Pollen and other airborne particles
Introduction
We are constantly breathing in a complex mixture of particulate matter that is present in the
atmosphere. Most of this is filtered out by the cilia in our upper respiratory tract, so no harm is
done. However cigarette smoke and other airborne chemicals can interfere with this process,
giving rise to respiratory disease. Also some organic material can cause respiratory illness such
as hay fever and asthma1,2.
The National Pollen and Aerobiology Research Unit (NPARU) produce a regular bulletin of the
pollen index drawing its measurements from a network of 33 monitoring sites in the UK3.
Measurements are taken from pollen traps usually sited on the roofs of buildings.
Melted Vaseline is smeared onto
a melinex film on a circular drum:
movement of the drum is
controlled by a clock mechanism.
After a fixed time, the melinex
strip is removed and cut into
lengths to fit onto microscope
slides. With care, it is possible to
monitor three minute intervals in
time on the strips, i.e. sudden
peaks in pollen or spore
concentration, caused by local
grass cutting or harvesting on a
nearby farm. Usually, twelve
sweeps are made of a slide to
estimate pollen and spore concentrations. These are then forwarded to the NPARU. Together
with the weather forecast, this results in the pollen forecast. 4
The trapping system is contained within an apparatus with a constant airflow intake of 10 dm3
min-1.
The one at Cambridge University is shown above.
Making a pollen trap
All that is needed is a cover to prevent rain spoiling the slides,
and a surface to place the slides on. A two litre or five litre plastic
water bottle can be cut to form a tubular cover and a piece of
plastic or wood can be placed inside to form the base. A slightly
more sophisticated example is shown on the left.
To trap pollen and other material, the surface of the slides must
be sticky. We have used concentrated sugar solutions, glycerol,
or even Golden Syrup. We have not used the Vaseline system
described above (because it is difficult to stain the pollen
afterwards), but prefer to use the basic fuchsin solution that is
used to stain the pollen.5 This method seems less affected by
early morning dew than the other solutions. Once warmed to about 50°C, a drop can be applied
to a slide and another slide drawn across the surface, much as is done to make a blood smear. A
piece of Blutac® underneath the slide holds it 2 in place. After sufficient time under the cover (2 Science & Plants for Schools: www.saps.org.uk
Pollen and other airborne particles: p. 1
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7 days, depending on the time of year and the weather conditions), the slide needs to be warmed
to melt the stain and then a
cover slip placed on (it can be helpful to place another drop of stain on, before the cover slip is
lowered). Keep the slide warm for a few minutes to allow the stain to spread out, and then use a
toothpick with gentle pressure to remove any air bubbles. Once cool, the excess stain around the
edges of the cover slip can be removed with a scalpel, or by running hot water over the edges of
the slide to dissolve the stain (keep a finger firmly on the cover slip while doing this).
Application
This method can be used to study the pollens that
are present in the atmosphere and which may
cause hay fever. Students who suffer from these
conditions will find it interesting to identify what is
actually causing their symptoms. Commonly it is
the pollen from trees that sets off hay fever in
Spring, that of grass in summer, and then of
various weeds in late August and early Autumn 6.
Those students suffering from asthma could also
identify common fungal spores in the Autumn,
which are often the causative agents of asthma. In
addition, traps set up in different places may be
used to demonstrate air pollution. A school in a
rural environment may show quite different results
from one in a town.
Pine pollen
Results
Common airborne pollen such as a pine, cedar,
hazel, and grass are likely to be found in
abundance. In addition there will be a number of
fungal spores, such as Alternaria 7,8. Small bits of
leaves (probably from grass cuttings, as they
usually show the typical cell structure of a
monocotyledonous leaf) may be present as
well as trichomes 9,10.
Alternaria
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Pollen and other airborne particles: p. 2
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Part of a monocotyledonous leaf
Trichomes
Appendix
Pollen and hay fever
Many plants have evolved to depend on insects and other animals for pollination. These
entomophilous plants have highly coloured flowers (often with nectar guides) and nectar to attract
the pollinators. Generally the stamens are enclosed within the flower so little pollen is lost to the
atmosphere. Also, the more perfect the association between flower and pollinator, the less pollen
is produced, although there are exceptions to this generalisation.
As wind pollination (by anemophilous plants) is much more haphazard, so pollen production is
much greater. It also needs to remain in the atmosphere to stand a chance of reaching another
plant of the same species. Pollen carried in the wind is often small, rounded and with a smooth
surface, or in the case of the Pinaceae having air bladders – all features to make them
aerodynamic.
To cause hay fever, pollen must have a combination of the following characteristics:buoyancy,
abundance and allergenic toxicity. To complicate the issue, some people are allergic to one type
of pollen only, others to several types.
Spores
In the late Summer and early Autumn, fungal spores are produced in abundance.
Commonly these are from Cladosporium, Aspergillus, and Alternaria, as well as from the
Basidiomycetes (mushrooms and toadstools) 7. These are often responsible for respiratory
illness. Alternaria can even be found growing indoors on carpets!
Trichomes
These are hair like protrusions from the epidermis of
plant leaves and stems. They help plants ward off insect
attack. Some types make it difficult for an insect to
penetrate the leaf surface (Hibiscus): others trap
predators by secreting sticky fluids and may also
produce toxins to kill the insect (incidentally it is the
secretions from trichome glands that give rise to the
characteristic smell of tomato plants). These glandular
trichomes break down easily and so are unlikely to
show up on pollen traps.
Glandular trichome from a tomato leaf
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Pollen and other airborne particles: p. 3
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Glandular trichomes are common in plants used in perfumes and cookery, e.g. lavender and
rosemary. Usually it is the more robust, spiny type that are found in pollen traps. Perhaps the
most well known trichomes are those of stinging nettles, which penetrate the skin, releasing a
mildly poisonous substance 9. The ability of plants to ward of insect attack by their trichomes is
the subject of gene technology. Work on Arabidopsis has established genes which bring about
trichome initiation and development. Tobacco plants with a modification to the gene responsible
for the metabolism of the active principle in trichomes, show a greater resistance to aphid attack
11.
References and Websites
1. http://www.worcester.ac.uk/discover/national-pollen-and-aerobiology-research-unit.html
The National Pollen and Aerobiology Research Unit
2. www.users.globalnet.co.uk/~aair/hayfever.htm
3. www.pollenuk.co.uk/aero/ABPollenUK.html
How the pollen count is put together
4. www.bbc.co.uk/weather/ukweather
The UK pollen index, maintained from Spring to Autumn by the BBC
5. http://www.saps.org.uk/library/555
Forensic Palynology in Osmosis 26 (Autumn 2004) giving the recipe for basic fuchsin
6. http://www.zirtek.co.uk/hayfever/pollen_calendar.php
A useful table showing the pollens and spores in the atmosphere, month by month
7. http://pollen.utulsa.edu/Spores/Alternaria.html
Alternaria and other airborne spores
8. www.mold-help.org/index.php
Molds and disease. Go to ‘Mold Types and Images’ and search for Alternaria
9. www.biologie.uni-hamburg.de/b-online/e05/05a.htm#trichome
Trichomes and other specialised leaf structures
11. www.nature.com/nbt/journal/v19/n4/full/nbt0401_371.html
Suppression of cytochrome P450 hydroxylase in tobacco trichomes
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Pollen and other airborne particles: p. 4
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2012.