The use of a rootstock or interstock to dwarf a scion variety is an age-old technique. Grafting a vigorous cultivar (the scion) on the top of a certain tree (the stock) is known to produce various degrees of dwarfing. The degree of dwarfing achieved depends on the rootstock but may also be influenced by other factors, such as the natural vigor of the scion, the soil, and cultural practices. The rootstock may be the same species as the scion, as in apple, or a different species, as in pear (quince root). More research has been conducted on rootstocks and their effect on the scion than any other aspect of dwarfing. Still, the mechanisms for dwarfing by the rootstock are not well understood. In 1956 Beakbane stated, "we know much about the extent, something about the duration, but we still have some way to go before we can say that we fully understand the fundamental nature, or mechanism, of rootstock influence." Over 40 years later we have a better understanding of the dwarfing influence of rootstocks, but we still do not have a full understanding of how rootstocks dwarf the scion variety.
For certain, no one mechanism is responsible, and it is likely that several mechanisms, working concurrently, in tandem, or independently, are responsible.
One early theory suggested that the roots of a dwarfing rootstock occupy a smaller volume of soil, grow to less depth, and absorb less nitrogen than roots of a standard size rootstock, thereby resulting in a smaller scion. While the root system of a dwarf tree does occupy less soil volume than a vigorous seedling (the roots are in equilibrium with the top), research has failed to fully support this theory. Nutrient uptake, especially phosphorus, has been implicated as a growth-controlling mechanism in dwarf trees; however, results of numerous studies have been inconsistent in establishing nutrient uptake as a dwarfing mechanism. Evidence is available that demonstrates that restricted nutrient translocation across the graft union between root-stock and scion plays a role in the dwarfing mechanism. Anatomical changes within the xylem and other tissues at the graft union that affect the movement of nutrients and water from rootstock to scion have been implicated as a dwarfing mechanism (Simons, 1987). Hormones, especially auxins that are translocated from shoot tips to roots, are thought to act as "regulators" in nutrient and water flow across the graft union. Unfortunately, the extent of influence by these factors in the dwarfing process is unclear. Recent work by Atkinson and Else (2001) indicates that a gradient in hydraulic resistance exists within the rootstock union, ranging from high to low for dwarf, semidwarf, and vigorous rootstocks, respectively. Resistance to water flow at the graft union affects sap flow rate and the concentration of hormones and other solutes in the sap, which affects growth of the roots and shoots.
The primary plant hormones—auxins, gibberellins, and cytokinins— have been studied and implicated in the dwarfing mechanism. However, their precise role, alone or in combination, remains unclear. Auxins promote root growth, and studies show that dwarfing root-stocks have lower levels of auxin than more vigorous stocks. Auxin levels affect differentiation of xylem and phloem tissue, thus affecting the flow of nutrients, water, and assimilates (plant food). Gibberellins (GAs) affect cell elongation and have a major effect on shoot growth. There is no evidence that root-produced GAs influence shoot growth, and most researchers feel they play a minor role, if any, in the dwarf rootstock effect. Cytokinins, on the other hand, are produced in great quantities in the roots and translocated upward, where they influence shoot growth. Dwarfing rootstocks show high levels of cyto-kinins accumulating at the graft union, which indicates that root-stocks may affect growth by reducing upward movement of this vital growth hormone. Evidence has also been presented that dwarfing rootstocks have significantly higher levels of abscisic acid (ABA) than more vigorous stocks.
In most tree fruit that respond to grafting, dwarfing can be obtained by grafting a piece of stem from a dwarf rootstock between a more vigorous stock and a scion variety. This suggests that the mechanism for dwarfing is associated with the rootstock stem piece and not the root system. The longer the interstock stem piece or the higher a scion is budded on the rootstock, the greater the dwarfing effect. Much of this effect is attributed to bark phenols and their effect on auxin (indoleacetic acid) metabolism. It is hypothesized that as auxin moves basipetally, it is degraded and the concentration decreases. The thicker bark and much higher starch levels in dwarf rootstocks indicate a low level of auxin in these tissues, lending support to the bark phenol hypothesis.
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