1. Introduction
Vegetable grafting was initially promoted to meet the restrictions to soil disinfestation against soil-borne pathogens and pests, but over time, it has become a strategic tool for improving the crops’ performances under a wide array of suboptimal growth conditions [
1,
2,
3], becoming a cornerstone of the modern sustainable horticulture. Nowadays, grafting is recognized as a pivotal means to modify the vegetative vigor and maximize the yield of several horticultural crops [
4], but such increases can be accompanied by variable effects on fruit quality [
5].
Tomato (
Solanum lycopersicum L.) is one of most important greenhouse crops in the Mediterranean Basin [
6,
7], for which grafting is considered a standard practice in commercial greenhouse cultivations [
8]. An improved yield is commonly reported in tomato grafted onto appropriate rootstocks as a result of an increased fruit size or number of fruits per plant, the former feature typically recognized also in scions with small-sized fruits, such as cherry tomato [
5,
9]. It has been reported that vigorous rootstocks tend to increase fruit yield probably by enhancing water and nutrient uptake and transport to the scion [
10]. As a consequence of the central contribution of both processes to fruit metabolism, several authors have noticed rootstock-mediated effects on many tomato traits such as shape [
11], color [
12], texture [
13], taste and aroma components [
14,
15] or ascorbic acid content [
16]. However, these effects have been even conflicting among authors, making impossible to draw sound conclusions about the overall effects of this technique on tomato quality. This is primarily due to the existing complex interactions involving the genetic background of the grafting partners and the surrounding growth environment in determining the tomato phenotype [
17]. Moreover, the researches in this field have been conducted using different scion cultivars and fruit typologies, making impossible to compare indiscriminately the relative findings.
To date, the most popular approach has been to examine the effects of different rootstocks on one or few scions. Although giving useful information, this may not consider the typical characteristics of the scion, which actively exchanges photosynthates and metabolic messengers with the rootstock [
17]. To observe the rootstocks-induced changes in tomato yield and fruit quality without missing the active role of the scion and rootstock characteristics, large experimental designs including many rootstocks-scion combinations are required. With this in mind, the present experiment was designed to examine, in a large set of grafting combinations (7 scions × 8 rootstocks, plus ungrafted controls), the effects of grafting on growth, yield and quality of greenhouse cherry tomato, with particular focus at elucidating (i) the possible predominance either of the scion or of the rootstock in influencing the response of the grafted plant, and (ii) whether these results could be generalized on the basis of the fruit size of the scion or of the rootstock genetic background.
4. Discussion
Under the experimental conditions in which we operated, ‘S’ explained the highest variability of both whole plant and vegetative biomass, whereas the influence of both ‘R’ and ‘R × S’ prevailed on harvest index. When the grafting part classes were considered, the scion group did not influence the growth and developmental variables, which, instead, were under the influence of the rootstock genetic background. It has been suggested that the improved plant growth in response to grafting flows from a greater root development, enabling plants for a better absorption of water and minerals from the growth substrate, thus maximizing the photosynthetic gain of the scion [
1,
21]. Among the rootstock classes considered in the present experiment, the
S. habrochaites hybrids maximized both whole plants and vegetative biomass accumulation (with a 10 and 22% increase, respectively, when compared to the ungrafted class), whereas a significant reduction of both variables was recorded in the
S. pimpinellifolium class in comparison to the most vigorous rootstocks (by 11 and 13%, on average). Accordingly, among the studied rootstocks, ‘Dynafort’ (
S. lycopersicum ×
S. pimpinellifolium) proved to have the lowest plant and vegetative biomass and related σ
2E values, meaning that its tendency to reduce tomato plant growth was poorly affected by the scion cultivar.
S. habrochaites is a highly vigorous species, adapted to thrive in a wide latitudinal distribution, showing characteristics promoting growth in grafted tomato [
22,
23]. On the other hand,
S. pimpinellifolium is the closest wide relative to the cultivated tomato, and has been used to improve tomato resistance to several biotic stressors [
24] or fruit quality traits [
25]. Recently, Mata-Nicolás et al. [
26], in a wide
Solanum germplasm collection including
S. pimpinellifolium and
S. lycopersicum, found that the
S. pimpinellifolium accessions were the least vigorous in terms of stem width and leaf length. Therefore, our results suggest that this low vigor tendency could be at the base of the low growth induction in grafted tomato scions. Interestingly, these two highly differentiated rootstock classes in terms of plant growth performances shared a decreased fruit biomass production when compared to the other graft combinations (−10%, on average). Concerning the
S. habrochaites class, the reduction of fruit biomass acted to significantly lower the harvest index. This suggests that the enhanced plant growth was accompanied by an increased sink strength of the vegetative fraction. Indeed, it has been reported that vigorous rootstocks may act as additional sinks, exacerbating the competition for photosynthates allocation with other plant fractions [
27]. The ‘S
class × R
class’ interaction revealed that the fruit size of the scion contributed to buffer the sink strength of the rootstock. Indeed, when the ungrafted control class was considered, both large- and small-fruited scions showed the highest fruit biomass, whereas in three groups (
S. habrochaites,
S. peruvianum and intraspecific hybrids) grafting penalized the fruit biomass of both medium- and small-fruited scions, with the large-fruited one always showing the highest fruit biomass value. In this respect, the large-fruited class of our experiment comprised only one cultivar (‘Porpora’); for this reason, we are aware that larger experimental classes would be needed to parameterize the possible relationship we noticed among fruit size and biomass in grafted tomato plants. Interestingly, this reduction was restrained by the
S. pimpinellifolium class, which conferred the least vegetative vigor to the scions. Recently, Grieneisen et al. [
28], in a meta-analysis involving a large set of literature data, reported that, in the majority of the cases examined, tomato grafting had no effect (58% of the cases) or a negative effect (6%) on fruit yield. For the authors, it was impossible to separate the effects of the scion cultivar to explain the yield response included in the dataset. This is in agreement with our results, since ‘S’ was by far the main source of yield variation, followed by the ‘S × R’ interaction, meaning that under unstressed growth conditions like those of our experiment, the centrality of the rootstock per se in determining yield is at least partially lost. This implies that despite its central importance, the yield outcome of tomato grafting still remains poorly predictable and to be accessed case by case. However, when clustered by class in the ANOVA, the yields we recorded were proportional to the fruit size of the scions, denoting that medium- and small-fruited cherry tomatoes put a penalization on crop yield when grafting is concerned.
According to Schwarz et al. [
13] and Kyriacou et al. [
5], the mean fruit weight is one of the primary traits that is influenced by grafting. In our study, beyond the largely prevalent effect of ‘S’, the average fruit weight was significantly affected by ‘R’ and ‘R × S’ too, whereas all the interspecific hybrid classes promoted such trait, without interactive effects.
When positive, higher
a* values refer to an increased intensity of the red hue, representing in tomato the top contributor to the lycopene-derived color [
29]. In red-ripe tomatoes, both
a* and
b* have positive values, but the higher the
b* value the more perceived color turns to orange, through progressive yellow addition. Higher
L* values represent a transition toward lighter colors, potentially representing a deterioration of tomato pigmentation too. Graft-induced worsening of tomato color has been sometimes reported as a result of a reduced carotenoid concentration by grafting onto vigorous rootstocks [
12,
30], though this finding has not always been confirmed [
13]. In the present study, the fruit chromatic coordinates were mostly ‘S’-dependent, though a significant ‘R × S’ interaction was recorded for
a*. Accordingly, for all these chromatic coordinates, the scion genotypes proved lower σ
2E values than the rootstock ones, with
a* showing higher σ
2E values than
L* and
b*. ‘Porpora’ (large-fruited) and ‘Caprice’ (small-fruited) exhibited the highest stability related to
a*, meaning that the ability of the scion to superimpose the red hue was independent from the fruit size. In contrast, ‘Bental’ and ‘Pittam’ (both deriving from
S. peruvianum) showed the highest σ
2E, and the ANOVA by classes revealed that they acted to reduce
a*, together with
S. habrochaites hybrids (−9%, on average). It is interesting to note that both rootstock classes were characterized by the highest vegetative biomass, so we cannot exclude that their depressive effect sometimes recorded on
a* was not associated with a heavier fruit shading and subsequent lower fruit temperature, since both optimal light and temperature are key promoters of lycopene accumulation in tomatoes [
31,
32].
Fruit shape modifications accompanying the increased fruit size have been reported in grafted tomato [
5]. However, we did not observe any shift in fruit shape in response to grafting, though some fruit weight increases were observed. This indicates a strong maintenance of the typical shape in all the studied scions, regardless of the grafting combinations, consistent with the known poor dependence of this trait on non-genetic factors [
33].
Texture, often described through firmness, is a key quality trait of tomato, as it influences postharvest transportability, shelf-life and even flavor perception [
34,
35]. Although tomato firmness is often reduced by vigorous rootstocks, the results in this regard are sometimes contrasting [
36,
37]. In our experiment, fruit firmness was not affected by ‘R’ nor by ‘S × R’, being ‘S’ the only contributor to the experimental variability, though no significant differences emerged in relation to fruit size. Accordingly, for all the taste variables, the scion genotypes proved lower σ
2E values when compared to rootstocks.
Soluble sugars (mainly glucose, fructose and sucrose) and organic acids (mainly citric and malic) are key taste-compounds of tomato fruits, whose amounts are commonly measured through total soluble solids (TSS) and tritatable acidity (TA), respectively [
38,
39]. When the organoleptic quality of tomatoes is concerned, their measure is referred to the perceived sweetness (TSS) and sourness (TA), whereas the TSS/TA ratio describes the overall balance among them in the perceived taste [
12]. Although fruit itself can partially contribute to carbon fixation, most of this element (85–90%) needed for fruit growth is imported from leaves through the phloem tissue, in the form of carbohydrates [
33,
40]. In contrast, despite a certain amount of organic acids being able to be supplied through the sap, their accumulation in fleshy fruits is primarily due to the metabolism of citrate and malate in the fruit itself [
41,
42,
43]. In our experiments, both TSS and TA peaked in the small-fruited scions. For TSS this is consistent with its inverse proportionality with the fruit size [
39]. However, grafting onto the most vigorous rootstocks (i.e., the
S. habrochaites and
S. peruvianum classes) acted to limit TSS, irrespective of the scions’ characteristics. This seems to reinforce the hypothesis of a limited photosynthates flow toward fruits, due to the modified source-sink edaphic relationships induced by vigorous rootstocks [
5,
27]. Additionally, efficient rootstocks in water absorption may increase fruit water content, leading to a decreased fruit sugars concentration [
11,
44]. This hypothesis is corroborated by observing that, despite a lower fruit biomass in the most vigorous rootstocks, no grafting effect was recorded on scions’ yield, likely deriving from an increased fruit size due to a higher water content. On the contrary, all the rootstocks under study promoted TA in small-fruited scions, generating a lower TSS/TA ratio, especially in the
S. habrochaites and
S. peruvianum hybrid rootstocks (−8%, on average). Organic acids (in particular citrate) tend to substitute sugars as a respiratory substrate in the case of cytosolic carbohydrates shortage into the fruit [
45,
46]. This hypothesis seems to be corroborated by the highest TA increase in response to grafting recorded in the small-fruited scions, i.e., those most suffering the sink strength imposed by the rootstocks.
Beyond its importance as antioxidant in the human diet [
47] ascorbic acid (AsA) content plays a pivotal role for the plant, being involved in cell division, cell wall synthesis and in the interaction of the plant with the surrounding environment [
48]. According to Massot et al. [
49] the AsA concentration in tomatoes is the complex result from its import (or the import of precursors) from leaves, its synthesis and recycling inside the fruit and its export outside the fruit. In the present study, the AsA content proved to be mostly under the scion influence, but when these were clustered in the ANOVA, it was impossible to establish differences on the basis of the fruit size. Accordingly, there were huge differences among scions in terms of σ
2E, whose highest values were recorded both in large-fruited (‘Porpora’) and small-fruited scions (‘Caprice’ and ‘Beka’). In contrast, despite the significant ‘S × R’ interaction, the ANOVA for rootstock classes revealed that all the heterograft combinations dramatically lowered the fruit AsA content, particularly in the highly vigorous
S. habrochaites hybrids (−45%). The lowest fruit AsA content characterizing the most vigorous grafting combinations has been explained through their higher vegetative biomass, resulting in a redistribution or accumulation of this metabolite in other plant fractions [
50]. However, this hypothesis can only partially explain our data, since the low fruit AsA content recorded within the
S. pimpinellifolium class (i.e., the least vigorous one). The analysis of correlation (data not shown) revealed that fruit weight and AsA content were negatively related (−0.969 *), indicating the existence of a dilution effect altering this important compositional trait even in the least vigorous rootstocks.