Conclusion
Our results demonstrate that
pine
mistletoes are fully carbon-autonomous: they do not provide carbon to
the host and are also not supplied with carbon compounds by the host. We
also observed that mistletoes are constrained in their photosynthesis by
the host when soil water availability is low, most likely due to
competition for xylem water – when the competition is released by
removing the host needles, the 13C assimilation of the
mistletoe increases. This result provides physiological evidences that
mistletoes do increase the drought stress of their hosts, resulting in
an increased mortality risk during severely dry periods previously
proposed. We, therefore, conclude that the hemiparasites live on their
own in terms of carbon gain which, however, depends on the water
provided by the host tree.