No transport of newly assimilated carbon from host to mistletoes
The wrapping experiment showed that non-wrapped mistletoes efficiently assimilate carbon (Fig. 3), although the incorporation of13C originating from the labeled CO2in mistletoe leaves was only half of that in pine needles (Figs. 3, 4). We did not find a strong effect of irrigation on the13C incorporation in mistletoe leaves or shoots (Table 1), indicating that the carbon assimilation capacity of mistletoes was not affected by restricted soil water availability, although mistletoes are known to rely on acquiring water resources via the xylem of the host tree (Glatzel & Geils, 2009; Zweifel et al., 2012). M. C. Scalon and Wright (2017) investigated 42 mistletoe–host species pairs sampled from 5 sites in Australia and Brazil under different soil water availability and found that the photosynthetic capacity of mistletoes and their hosts were on a similar level, but that mistletoes had leaf dark respiration rates that were twice that of the hosts at a given photosynthetic capacity, resulting in higher leaf maintenance costs for these hemiparasitic plants. In our study, it is possible that higher respiration rates, and thus loss of previously fixed13C, contributed to the lower overall incorporation of13C in bulk organic matter of mistletoe leaves compared with pine needles.
In contrast, wrapped mistletoes were not able to assimilate new carbon assimilates after the labeling event (Fig. 3). This clearly shows that new carbon assimilates are not transported from the host to any mistletoe tissue in significant amounts, which is consistent with the concept suggested in previous studies assuming that no phloem connection is established between hemiparasite and host (Glatzel & Geils, 2009; M. C. Scalon & Wright, 2015; Těšitel et al., 2010). In contrast, Popp and Richter (1998) compared the theoretical values predicted from gas exchange measurements with calculations of the amount of heterotrophic carbon gain and observed that only part of the carbon in the biomass ofV. album originates from its own photosynthesis activities. Escher, Eiblmeier, Hetzger, and Rennenberg (2004a) also provided indirect evidence of heterotrophic carbon gain of pine mistletoes via the xylem sap from the host by comparing the carbon concentrations in tissues in different seasons. However, even after 180 d, we did not find any 13C signal in the wrapped mistletoe clusters, indicating that also in the longer term no labeled carbon was obtained from the host. Within the same whole-tree labeling experiment, Gao et al. (2021) showed that even 10 months after labeling, tree respired CO2 still had δ13C values of up to 25‰. This indicates that significant amounts of label were still present in the host’s tissues and transport systems on our final sampling date, yet not transferred to the mistletoes. We conclude that mistletoes are complete carbon autotrophs and do not receive significant amounts of carbon directly from the host; thus, we can reject H1 .