A global analysis of water and nitrogen relationships between mistletoes and their hosts: broad‐scale tests of old and enduring hypotheses

Abstract

Summary 1.Mistletoes use far more water per unit carbon fixed during photosynthesis than their hosts (i.e. they have lower 'water use efficiency', WUE). The widely cited 'nitrogen-parasitism hypothesis' posits that N is the most limiting resource for mistletoes and that they use their faster transpiration rates to acquire sufficient N from the host xylem. In a rather different context, the 'mimicry hypothesis' arose in the literature suggesting that some mistletoes mimic the morphology of host leaves in order to deploy higher N leaves without suffering higher levels of herbivory. These two non-exclusive hypotheses share the common goal of trying to explain patterns of mistletoe leaf N concentration. 2. We set out to test the generality of both hypotheses at broad geographic scale using data for 168 mistletoes-host pairs, from 39 sites, encompassing all continents except Antarctica. We drew together data from published literature and our own field data on two key plant functional traits, leaf N concentration (N mass ) and leaf carbon isotopic composition (d 13 C) (representing long-term WUE and degree of stomatal control over photosynthesis).3. Key findings included (i) little or no support for the N-parasitism hypothesis: differences in mistletoe and host N mass explained only 3% variation in differences in leaf d 13 C, and mistletoe-host differences in leaf d 13C were unrelated to whether or not the hosts were N-fixers (presumed to have higher N concentration in xylem sap); (ii) partial support for the mimicry hypothesis: mimic mistletoes generally had higher N mass when associated with N-fixing hosts (but, on non-N-fixing hosts there was no such pattern); and (iii) more broadly, mistletoes showed similar trait responses as their hosts to environmental drivers; for example, they showed similar-magnitude shifts in N mass and d 13 C in relation to site aridity.4. Contrary to current belief, our findings suggest that nitrogen is not the limiting nutrient for mistletoes, at least not the main component driving the faster transpiration rates. Our results also give insight into the evolution of mimicry in mistletoes and show, for the first time, that mistletoes are also constrained by local water availability, exhibiting clear trait adaptations to environmental gradients. By reconsidering these issues at broad geographic scale and across a large number of species, our findings substantially modify current knowledge on the ecology and physiology of mistletoes and their hosts.

To recycle or steal? Nutrient resorption in Australian and Brazilian mistletoes from three low‐phosphorus sites

“…A1). The correspondence between host and mistletoe leaf N concentration was already reported in a global analysis considering more than 160 mistletoe–host pairs (Scalon and Wright ), but the fact that leaf P concentration of mistletoes does not reflect that of their hosts suggests that the maintenance of higher P concentration may be achieved through the resorption process.…”

Leaf morphophysiology of a Neotropical mistletoe is shaped by seasonal patterns of host leaf phenology

“…The authors suggested that this correspondence could occur because of similar responses to external factors, such as the water VPD (Ullmann et al 1985). In a global analysis, Scalon and Wright (2015) showed that mistletoes become more conservative in their water use with increasing aridity, suggesting that they respond to environmental constraints in a manner similar to that of their hosts. Alternatively, mistletoes could be responding to an internal mechanism driven by the host, such as hormonal control, as suggested by Ullmann et al (1985).…”

Mistletoe generates non‐trophic and trait‐mediated indirect interactions through a shared host of herbivore consumers

“…The changes caused by mistletoe parasitism in pine (Zweifel et al. , Scalon and Wright ) transform pine tissues into a worse food for the insect due to stronger induced defenses (Lázaro‐González et al. ) and a reduction of mineral nutrients (Mutlu et al.…”

“…We identified the indirect effects of mistletoe on arthropod herbivores as a TMII, via changes in the quality (trait) of the pine as food. The changes caused by mistletoe parasitism in pine (Zweifel et al 2012, Scalon andWright 2015) transform pine tissues into a worse food for the insect due to stronger induced defenses (L azaro-Gonz alez et al 2019) and a reduction of mineral nutrients (Mutlu et al 2016). These negative effects of mistletoe on pines prove especially stressful at a site such as Sierra de Baza, with poor soils ) and a severe summer drought, where high mistletoe load inevitably leads to the death of the pine .…”