Research paperEffect of scion and graft type on transpiration, hydraulic resistance and xylem hormone profile of apples grafted on Geneva®41 and M.9-NIC™29 rootstocks

- Graft union strength in apple trees is not correlated with whole tree transpiration rate.
- Hydraulic resistance is not associated with graft union strength in similar vigor rootstocks.
- Xylem exudate hormone profiles of apple rootstocks are genotype and graft type dependent.

New highly productive apple rootstocks resistant to fire blight and crown rot can improve orchard profitability. However, some of the most promising rootstocks appear to have weak or brittle graft unions that are susceptible to breakage in the nursery and in the field. Flexural strength of graft unions may be related to the amount of vascular connections between graft partners, and poor vascular connections may cause increased hydraulic resistance. We sought to determine if transpiration rate and hydraulic resistance were correlated with graft union strength by comparing Geneva® 41 with the strong graft-forming Malling 9 selection NIC™ 29. The primary goal of this study was to compare weak and strong graft unions between similarly vigorous rootstocks by measuring whole-tree transpiration, as well as hydraulic resistance using the pressure chamber method. The effects of rootstock and grafting method on xylem hormone concentrations were also compared. There was no correlation between graft union strength and whole tree transpiration rate or leaf area-specific transpiration rate. The hydraulic resistance of the weaker forming rootstock was not significantly different than the strong graft union rootstock and was more closely correlated with overall size of the root system. However, hydraulic resistance of the scion and graft union were small relative to root system resistance, making it difficult to detect differences in graft union resistance among scion-rootstock combinations. Conversely, we observed differences in the hormone profile of xylem exudate among rootstock genotypes and among graft types.