The photosynthetic capacity of pine mistletoes is suppressed by
the host trees under drier conditions
Although we did not find a significant effect of the irrigation
treatment on mistletoe 13C uptake and incorporation in
the wrapping experiment (Fig. 3), mistletoe leaves and shoots
accumulated more13C-labeled assimilates in the control trees than in
the irrigated trees, but only when the needles were removed from the
girdled branches (Table 2, Fig. 4). This is surprising,
as photosynthetic carbon
acquisition is normally greater in an environment with higher soil
moisture (Joseph et al., 2020; Reich et al., 2018; Wang, Hawkins, &
Letchford, 1998). Since we have shown in Exp. 1 that no direct carbon
transport occurred from the host to the mistletoes, this phenomenon
needs to be attributed to the mistletoe photosynthetic activity. We need
to consider that mistletoe leaves were larger and had a higher water
content under the dry control conditions (Fig. 2). This might indicate a
higher photosynthetic potential of mistletoe leaves in drier sites,
which is, however, only expressed when the host’s needles are removed.
These results further signify that needle removal not only affected
source–sink carbon relationships in the girdled branch but might also
result in more available water for the mistletoe due to discontinued
host transpiration. The reduced competition for water may allow the
mistletoes to keep stomates more open, thus allowing for higher
photosynthesis rates.
We also found that mistletoe leaf NSCT concentrations
were significantly higher in the control trees compared with the
irrigated trees when pine needles were not present anymore (Table 2,
Fig. 5), which corroborates our assumption of higher assimilation by
mistletoes when there were no needles to demand water under dry
conditions. The high carbon accumulation capacity of mistletoes in a
stressful environment also demonstrates the competitive ability of the
hemiparasite. However, it seemed that mistletoes with bigger leaf size
and higher assimilation potential benefit more from pine needle removal
than the pine needles benefit from mistletoe removal (see discussion
below). Still, mistletoes are known to exacerbate tree mortality in
drought-exposed regions (M Dobbertin & Rigling, 2006; Durand-Gillmann,
Cailleret, Boivin, Nageleisen, & Davi, 2014; Rigling et al., 2010;
Sangüesa-Barreda et al., 2013; Zweifel et al., 2012). Our findings
support our hypothesis H2b that expected an increased carbon
level in mistletoes due to increased carbon assimilation by the
hemiparasite itself as a result of decreased competition for water with
the host after host needle removal. In contrast to our hypothesisH2a , an increase in carbon level and carbon assimilation of
mistletoes after host needle removal was found, which is not related to
a change in source activity of the host but is rather due to the
released water restrictions of the mistletoe by removing the transpiring
host needles under dry conditions.