Mistletoe ecophysiology: host–parasite interactionsThis review is one of a collection of papers based on a presentation from theStem and Shoot Fungal Pathogens and Parasitic Plants: the Values of Biological Diversitysession of the XXII International Union of Forestry Research Organization World Congress meeting held in Brisbane, Queensland, Australia, in 2005.

Abstract

Mistletoes are highly specialized perennial flowering plants adapted to parasitic life on aerial parts of their hosts. In our discussion on the physiological interactions between parasite and host, we focus on water relations, mineral nutrition, and the effect of host vigour. When host photosynthesis is greatest, the xylem water potential of the host is most negative. To maintain a flux gradient and avoid stomatal closure and wilting, the mistletoe must tolerate a more negative water potential than the host. Succulent leaves enhance water storage and allow mistletoes to rehydrate before their hosts rehydrate. Mistletoe infections may disrupt the host stomatal control system, causing early and oscillating closure of host stomata, thereby diminishing host photosynthetic gain. Mistletoes lack the active uptake of minerals of a typical plant root system and rely upon the haustorium to connect with the host for the essentially one-way flow of photosynthates and nutrients from host to parasite. Modest growth rates, tolerance, succulence, and rapid leaf turnover are some means by which mistletoes avoid mineral deficiency or excess. We propose high concentrations of some mobile elements in the mistletoe by comparison with the host result not from active uptake, but from the inevitable accumulation by a parasite that utilizes host phloem sap. The relationship between host condition and mistletoe performance varies by situation and over time. In some cases, the host can outgrow the mistletoe, but favorable host status can also accelerate mistletoe growth. A better understanding of the mistletoe–host interaction can be utilized in improved management of infested forest plantations for resource production as well as for conservation of biodiversity and endangered species.

Occurrence of European Mistletoe (Viscum album L.) on Forest Trees in Poland and Its Dynamics of Spread in the Period 2008–2018

“…[1] is an evergreen plant with functional chlorophyll, capable of fixing atmospheric carbon dioxide and, therefore, considered to be a semi-parasite (hemi-parasite) [2,3]. It penetrates the tree branches, taking up water and mineral nutrients [2,4,5] through the endophytic system which consists of the haustoria and sinkers that reach the host cambium, and cortical strands that provided lateral spread [6]. Mistletoe grows mainly on branches, and rarely on trunks of woody species [7].…”

Quantifying the effect of pine mistletoe on the growth of Scots pine

“…This exacerbates drought stress of trees especially in water-deficit areas (Glatzel & Geils, 2009), leading to growth loses and accelerated mortality by such secondary factors as fungi and bark beetles (Hawksworth & Wiens, 1996). Mistletoe can also be responsible for reduced seed production and low wood quality in the host plants (Hawksworth & Wiens, 1996;Muir & Hennon, 2007).…”

Leaf area – sapwood area relationship in Scots pine (Pinus sylvestris L.) under mistletoe (Viscum album ssp. austriacum) infection

“…Previous studies have clearly shown the effects of mistletoe on tree growth (Bilgili et al, 2020;Kanat et al, 2010), wood anatomy (Gol et al, 2018;Ozturk et al, 2019), biology (Mutlu et al, 2016b;Persoh et al, 2010), physiology (Ehleringer et al, 1986;Glatzel & Geils, 2009;Mutlu et al, 2016a) and morphology (Rigling et al, 2010) of host plants. In addition, the relationship between leaf area and sapwood area have been examined and documented in many broadleaved (Meadows & Hodges, 2002) and conifer species (Bancalari et al, 1987;Waring et al, 1982) including Scots pine (Albrektson, 1984;Berninger & Nikinmaa, 1994;Whitehead, 1978).…”

“…The transpiration rate and stomatal conductance of mistletoes are higher than those of hosts (Mathiasen et al, 2008;Ullmann et al, 1985). Drought stress exacerbated by mistletoes (Glatzel & Geils, 2009) and coupled by higher transpiration rate developing with decreasing xylem water potential in host branches lead to reduced photosynthetic rate of host (Mathiasen et al, 2008). Therefore, increased growth loss (Barbu, 2012;Bilgili et al, 2020;Bilgili et al, 2018;Ozturk et al, 2019), altered resource allocation (Tinnin & Knutson, 1980) and mortality (Rigling et al, 2010) are the typical results of mistletoe infection.…”