There is no carbon transfer between Scots pine and pine mistletoe but
the assimilation capacity of the hemiparasite is constrained by host
water use under dry conditions
Abstract
Pine mistletoe is a hemiparasitic shrub that can produce its own
photosynthates. There is a lack of knowledge about the interaction of
mistletoe and host under varying environmental condition that might
influence carbon gain and allocation. In a 13C-pulse
labeling experiment with mature Pinus sylvestris (pine) infected
by mistletoes grown in naturally dry or irrigated conditions, (1)
mistletoe clusters were shielded from 13CO
2 added , and (2) mistletoes or host
needles were removed to manipulate the local assimilate and water
availability. No 13C signal was found in shielded
mistletoes, indicating no carbon transfer from the host to the
mistletoe. When the pine needles were removed from girdled branches, no
13C signal was found in the host tissues, implying no
carbon transfer from mistletoe to the host. However, mistletoes on
needle-removed pine trees accumulated more labelled assimilates and had
higher non-structural carbohydrate (NSC) concentrations only under
naturally dry conditions but not in irrigated plots. Our results suggest
that mistletoes show full carbon autonomy, as they neither receive
carbon from nor provide carbon resource to the host trees. Moreover, the
high assimilation capacity of mistletoes seems to be constrained by the
host water use under dry conditions, suggesting that drought stress is
not only negatively impacting trees but also mistletoes. Therefore, we
conclude that the hemiparasites live on their own in terms of carbon
gain which, however, depends on the water provided by the host tree.