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.