How do dwarfing rootstocks control fruit tree vigor? (Is it about hormones, nutrients, carbohydrates or water?)

Most commercial temperate fruit trees are compound trees made up of two different genotypes; a scion and a rootstock. Increasingly, rootstock genotypes are chosen for their ability to reduce or control the growth of the scion. Although modern fruit production is becoming increasingly dependent on “size-controlling” rootstocks, there is still not unanimous agreement or clear understanding of the mechanisms involved in the size-controlling effects that specific rootstocks have on scion growth. There are several theories about the mechanisms involved in causing the size-controlling behavior of scions on dwarfing rootstocks. These theories were presented in a recent review by Basile and DeJong (2019), and the objective of this talk will be to review these theories and provide some clarity about which theories appear to be most likely to be involved. The most common theories fall into the following categories: semi-incompatibility, hormone/signaling, nutritional, water relations, carbohydrate storage/mobilization/transport, and competition/architecture. Most of these theories are not mutually exclusive, and several mechanisms may be active in a given rootstock/scion combination to a greater or lesser extent. The nutritional theory appears a bit weak because, if it is in play, it should be able to be overcome by increasing the supply of nutrients preferentially to either the rootstock or the scion to overcome the relative dwarfing effect. Although the hormone/signaling theory is probably the most widely accepted among horticulturists, it is fairly weak because most evidence for it comes from associative experiments in which hormone or growth regulator concentrations are associated with growth activity. However, it is not known whether the hormone concentrations cause the observed growth effects or are a result of them. A theory that has substantial empirical evidence to support it is based on differences in xylem anatomy that affect tree water relations.