Pollen, wind, allergies and biodiversity: how are they related?

Sneezing, watery eyes, runny nose, headache… These are some of the most common allergic reactions to pollen. Despite the nuisance pollen causes, it is essential to biodiversity and without the thousands of particles that, especially in spring, are carried by the wind, the continued existence of a large number of flower species would be at risk.

12% of flowering plants and a significant number of trees, including conifers (trees that produce cones), reproduce by “hitching a ride” on the wind. From small wild plants such as mugwort and the invasive common ragweed, to larger trees including olive, cypress and pine, there are many species of flora whose pollen needs the wind to disperse it.

These are the so-called anemophilous plants (or anemophilous reproducing species): “anem” for wind and “philia” for love or affinity, terms that come together to name plants who reproduce by their affinity for the dispersal effect of the wind. And the wind can carry their pollen over great distances, which encourages greater genetic and ecosystem diversity.

So what is pollen, anyway? Each tiny grain is actually the structure (microgametophyte) that produces the male reproductive cells (the male gametes) of plants. For reproduction to take place, pollen must be able to reach the female structures, whose configuration is well adapted to accommodating these tiny grains.

To be successful in propagating, anemophilous plants produce millions of microscopic pollen particles. They are about 10-50 micrometres across (one micrometre is one-thousandth of a millimetre), but may be smaller, as happens with the genus Myositis (2.5 micrometres), or larger as with the genus Zea (125 micrometres). Because they are so small, dry and light, these particles can remain suspended in the atmosphere up to 3,000 metres above sea level and travel on the wind for hundreds of kilometres. Only a tiny number reaches the female structures of plants.

When pollen is in the air it ends up contacting with our respiratory system and this is how allergic reactions and symptoms are triggered. These symptoms occur more intensely in spring because this is the time of year when most plants produce pollen, although different species produce it at different times of the year.

The higher concentration of particles and the hotter, drier spring days increase the amount of pollen in the air (rain and humidity make them settle), which can cause allergic rhinitis, allergic conjunctivitis, eczema, hives and asthma attacks. These are reactions we recognise in humans, but some are also common to other domestic species such as cats and dogs.

Did you know that…

• Contrary to what many people think, it is not the downy feathers of the dandelion (Taraxacum officinale) or the cotton-like tufts from poplars (Populus spp.) that are mainly responsible for springtime sneezing, as these filaments are too large to enter our respiratory system. The pollen particles responsible for allergic reactions are invisible to the naked eye.
• Gramineae or poaceae (grasses) are one of the families of angiosperms (plants with flowers whose seeds are surrounded and protected by fruits) that have the greatest reproductive success due to the action of the wind and are also one of the most allergenic. There are estimated to be over 10,000 species of grasses in the world (about 270 in Portugal) and they can live in almost any type of habitat. Over 50% of the calories we consume come from grasses, especially cultivated grasses such as barley, wheat, oats, rice, corn, sugar cane or bamboo, as well as plants we use in teas, such as lemongrass. Some are also an important source of nutrients for various herbivores.
• As they do not need to attract pollinating animals to achieve pollination, the evolution of flowers in these anemophilous species has favoured those that do not waste energy on elaborate aesthetics. In most, the flowers are small, not very colourful, lack perfume and do not produce nectar; nor do they have petals. In the female structures, the stigma (upper part of the stylet, which is the female part of the flowers) is prominent and broad to better accommodate the particles that will later germinate and fertilise the ovule. Moreover, this pollen is usually smaller, produced in larger quantities and is less nutritious (less appetising to pollinating insects).

Different forms of Pollination

These allergy-causing particles are typically smaller than the grains carried by pollinators. Pollination by bees and other animals – such as butterflies, beetles, flies, birds, bats and even ants – is called zoophilic or entomophilic.

This is the most common way to ensure the continuity of flora species as well as their genetic diversity. To aid successful transport, in addition to differences in size, this pollen has sticky or spiky structures, that enable it to adhere to insect hairs.

Additionally, there are species whose pollen floats on the surface of water and moves until it finds another plant. This form of pollination, called hydrophilous surface pollination, is rarer. It is limited to some aquatic plants and is estimated to occur in only 2% of cases. Even rarer are the cases where pollen “sails” underwater.