Plant Science Leaders - Mark Lefsrud, McGill University

Dr. Mark Lefsrud has been working on how to grow plants in Controlled Environment Agriculture (CEA) for his entire career. He obtained a B.Sc. and M.Sc. in Agricultural and Bioresource Engineering after which he worked developing plant growth systems and then decided to come over to the ‘other side’ and obtained a Ph.D. in plant physiology. Since he was a teenager, his dream was to grow plants in space, specifically the moon. He read a story in Popular Science that said plants can indeed be grown in space, it was called the ‘Salad Machine Project’. His ultimate goal since then became to figure out the best way to grow plants, and he has been working on that ever since in industry, at NASA, and now as a Professor at McGill University in Montreal.

Dr. Lefsrud runs a large laboratory known as The Biomass Production Lab. The current research is focused on LED characterization, plant growth responses, vertical farming, biocompatible concrete among others. Overall, his research program involves the development of bioprocesses, improvements to plant growth in CEA and energy use. Dr. Lefsrud emphasizes that much of his research would not be possible without the contributions of Bo-Sen Wu and the talented researchers, students, and staff of the Biomass Production Laboratory, whose collective efforts have helped advance the lab's work in controlled environment agriculture, plant physiology, and lighting research.

What inspired you to revisit the McCree Curve?

When I was a graduate student in plant physiology, I saw that the maximum photosynthetic potential of the McCree curves was at 595-600 nm and in this classic paper he suggested that there was an inverse relationship between chlorophyll concentration and photosynthetic rates. This interested me as there are no known primary plant pigments that absorb light between 595-620 nm (amber). We have however observed that there is light absorption in the amber region by degraded primary pigments which seem to play a role in excess energy dissipation. Light is energy but it is also damaging and plants are dynamic systems constantly balancing light absorption and energy use in photosynthesis at the molecular level. The paradox of the amber gap in light absorption and higher photosynthetic rates under amber light in the McCree curve is what led me to investigate this further.

What was the most interesting result from this updated McCree curve?

As we hypothesized, the peaks of photosynthesis did not correlate to pigment absorption peaks but rather to the lack of pigments. The net photosynthetic rates in tomato and lettuce plants responded similarly to wavelengths between 425-600 nm. Compared to tomato, lettuce had higher photosynthetic rates from 400-425nm and lower rates from 600-680 nm. Relative quantum yields responded to spectrum a little differently in the two species. Tomato had many small peaks and valleys across 425-680 nm while lettuce had peaks at 423 nm and 575 nm with low quantum yields between 450-550 nm and 650- 680 nm. We think that the differences we observe between lettuce and tomato is due to lower chlorophyll and carotenoid concentrations, higher succulence and their energy dissipation capacity in lettuce plants. On the other hand, tomato has narrower leaves which allows for better gas exchange and the pigment concentration is high to regulate energy imbalances in the plant cells.

Technically how did you measure action spectra with such precision?

It took us five years to develop this system. The main challenges were finding the large array of LEDs that we wanted to use, an appropriate cooling system and the ability to get a high enough photon flux density. McCree measured photosynthesis every 15-30 nm with Full Width Half Maxima (FWHM) at between 150-200 nm. We wanted to measure the action spectra more precisely under more discrete and narrow wavelengths and were able to bring the FWHM down to 10 nm with a resolution of 1 nm. The McCree curve was thus reinterpreted by measuring photosynthesis under distinct wavelengths that were buried under McCree’s broader spectra.

From your experience growing plants in CEA and from these data do you think tunable lights would be beneficial?

It seems to be going in that direction. Plants are adaptive, there seems to be evidence that adjusting the spectra can be beneficial for yields and energy use. As I mentioned above, light is harmful and detrimental. Looking at light from that direction, where the plants put up barriers to prevent different wavelengths from entering the leaf, a tunable spectrum could be beneficial. If we can figure out what the plant is reacting to, we would be able to regulate the defense mechanisms to increase yields and potentially reduce energy use. With this in mind, I say yes.

Reference to Dr. Lefsrud’s article updating the McCree curve for tomato and lettuce in the link below: