Grasses—what they are

When speaking of grasses, it is common that pastures, meadows and maybe decorative plants in the city are visualized. They may evoke holiday memories (e.g., pampas grass, Cortaderia selloana) or represent the main element of a landscape (grasslands, crop fields). However, botanically speaking they are a family, called Poaceae belonging to the order Poales within the class Magnoliopsida. Poaceae include annual and perennial species and are mostly wind pollinated. They are characterized by producing a high number of pollen grains that can be transported great distances. Grasses are vast in number and nearly everywhere—or more precisely, they are one of the largest plant families and comprise over 10,000 species in about 780 genera and occur globally in all climatic zones [1]. In addition, they include the most important crops used by humans, among them wheat (Triticum aestivum), maize (Zea mays), and rice (Oryza sativa). Thus, Poaceae dominate large areas across the globe.

It is probably their nearly ubiquitous occurrence that makes them the most frequent source of pollen allergy around the world. Grass pollen produce 11 different groups of major and minor allergens, and are responsible for sensitization rates of up to 30% depending on climate and region [2]. Extensive cross-reactions among allergens of different grasses are another problem for grass pollen allergy sufferers [3]. This cross-reactivity may extend the time of burden for concerned persons and additionally complicates allergen avoidance. However, single grass species show a different allergenicity [4]. Therefore, the question how (much) particular grass species contribute to the grass pollen season and/or the burden of grass pollen allergy sufferers is still not finally clarified or rather not well enough examined for many regions of the world.

Grass pollen season—a season unlike other pollen seasons

The grass pollen season is special. It is longer in duration than other pollen seasons, composed of multiple peaks and it is also of relevance at higher altitudes. Usually, one dominant species (e.g., ragweed; Ambrosia artemisiifolia) or only a couple of species (e.g., alder; Alnus glutinosa, Alnus incana, Alnus × spaethii and Alnus viridis for Middle Europe) govern the pollen season. Furthermore, the progress of the pollen season is mostly the same: start, peak and end of the pollination period follow to a greater or lesser extent a smooth curve depending on the weather conditions during the time of pollen release.

This situation is completely contrasted in the case of the Poaceae family. Different taxa have different time periods of start, peak, and end during the pollination period. This results in multiple peaks and an irregular progress of grass pollen concentrations during the season. This impacts the burden of concerned people with different times and severity grades throughout the season [5, 6].

Neither standard nor new methods used to evaluate air samples and measure pollen concentrations have the possibility to discriminate between grass pollen from different species or even taxa. “Grass pollen” is just counted as grass pollen without more information. This is caused by the nearly identical morphology of grass pollen [7] as illustrated in Fig. 1a–d. Therefore, other methods are required to retrieve information on the contribution of grass taxa to the pollen amount, the grass pollen season, and the burden of persons concerned.

Fig. 1
figure 1

Important Poaceae (Habitus © Katharina Bastl, scanning electron microscope [SEM] picture from PalDat as cited in the following): Poa sp. A Habitus, a SEM pollen morphology [14]; Dactylis glomerata B Habitus, b SEM pollen morphology [15]; Festuca sp. C Habitus, c SEM pollen morphology [16]; Lolium perenne D Habitus, d SEM pollen morphology [17]; Scale bar equals 10 µm

Full size image

Studies using phenology

Phenology is the study of the life cycle of plants, such as flowering (start, full flower, end) among other possible seasonal events such as budburst, hibernation or dormancy. It depends on field observations, in the case of grasses covering larger areas and taken out on a regular basis. Studies using phenology to examine the grass pollen season are still rare. A total of 36 publications are shown when entering the keywords “phenology” and “grass pollen” in PubMed (9 May 2023), but most of them do not deal with phenology. In the following, the most important studies are summarized.

The first publication is a pilot study that was conducted in Vienna throughout the 2014 grass pollen season [5]. It combined aerobiological pollen measurements and symptom data with phenological data. Large observation sites were included, covering 40,000 m2 and phenological phases for grasses were adapted from [8] and concern the definitions of the flowering phases. Hence, the phenological approach was reduced to four phases to simplify the approach. The full flowering phase is defined as 50% or more individuals showing extruded or fully developed anthers, whereas the start of the pollination period is reached at 25% and the end of the flower at 75% of the population revealing open florets. Observed grass species included Alopecurus pratensis, Anthoxanthum odoratum, Poa pratensis, Dactylis glomerata, Bromus hordeaceus, Arrhenatherum elatius, Festuca sp., Holcus lanatus, Lolium perenne, Phleum pratense, Agrostis capillaris and Cynodon dactylon. The statistical analysis found the five most important grass species were also those with the highest distribution: Poa pratensis (Fig. 1A), Dactylis glomerata (Fig. 1B), Arrhenatherum elatius, Festuca sp. (Fig. 1C), and Lolium perenne (Fig. 1D).

The same study design was later applied in a larger study including Berlin (Germany), Turku (Finland), and Vienna (Austria) in the 2015 grass pollen season [9]. The observed grass species varied with the region. For example, Cynodon dactylon was observed in Vienna and Berlin, but not in Turku, while two species of Deschampsia were observed in Turku, but not in Vienna and Berlin. Differences are explained by a regional different importance of grass taxa. Finland is much further north and in a different climate zone resulting in a later flowering start compared to Central Europe (see Fig. 2 in [9]). This study found that for all sites the taxa Poa pratensis and Festuca sp. were important contributors, whereas Dactylis glomerata and Arrhenatherum elatius are of greater importance in Vienna and Berlin only and a wider spectrum of grass species contributed to the main grass pollen season in Turku.

A set of three years (2014–2016) of symptom, pollen, and phenological data in Vienna (Austria) showed that there is variability throughout the grass pollen season [6]: the largest fraction reacts in the second part of the grass pollen season (70%), followed by the first part (20%), and the third part (10%). This pattern was stable for all observed grass pollen seasons. Moreover, certain grass taxa could be associated with the highest symptom severities. The taxa of highest relevance are Poa and Dactylis for the first section of the season, Arrhenatherum, Festuca and Lolium for the second section, and Phleum and Cynodon for the third section. Individual reaction patterns were found and explain the divergent reports of grass pollen allergy sufferers upon the timing and severity of the grass flower.

A study in Italy used an area of 25 km2 and included the following grass species [10]: Agropyron, Alopecurus mysuroides, Avena fatua, Brachypodium spp., Bromus spp., Cynodon dactylon, Dactylis glomerata, Digitaria spp., Echinocloa crus-galli, Eragrostis cilianensis, Festuca spp., Holcus lanatus, Hordeum murinum, Lolium perenne, Phalaris spp., Poa annua, Poa pratensis, Poa trivialis, Setaria spp. From these Dactylis glomerata, Lolium perenne, Poa trivialis and Cynodon dactylon were identified as the most important contributors to the ambient grass pollen concentrations and Dactylis glomerata and Lolium perenne were suspected to be of highest relevance for allergology.

A study in Spain examined even 33 Poaceae species and their phenological phases [8]: they revealed that Dactylis glomerata, Lolium rigidum, Trisetaria panicea and Vulpia geniculata were the major contributors to grass pollination.

Another study integrated remote sensing techniques to link the vegetative and reproductive development and the pollen emission in the Mediterranean area [11]: the highest grass pollen emission was produced when the major grass-dominated vegetation types were in the final phases of vegetative development. Lolium rigidum, Aegilops geniculata, Trisetaria panicea, Dactylis glomerata and Melica ciliata were identified as most important taxa.

It should be mentioned that also new techniques in aerobiology like environmental DNA (eDNA) and molecular ecological techniques (DNA metabarcoding, quantitative polymerase chain reaction [qPCR] and enzyme-linked immunoassay [ELISA]) need phenological data as well to understand the connection between species-specific atmospheric eDNA and flowering phenology [12].

Impact of phenological data

The following conclusions can be drawn based on the above reviewed studies on the grass pollen season in Europe:

  • The composition of (the most important) grass species varies regionally,

  • Certain grass species are of high relevance, and

  • Phenology remains a key method.

Especially Poa (Fig. 1A) and Dactylis glomerata (Fig. 1B) play an important role at the begin of the grass pollen season, followed by Festuca (Fig. 1C) and Lolium (Fig. 1D) in nearly all European regions.

Detailed information on the flowering behaviour of Poaceae is important for:

  • Correct choice and timing of immunotherapy,

  • Component-resolved diagnosis,

  • Accurate support for persons affected by grass pollen allergy (there are differences in grass pollen allergy patients and timing of their burden),

  • Climatic studies (whether and how species respond differently to climate change),

  • Development of enhanced forecasting models and systems for monitoring as well as remote sensing, and

  • Reveal local phenomena like a late flower of reed plants (Phragmites australis) in September that extend the grass pollen season (e.g., in Austria [13]).

Therefore, phenology as a traditional study method will also play an essential role in future studies on the grass pollen season.