Wrapping effects on 13C assimilates in mistletoe tissues (Exp. 1: wrapping)
Irrigation had no significant effects on the 13C assimilates of mistletoe tissues in the wrapping experiment, whereas the wrapping treatment had a strong effect on the 13C accumulation in mistletoe tissues (Table 1). Strong labeling signals were found in the non-wrapped mistletoe leaves and shoots, while in the wrapped mistletoe clusters no 13C signal was found in leaves or shoots (Fig. 3). The peak value of Δδ13C in non-wrapped leaves occurred approximately 8 h after the labeling started, after which point Δδ13C values decreased gradually (Fig. 3a). Δδ13C values reached a peak in wrapped mistletoe shoots at the first sampling time after labeling, remained at a relatively stable high level until 15 d after labeling, and decreased slowly thereafter (Fig. 3b).
Removal effects on carbon assimilates in pine and mistletoe tissues (Exp. 2: girdling and removal)
The irrigation treatment and its interaction with other factors (time or removal treatment) did not affect Δδ13C or NSC concentration (NSCT, sugars, starch) in the pine tissues (Table 2). However, there was a direct effect of the removal treatments on the carbon assimilates in host tissues (Table 2). Mistletoe removal resulted in significantly lower needle Δδ13C at 24 h after labeling (Fig. 4a), but did not affect needle NSC concentrations (Table 2, Fig. 5a). Mistletoe removal did not affect new carbon assimilates in the host phloem (Figs. 4b, 5b) but led to significantly lower Δδ13C in the pine xylem tissue at 3 and 15 d after labeling (Figs. 4c, 5c). When the needles were removed, no significant 13C signals were found anymore in host phloem or xylem (Fig. 4b, c). Needle and mistletoe removal did not affect the NSC concentrations in pine phloem (Fig. 5b). Needle removal decreased the host xylem NSC concentrations, while such effects were not observed in the mistletoe removal treatment (Fig. 5c).
Irrigation significantly (P< 0.05) or marginally significantly (P<0.10) influenced the Δδ13C and NSC concentrations (except sugars) in both mistletoe leaves and shoots (Table 2), and host needle removal also significantly affected the carbon assimilates and NSC concentrations in both mistletoe leaves and shoots (Table 2). In addition, host needle removal interacted with irrigation to significantly affect Δδ13C and NSC concentrations in mistletoe leaves and Δδ13C in mistletoe shoots (Table 2). Mistletoes on trees grown under dry control conditions tended to have higher Δδ13C levels in leaves and shoots than those on irrigated trees, irrespective of host needle removal, but the higher Δδ13C levels of mistletoes were found when needles were removed in trees grown under dry conditions (Fig. 4d, e). Similarly, NSC concentrations in mistletoe leaves were higher in the control compared with the irrigated trees, but mainly when the pine needles were removed (Fig. 5d, e).
Table 1: Results of linear mixed models for Δδ13C values (uptake and incorporation of 13C) of bulk material in different tissues of Viscum album ssp. austriacum in the wrapping experiment (Exp. 1). Degrees of freedom (df) and F-values are given for time, irrigation treatment and wrapping treatment (i.e. wrapped vs non-wrapped; n = 3).