The photosynthetic shortcut ensures that the Christmas trees stay green in the winter

Pine tree in winter. Credit: Assembly of Stefan Jansson and Pushan Bag

For example, how can a coniferous tree used as a Christmas tree keep its green needles in the winter when most of the conifers shed their leaves? Science has not given a good answer to this question, but now an international team of scientists, including researchers from Umeå University, says that the short path in the photosynthetic machine allows the needles of pine trees to stay green. The study was published in the journal Nature Communication.

In winter, light energy is absorbed by green chlorophyll molecules, but low reactions in the photosynthetic apparatus cannot be used because freezing temperatures stop most biochemical reactions. This is a problem, especially in early spring, when temperatures can still be very low, but sunlight is already strong and excess light energy can damage photosynthetic machine proteins. Researchers have shown that the photosynthetic device is wired in a special way that allows the pine needles to stay green all year round.

Under normal conditions, two photosystems, two functional units in which light energy is absorbed and converted into chemical energy, are separated from each other and prevent a short path and allow efficient photosynthesis. In winter, the structure of the thyrocoid membrane, where the two photosystems are located, is reconstructed, which brings the two photosystems into physical contact. Researchers have shown that Photosystem II provides energy directly to the energy system, and that this shortcut protects green chlorophyll and needles when conditions harden.

“Over the course of three years, we observed several pine trees growing in Umea in northern Sweden,” said Pushan Bag, a doctoral student at Umeå University, who collected samples throughout the year and conducted many analyzes. “Before analyzing the needles with the electron microscope we used to visualize the structure of the thyroid membrane, for example, it was important to work ‘outdoors’ to prevent them from adapting to high temperatures in the laboratory environment.”

All plants have safety valves to combat excess light energy emitted, such as heat or fluorescent light. However, only conifers have strong enough valves to keep the photosynthetic apparatus intact in extreme boreal winters. The research team combined biochemistry and ultrafast fluorescent analysis, which is a very advanced method that can resolve chlorophyll fluorescent light in picosane time. In this way, they can demonstrate how candle needles are already dealing with light energy to protect their sensitive photosynthetic apparatus from damage.

“To trap the rare mechanism, we had to adjust the devices to study the pine needles in the cold heat,” explains Volha Chukhutsina of Amsterdam, Vrije University, which does most of the ultrafast fluorescent analysis. “We also tried spruce needles, but it was difficult for them to adapt well to the device.”

Alfred Holzwarth, who developed fluorescent measurements over time, adds: “Because pine needles have shown this extreme adaptation, they have given us the opportunity to study this short mechanism, also known as shedding.”

The study was conducted with pine trees, but researchers believe that the mechanism is similar to that of other conifers, such as the typical Christmas trees spruce and fir.

“This remarkable adaptation not only makes us happy at Christmas, in fact it is extremely important for humanity,” says Stephen Jansson, a professor at Umeå University. “Accidental conifers have survived harsh winters, and large areas of the northern hemisphere may not have been colonized because conifers provide firewood, housing and other necessities. They still form the basis of the economy in most of the circular taiga region today. ”

Reference: “Direct energy transfer from Photosystem II to Photosystem ensures winter sustainability in Scottish Pine” Pushan Bag, Volha Chukhutsina, Zishan Zhang, Suman Paul, Alexander G. Ivanov, Tatyana Shutova, Roberta Croce, Alfred R. Holzwarth and Stefan Jansson, 15 December 2020, Nature Communication.
DOI: 10.1038 / s41467-020-20137-9

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