Portrait of Pushan Bag sitting on a swing in front of a lakePhD student Pushan Bag at Nydalasjön in Umeå (Photo: Jenna Lihavainen)

What allows conifer trees to stay green during winter when temperatures are low but solar radiation is high? Pushan Bag, PhD student in Stefan Jansson’s group at Umeå Plant Science Centre, showed that conifers have evolved special mechanisms that prevent damage to their photosynthetic machinery. He will defend his PhD thesis at the Department of Plant Physiology, Umeå University, on the 20th of May.

What led you come to Umeå and start your PhD on winter acclimation of conifer trees in Stefan Jansson’s group at UPSC?

Pushan Bag: I was working on photosynthetic acclimation of algae in my master’s at the University of Hyderabad in India. We were investigating the effect of salt stress on photosynthesis by using different salts and growing algae under controlled conditions. For my PhD, I did not want to work with “artificial” controlled conditions but instead I wanted to understand natural adaptations under “real” conditions. At that time, Stefan had a PhD position focussing on conifer trees growing in boreal forests. These forests are one of the harshest environments for plants and that was interesting me. So, I applied and came here in 2017.

How did you acclimatise to the Nordic winter?

Pushan Bag: Well, I arrived here on the 17th of February 2017, and February is the coldest time of the year in Umeå. On the very next day after I arrived, I had to collect samples from the forest behind the SLU building. It was fun! To be honest, I love the Nordic winter and the snow, probably because I am from Kolkata and we do not see winter temperature dropping below 5°C. Here in Umeå, I can do some winter sports, which I could not do back in Kolkata.

You worked with Norway spruce and Scots pine and followed the changes that happen in their needles throughout the year, but you set a special focus on spring. What is so special with this season?

Pushan Bag: Spring in the Nordic climate is very different than spring in any other parts of the world. Air temperatures remain well below zero – we measured even -25 °C in February and March 2018 -, while the sun shines very bright. This makes acclimation extremely tough. Sun light drives photosynthetic reactions, but freezing temperatures make it difficult to conduct those photosynthetic reactions. Another problem the plants face is that reactive oxygen species are generated under such high solar radiation and they can damage the photosynthetic machinery.

What is the major outcome from your studies? Can you explain why conifer needles can stay green throughout the year?

Pushan Bag: We discovered that conifers possess a kind of “spill-over” mechanism to protect their photosystems from energy overload. These are the functional units where photosynthesis takes place. The structure of the inner membranes in the chloroplast is changed during winter so that the two photosystems are in physical contact with each other. This is a kind of short cut that allows direct energy transfer from photosystem II to I which is normally not possible. As far as I know, this “spill-over” mechanism was not reported before for any other vascular plant. Conifers are pretty unique in this sense!

What was the most unexpected result you got during your PhD?

Pushan Bag: We found another mechanism that conifer needles have developed to protect their needles from damage by reactive oxygen species. These results are not published yet but they were really unexpected. We hope the paper will be accepted soon.

What was the biggest challenge you faced during your PhD?

Pushan Bag: I think the biggest challenge for me was to learn that in science multiple possibilities can be correct at the same time. Plant species are very different from each other. They have evolved different regulatory ways and they can respond to the same stimuli in different ways. We often tend to generalise mechanisms and functions based on results from one model plant. However, like in our case, it can help to look on different contrasting possibilities to understand a natural phenomenon and maybe discover some new mechanisms.

You recently received a long-term postdoctoral fellowship from the International Human Frontier Science Program Organization and will start working with Professor Barry Bruce at the University of Tennessee in Knoxville. What do you think will you miss most from UPSC and Umeå?

Pushan Bag: Everything!!! I will most certainly miss my friends here in Umeå and also the working culture at UPSC, but I will take all my memories with me and I am looking forward to my new project.

About the public defence:

Pushan Bag, Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, will defend his PhD thesis on Friday, 20th of May 2022. Faculty opponent will be Francis Andre Wollman from the Institute of Physico-Chemical Biology (IBPC), Laboratory of Membrane and Molecular Physiology of Chloroplast at Sorbonne Université in Paris, France. The thesis was supervised by Stefan Jansson.

Title of the thesis: How could Christmas trees remain ever green? Photosynthetic acclimation of Scots pine and Norway spruce needles during winter

Link to Pushan Bag's PhD thesis

Further reading

More information about Pushan Bag’s long-term postdoctoral fellowship from the International Human Frontier Science Program Organization

Link to the press release about the Nature Communication article in which the findings about the photosynthetic short-cut in conifer trees were published in 2020

For more information, please contact:

Pushan Bag
Umeå Plant Science Centre
Department of Plant Physiology
Umeå University
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Twitter: @BagPushan