Summary: Forest Tree Pollen Dispersal via the Water Cycle
Introduction
A large hailstone found in Kansas contains hundreds of biological particles, 5% of which were pine pollen
(Mandrioli et al, 1973). This percentage shows evidence that pollen dispersal is possible via the water
cycle. Pollen dispersion happens through the water cycle in the following manner: Single pollen grains
are first released, and then are transported vertically into the atmosphere through the turbulence of the
air. Once in the upper atmosphere, these pollen are captured in ice at sub-zero temperatures, through a
process called ice nucleation. As individual pollen grains covered in ice collide with one another, they
form into larger droplets which then fall as precipitation. As the article states: “Direct evidence for
aerodynamic capture and its role in water-cycle dispersal comes from studies of ice nucleating bacteria…
[and][e]merging evidence points to a similar dispersal path via the water cycle for temperate-forest tree
pollen” (Williams, 1184).However, since the pollen has been frozen during ice nucleation, researchers
seek to determine if pine pollen, specifically, can still germinate after its exposure to immersion freezing.
The researchers are determining whether a specific species of pine is more likely to use the water cycle
for germination than other conifers. They are basing their theory off of the pollen’s certain biological
properties that would influence its ability to germinate after having been frozen. Some of these
properties include a thick exine and a thin intine, which allows the pollen to resist rupture when
exposed to freezing, as well as the fact that pine pollen is dehydrated when it is released.
They chose to test the ability of pollen from pine trees from around the world to germinate after
undergoing the freezing process. If the pollen is found to be viable and able to germinate after having
been frozen in a manner like unto the freezing that occurs in the upper atmosphere, then it is pertinent
to the ability of forests to use the water cycle to pollinate over a greater distance than previously
thought.
In this experiment, the researchers are trying to show that pine pollen can successfully germinate after
immersion freezing, and that dehydrated pollen has a higher germination rate than saturated pollen
after the samples have been frozen.
Materials and Methods
For their experiment, the researchers subjected pollen samples from six species of pine tree-- from
various parts of North America, Asia, and Europe-- to immersion freezing, then tested each one for
germination. Each sample contained two pollens from the same species of tree-- a dehydrated and a
water saturated pollen. Each sample also had separate treatment durations, either 15, 60, or 120
minutes of immersion-freezing, totaling in 6 samples of pollen per species, and 36 samples in all.
Afterward, each sample was thawed and spread over a petri plate and incubated for the germination
assay to be compared to a control group of germinated pollen.
The techniques used to execute this study started with the pollen being collected at an arboretum from
six different species of pine. Then, the control pollen was incubated in a petri plate at 28 °C for 48 hours,
and its germination tubes were counted, giving the researchers a baseline of germination to compare to.
After the control group was separated and germinated, the collected pollens used for the test group
were dried in an oven until the pollen had reached a 0% moisture content. Each sample was then
separated into two tubes per sample. One tube was filled with dehydrated pollen, and one tube filled
with pollen that had been completely saturated by water after dehydration. The experiment was set at
-20° C, which is the temperature that the researchers determined 100% of pollen would form ice nuclei,
and “immersion freezing was conducted in an insulated beaker filled with ethelyne glycol chilled to -20°
C” (1187). To clarify, each immersion-freezing sample contained 2 tubes, one dehydrated and one
hydrated. (2 tubes x 6 species= 12 tubes) and each timed duration of freezing was conducted
successively (12 tubes x 3 intervals = 36 tubes) After freezing, the experimental samples were thawed,
spread in a petri plate, and incubated for 48 hours at 28°C. After allowing the frozen pollen to germinate
for the same length of time and at the same temperature as the control, the sets were compared to the
control by counting the number of germination tubes per sample.
Results
Pollen that had been dehydrated first and then frozen immediately via the immersion-freezing process
was found to have an average of 43.3% germination rate, whereas pollen that had been dehydrated and
then water saturated before freezing only had a germination rate of 7.6% after having been exposed to
immersion-freezing. The researchers found that in the combined analysis of all the samples, these
results were statistically significant.
Germination rates for the water saturated pollen were found to be similar throughout the samples,
whether the sample was treated for 15, 60, or 120 minutes, whereas the germination rates for the
dehydrated samples were varied based on time spent frozen, and thus were averaged for the results.
Discussion
Pine pollen was found to be viable after immersion freezing. Germination was higher for dehydrated
pollen, which supports the idea that being dry before freezing protects the pollen from being damaged
from the ice. Interestingly, the length of time spent frozen had no effect on the water saturated pollen,
suggesting that hydrated pollen was damaged rapidly when frozen, and not damaged slow and
cumulative as expected.
That pollen can still germinate after having been frozen provides early evidence in favor of the idea that
pollen dispersion can happen successfully through the water cycle, and gives credence to the idea of
long distance dispersion of pollen as well.
Limitations were stated as showing that the experimental design was “stronger for discerning
germination between dehydrated and water saturated pollen than for discerning germination of a
treatment in relation to its control.” (1188). Limitations also mentioned that performing all of the tests
at -20°C was an extreme simplification of the cloud formation process at high altitudes, and that
measuring each species sample across a temperature gradient from 0°C to -20°C would show the
adaptive response of each species, per se. In closing, this experiment shows that pine pollen retains
some of its viability to germinate after having been frozen, adding to the evidence that pollen is
dispersed via the water cycle.
Citations
Mandrioli, P., G. Puppi, N. Bagni, and F. Prodi. 1973. “Distribution of Microorganisms in Hailstones”
Nature 246: 416-417.
Williams, Claire G. 2013. “Forest Tree Pollen Dispersal via the Water Cycle.” American Journal of Botany
100 (6): 1184-1190.