**5. Requirements and graft union formation**

There are five requirements critical to achieve a successful graft union: (1) the scion and rootstock should be compatible, (2) proper cambial alignment between scion and rootstock, (3) enough pressure to keep the cut surfaces firmly together, (4) avoidance of desiccation by maintaining high humidity around the cut surface, and lastly (5) both plants should be at the proper physiological stage for grafting to occur [29]. Good craftsmanship is an important requirement that brings the five requirements together. Graft union formation in compatible species involves a number of stages. In the first stage, parenchymatous cells are formed on the cut surfaces of the scion and rootstock followed by the interlocking of the callus between scion and rootstock leading to the formation of a callus bridge. This is followed by the differentiation of cells and the formation of the vascular cambium across the callus bridge between the scion and the rootstock and the eventual connection between phloem and xylem of the scion and rootstock to form a composite plant. The vascular connection lays the foundation for the transport of nutrients and water [30]. In tomato grafting, the formation of the xylem and phloem vessels occurs 8 days after grafting is performed [31].

Graft incompatibility refers to the inability of a graft union to form or grow properly between a scion and a rootstock, because of certain physical or chemical characteristics of the scion and rootstock. This leads to major setbacks in grafting operations, which may have economic implications in terms of grafting percentage and fruit yield. The response of Solanaceous plants to graft incompatibility may differ based on the combination of the scion and the rootstock selected. Severe incompatibilities have been observed in, for example, tomato/pepper (scion/rootstock) grafts, while moderate incompatibilities have been observed in eggplant and tomato (scion/rootstock) grafts. This is related to yield and the number of grafted plants that survived after grafting [32].

rates [33]. High humidity within the grafting chamber can be achieved by misting the chamber regularly with water; the use of plastic polythene to cover the grafting chamber acts as an insulator, which shields the plants from the changes in temperature and other weather

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**Figure 2.** Grafted tomato plants showing adventitious bud regrowth (red arrow). Picture by Charles Agyeman.

An ideal post-grafting operation should therefore include the maintenance of an ideal air temperature and relative humidity of 25–28°C and 80–90%, respectively, which will promote a higher survival rate and quality of grafted seedlings [34]. In situations where temperature levels have exceeded 30–32°C, the leaf weights (dry weight and fresh weight) have been

Quality has become the hallmark of consumers who purchase vegetables as part of their daily dietary requirements; consumers therefore use certain visual and nonvisual attributes to determine the quality of vegetables and fruits in general. Consumers determine the quality of tomato fruits based on their appearance (size, color, and shape) and texture (firmness, mealiness, and juiciness) as well as their flavor and nutritional content [36]. However, different

reported to reduce significantly in watermelon [35].

**6. Fruit quality of grafted plants**

conditions.

Rootstock regrowth, also referred to as "suckering" or adventitious bud growth, usually occurs about 14 days after grafting success. The regrowth becomes vigorous and occurs beneath the graft union on the rootstock. Usually both rootstocks (*S. macrocarpon* and *S. aethiopicum*) exhibit adventitious bud regrowth (**Figure 2**).

Monocotyledonous plants cannot be grafted because they lack the ability to form cambium layers, compared to dicotyledonous plants. Temperature and relative humidity levels are crucial environmental factors for graft union formation, and acclimatization of grafted plants. The regulation of these post-grafting factors will influence the survival rates of the grafted plants, grafting success, and yield. Generally, a higher relative humidity in the grafting chamber tends to favor grafted tomato plants, as grafted plants do not lose moisture at higher Grafting: An Effective Strategy for Nematode Management in Tomato Genotypes http://dx.doi.org/10.5772/intechopen.82774 9

**Figure 2.** Grafted tomato plants showing adventitious bud regrowth (red arrow). Picture by Charles Agyeman.

rates [33]. High humidity within the grafting chamber can be achieved by misting the chamber regularly with water; the use of plastic polythene to cover the grafting chamber acts as an insulator, which shields the plants from the changes in temperature and other weather conditions.

An ideal post-grafting operation should therefore include the maintenance of an ideal air temperature and relative humidity of 25–28°C and 80–90%, respectively, which will promote a higher survival rate and quality of grafted seedlings [34]. In situations where temperature levels have exceeded 30–32°C, the leaf weights (dry weight and fresh weight) have been reported to reduce significantly in watermelon [35].

### **6. Fruit quality of grafted plants**

when grafting cucumbers because the seedlings of cucumber are large (hypocotyl length and

The tube grafting method also has a high percent graft rate. The grafting of two tomato cultivars ("PG3" and "Beaufort") using the tube and the cleft graft methods resulted in a high-percentage graft rate (79–100%), an indication of the suitability of both methods for tomato grafting [28].

There are five requirements critical to achieve a successful graft union: (1) the scion and rootstock should be compatible, (2) proper cambial alignment between scion and rootstock, (3) enough pressure to keep the cut surfaces firmly together, (4) avoidance of desiccation by maintaining high humidity around the cut surface, and lastly (5) both plants should be at the proper physiological stage for grafting to occur [29]. Good craftsmanship is an important requirement that brings the five requirements together. Graft union formation in compatible species involves a number of stages. In the first stage, parenchymatous cells are formed on the cut surfaces of the scion and rootstock followed by the interlocking of the callus between scion and rootstock leading to the formation of a callus bridge. This is followed by the differentiation of cells and the formation of the vascular cambium across the callus bridge between the scion and the rootstock and the eventual connection between phloem and xylem of the scion and rootstock to form a composite plant. The vascular connection lays the foundation for the transport of nutrients and water [30]. In tomato grafting, the formation of the xylem and

Graft incompatibility refers to the inability of a graft union to form or grow properly between a scion and a rootstock, because of certain physical or chemical characteristics of the scion and rootstock. This leads to major setbacks in grafting operations, which may have economic implications in terms of grafting percentage and fruit yield. The response of Solanaceous plants to graft incompatibility may differ based on the combination of the scion and the rootstock selected. Severe incompatibilities have been observed in, for example, tomato/pepper (scion/rootstock) grafts, while moderate incompatibilities have been observed in eggplant and tomato (scion/rootstock) grafts. This is related to yield and the number of grafted plants

Rootstock regrowth, also referred to as "suckering" or adventitious bud growth, usually occurs about 14 days after grafting success. The regrowth becomes vigorous and occurs beneath the graft union on the rootstock. Usually both rootstocks (*S. macrocarpon* and *S. aethi-*

Monocotyledonous plants cannot be grafted because they lack the ability to form cambium layers, compared to dicotyledonous plants. Temperature and relative humidity levels are crucial environmental factors for graft union formation, and acclimatization of grafted plants. The regulation of these post-grafting factors will influence the survival rates of the grafted plants, grafting success, and yield. Generally, a higher relative humidity in the grafting chamber tends to favor grafted tomato plants, as grafted plants do not lose moisture at higher

diameter), making the grafting process easy [27].

8 Recent Advances in Tomato Breeding and Production

**5. Requirements and graft union formation**

phloem vessels occurs 8 days after grafting is performed [31].

*opicum*) exhibit adventitious bud regrowth (**Figure 2**).

that survived after grafting [32].

Quality has become the hallmark of consumers who purchase vegetables as part of their daily dietary requirements; consumers therefore use certain visual and nonvisual attributes to determine the quality of vegetables and fruits in general. Consumers determine the quality of tomato fruits based on their appearance (size, color, and shape) and texture (firmness, mealiness, and juiciness) as well as their flavor and nutritional content [36]. However, different market players along the vegetable value chain their standard for quality. The quality of tomato is based on soluble solids, acidity, sugars, pH, and shelf life [37].

Vegetable farmers and traders prefer tomato cultivars which exhibit firmness and can withstand mechanical damage, whilst in transit to various market centers [38]. The term fruit quality, which can be defined based on the visual and sensory properties such as color and sweetness, has been found to be controlled by certain inherent genes in some plant cultivars; some of these genes or genetic traits can be bred into new genotypes from other wild species [39].

Conflicting reports on the influence of grafting on fruit quality in vegetables exist. Positive and negative influences of grafting have been documented [40]. In their review of the impact of grafting on fruit quality in vegetables, Rouphael et al. [40] attributed these conflicting results to the differences in environments, production methods, scion/rootstock combinations, and harvest dates.

In an experiment conducted by Matsuzoe et al*.* [41], where tomatoes (Momotaro) were grafted on three *Solanum* species (*S. torvum, S. toxicarum* and *S. sisymbriifolium*), there were, however, no significant differences in the quality of grafted and ungrafted tomatoes in relation to the amount of sugars and their organic acid contents.

> *Physalis peruviana,* and TNAU Tomato Hybrid CO-3 and US-618) consisting of eight wild species and two F1 cultivars were evaluated for their resistance to root-knot nematode over a 60-day period, and the results showed that *S. sisymbrifolium* rootstock had the highest shoot

**Table 3.** Comparison of grafted rootstocks and inoculum level interaction on TSS, TSS/TA, pH, and TA.

P/SA = Pectomech grafted onto *Solanum aethiopicum*; P/SM = Pectomech grafted onto *Solanum macrocarpon*; TSS = Total soluble solids; TA = Titrable acidity; LSD = Least significant difference; ns = no significant difference.

**Treatments Inoculum levels TSS TSS/TA pH TA** P/SA 0 5.42 3.04 4.47 1.87 P/SM 0 5.99 4.02 4.36 1.65 P/SA 500 6.27 4.01 4.55 1.67 P/SM 500 6.33 4.40 4.79 1.62 P/SA 1000 6.67 4.27 4.42 1.65 P/SM 1000 5.78 6.44 4.72 1.22 P/SA 5000 6.3 4.25 4.45 1.45 P/SM 5000 6.28 3.81 4.43 1.66 LSD(P = 0.05) ns ns ns ns

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The rootstocks, *S. sisymbrifolium, Physalis peruviana*, and *S. torvum* recorded the least nematode population of 39, 40 and 43 per 200 cc of soil and a reproductive factor of 0.71, 0.74, and 0.84, respectively. *Solanum sisymbrifolium*, *P. peruviana,* and *S. torvum* were resistant to root-knot nematode (*Meloidogyne incognita*), and *S. incanum* and *S. aethiopicum* were found to

**8. Screening of rootstocks for the Mi gene using molecular markers**

The resistance offered by plants to the damage caused by root-knot nematodes have been well researched and attributed to the presence of a single dominant gene (Mi gene). The Mi gene confers resistance to various root-knot nematode species (*M. incognita*, *M. javanica,* and *M. arenaria*) in addition to whiteflies and aphids [46]. *Solanum* spp., for example, *Lycopersicon peruvianum* and *S. torvum* have been reported to have this resistant gene, which enables the plant to tolerate the feeding activities and the reproductive abilities of root-knot nematodes [47]. The Mi gene was first discovered in an accession of a wild *L. peruvianum* in South America from which commercial F1 varieties were introgressed with the gene [48]. This process involves the extraction and detection of the gene using DNA markers and subsequent isolation of the gene for introgression. In other related research conducted using the positional cloning approach to isolate gene with linked traits and the subsequent sequencing of the DNA, Kaloshian et al*.* [49] reported that the

fresh weight and dry weight of 103.87 and 10.44 g, respectively.

be moderately resistant to *Meloidogyne incognita*.

Agyeman [19].
