Graft Union Strength of Apple Rootstocks and Hormone Flux through Graft Unions

Certain apple rootstock and scion combinations may form weak graft unions. We tested whether applications of exogenous plant growth regulators (PGR) can influence vascular differentiation and wood formation, to improve graft union strength. Several commercial and experimental PGRs were applied on replicate graft unions of nursery trees or on apical meristems over two seasons. Two of the treatments (benzyl adenine and prohexadione calcium) significantly increased the flexural strength per scion cross-sectional area and the flexibility of the union. Apple tree graft specimens of these unions were imaged using a Zeiss Versa XRM-520 CT to observe graft union morphology associated with treatments. In a separate grafting experiment with similar scion rootstock combinations we monitored endogenous plant hormones in grafted trees and found different hormone concentrations based on the type of graft in the same rootstock scion combination. An additional grafting experiment monitored endogenous auxin levels and gene expression of auxin related transport genes (auxin influx AUX1, efflux PIN1, PIN2, ABCB1 and intracellular homeostasis PIN5) in xylem and phloem near or within the graft of M.9 (dwarfing) and MM.111 (vigorous) rootstocks. Auxin concentrations were higher above the graft union of scions on M.9 rootstocks than on MM.111 rootstocks. Significant rootstock and scion-rootstock interaction effects were found for AUX1 expression, elevated RNA detected only for self-grafted M.9 in xylem below the graft. All these experiments indicate yet to be unraveled complex interactions at the site of the graft that influence hormone transport, wood formation and gene expression. Although complex, these composite systems can be utilized to understand communication between tissues of two distinct genotypes.