PhD student Sanchali Nanda (front) and her supervisor Stefan Jansson (back) at the ChloroSpec instrument at Umeå Plant Science Centre (photo: Anne Honsel).
Light drives photosynthesis, but excessive light can be harmful. Plants protect themselves by converting surplus energy in their chloroplasts into heat for dissipation. PhD student Sanchali Nanda helped validate a novel instrument that monitors the stress levels of plants and used it to gain new insights on their energy dissipation mechanisms.
- You did your PhD in Stefan Jansson’s research group at Umeå Plant Science Centre. Coming from India, how was it for you to work and live in Northern Sweden?
Sanchali Nanda: I enjoy challenges, and this was a fun challenge. On my first night in Umeå, seeing the greenhouse lights glowing was an enlightening moment - I was thrilled from the start. I arrived in August when it was still bright and sunny, but everything was new: the environment, the people and the culture. I experienced my first snowfall, skied for the first time, and even learned to cycle through winter. Coming from a rather pampered background as an Indian student, I had to become more self-reliant, managing things independently and taking responsibility not just professionally but also personally. I think these are the core lessons that I carry from my five and a half years of experience in Umeå.
- You studied how plants protect themselves from too much light. Why is this important?
Sanchali Nanda: Photosynthesis is the most important process on Earth, driving oxygen evolution, food production and sustaining life - yet it remains not fully understood. While its core fundamentals are known, much more is at play. Light powers photosynthesis, but plants cannot use all of it. Once saturated, excess energy can lead to radical formation, oxidative stress and damage to the photosynthetic machinery. Such damage is costly, so plants have evolved mechanisms to dissipate surplus energy and prevent such inadvertent effects.
- What kind of mechanisms have plants developed to protect themselves?
Sanchali Nanda: Various mechanisms regulate light energy. My PhD research focused on non-photochemical quenching processes, which are photoprotective mechanisms whose molecular characterization began in the 1990s. One highly debated mechanism in the field is direct energy transfer between the two photosystems in the thylakoid membrane inside the chloroplast, commonly referred to as spillover. First described in the literature in 1976, this phenomenon is now supported by evidence from our experiments.
During photosynthesis, light is absorbed by the light-harvesting complex, which consists of proteins, chlorophylls and other pigments within the chloroplasts of plant cells. This energy is then transferred via several steps to fuel carbon fixation and sugar synthesis. When this pathway becomes saturated, excess energy can be released either as light of different wavelengths or as heat through non-photochemical quenching. There are different processes, some slower and some faster ones, involving different molecular players. The aim of my thesis was to propose a more holistic approach to describing non-photochemical quenching by integrating data from fluorescence spectra obtained from in vivo samples at physiological temperature for the first time.
- When summarising your thesis, what do you consider as the major outcome?
Sanchali Nanda: I used a novel chlorophyll fluorescence measuring technique with an instrument called ChloroSpec, which is currently exclusively available in Umeå. This instrument measures the fluorescence emitted by chlorophyll as a proxy for estimating the intensity of non-photochemical quenching processes. Since fluorescence and non-photochemical quenching are competitive processes, greater energy dissipation as heat results in reduced fluorescence emission. However, non-photochemical quenching not only affects the overall fluorescence intensity but also alters the spectral properties of the emitted light, depending on the quenching process involved.
The ChloroSpec instrument combines two functions in one: it measures fluorescence at key wavelengths with high temporal resolution while simultaneously detecting the full light spectrum. Its greatest advantage is the ability to perform these measurements on living plants, in real time and at physiological temperatures. To validate the instrument, we compared our measurements with fluorescence parameters described in the literature. I was heavily involved in troubleshooting, testing the software and contributing to its further development. This required significant effort but progressed surprisingly well. Now, we can characterise multiple fluorescence parameters with a single measurement in living plants, enabling a more holistic assessment of plant stress. We hope that many other researchers will benefit from this advancement.
Using ChloroSpec, we re-examined the roles of two key molecules, in non-photochemical quenching: PsbS, a protein that is part of photosystem II, and zeaxanthin, a yellow pigment that is part of the light-harvesting complex. Our findings revealed that PsbS facilitates quenching in the light-harvesting complex, while zeaxanthin regulates energy transfer between the two photosystems, working together to protect plants from light damage. Electron microscopy also showed how non-photochemical quenching causes structural changes in plant cells, providing a clearer understanding of how plants adapt to light stress. We expanded our model by testing hybrid aspen mutants, validating our results. Overall, our work offers new insights into the dynamic, time-dependent nature of non-photochemical quenching, extending beyond previous research focused on steady-state conditions.
- Were there any results that you did not expect or that were astonishing you?
Sanchali Nanda: What surprised me the most were the results we obtained when comparing naturally occurring aspen varieties from different latitudes in Sweden. While we theoretically expected variation, seeing up to two-fold differences in non-photochemical quenching and plant height was truly astonishing. This finding raised exciting questions about the genetic basis of this variation, and we are now exploring whether these differences are linked to specific genetic variations, though we don't have those results yet.
- Doing a PhD is often very challenging. Would you like to share some of the challenges that you had to overcome?
Sanchali Nanda: Ultimately, everything turned out positively, but I’d be remiss not to acknowledge the challenges that came along the way, both personally and professionally. I had to switch supervisors, a difficult decision, but the department was incredibly supportive, for which I am deeply grateful. I am fortunate to have found mentors who believed in me and understood my journey at a crucial time. Of course, there were also the inevitable challenges of failed experiments, modifying plans and adjusting projects. Flexibility and continuous evaluation of my work were key. Throughout this process, I discovered more about myself - my strengths and limitations - and learned how to cope with setbacks, how to talk about them openly, and how to take responsibility. It feels like a mental metamorphosis, like transitioning from unquenched fluorescence to regulated non-photochemical quenching - learning to channel energy efficiently under pressure. Now, as I near the end of my PhD, I am exhausted but content with the progress I’ve made, and I have no regrets.
- What are your plans now?
Sanchali Nanda: I am currently seeking my next challenge - a postdoc abroad. I am actively applying and have connected with researchers interested in my profile. I have identified a couple of labs that I would like to work with and am exploring ways to make this possible. In the coming months, I will remain at UPSC to wrap up some unfinished work. Two of my papers are still in progress: one has already been submitted, while the other awaits further analysis of data. I plan to take a break after my defence before diving back in to finish my final chapter in Umeå.
About the public defence:
Sanchali Nanda, Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, had defended her PhD thesis on Thursday, 6th of March 2025. Faculty opponent was Matt Johnson, Professor at the University of Sheffield, United Kingdom. The thesis was supervised by Stefan Jansson.
Title of the thesis: New light on photoprotection: Spectral resolution of non-photochemical quenching
Link to Sanchali Nanda’s PhD thesis
For more information, please contact:
Sanchali Nanda
Umeå Plant Science Centre
Department of Plant Physiology
Umeå University
Email: