as promising as both fruit yield and quality were increased in this graft combination under a saline condition (25 and 50mM NaCl) [50]. The inference drawn from these is that the combination of scion and rootstock should be selected carefully to get the maximum benefit of the grafting technique.

#### **4. Mechanisms of Salt Tolerance in Grafted Plants**

In order to overcome the harmful effects of salinity, grafted tomato plants employ certain adaptive strategies such as salt exclusion or retention, osmotic adjustment, activation of antioxidant defense system, nutrient homeostasis, plant hormonal balances and a gene expression led favorable response. Roots being the primary plant organ have to face any soil related stress (e.g., salinity); their intrinsic characteristics would determine overall plants performance.
