Control of fruit tree vigor induced by dwarfing rootstocks

Grafting of fruit trees has been practiced for more than two millennia, and dwarfing rootstocks have been used to control scion vigor for several centuries. The fact that most commercial temperate fruit trees are compound plants composed 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. 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, different hypotheses focusing on various aspects of plant function have been proposed to explain the dwarfing mechanism. This review discusses the concepts and the experimental support for six of the primary theories proposed to explain the dwarfing mechanism induced by rootstocks in fruit trees. These theories are based on the idea that dwarfing is caused by semi-incompatibility between the rootstock and scion or anatomical, morphological, or physiological characteristics of the rootstock or graft union that affect tree water relations, tree nutrition, scion–rootstock hormonal and metabolic signaling, carbohydrate storage and mobilization, and/or the relative abilities of the scion and rootstock components of the tree to compete for resources. Each of the proposed 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. None of the theories have been documented to be involved in all rootstock–scion combinations or to be solely responsible for the dwarfing response of scions associated with specific rootstocks. However, there is good evidence that changes in water relations are related to the dwarfing response in several species. In retrospect, it is perhaps unrealistic to think that any single one of these or other mechanisms could be solely responsible for orchestrating all of the complex interactions involved in rootstock-induced vigor reduction of the scion