Hormonal factors involved in the control of vigour of grafted peach [Prunus persica (L.) Batsch] trees and hybrid rootstocks

Tree vigour is regulated through complex mechanisms, whose exploitation might allow more efficient management of fruit orchards. Vigour depends on several factors, but can be controlled by grafting the cultiva- ted species on suitable rootstocks, that influence tree growth. According to a widely accepted hypothesis, root- stock and scion reciprocally control their growth through the exchange of hormonal signals. Among these, cytokinins are thought to enhance shoot vigour, being rootstock growth correlated with cytokinin export from roots. Data available forauxin are less clear, therefore we decided to investigate the hormonal flow in xylem exu- dates of field-grown Prunus rootstocks and peach scions grafted on them. Data were compared with primary growth parameters, representative for vigour, thus showing that auxin and cytokinin sap flows are positively related with tree vigour. Consequently, we hypothesize that the supply of indoleacetic acid and zeatin riboside through the xylem enhances shoot development. The hormone flow rate depends on rootstock genotype, which may modulate hormone metabolism and transport, as well as gene expression in the scion. In this view, the xylem flow of auxin and cytokinin from the rootstock would represent a basic chemical signal regulating gene expres- sion in the scion, thereby controlling tree vigour. The growth of the crown of peach scions of grafted trees was lower than that of Prunus rootstocks: this may be caused by the restriction imposed by the graft union on the acropetal hormone transport.