SAXifraga oppositifolia: at the edge of the vascular plant´s LIFE

Let us tell you a story about one of the northernmost vascular plant species that also lives in the coldest place ever recorded for angiosperm life: the purple saxifrage, Saxifraga oppositifolia L. (“Saxops” for friends ;).

This is a well-known species in the Arctic thanks to its purple impressive flowering that occurs sooner after the snow melting, easily capturing the attention of the wildlife observers. In fact, 30 years ago, in one of the first ecophysiological papers of this species (developed in Ny-Alesund, Svalbard), Crawford and colleagues (1995) beautifully described its performance as “an unsurpassed pioneer species in arctic and alpine habitats demonstrated by a widespread circumpolar distributions” and also “unequivocally demonstrates its supreme ability to adapt to short, cold, growing seasons”.

In Greenland this species reaches the northernmost latitude recorded for angiosperms (sharing this “Guinness” world record with the arctic poppy Papaver radicatum Rootb.), at 83º40´N in Kaffeklubben Island, east of Kape Morris Jesup. However, not just lives around the whole Arctic, as well reaches southern locations through the highest mountain ranges in the northern hemisphere as the Rocky mountains, Alps, Pyrenees, Sierra Nevada, Tatra or the Karakorum-Himalayas. Its adaptation to cold environments is so remarkable that Christian Körner (Univ. Basel, Switzerland) found this species growing at 4,505 m at the Dom summit (Swiss Alps). Through dataloggers placed in the vicinity of the plants they recorded the climate conditions there, observing during the short growing season an impressive low average air temperature, just 2.6ºC. Last summer 2022, Martin Macek & Martin Kopecky also observed it at another impressive altitude data of 5,450 m.a.s.l. in a morraine of the Saser Kangri group, Karakoram range (pers. comm.) (Fig. 2), in a field-campaign developed by Jiri Dolezal´s group of the Univ. of South Bohemy and the Czech Academy of Sciences.

Figure 2. Saxifraga oppositifolia growing at 5,450 m.a.s.l. (a) in a morraine at the Saser Kangri group, Karakoram range (b and c), Ladahk, India. Pictures by Martin Macek and Martin Kopecky.

Altogether, evidence indicates that this species is a successful specialist in the most extremely cold, dry and low nutrient environments, and as an autotrophic living being, able to fulfil their carbon requirements in the really short growing seasons of these tough places. This species earlier got the attention of the polar scientists being described its elevated desiccation tolerance in 1973 by J.A. Teeri in a Science paper developed in Devon island (76ºN, Canada). Some years later, it was investigated by different researchers as R.M.M. Crawford or A. Kume (between others scientists), another amazing point of Saxops: it can show two contrasted growth-forms inhabiting together in the High Arctic; some individuals showed a compact cushion-like form (Fig. 3left) meanwhile others show a disperse trailing form (Fig. 3right).

More recently studies developed by Pernille Bronken (UNIS Centre, Svalbard) offer much more light to understand why these species show so contrasting growth-forms: each form was related to the individual´s ploidy level. Polyploidy, is a biological process that includes the whole genome duplication (produced by a failure in the meiosis). This phenomenon is also common in Saxops. Specifically, in Svalbard and in Greenland have been observed tetraploids (individuals with double the number of chromosomes than parental diploids). These individuals always show trailing-forms, meanwhile diploids would frequently shown cushion-like (but also in some cases trailing forms, of course, in biology will not be only one explanation, but this is another story).

Further than affect the growth-forms of the individuals, polyploidy also drives another major difference: diploids show an amazing flowering, however tetraploids individuals do not flower or just show a low number of them (Fig. 3).

Of course, this behaviour will have important consequences in terms of costs. It is calculated that sexual reproduction is 10,000 times more costly than vegetative propagation. Obviously, the clonal propagative strategy reduces individual biological variability, but maybe this sacrifice helps them to save essential resources to survive in these tough environments, and thus could be a key-point to explain the ecological success of tetraploids compared to diploids under certain conditions. Additionally, earlier observers more than 40 years ago notice that both forms show a slightly different ecological niche, meanwhile cushions were frequent in rocky areas, slope and cliffs, tetraploids were usually located at the bottom of the valleys, in sandy highly fluctuating soils that tend to experience flooding events after snow melting during the growing season.

But, should ploidy level also affect their photosynthetical capacity and stress tolerance? Obviously, photosynthesis, the basis of the primary productivity of the planet, as any other biological process is highly dependent of temperature, and the rest of environmental factors as light, water and nutrient availability… thus, Saxops should display impressive adaptations at leaf structural and biochemical level to ensure a net positive carbon gain in those short and tough growing seasons without sacrificing leaf functional traits that let it to tolerate the polar environment. We were studying the major mechanisms that could drive a trade-off between maximal photosynthesis and stress tolerance at leaf level. This is relevant because if we understand the mechanisms behind this trade-off we could improve our knowledge about the more tolerant species in the High Arctic and other polar deserts, plus as well this information also is valuable for new crop breeding strategies in agriculture.

We flied to Villum Research Station (Aarhus University, Danmark), the scientifical facility in the military outpost Station Nord in Greenland (81°36′ N, 16°40′ W), from Longyearbyen airport in Svalbard (Norway) crossing the impressive icy Greenlandic sea (Fig. 4).  Once there, and after knowing the function and the organization of the station, in our first walk around the station we found “huge” Saxops individuals (Fig. 5). All the individuals that we observed previously in other locations as in Svalbard (Longyearbyen) or Pyrinees and Sierra Nevada (3,000 m.a.s.l. Spain) where we also observed this species in our studies).

Here, in one of the most remote and extreme places that we ever work (Fig. 6), this amazing species found an optimal place to live.

Fortunately, we found what we need there. Just walking around the vicinity of the station, we got several sites, with slightly different soil type and alteration level, and including individuals with the different growth forms, exactly what we were looking for. The first day it was an amazing sunny-clear-sky 13ºC warm-day there, actually, it was the first and the last sunny day, for our campaign there. The rest of our week campaign was cloudy and windy, including 4 snowy days, one of them 24 h without stop (this was end of July- beginning of August). When we woke up that morning we see that a north-west gusty wind was fiercely frozen everything in its way, with the different species covered by an ice layer, even the beautiful flowers of the arctic poppy (Fig. 7).

Figure 7. Frozen view of the surroundings of the research facility at Villum Research station, details of frozen individuals of Saxifraga oppositifolia,, Oxyria dygina, Cerastium sp. and Papaver arcticum.

Frozen plants were not really helpful to perform our measurements, we had to wait to the late afternoon, that become sunny and clear sky to get the plants unfrozen and active again. Those tiny leaves characteristic of Saxops will also not make things easy so with a lot of careful we started to measure their photosynthetical capacity. It was surprisingly that in general individuals from the different sites did not show water stress at all. In agreement, photosynthetical measurements showed that plants were enjoying the greenlandic summer growing season with impressive assimilation rates ranging from 4-7 μmol CO₂ m^-2 s^-1 for this tough environment (average for angiosperms ranges ca. 10 μmol CO₂ m^-2 s^-1).

We also collect samples to measure C and N use efficiency, leaf ionomic profile, leaf and stem anatomy, their primary and secondary metabolism and, of course, their ploidy levels. Independent of the growth-form, the assessment of the ploidy levels of each of the individuals measured in this project is essential to can subsequently relate to its molecular and ecophysiological performance. However, at remote environments like this one, everything becomes much more difficult. Proper biochemical analysis rely into the avoidance that samples do not suffer any biological degradation during the sampling, for this purpose in the lab they are immediately frozen in liquid nitrogen (>-195ºC). Unfortunately, liquid nitrogen is not easily available out of the research centers and of course impossible at remote locations. To avoid this problem and got the opportunity to characterize deeply its molecular mechanisms we took with us a dry-shipper, a container that keeps the liquid nitrogen with a foam inside (up to 21 days), thus providing ultra-frozen temperatures there for keeping perfectly our samples, plus it can be safety transported by plane (Fig. 8), that basically makes our sampling (and life) easier (Fig. 9).

Combining ecophysiologal in situ field measurements with biochemical sampling open us new opportunities to understand the performance of this species, and how the ploidy phenomenon affected it. Molecular mechanisms behind the ecophysiological responses between diploids and tetraploids can be investigated by several high-throughput “omic” technologies as metabolomics; letting us to establish the major primary and secondary biochemical routes that can be associated to the differences between ploidy levels and its impressive stress tolerance. We hope that this knowledge as well give us more information about the future of this species facing the global warming that will dramatically affect the Arctic region.

Acknowledgments

Pernille Bronken (University of Oslo) for her kindly review of this text and Jørgen Skafte (Villum Research Station) for his support organizing the logistics and facilities for this project. This campaign was funded thanks to INTERACT EU Call and the spanish national project MCIN/AEI/10.13039/501100011033, national call ‘Generación de Conocimiento’ 2019–2020, project number PID2019-107434GA-I00 and by European Union ‘NextGenerationEU/PRTR’.