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 .