Pollen: Structure and Function.

Sunflower Pollen.

Inside The Hive TV’s Dr Humberto Boncristiani recently dissected a research paper which revealed that varroa levels were lower in colonies foraging on sunflower pollen. However, there was no clear evidence of the mechanisms at work here, but this week Dr Boncristiani suggests that sunflower pollen is not good as nutrition for honey bees and that their brood might not supply enough nutrition for varroa to feed sustainably on. This could account for lower levels of varroa rather than a substance in sunflower pollen harming varroa. An interesting perspective and worth considering before turning your land over to growing sunflowers. https://www.youtube.com/watch?v=sQSiuDoqt64

Melissopalynology.

This article aims to share more information about the fascinating subject of pollen and why we should know what our bees are foraging on. Look out for a follow up article giving advice on finding and recording local forage.

Melissopalynology is the study of pollen grains and comes from the Greek language; melisso means honey, palyn refers to small parts, and ology means the study of. Pollen grains are indeed small; so small that a compound microscope or scanning electron microscope is required to see individual grains. Forget-me-not flowers produce the smallest grains of around 6-7µm (microns, and one micron is a thousandth of a milligram) whilst crocus and mallow have some of the largest of over 100 µm. Most pollen grains lie in the middle with a diameter of between 30-50 µm.

To get the most out of studying pollen you’ll need to know how to use a microscope, make pollen slides, and identify individual flower pollen grains. I’ve included guidance on where to study this in more depth within the UK, and there’s a resource list at the end of this article.

Diversity.

There is vast diversity amongst all groups of living things, and plants are no exception. It may amaze you, yet come as no surprise, to learn that pollen grains are so varied though science has not yet discovered exactly why this is. Some plant families such as Asteraceae have flowers, like the dandelion and sunflower, with similar pollen structures and spiky surfaces making it easier for us to identify them. With a bit of experience, we can often identify families if not individual species by the shape and size of a pollen grain.

Sex Cells.

Pollen is the dust-like material which carries the male sex cells, or gametes, deep inside its complicated structure. One of the phenomenal characteristics of angiosperms, or flowering plants, is that their pollen tubes carry two gametes so that one will fuse with an ovule, and the other will fuse with a different type of cell to become an endosperm or food package. So, the offspring arrives with a food parcel to consume during its early days.

Chromosomes.

Unlike humans who have 23 pairs of chromosomes, and female honey bees who have 16 pairs, plants have no set numbers of chromosomes. Again, there is marked variation with hawkweed having 8 chromosomes and black mulberry trees containing 308. Plants also have the largest genomes amongst all organisms with wheat having 94,000 genes which is five times more than our own compliment of genes.

Why Study Pollen?

Why would we want to study pollen? The word, by the way, comes from Latin meaning fine dust or flour. Well, apart from enjoying great beauty under the microscope, we need to know what forage is available for our bees at all times of the year and these plants can be identified from pollen analysis. The reason for knowing this is so that we can supplement a colony’s food stores if there is a dearth of nectar income. Sometimes plants are flowering but the temperature may not be warm enough for them to secrete nectar and if we learn more about plants and botany we have a better understanding of honey bee nutrition and when to supplement . If we build up a forgage profile over a year we can use our records as management tools. We might also want to know the nectar source of a particular honey that we wish to advertise for sale.

Honey bees must have pollen because it is their only source of protein, and they need large amounts annually. They need protein for bee-growth and development of the brood as well as tissue repair for adult bees. It is essential for development of the hypopharyngeal glands without which brood food could not be produced.   A colony of around 20,000 bees might collect about 57 kg pollen in a year. Pollen also provides minerals, lipids, vitamins and essential amino acids, and if there is not enough pollen, or it is of insufficient quality, then brood rearing decreases and workers live shorter lives. Inadequate pollen income can the herald the death knell for a colony.

Using Pollen to Solve Crimes.

Palynology has a great role in the wider world of archaeology, anthropology, criminology, forensic science and medicine. Crimes may be solved by identifying pollen on a victim’s body and locating the crime scene, or where the criminal came from. Hay fever levels can be predicted by studying atmospheric pollens.

One of the biggest crimes being committed around us today is food fraud. Did you know that it is worth around $50 billion worldwide and honey fraud ranks third after adulterated milk and olive oil? Often, “honey” is so highly filtered that all traces of pollen are removed and so it cannot be identified. Or, pollen may be added to honey to trick people into believing that it comes from a particular source. In some countries, bees are fed corn syrup so they don’t actually produce real honey. Honey fraud also occurs when it is labelled as being a product of a particular country or region and yet the pollen does not match the area. For example, if citrus pollen was to be found in honey produced in Devon.

Honey Fraud.

The Chinese use a highly sophisticated resin technology to mask antibiotics in honey and so they have been able to export tonnes of contaminated honey to the US in the past. Fortunately, the world is onto the fraudsters and we are much more aware than we were following the exposure of fraud at the 2019 Montreal Apimondia Honey Show. Lynne Ingram, NDB, formed The Honey Authenticity Network UK and you can find more information about their role and work on their Facebook page: https://www.facebook.com/groups/588674292271458

Identifying pollens in adulterated honey is complicated and requires more than our compound microscopes in dealing with the problems. Fortunately, nuclear magnetic resonance profiling is now a powerful tool because it can detect the key modes of adulteration of honey and currently has allowed for the collection of a large global data base of 19,000 honey samples (1).

As you can see, most pollen studies depend upon much more sophisticated equipment than our simple compound microscopes. However, beekeepers can do a good job of studying pollen in relation to their own local areas, seasons and weather conditions for the benefit of their bees. So, let’s now take a look at the structure of a pollen grain.

Structure.

If you study the picture of a mature grain in figure 1, you will notice that the two important male sex cells containing genes are contained within the generative nucleus which will divide and migrate towards the pollen tube nucleus which forms the tube to carry these sex cells to the female ovules in the ovary. These nuclei are surrounded by a protective watery nutritive fluid called cytoplasm which also supports other parts of a functioning cell.

These crucial cells and cytoplasm are double-wrapped. Firstly, by the soft inner wall known as intine comprising mostly cellulose which covers the pore and bursts open under pressure (like a human ear drum rupturing when a water skier falls over and hits the water side-on) when the pollen tube pushes through the aperture on the first part of its important journey. When you stain pollen grains for examination under a microscope the intine will not stain and so pores are more easily identified.

Tollund Man.

The outer layer or exine is so tough it is almost indestructible and impervious to acids which is why it is poorly digested by humans and this has allowed pollen grains to remain intact in preserved bog bodies for thousands of years. Tollund man was found in Denmark in the 1950’s during excavation of a peat bog and was perfectly preserved thus allowing his stomach contents to be studied. Following his murder, Tollund man had been lying in a peat bog since Pre-Roman Iron Age times in the 4^th century BC and it was possible from pollen analysis to ascertain that his last meal was porridge made from barley, flax, false flax and knotgrass.

So, pollen is easily utilised by honey bees but is not well digested and absorbed by humans and other animals which is a conundrum given the money made in the health food industry from pollen sold as human dietary food supplements.

The exine is made from sporopollenin which is the diamond of the plant world being the toughest plant substance known. Its exact chemical nature and ratio of elements is still unknown but it comprises; carbon, hydrogen, oxygen, fatty acids, and aromatic and carbolic acids. Surrounding the exine on most pollen grains, is a very sticky substance called pollenkitt which is made from oily lipids produced in the inner layer of the male anther. Pollenkitt has an important role in attracting insects and other animals such as birds, bats and butterflies to pollen, and in the adherence of pollen to these animals.

Pollenkitt acts as a protective coat preventing desiccation of the grain and solar damage to the cytoplasm. It maintains the important pollen proteins inside exine cavities. If you want to see a fine example of pollenkitt make a microscopy slide of dandelion pollen and marvel at the copious oily yellow material in your sample.

Apertures are one of the key characteristics alongside shape, size, surface and colour that we examine when identifying pollen. The earliest fossil pollen grains from 120-130 million years ago have just one elongated slotted aperture and many flowering plants like magnolias and palms still carry this trait. Flowers evolved to have as many as 12 apertures but this is unusual, and 3 apertures is the average though pollen milkwort (Polygala spp.) is an interesting pollen with 5/6 apertures.

Figure 2 demonstrates orientation of a pollen grain like hawthorn, Cratagaegus monogyna. When viewed end-on it looks circular, but a side-on view makes it appear ovoid and so pole-to-pole distance will be different from the diameter or equatorial distance. Usually we measure pole-to-pole distance. This means that using Sawyers key (2) may be difficult if your measurements of the grain are incorrect due to your choice of orientation of specimen (3). You need to study many of the key features like size, grain surface, pores, furrows and apertures before you can identify pollen grains correctly so be aware of the orientation and search carefully in your slide for a suitable grain. Bear in mind that pollen grains swell in glycerine jelly during slide preparation and will be larger than dry grains straight from the flower.

I hope that this study of pollen will help those of you sitting exams later on this month and be useful in your beekeeping management.

References:

(1) Phipps, R., International Honey Market. American Bee Journal September 2018, Pp.997-981.

(2) Sawyer, R., 2006, Pollen identification for Beekeepers. Northern Bee Books, Yorkshire.

(3) Leitch, A., Salvage, J., Pollen: Grain Orientation is Key. BBKA News November 2018, Pp. 381-383.

Resources:

Adams, M, A, 2021, Pollen Grains & Honeydew a guide for identifying the plant sources in honey. Northern Bee Books.

Aston, D., Bucknall, S., 2004, Plants and Honey Bees. Northern Bee Books, Yorkshire.

Hodges, D., 2009, The Pollen Grain Drawings of Dorothy Hodges. IBRA.

Maurer, B., 2012, Practical Microscopy for Beekeepers. BeeCraft.

Kesseler, R., & Harley, M., 2004. Pollen- The Hidden Sexuality of Flowers. Papadakis in collaboration with the Royal Botanical Gardens, Kew.

NDB Short Course: Microscopy Course https://national-diploma-beekeeping.org/

Sawyer, R., 1998, Honey Identification, Cardiff Academic Press, Cardiff.

SBA: Microscopy Course https://scottishbeekeepers.org.uk/learn/exams-dates-fees/microscopy

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Thank you, Ann 🐝