Text by Liisa Kübarsepp
Recently our lab published a new paper in New Phytologist on the stomatal reactions of ferns and allies. In this paper, we question the overall sluggishness that is assumed of evolutionarily older plants, and study thoroughly the reactions of many fern species both from physiological, morphological and evolutionary perspective.
Stomata – microscopic pores on the leaf surface – are highly significant in the functioning of a plant. Stomatal pores enable gas exchange between the leaf inner structures and the atmosphere, which is otherwise minimal. The epidermal cells covered by a cuticular layer protect the plant from desiccation, pathogens, pollutants etc. however, some gas exchange between the plant and outside environment is crucial. The gradient between water concentrations in the intercellular airspaces inside the leaf and the atmosphere is one of the important factors creating the water flow from the roots to the leaves. Closed stomatal pores also prohibit CO2 from reaching the mesophyll cells – thus inhibiting photosynthesis. The opening and closing of stomatal pores are controlled by stomatal guard cells, which respond to different environmental cues such as light, CO2 concentration, and humidity (both in soil and air).
Stomatal reactions have been studied thoroughly, however, most studies focus on the model species Arabidopsis thaliana or other angiosperms. More recently other plant species have caught more attention, and it has been demonstrated these reactions can significantly differ even within one genus. Digging even deeper and comparing angiosperms to phylogenetically more distant species, like gymnosperms, ferns or lycophytes, the differences are even more drastic. Some studies have even found no or very slow stomatal responses to factors like CO2 concentration and light in these older plant groups. Therefore, it has long been thought that the stomata of evolutionarily older species, like ferns, are sort of sluggish when it comes to any other environmental factor apart from water deficiency. However, when giving a closer look some studies have shown that it might not be quite that simple. Stomatal reactions of evolutionarily older species are controlled by other mechanisms, not just lack of water, which causes water potential decrease in the plant and thus induces the stomatal closure.
In our study, we focused on ferns and allies (altogether 29 species), and their stomatal reactions to environmental factors. We compared their reactions also to two angiosperm species, one woody and one herbaceous (Alnus subcordata and Phaseolus vulgaris). In our experiment, we used the same set of species to measure reactions to different environmental stimuli: changes in CO2 concentration, air humidity and light. Most studies only focus on one environmental factor at a time, making it difficult to make generalizations about the stomatal reactions. Indeed, we discovered that the stomatal reactions of ferns and allies are dependent on the environmental conditions, the reactions were fastest in response to low air humidity, and slower in response to CO2 concentration and light. What was also very interesting – in many cases, the stomata of ferns are not sluggish at all, in fact, they are often quite similar to angiosperms.
One important part of this study is also looking for the explanations behind the stomatal reactions. Why are some stomata reacting faster than others? Mainly we looked at morphology – the size and density of stomata. It appears that in some cases, the reactions of smaller stomata are indeed faster like many studies have previously found. These include reactions to low CO2 concentration, high light intensity and low air humidity. However, this is not universal to all reactions leading to the conclusion that different mechanisms should control the stomatal reactions in ferns – not only passive regulation by water content in the plant as many other studies have stated.
To search for the reasons behind the differences in stomatal reactions we went even further and used the phylogenetic signal analysis to see if perhaps there is a relationship between the phylogenetic distance of species and the differences in stomatal reactions. However, we did not manage to find such relations in our species set for any of the environmental condition changes. This would not necessarily mean that such a correlation could not exist, however, a much broader set of species would be needed to perform such analysis in the future.
To read further see the paper: Kübarsepp, L., Laanisto, L., Niinemets, Ü., Talts, E., & Tosens, T. (2019). Are stomata in ferns and allies sluggish? Stomatal responses to CO2, humidity and light and their scaling with size and density. The New phytologist, https://doi.org/10.1111/nph.16159 (link to full text)
Fast stomatal reactions enable plants to successfully cope with constantly changing environment yet there is an ongoing debate on the stomatal regulation mechanisms in basal plant groups.
We measured stomatal morphological parameters in 29 fern and allies species from temperate to tropical biomes and two outgroup angiosperm species. Out of these, stomatal dynamic responses to environmental drivers were measured in 16 ferns and the two angiosperms using gas‐exchange system. PCA analyses were used to further reveal the structure ‐function relationships in stomata.
We show more than 10‐fold variation for stomatal opening delays and 20‐fold variation for stomatal closing delays in ferns. Across species, stomatal responses to VPD were the fastest, while light and [CO2] responses were slower. In most cases the outgroup species’ reaction speeds to changes in environmental variables were similar to those of ferns.
Correlations between stomatal response rate and size were apparent for stomatal opening in light and low [CO2] while not evident for closing reactions and changes in VPD. No correlations between stomatal density and response speed were observed. Altogether, this study demonstrates different mechanisms controlling stomatal reactions in ferns at different environmental stimuli, which should be considered in future studies relating stomatal morphology and function.