New paper accepted – Extremely thick cell walls and low mesophyll conductance: welcome to the world of ancient living!

Text and pics by Linda-Liisa Veromann-Jürgenson

I am overjoyed that I have my first article for my PhD accepted! It is now published in Journal of Experimental Botany.

a day in the lab

Linda-Liisa in the lab

As everyone in our group knows, mesophyll conductance is a key player in net assimilation rates in plants, but it’s not too well studied. Recent research, several from this department, has highlighted the importance of mesophyll structure in determining its conductance. Although several papers have been published on the mesophyll conductance of angiosperms and its underlying ultrastructural characteristics, only two studies had been conducted on gymnosperms that consider gas-exchange as well as mesophyll anatomy. We studied evolutionarily old plants including three divisions of gymnosperms, a whisk fern and a clubmoss to understand the role of mesophyll conductance on net assimilation rate as well as the impact of mesophyll anatomy on gas-exchange with an evolutionary perspective. Additionally, we examined the leaf economics spectrum (LES) correlations in these species as gymnosperms have been found to differ significantly from angiosperms in the LES parameters. We got several important results. Firstly, mesophyll conductance is a very important limiter of photosynthesis in gymnosperms and it depends on mesophyll anatomy. Secondly, the cell wall thickness as well as chloroplast area exposed to intercellular airspaces influence mesophyll conductance significantly and in opposite directions. Furthermore, chloroplast shape and size can play an important role in limiting CO2 diffusion in the liquid phase. Thirdly, although anatomy plays a key role, high leaf mass per area did not relate low net assimilation or high cell wall thickness. Interestingly, we measured extremely thick cell walls in several species, which may indicate a preservation of ancient traits through time due to some evolutionary constraints.

Citation: Veromann-Jürgenson, L. L., Tosens, T., Laanisto, L., & Niinemets, Ü. (2017) Extremely thick cell walls and low mesophyll conductance: welcome to the world of ancient living!. Journal of Experimental Botany, DOI: (link to full text)

Psilotum nudum TEM

Psilotum nudum transmission electron microscopy photograph


Mesophyll conductance is thought to be an important photosynthetic limitation in gymnosperms, but they currently constitute the most understudied plant group in regard to the extent to which photosynthesis and intrinsic water use efficiency are limited by mesophyll conductance. A comprehensive analysis of leaf gas exchange, photosynthetic limitations, mesophyll conductance (calculated by three methods previously used for across-species comparisons), and the underlying ultra-anatomical, morphological and chemical traits in 11 gymnosperm species varying in evolutionary history was performed to gain insight into the evolution of structural and physiological controls on photosynthesis at the lower return end of the leaf economics spectrum. Two primitive herbaceous species were included in order to provide greater evolutionary context. Low mesophyll conductance was the main limiting factor of photosynthesis in the majority of species. The strongest sources of limitation were extremely thick mesophyll cell walls, high chloroplast thickness and variation in chloroplast shape and size, and the low exposed surface area of chloroplasts per unit leaf area. In gymnosperms, the negative relationship between net assimilation per mass and leaf mass per area reflected an increased mesophyll cell wall thickness, whereas the easy-to-measure integrative trait of leaf mass per area failed to predict the underlying ultrastructural traits limiting mesophyll conductance.

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Plant physiologists defeating stress in 7 km run

Text by Kaia Kask, pics by Steffen Noe and Kaia Kask

On the 8th of April 2017 Ülo Niinemets’ lab members went to run 7 km in Parkmetsa running competition in Tartu, Estonia. Our crew members were: Ülo Niinemets, Steffen M. Noe, Tiina Tosens, Kaia Kask, Leila Pazouki, Arooran Kanagendran, Bin Liu, Valentina Zolotarjova, Chickodinaka Okereke and Evi Vaino. Main purpose was to have fun and not to stress about the time. Weather was sunny and team was ready to give 100%!

Next running competition is in August 2017. So we have time to prove our running results!


Kaia and Tiina (65) running


Ülo (64) finishing


Valentina (73), Tiina (65), Steffen (71) and Bin (69) after the run


Chika mitigating stress with water


Kaia (66), Bin (69), Eevi (74), Arooran (70), Ülo (64) and Steffen after finishing

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New publication – Spectacular oscillations in plant isoprene emission explain the enigmatic CO2 response

Text by Lauri Laanisto

Well, I do love a paper which says both spectacular and enigmatic in its title! That should be quite a hook for news agencies? Ok. As the one sentence summary says: Oscillations in isoprene emission demonstrate that the emission is controlled by chloroplast reductant status. There! I think this summarizes this paper quite nicley, without the spectacularity of pertinent enigma(s). (I´m not particularly competent in this topic, so I´ll withhold further comment…)


Figure 9 from the paper – Simple scheme of photosynthetic energy-metabolite supply for isoprene synthesis and processes inducing oscillations in photosynthesis and isoprene emission.

Citation: Rasulov, B., Talts, E., & Niinemets, U. (2016). Spectacular oscillations in plant isoprene emission explain the enigmatic CO2 response. Plant Physiology, . (link to full text)


Plant isoprene emissions respond to light and temperature similarly to photosynthesis, but CO2-dependencies of isoprene emission and photosynthesis are profoundly different with photosynthesis increasing and isoprene emissions decreasing with increasing CO2 concentration due to reasons not yet understood. We studied isoprene emission, net assimilation rate and chlorophyll fluorescence under different CO2 and O2 concentrations in the strong isoprene emitter hybrid aspen (Populus tremula x P. tremuloides), and used rapid changes in ambient CO2 or O2 concentrations or light level to induce oscillations. As isoprene-emitting species support a very high steady-state chloroplastic pool sizes of the primary isoprene substrate, dimethylallyl diphosphate (DMADP), that can mask the effects of oscillatory dynamics on isoprene emission, the size of DMADP pool was experimentally reduced by either partial inhibition of isoprenoid synthesis pathway by fosmidomycin-feeding or by changes in ambient gas concentrations leading to DMADP pool depletion in intact leaves. In feedback-limited conditions, oscillations in photosynthesis and isoprene emission were repeatedly induced by rapid environmental modifications in both partly fosmidomycin-inhibited leaves and in intact leaves. The oscillations in net assimilation rate and isoprene emission in feedback-inhibited leaves were in the same phase, and relative changes in the pools of photosynthetic metabolites and DMADP estimated by in vivo kinetic methods were directly proportional through all oscillations induced by different environmental perturbations. We conclude that the oscillations in isoprene emission provide direct experimental evidence demonstrating that the response of isoprene emission to changes in ambient gas concentrations is controlled by the chloroplastic reductant supply.

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New paper published – Cell‐level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks

Text by Lauri Laanisto

Another study by from the Mediterranean. Peguero-Pina has collaborated with Ülo seveal times before, and also regarding the physiology of oaks (link to blog post). This time the study does not focus on a single species, but seven Quercus species; and not all of them from the the Mediterranean, but some from Mediterranean as well. The importance of mesophyll conductance in photosynthesis is a topic that has been covered a lot in this blog, so I will not go deep into it here (click on the relevant tags if you´re interested). It seems that from the theoretical and methodological point of view this paper here did not do anything spectacular. The novel part was that they measured the traits from not one, but several species. At least that is what they conclude themselves: “To the best of our knowledge, this is the first interspecific comparison of photosynthetic characteristics and underlying leaf anatomical traits among Mediterranean oaks from Europe/North Africa and North America.” Of course, it was rather surprising that species from the same genus had such differences between these traits. I would suggest that in addition to interspecific comparison, these things need to be studied also in the context of intraspecific variability (which seems, at least for me, to be one of the understated areas in ecophysiological and physiological studies…) – just a single population (10 individuals) from each species was studied in this case.

Citation: Peguero‐Pina, J. J., Sisó, S., Flexas, J., Galmés, J., García‐Nogales, A., Niinemets, Ü., … & Gil‐Pelegrín, E. (2017). Cell‐level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks. New Phytologist, DOI: 10.1111/nph.14406 (link to full text)


Dehesa – Oak-dominated savanna-like semi-natural ecosystems in the Mediterranean (pic from here)


  • Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (gm), which is the most limiting factor for area-based photosynthesis (AN) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar AN to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (Sm/S and Sc/S), may offset the restrictions imposed by high LMA in gm and AN in these species.
  • Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits.
  • Mediterranean oaks showed specific differences in AN that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in gm, the most limiting factor for carbon assimilation in these species.
  • Species with higher AN showed increased Sc/S, which implies increased gm without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach AN values comparable to congeneric deciduous species despite their higher LMA.
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Upcoming seminar – Julie Gray about stomata

Tomorrow! Guest seminar by Julie E. Gray – March 17, at 14:15, Nooruse 1, first floor auditorium!

You are warmly welcome to the seminar by prof. Julie Gray from The University of Sheffield.

She  will be speaking about her studies on the regulation of stomatal function and density in changing environment. Within past years she has been excellent in transferring knowledge gained in model species (Arabidopsis) to crops.


Stomata of Lavendula dentata (pic from here)

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Visiting scientist – Jeroni Galmes

Text and pic by Lauri Laanisto

Last week Jeroni Galmes visited our lab. He somehow opted to come from the Balearic to slushy Tartu and stay here for the entire week. I guess this might indicate the quality of the research that we do here…

Jeroni has been collaborating with Ülo already for years – the last published paper, about the functionings of Rubisco was also mentioned in this blog (link to blog post). This research direction still seems promising, and that is the reason why Jeroni came. There are quite a few papers planned based on Rubisco data acquired from the literature. (I have also a little role to play here – to filter in the phylogenetic data.)


“Old school” way of extracting meta-data (Jeroni on the left and Ülo on the right)


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New projects funded!

Text by Lauri Laanisto, Linda-Liisa Veromann-Jürgenson and Tiina Tosens

New Year begun with some new funding! This post will give a short overview of what some of our lab members will be doing in upcoming years.

So, Tiina Tosens and Lauri Laanisto both got the personal research grants (for four years) from the Estonian Research Council. Tiina got the start-up research grant, which is meant to support young researchers who wish to start their independent research career and to establish a research group. And Lauri got the exploratory research grant, which is meant to support an innovative high-risk research project which may involve high risk at a high international level. So at least one of us has to do his risk assessment soon…

Competition for this funding is always pretty strong. This year there were altogether 365 applications and 89 of them were eventually funded, so around 25% funding rate. You can see the list, unfortunately in Estonian only, of all funded projects here (link).


Ye should have faith! (pic from here)

Tiina’s project is titled: “Window through the past: the effect of mesophyll CO2 diffusion conductance on plant performance across 120 extant and 13 extinct gymnosperm species.”

Climate change leads to the alteration of multiple environmental factors simultaneously and, what is more, is expected to result in amplifying the frequency as well as the severity of both environmental and biotic stresses, and stress interactions worldwide. The responsiveness, tolerance and acclimation capacity to stress differs in and between plant species. Furthermore, multiple stresses can interact and alter plant functions resulting in greatly more complicated predictions of climate change effects on vegetation. As recently as about ten years ago, plant acclimation and plasticity were not taken into account in modelling vegetation trends and still mostly model angiosperms are used. Today, we have more advanced technology and therefore, better opportunities for detailed research. Fundamental knowledge about what photosynthesis is influenced by in various plant groups and how plants in all taxa react, acclimate and adapt is crucial to predict global vegetation responses to the changing climate.

A significant part of this response is the change in photosynthetic rate (An) and its underlying characteristics: biochemistry, stomatal and mesophyll conductance. Mesophyll conductance (gm) has now been estimated for more than 100 species, of which the vast majority are angiosperms. The photosynthetic limitation by gm varies strongly with mesophyll structure. The reduction of photosynthesis due to low gm is 10-30% in species with thin mesophytic leaves (agricultural plants, annuals, temperate deciduous tree species), but it is much larger (50-75%) in evergreen sclerophyllous leaves and in evolutionary old species (e.g. conifers and ferns). Only twenty gymnosperm species have been studied in terms of gm so far (most are studied by members of our group) and only three studies have considered the underlying structural correlates. Yet, there are almost 900 species of gymnosperms in existence and, consequently, this project will provide information of about 14% of them. Conifers show the lowest values of gm among spermatophytes. This constitutes a serious gap in our knowledge that precludes driving any broad conclusions as for the evolution of mechanisms controlling gm. In the next four years, this project aims to uncover the variation and consequences of mesophyll conductance in evolutionarily old plants by studying a large sample of Cycadaceae across a environmental water gradient, 80 conifer species and gymnosperm fossils to study the importance of gm as a limiting factor of photosynthesis across ancient gymnosperms at different geological time points. It is imperative to know how much photosynthesis is limited by gm in conifers compared to stomatal and biochemical limitations, if there is an evolutionary advancement for lower mesophyll limitations in evolutionarily younger species, and whether low WUE and PNUE are consequences of low gm. Furthermore, it is important to find out which structural traits are responsible for low gm in conifers for both modelling and modification purposes.

For more about mesophyll conductance in ecolutionary old species, see also this blogpost.


Lauri´s project is titled: “Patterns and mechanisms of woody species stress polytolerance on global scale”.

Like the title says, it focuses on the global patterns of both abiotic and biotic stress polytolerance in woody species. According to traditional ecophysiological theories plant stress tolerance is determined by universal physiochemical constraints that don´t allow plants to successfully tolerate several abiotic stress factors simultaneously. Yet, recent studies have shown these trade-offs to be less fundamental, leaving more wiggle room for gaining polytolerance through adaptations. Various functional and phylogenetic traits, but also climatic factors have been proposed as the main mechanisms behind the variability of woody species to various abiotic stress. Moreover, some of the alternative explanations, like intraspecific variability – either ontogenetic development, or locally climate induced plasticity –, and biotic stress tolerance has been overlooked in such studies. The core of this project is to test: 1) global patterns of abiotic stress trade-offs and mechanisms shaping polytolerance; 2) intraspecific variability in polytolerance; 3) trade-offs between abiotic and biotic stress tolerance.

This project is a continuation of Lauri´s previous personal grant on similar topic, which was only given for two years, and therefore yielded just the most basic and initial results saying that indeed: both cold and waterlogging tolerance are negatively related to species capabilities of simultaneously tolerating low-light and low-water conditions. Plus, this pattern was different in angiosperms and gymnosperms, species region of origin and leaf type had no effect on this relationship. So now the idea is to take a deeper look into the causes and also effects of these complex and in some ways curiously contrasting patterns. Whether the intraspecific variability, or trade-offs between abiotic and biotic stress could explain these results in a more mechanistic way.

For more about polytolerance, see also this blogpost, and this one.

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