# Hormonal Interactions Underlying Rootstock-Induced Vigor Control in Horticultural Crops
## 2 Rootstocks Control Scion Morphology  
Rootstocks have a noticeable effect on the morphological characteristics of the scion part. Dwarfing rootstocks have varying effects on the vegetative growth of grafted plants, including plant height, primary shoot length, number of nodes, internodal length, branch composition, trunk cross-sectional area (TCSA), and yield [6; 10; 22; 31]. Likewise, [PERSON] et al. [32] showed that 'Golden Delicious' apple trees grafted onto vigorous P.22 rootstock had greater tree vigor and canopy volume than those grafted onto dwarfing 'M-9' rootstock. [PERSON] et al. [33] examined the growth characteristics of sweet cherry grafted onto several rootstocks. In addition, they noticed that rootstocks had a considerable  
Figure 1: An illustration of various plant hormones and their role in dwarfing mechanisms.  
influence on the shoot length, internode length, and trunk cross-sectional area (TCSA). Moreover, 'Shatangju' mandarin grafted onto 'Rough lemon' and 'Canton lemon' rootstocks had greater shoot length, trunk diameter, and internodal length; however, trees grafted with 'Fragrant orange' and 'Red tangerine' rootstock had the lowest values of morphological traits and weakest growth vigor [34]. Likewise, [PERSON] et al. [10] reported that scion cultivar grafted onto dwarfing 'M-9' rootstock produced dwarf size trees, whereas taller rootstocks, such as 'Baleng' and 'Christock-1', increased primary shoot length and other vegetative traits. In addition, the root system of the 'Baleng' rootstock was more vigorous, with greater root volume, root length, surface area, number of forks, and number of tips. The root is the primary organ that transports water and nutrients from the soil medium to the aerial portion, thus the root morphology has a significant impact on growth vigor and yield [35].  
Dwarfing, or vigor reduction phenomena, are typically linked with short internodes [36]. Additionally, plant growth regulators promote shorter internodes than untreated trees [37]. Furthermore, [PERSON] and [PERSON] [38] reported that the trunk cross-sectional areas of trees grafted onto M46 and _P. divariatica_ rootstock were much more significant than other rootstocks. On the other hand, trees grafted with the 'Wangenheim Prune' rootstock displayed lower morphological values and dwarfing characteristics. Salsutiaan scion cultivar grafted on 'Rough lemon' rootstock produced the most extended primary shoot length, largest trunk diameter, and vigorous root system [6]. In a recent study, [PERSON] et al. [39] found that 'Shatangju' mandarin scion grafted onto 'Flying Dragon' rootstock produces dwarf plants. Moreover, vigor reduction in the 'Flying Dragon' rootstock is attributed to a lower node number, shorter internodal length, and a smaller trunk diameter of the scion. The relationship between a genotype's dwarfing potential and a genetic, physiological feature would benefit the early selection of dwarf rootstocks [40]. Therefore, it can be concluded from the literature that size-controlling rootstocks significantly affect scion morphological traits, including plant height, primary shoot length, internodal length, leaf size, number of nodes, number of branches, TCSA, and weight of the overground part.