Hormones
Since hormones mediate many physiological changes (Ghanem et al.2008; Albacete et al. 2008a), we profiled several root and shoot tissues and xylem and phloem exudates of grafted plants and measured their hormone levels (Figures 3, 4; Table S1).
Generally, NCED OE grafts had relatively few significant effects on ABA concentration in tissues and transport pathways compared to the WT rootstock (Figure 3a). Interestingly, the NCED OE rootstocks significantly increased ABA concentrations in the xylem sap of a flowering truss 180 days after transplanting, but those differences decreased during green fruit stage and disappeared at maturity stage. Moreover, mature fruit (juice) ABA concentration of plants grafted onto SP12 rootstocks was more than 2-fold higher than in plants grafted on WT rootstocks. Leaf phloem exudate ABA concentrations decreased in plants grafted on NCED OE rootstocks (Figure 3a). SP12 rootstocks had higher root and root xylem sap concentrations of the ABA catabolites phaseic acid (PA) and dihydrophaseic acid (DPA) respectively, with leaves of plants grafted on SP12 having higher DPA concentrations (Figure 3b). Thus, rootstock NCED OE had significant effects on ABA (and metabolites) concentrations only in some shoot tissues.
Plants grafted onto NCED OE rootstocks had lower total CKs (t -Z and iP type) in the xylem sap of roots and flowering truss, as well as in leaf tissue and green fruits mainly due to lower t -Z levels (Figure 4; Table S1). The different graft combinations had similart -Z and iP concentrations in leaf xylem sap and root tissues. However, iP type CK concentrations on leaf tissue (130 DST) and leaf phloem exudate were 5-14-fold higher in plants grafted on NCED OE rootstocks than on WT rootstocks, with iP the only hormone increasing in leaf phloem exudate (Figure 4; Table S1). Thus, rootstock NCED OE significantly affected CK concentrations in root xylem sap and shoot tissues.
Rootstock genotype also significantly affected auxin (IAA) and ethylene precursor (ACC) measurements. Leaf phloem exudate and root tissue ACC concentrations were 3-25 times lower in plants grafted on NCED OE rootstocks, while they had a higher ACC concentration in xylem sap of a mature fruit truss (Figure 4; Table S1). Leaf phloem exudate and xylem of mature fruit truss had up to 6-fold lower IAA concentrations when grafted on the SP5 rootstock (Figure 4; Table S1), otherwise there were no significant rootstock impacts on IAA levels. Similar to ABA, xylem sap of trusses at flowering and green-fruited stages had 7.5 to 4-fold more GA3 when grafted on NCED OE rootstocks, with these differences disappearing at fruit maturity (Figure 4). However, leaf xylem GA3 concentration of plants grafted on NCED OE rootstocks was 65-80% lower than when grafted on WT rootstock. Furthermore, root xylem JA concentration of plants grafted on SP5 was lower, even though plants grafted on NCED OE rootstocks had leaf JA concentrations that were more than twice that of plants grafted on WT rootstocks at 80 DST (Table S1); however, these differences disappeared at 130 DST (Figure 4). No significant rootstock differences in JA concentrations occurred in other tissues at the time points analyzed (Figure 4). The NCED OE rootstocks had few significant impacts on SA, except for 3-10 fold lower concentrations in leaf xylem and phloem exudates and a similar increase in ripe fruits (Figure 4, Table S1). Thus, NCED OE rootstocks also occasionally affected tissue and transport fluid concentrations of other acidic hormones.