How can conifers used, for example, as Christmas trees keep green needles during the boreal winter when most trees 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, has deciphered that a shortcut in photosynthetic machines allows pine needles to stay green. The study was published in the journal Nature Communications.
In winter, green chlorophyll molecules absorb light energy, but it cannot be used by downstream reactions in photosynthetic machines, because freezing temperatures stop most biochemical reactions.
This is especially a problem in early spring when temperatures can still be very low, but sunlight is already strong, and excess light energy can damage the proteins of photosynthetic machinery. Researchers have shown that the photosynthetic device is wired in a special way that allows 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 kept separate to prevent shortcuts and allow efficient photosynthesis.
In winter, the structure of the thylakoid membrane, in which the two photosystems are located, reorganizes, bringing the two physical systems into physical contact. Researchers have shown that Photosystem II donates energy directly to Photosystem I, and this shortcut method protects the green chlorophyll and needles when conditions become harsh.
“We tracked several pine trees growing in Umeå in northern Sweden over three seasons,” says Pushan Bag, a doctoral student at Umeå University, who collected samples throughout the year and did many analyzes.
“It was necessary to be able to work on the needles” directly from the outside “to prevent them from adapting to higher temperatures in the laboratory environment before we analyzed them, for example by electron microscopy to visualize the structure of the thylakoid membrane.”
All plants have safety valves to suppress excess light energy that is wasted as heat or as fluorescent light. However, it seems that only conifers have such powerful valves that they can keep the photosynthetic apparatus intact during the extreme boreal winter.
The research team combined biochemistry and ultrafast fluorescence analysis, a very sophisticated method that can solve the fluorescent light of chlorophyll in a picosecond time scale.
Thus, they could show how pine needles cope with excess light energy to protect their sensitive photosynthetic apparatus from damage.
“We had to adjust the equipment for studying pine needles in cold temperatures in order to capture the unique mechanism,” explains Volha Chukhutsina from Vrije Universiteit Amsterdam, who performed most of the ultrafast fluorescence analysis. “We also tried spruce needles, but it was hard to fit them well into the equipment.”
Alfred Holzwarth, who developed time-resolved fluorescence measurements, adds: “Pine needles have given us the opportunity to study this shortcut mechanism – also called overflow – because they really show extreme adaptation.”
The study was done on pine trees, but the researchers believe that the mechanism is probably similar to other species of conifers – such as typical spruces and firs – because their photosynthetic apparatus is similar.
This remarkable adaptation not only enjoys us at Christmas time, but is actually extremely important to humanity. Could the conifers not survive in an extremely harsh winter climate, large areas in the northern hemisphere may not have been colonized because the conifers provided firewood, shelter and other necessities? Even today, they form the basis of the economy in most of the circumpolar region of the taiga. “
Stefan Jansson, professor at the University of Umeå
Bag, P., and others. (2020) Direct energy transfer from photosystem II to photosystem I provides sustainability in winter in Scots pine. Nature Communications. doi.org/10.1038/s41467-020-20137-9.