Mistletoes cannot act as a carbon provider, even when the host
is carbon limited
In the needle removal treatment, no strong 13C signal
was found upon 13C labeling in the pine sink tissues
(i.e. phloem and xylem). The minimal 13C traces
detected (Fig. 4b, c) might be due to bark photosynthesis (Aschan &
Pfanz, 2003; Simkin, Faralli, Ramamoorthy, & Lawson, 2020). This
finding suggests that mistletoe assimilates do not act as a significant
carbon source for the host sink tissues. We thus conclude that there is
no exchange of carbon assimilates between mistletoe and host. Neither do
mistletoes use host carbon resources nor do they provide any carbon to
the host, even when the host is carbon limited in special situations
(needle removal treatment, drought). These results support H3and prove that mistletoes and hosts are carbon-independent and that only
water and nutrients are transported from the host to the mistletoes.
Meanwhile, needle removal also resulted in a decrease in
NSCT concentrations in pine xylem tissue after one week,
which can be explained by the lack of delivery of new assimilates to the
sink tissues. Pine xylem, however, also accumulated less13C-labeled carbon assimilates when mistletoes were
removed from the branches (Fig. 4). As there is no transport of
assimilates from the mistletoe to the host, we propose the following
explanation: mistletoe tissues have lower water potentials compared with
host tissues, which ensures
continuous water uptake from the host xylem (M. C. Scalon & Wright,
2015; Schulze, Turner, & Glatzel, 1984; Zweifel et al., 2012). Thus,
xylem parenchymatic tissues compete for water with the hemiparasite.
Removing the mistletoe might reduce the need to incorporate large
amounts of osmotically active compounds in the woody tissues and thus
decrease the transport of new 13C-labeled soluble
carbon compounds to the host xylem.