A thirst for more:
Morphological changes in Arceuthobium americanum
indicate water theft from its host, Pinus contorta
Dylan Ziegler and Cynthia Ross Friedman

Introduction
• Dwarf mistletoes (DMs) are a group of dioecious
parasitic flowering plants1
• Arceuthobium americanum (lodgepole pine DM)
infects Pinus contorta (lodgepole pine) (Fig. 1)
and stunts the host’s growth
• DM infection increases the host’s vulnerability
to abiotic stresses (such as drought)1
• A. americanum invades ray parenchyma of the
tree and consequently steals the host’s water1
• A. americanum explosively-discharges its seeds
• This process is dependent on a buildup of
hydrostatic pressure within the fruit
• Objective: As stomata are involved in water
conductance, we aimed to gain a greater
understanding of their role in DM pathology

Th
.

M.

Figure 1. Female A. americanum parasitizing Pinus contorta. The aerial shoots
erupt from the bark after 4-5 years of incubation within the host’s wood.
Flowers are pollinated in their first year, and fruits explosively discharge
their seeds the following year. Bar: 2 cm

Methods
Female DM shoots from the same five trees were
collected once per week from April-August 2015
from Stake Lake, BC
Second-year fruits were mounted on aluminum
stubs and visualized using environmental
scanning electron microscopy (eSEM)

Micrographs were analyzed with ImageJ to
determine stomatal density and fruit volume;
Polynomial regression was used to reveal trends
References
1. Hawksworth, F.G., and Wiens, D. 1996. Dwarf Mistletoes: Biology, Pathology, and Systematics. Washington, DC: United States Department of
Agriculture.
2. Hinds, T.e., Hawksworth, F.G., and McGinnies, W.J. 1963. Seed discharge in Arceuthobium: a photographic study. Science. 140(357): 1236-1238.
doi:10.1126/science.1403572.1236.
3. deBruyn, R.A.J., Paetkau, M., Ross, K.A., Godfrey, D.V., and Ross Friedman, C. 2015. Thermogenesis-triggered seed dispersal in dwarf mistletoe.
Nat. Commun. 6: 6262.

Figure 2. Second
year DM fruit eSEM
images taken in 2015
on (a) April 29 (b)
May 28 (c) June 30
(d) July 28 (e)
August 25 and (f)
September 1. The
fruit body (Fr) is
shown in green,
sepals in beige, and
subsidiary cells of
each stoma in
orange, An
epidermal fold
occurs (Ef) where
the sepals are
inserted at the
ovary. Visually,
development is
expressed by an
expansion in
diameter and an
increase in length
with a declining
stomatal density and
loss of the Ef by
May. Floral organs
persist through the
growing season.
Bar: 400 µm.

Figure 3. Stomatal density (blue) and fruit volume (red) relationship
over time in second-year DM fruits. The stomatal density levelled at the
maintenance point (M), at 27 stomata/mm². The difference between
density and volume when discharge occurs is the threshold (Th).

Results & Discussion
• Stomata were found only on the flowers, stems,
and reduced leaves, never on the fruit body (Fig. 2)
• Stomatal density (p<0.001) and fruit volume
(p<0.001) changed significantly through the season
(Fig. 3)
• Initial floral stomatal density (55 stomata/mm²) is
greater than the abaxial surface of the leaves (38
stomata/mm²)1
• Flowers may provide water conducting force for
the DM, analogous to leaves of other angiosperms
• A. americanum uses thermogenesis for seed
dispersal3 and requires exogenous heat2
• We propose stomatal density declines to the
minimum point required for homeostasis after
which the difference between stomatal density and
volume reaches a threshold, inducing discharge
through lack of transpiration-driven cooling
(activating thermogenesis) and by excessive
accumulation of hydrostatic pressure
Acknowledgements

The authors are grateful for Mark Paetkau’s and Dave Pouw’s assistance with data analysis and the eSEM, respectively. DM
photographs were taken by Amanda Ziegler, who also provided valuable insight on usage of Adobe Photoshop©. Joanna Urban,
Nancy Flood, and Lyn Baldwin were committee members over the duration of this honours project for upon which this poster is based
are gratefully acknowledged.