Eggplant relatives as sources of variation for developing new rootstocks: Effects of grafting on eggplant yield and fruit apparent quality and composition
Research highlights
▶ We propose using the vigorous interspecific hybrids of eggplant with Solanum incanum and Solanum aethiopicum as new rootstocks for eggplant. ▶ These hybrids have better germination than the currently used rootstock Solanum torvum and have full grafting compatibility with eggplant. ▶ ‘Black Beauty’ eggplants grafted on interspecific hybrids have higher yield and earliness and lower plant mortality than other rootstocks. ▶ Fruit apparent quality and composition are not affected by grafting on these interspecific hybrids. ▶ Out of all the materials studied, the interspecific hybrid S. incanum × S. melongena proved to be the best rootstock for eggplant grafting.
Introduction
Grafting of vegetable crops is used to provide resistance to soil pests and pathogens, to increase the tolerance to abiotic stresses, to improve water or nutrient uptake, or to enhance the vigour of the scion (Davis et al., 2008a, Davis et al., 2008b, King et al., 2008, King et al., 2010, Lee, 1994, Lee and Oda, 2003, Rivero et al., 2003). Lack of cultivars tolerant or resistant to increasingly important soil biotic and abiotic stresses, together with the prohibition of the use of methyl bromide for soil disinfestations, have led to a worldwide renewed interest in vegetable crops grafting (Bletsos, 2005, Davis et al., 2008a, Davis et al., 2008b, King et al., 2008, Miguel et al., 2004).
Eggplant (Solanum melongena L.) is widely cultivated in tropical and temperate regions around the world and is amenable to grafting (Bletsos et al., 2003, Daunay, 2008). Because soil pathogens can cause important loses in eggplant production, several rootstocks reported to be resistant or tolerant to soil pathogens, or that induce vigorous growth of the scion are used for improving eggplant production (Daunay, 2008). The wild relative Solanum torvum Sw., which has resistance to a wide range of soil borne pathogens (Verticillium dahliae Klebahn, Ralstonia solanacearum (Smith) Yabuuchi et al., Fusarium oxysporum (Schlechtend:Fr.) f. sp. melongenae Matuo and Ishigami, and Meloidogyne spp. root-knot nematodes), is recommended for eggplant grafting (Bletsos et al., 2003, Daunay, 2008, Singh and Gopalakrishnan, 1997, King et al., 2010). However, its use is limited by difficulty in getting rapid and homogeneous seed germination (Ginoux and Laterrot, 1991). Some tomato (Solanum lycopersicum L.) hybrids (e.g., ‘Energy’, or ‘Kyndia’) as well as tomato S. lycopersicum × S. habrochaites S. Knapp and D.M. Spooner interspecific hybrids (e.g., ‘He Man’, ‘Beaufort’) are also commonly used as rootstocks for eggplant (Bletsos et al., 2003, Miguel et al., 2007, King et al., 2010). However, specific tomato–eggplant rootstock–scion combinations are only moderately compatible (Kawaguchi et al., 2008), and without an adequate selection of rootstock–scion combinations, deleterious effects may appear (Kawaguchi et al., 2008, Leonardi and Giuffrida, 2006, Oda et al., 1996). Also, the wild species Solanum sisymbriifolium Lam. and the hmong eggplant Solanum integrifolium Poir. (=Solanum aethiopicum L. Aculeatum group) have been tested as rootstocks for grafting of eggplant, although the results were not very promising due to poor performance (Rahman et al., 2002, Yoshida et al., 2004).
Other Solanum species and materials, as well as interspecific hybrids, could increase the sources of variation for developing eggplant rootstocks that are tolerant or resistant to biotic and abiotic stresses, or to enhance nutrient uptake and vigour. In this respect, the scarlet eggplant (S. aethiopicum Gilo, Shum, or Kumba groups) and the gboma eggplant (Solanum macrocarpon L.) are cultivated species of economic importance in Western Africa (Schippers, 2000). Both species are phylogenetically close to S. melongena (Furini and Wunder, 2004), are propagated by seed, and their germination is more uniform than that of the wild S. torvum (Ginoux and Laterrot, 1991). Materials of both species have been described as tolerant to F. oxysporum f. sp. melongenae and resistant to R. solanacearum (Cappellii et al., 1995, Daunay et al., 1991, Hébert, 1985). Resistance to root-knot nematodes (RKN) has also been reported in S. aethiopicum Gilo group (Cappellii et al., 1995, Hébert, 1985). Another species of interest as a source of variation for developing new eggplant rootstocks is Solanum incanum L., which is the putative ancestor of eggplant (Lester and Hasan, 1991), and which has been reported as resistant to F. oxysporium f. sp. melongenae (Yamakawa and Mochizuki, 1979). Furthermore, these species could provide tolerance to abiotic stresses such as drought and low or high temperatures, which are important breeding objectives in S. melongena (Daunay, 2008).
Interspecific hybrids are used as rootstocks in many vegetable crops since they can contribute several advantages including pathogen resistances from both parents, vigourous growth, and, in the cases where one of the parents is from the same species as the scion, a greater degree of rootstock–scion compatibility (Daunay, 2008, Lee and Oda, 2003, Miguel et al., 2007). Interspecific hybrids of S. aethiopicum, S. macrocarpon, and S. incanum with S. melongena have been obtained with different degrees of success (Behera and Singh, 2002, Bletsos et al., 2004, Daunay, 2008, Lester and Hasan, 1991, Schaff et al., 1982). In this respect, S. melongena and S. incanum are easily crossed and the fruit resulting from the crosses bear many seeds with high viability (Lester and Hasan, 1991). Hybrids of S. melongena with S. aethiopicum are more difficult to obtain by sexual crosses than those with S. incanum, but viable seeds are produced (Behera and Singh, 2002). On the contrary, hybrids between S. melongena and S. macrocarpon are difficult to obtain and few viable seeds are obtained per cross (Bletsos et al., 2004, Schaff et al., 1982). This suggests that while S. melongena × S. incanum and S. melongena × S. aethiopicum hybrids might be of interest as eggplant rootstocks, the use of S. melongena × S. macrocarpon hybrids as rootstocks does not seem to be economically viable at this time.
Apart from the productive advantages offered by grafting, a very important issue, which on many occasions remains overlooked, is the effect of grafting on fruit quality (Davis et al., 2008a). In this respect, the apparent quality characteristics and composition of the final product of grafted plants should remain unchanged or improved with respect to the non-grafted plants. In some cases, an improvement in fruit composition has been reported. For example, mini-watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai) fruit from plants grafted onto a Cucurbita moschata Poir. × Cucurbita maxima Duch. interspecific hybrid rootstock had higher levels of K, Mg, lycopene and vitamin C in comparison to their respective control plants (Proietti et al., 2008). Deleterious effects may also appear as a consequence of grafting. For example, an enhanced incidence of fruit blossom end rot in tomato grafted onto S. integrifolium rootstocks (Oda et al., 1996) and the accumulation of high amounts of nicotine in tomatoes from plants grafted onto Nicotiana tabacum L. have been reported (Yasinok et al., 2009). In the specific case of eggplant grafted onto Datura inoxia P. Mill., scopolamine and atropine were accumulated in fruit at levels sufficient to cause poisoning (Oshiro et al., 2008).
In this work, we assess the potential vigour and influence on eggplant yield and fruit quality traits of S. incanum × S. melongena and S. aethiopicum × S. melongena interspecific hybrid rootstocks, as well as of S. macrocarpon rootstocks. The results are compared with those obtained from non-grafted, self-grafted, and S. torvum rootstock grafted plants. Our objective is to identify new potential rootstocks for eggplant as well as to validate our hypothesis that using interspecific hybrid rootstocks may be a good strategy for improving eggplant production.
Section snippets
Plant material
The eggplant cultivar Black Beauty (B and T World Seeds, Aiguesvives, France) was used as the scion variety as well as the ungrafted control. Five rootstocks that included materials corresponding to the three species S. melongena, S. torvum, and S. macrocarpon and to two interspecific hybrids, S. incanum × S. melongena and S. aethiopicum × S. melongena, were evaluated (Table 1). Hybridity of the interspecific hybrids was confirmed by evaluation with five SSR markers: CSM7, CSM12, CSM21, CSM40, and
Seed germination and graft success
Germination of seeds sown in Petri dishes with GA3 containing medium could be observed at 3–4 d after sowing for ‘Black Beauty’, SI × SM, and SM × SA, and at 8 d after sowing for SMA. At 15 d after sowing, ‘Black Beauty’ and the interspecific hybrids SI × SM and SM × SA exhibited high percent germination (≥90%) (Table 3). SMA displayed significantly lower germination (58%), and no germination was obtained with this protocol for STO. However, it was possible to obtain the necessary number of STO plantlets
Discussion
Grafting has proved to be an efficient tool for increasing the yield, disease resistance and quality of a number of vegetable crops (Davis et al., 2008a, Davis et al., 2008b, King et al., 2008, King et al., 2010, Lee, 1994, Lee and Oda, 2003, Rivero et al., 2003). Ideally, rootstocks should improve the yield and/or quality of the produce. This can be achieved by using rootstocks that have resistance to soil diseases or pests, tolerance to abiotic stress, selective absorption of available soil
Conclusions
Interspecific hybrids SI × SM and SM × SA exhibited high and uniform seed germination and eggplant scions grafted onto them displayed good vigour, excellent survival despite nematode soil infestation, and high yield. These results, together with the lack of deleterious effects on apparent fruit quality traits or fruit composition from SI × SM and SM × SA rootstocks, indicates that both hybrids are an advantageous alternative to the presently used S. torvum rootstock. In particular, given the fact that
Acknowledgements
This work was partially financed by the Ministerio de Ciencia y Tecnología (AGL2009-07257 and RF-2008-00008-00-00). The technical assistance of Nuria Palacios and Mariola Plazas is gratefully acknowledged.
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