Active support of mistletoe to the host tree with carbon assimilation under source-limitation – results from a 13C labelling experiment

Pine Mistletoe (Viscum album ssp. austriacum) is a hemi-parasite shrub species, whose survival and development rely on water and mineral resources obtained through the xylem sap from the host tree. Mistletoe can produce photosynthates in its green organs on its own. On the other side, as they are connected to the phloem of the host tree, they may also be able to retrieve  carbon assimilates from their host and/or vice versa. However, the dynamics and the steering factors of this relationship remains unclear. We conducted ¹³C- labelling experiments with mature Scots pine (Pinus sylvestris) infected by mistletoes in a long-term (15 years) irrigation experiment in Switzerland (Pfynwald, Valais) to investigate the transport of carbon assimilates and nutrients between the host and the parasite under different soil moisture (600 mm vs 1200 mm of precipitation per year). Three irrigated and three control (natural xeric) pine trees infected by mistletoes were ¹³C labelled for 4 hours. During the ¹³C labelling of the trees, the following two experiments were carried out. 
    1) Wrapping experiment: 3-4 clusters of mistletoes on the labelled trees were wrapped with aluminium foil and enclosed in plastic bags to prevent mistletoe photosynthesis using ¹³C-enriched CO₂ and to investigate a potential host-mistletoe carbon transfer.
    2) Girdling and removal experiment: The phloem of 12 host tree branches (6 control & 6 irrigated) infected by mistletoes was manually removed (ca. 2 cm in width, basipetally of the mistletoes) to stop the downward transport of photosynthates from the girdled branches. Additionally, host needles or mistletoes were removed from the girdled branches to investigate the respective contribution of photosynthesis by the host needles vs. mistletoes to the host branch carbon balance.
     In the wrapping experiment, wrapped and unwrapped mistletoe leaves and stems were harvested at 10 times points over 6 months. In the girdling and removal experiment, needles, twigs (i.e. xylem and phloem) and mistletoe leaves were harvested at 7 time points over 14 days. In both experiment, bulk organic matter of each tissue was analysed to trace the ¹³C signal.
     We found that there was no ¹³C signal in the wrapped mistletoe leaves, indicating that there was no C-transfer from the host to the mistletoes via the phloem sap. Mistletoe removal from girdled branches decreased ¹³C-labelled carbon assimilates in host xylem and phloem. Meanwhile, when needles were completely removed from the girdled branches, host xylem and phloem were still able to acquire ¹³C from the mistletoes. These results suggest that mistletoes can support the host with carbon resources, which might be especially important when the host is C resource limited.
Our study provides new insights into the relationship between the hemi-parasite and its host tree. Mistletoes may play a role as C source providers to support the host and maintain a symbiotic relationship to survive.