Vegetative propagation of fruit trees has been practiced for several millennia and dwarfing rootstocks have been used to control scion vigor for more than five centuries. The fact that most commercial fruit trees are compound plants comprised of two separate genotypes (the scion selected for fruit and shoot characteristics and the rootstock selected for root characteristics) makes them interesting objects for studying root/shoot interactions in plants. While many intrinsic characteristics of each genotype are maintained even when the two are physiologically and anatomically joined through grafting, there are clear dependencies between roots and shoots of trees and there is much research that documents that the behavior of the shoot can be influenced by the root genotype. But there is less evidence of the root being strongly affected by the shoot except with regard to overall growth such as when the shoot carries a heavy crop load. The size-controlling effect of certain rootstocks in various fruit tree species is perhaps the most intriguing and commercially important example of how genetics of one part (the root) of compound plants can influence the behavior of the other part (the shoot). While there is no debate about the potential of rootstocks to affect the growth and productivity of the scion in compound fruit trees, there is no consensus on the physiological and/or anatomical mechanisms involved in the dwarfing phenomenon. Indeed several theories have focused on various aspects of plant function; the main ones being: scion/rootstock semi-incompatibility, water relations, hormonal interactions, competition for carbohydrates and nutritional interactions. Each of these theories has received some experimentally-based support in selected crops. Scion/rootstock semi-incompatibility is a general mechanism that may be involved in many scion/rootstock combinations and could cause responses that appear to involve the factors common to the other theories, but none have been documented to be solely responsible for the dwarfing response of scions associated with specific rootstocks. In retrospect it is perhaps unrealistic to think that any single one of these or another theory could be solely responsible for orchestrating all of the complex interactions involved in rootstock-induced vigor reduction of the scion.
Keywords: graft incompatibility, water relations, hormones, vegetative-reproductive growth competition, nutrients