Introduction
The relationship between a parasite and its host is important
ecologically and widely discussed in animal and plant pathology and
physiology. Most research on parasite–host relationships in plants has
concentrated on host responses to infections by parasites (Solomon,
James, Alphonsus, & Nkiruka, 2015; Streicker, Fenton, & Pedersen,
2013). In contrast, the interactions between plant hosts and plant
parasites, especially the effects of hosts on parasites in different
habitats and varying site conditions, have rarely been studied. However,
such parasite–host relationships, including a possible feedback system
between the host and parasite, are of central interest because they can
strongly affect the growth and survival of the higher plants serve as
hosts.
Mistletoes are well-known hemiparasitic plants that
maintain their own carbon
assimilation by photosynthesis and can infect many tree species in
various ecosystem types worldwide, making them an important and relevant
species in parasitism research (Glatzel & Geils, 2009; Zuber, 2004).Viscum
album ssp. austriacum (Santalaceae), pine mistletoe, is
the most widely distributed species across the European continent (M
Dobbertin & Rigling, 2006; Zuber, 2004). Pine mistletoe survival and
development in forest ecosystems mainly rely on water and mineral
resources obtained from the xylem sap of the host tree (M Dobbertin &
Rigling, 2006; Rigling, Eilmann, Koechli, & Dobbertin, 2010). If water
availability is high and nutrients are not limited, pine mistletoes and
their hosts co-exist for years without major restrictions for the host
tree (Solomon et al., 2015; Zuber, 2004). However, if water is limited
during dry periods, the high-water
consumption and low water-use efficiency of pine mistletoes may
exacerbate drought stress in the host tree, with negative consequences
on the host’s physiology and growth performance (M Dobbertin & Rigling,
2006; Rigling et al., 2010; Zweifel, Bangerter, Rigling, & Sterck,
2012). As a consequence, pine mistletoe infection leads to a reduction
of branching and of branch and needle growth(Rigling et al., 2010),
resulting in an increased risk of mortality for the host tree (M
Dobbertin & Rigling, 2006). This contribution of pine mistletoe to
drought-induced forest decline processes has been demonstrated in
several xeric forest ecosystems in Spain (Galiano, Martínez‐Vilalta, &
Lloret, 2011; Sangüesa-Barreda, Linares, & Camarero, 2012, 2013; M.
Scalon, Haridasan, & Franco, 2013) and inner-Alpine regions in
Switzerland and Italy (M Dobbertin & Rigling, 2006; Rigling et al.,
2010; Vacchiano, Garbarino, Mondino, & Motta, 2012).
Along with the negative effects of mistletoes on the host water balance,
pine mistletoes have also been found to affect the carbon balance of the
host in a variety of ways (Glatzel & Geils, 2009; Q. Le, K. U.
Tennakoon, F. Metali, L. B. Lim, & J. F. Bolin, 2016; M. C. Scalon &
Wright, 2015). High water use of
mistletoes and thus considerable water loss from the whole
host–parasite system may induce closure of the stomata in host trees to
save water (Rigling et al., 2010; Zweifel et al., 2012), resulting in
lower photosynthesis rates of the trees (M Dobbertin & Rigling, 2006;
Yan et al., 2016). Therefore, pine mistletoe can indirectly reduce the
host’s ability to acquire carbon resources, especially under
drought-stress conditions (Sangüesa-Barreda et al., 2013; Yan et al.,
2016).
Mistletoes perform photosynthesis at a rate similar to that of the host
(Lüttge et al., 1998; M. C. Scalon & Wright, 2017).
In some studies, however,
it has been reported that
mistletoes are able to additionally acquire organic carbon from the host
in the form of xylem-mobile organic acids and amino acids (Escher,
Eiblmeier, Hetzger, & Rennenberg, 2004b; Těšitel, Plavcová, & Cameron,
2010). Richter, Popp, Mensen, Stewart, and Willert (1995) estimated that
mistletoe leaves take up over 50% of its required heterotrophic carbon
from its host. Nevertheless,
according to Smith and Gledhill
(1983), the haustorium of V.
album grows only within the host’s xylem and does not connect to the
host’s phloem. This means that there should be only acropetal carbon
transport from the host xylem to the mistletoe via the transpiration
stream, with no basipetal carbon flow from the mistletoe to the host,
even under strong carbon limitation of the host (Glatzel & Geils, 2009;
M. C. Scalon & Wright, 2015). Hence, it remains unclear whether
mistletoes can directly absorb carbon resources from host tissues in
considerable amounts, in addition to their own photosynthetic
activities. By utilizing the stable 13C isotope tracer
technique, it is possible to determine the direction and quantity of
carbon assimilate flow between mistletoe and host, and also to assess
how this process depends on carbon and water availability.
Most studies on
mistletoe–host
relationships have been conducted by comparing trees infected by
mistletoes with non-infected trees growing under the same conditions (M.
Dobbertin et al., 2010; M Dobbertin & Rigling, 2006; Rigling et al.,
2010; Yan et al., 2016). Whether the host’s carbon resource
availability, which is strongly associated with its growth conditions
(e.g. soil water moisture), affects the mistletoe–host relationship has
only been investigated in a few studies, and these studies were only
focused on the response of hosts to mistletoe infection (Q. Le, K.
Tennakoon, F. Metali, L. Lim, & J. Bolin, 2016; Sangüesa-Barreda et
al., 2013; Zweifel et al., 2012).
It is still unclear if the carbon
dynamics in the mistletoe and in its host, as well as the potential
exchange of assimilates between the two, changes in response to the
local water availability of both the host tree and the mistletoe.
To address these unresolved questions, we conducted two separate
experiments under the umbrella of
a whole-tree 13C-pulse labeling experiment with mature
Scots pine (Pinus sylvestris ) trees infected by pine mistletoe
(V. album ). Host trees whose crowns were exposed to13CO2 were growing either in naturally
dry conditions (~600 mm precipitation per year) or in
irrigated areas (+700 mm per year, applied during the growing season)
for 15 years in the Swiss Pfynwald forest ecosystem experimental
platform (Joseph et al., 2020; Schaub, Haeni, Hug, Gessler, & Rigling,
2016).
In a wrapping experiment (Exp. 1), we shielded mistletoe clusters with
gas-tight plastic foil and darkened them with aluminum foil before the
whole-tree labeling to prevent 13C assimilation by
these clusters. We investigated the 13C values in both
wrapped and non-wrapped mistletoes, as well as in their host twigs, to
test the hypothesis (H1 ) thatV. album takes up carbon
resources from its host via the haustorium. Any signal in the wrapped
mistletoes (shielding from 13CO2 and
light exclusion) would originate from the host and we assumed the
contribution of the host (if any) to be higher in irrigated vs
drought-stressed trees due to increased assimilation rates in irrigated
trees (Schonbeck et al., 2021)).
To change source–sink carbon and water relationships, we performed a
tissue removal experiment (Exp. 2). We girdled pine branches infected
with mistletoes of drought-stressed and irrigated host trees to restrict
the phloem carbon translocation between the remaining tree and the
girdled branch (Andersen, Nikolov, Nikolova, Matyssek, & Haberle, 2005;
De Schepper & Steppe, 2013), while keeping a constant water and
nutrient flow. Beyond the girdling point, we then removed all pine
needles or all mistletoe tissues (including stem and leaves) from the
girdled pine branches to manipulate source-sink relationships and water
relations locally on the branch level. Through Exp. 2, we aimed to test
the hypothesis (H2a ) that
local changes in source–sink relationships by reducing assimilate
ability (i.e. host needle removal), would decrease the mistletoes’
carbon level due to lower amounts of carbon obtained from the host
(conditional H1 is supported). An alternative hypothesis (H2b )
is that needle removal increases the mistletoes’ carbon level due to
increased carbon assimilation by the hemiparasite itself as a result of
decreased competition for water with the host. This effect would be more
pronounced under the dry control conditions. Finally, we hypothesize
(H3 ) that mistletoes do
not provide any carbon to the host, even when the host is carbon limited
due to needle removal.