# Hormonal Interactions Underlying Rootstock-Induced Vigor Control in Horticultural Crops
## 4 Effect of Multiple Plant Hormones in Rootstock-Induced Dwaring
### Abscisic Acid (ABA)  
Abscisic acid (ABA) is a crucial phytohormone that controls plant growth, development, and stress responses [92]. ABA is usually produced in the roots and supplied to the leaves through the xylem or quickly synthesized in the leaves [93; 94]. It is crucial for numerous physiological functions in plants, including stomatal closure, leaf senescence, seed germination, cuticular wax accumulation, osmotic regulation, bud dormancy, and growth inhibition [95]. ABA can also cause dwarfism in higher plants by controlling plant growth and tolerance responses to various stresses [96; 97]. It is commonly believed that elevated ABA levels cause dwarfism because ABA acts as a growth inhibitor [98; 99]. Compared to vigorous rootstocks, dwarfing apple rootstocks have higher levels of ABA and lower ABA:IAA ratios [58]. The greater levels of ABA in the branch bark of size-controlling apple rootstocks can be used as a marker for selecting dwarfing rootstock.  
Additionally, _MdNAC1_ gene overexpression causes dwarfism in transgenic apples by regulating endogenous ABA and BR production [100]. In pear, the ultrastructural study reported that thinner or shorter stems cause size reduction because of a decrease in cell numbers. Mutant lines with high levels of endogenous ABA and ABA pathway gene transcripts showed that the _PbPAT14_ function is related to the ABA pathway [20]. Similarly, in _Arabidopsis_, elevated endogenous ABA concentrations inhibit shoot growth [101]. This implies that a rootstock with high amounts of ABA will decrease stomatal conductance and gas exchange, resulting in low yield and reduced growth vigor in grafted plants. Recently, [PERSON] et al. [102] reported a negative correlation between the ABA content of scion and rootstock with tree vigor, leaf gas exchange, photosynthetic pigments, and yield. Furthermore, Rough lemon, _Poncirus trifoliata_, and Fraser hybrid rootstocks had low concentrations of ABA, while Cox mandarin, Troyer citrange, and Benton rootstocks had significantly higher concentrations of ABA. These higher or lower ABA levels explain their attributes as vigorous or dwarfing rootstocks and greatly influence scion growth characteristics. Similar results were described by [PERSON] et al. [103], indicating a negative correlation between ABA levels and trees.  
According to [PERSON] et al. [104], citrus trees grafted on dwarfing rootstocks had greater ABA levels in their shoots compared with citrus trees grafted on taller rootstocks. The reduced plant growth was believed to be caused by the higher ABA levels. Similarly, [PERSON] and [PERSON] [96] found that increased levels of ABA cause lower hydraulic conductivity in dwarfing rootstocks, impacting tree development and yield. In addition, root growth inhibition is associated with high ABA levels in 'Jonathan' apple micro-cuttings. Further, high levels of abscisic acid (ABA) are believed to play a critical role in inhibiting the formation of adventitious roots (ARs) in apples [105]. In sweet cherry (_Prunus mahale_ L.), [PERSON] and [PERSON] [106] observed that vigorous rootstocks had the lowest ABA/IAA ratio compared with dwarfing rootstocks. They also found a strong correlation between growth vigor and ABA concentrations in shoot bark. Recently, root development was considerably suppressed in Qingzhen 1 apple plants after the exogenous application of ABA compared with control plants [44]. Conclusively, ABA levels were found to be higher (act as growth inhibitor) and impede growth features by reducing the accumulation of other hormones.